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Andres, Magdalena et al.: Denmark Strait Overflow Water observed at Line W: its evolution and mixing via deep cyclones.  Poster   View abstract

 
Denmark Strait Overflow Water observed at Line W: its evolution and mixing via deep cyclones
Magdalena Andres1*, John M. Toole1, Daniel J. Torres1, William M. Jr. Smethie2

* Presenting author

1) Woods Hole Oceanographic Institution, Physical Oceanography, USA
2) Columbia University, Lamont-Doherty Earth Observatory, USA

Shipboard velocity and property data from 18 transects across the North Atlantic Deep Western Boundary Current (DWBC) near 40˚N are analyzed to study the evolution of the Denmark Strait Overflow Water (DSOW) component of the DWBC and mixing between the DWBC and the interior. The transects were made between 1994 and 2014 and lie along Line W, which reaches from the continental shelf south of New England to Bermuda coincident with a Jason-2 satellite track. The shipboard data include measurements of velocity from lowered acoustic Doppler current profilers (LADCPs), CTDO profiles, and trace gas chlorofluorocarbon concentrations from bottle samples at discrete depths taken on the CTDO Rosette casts at 26 regular stations or a subset of these stations. In each transect, DSOW exhibits a distinct chlorofluorocarbon concentration maximum in the abyssal ocean (> 3000 m depth) along the sloped western boundary. Examination of the sea surface height (SSH) maps from satellite altimetry indicates that quasi-stationary meander troughs of the Gulf Stream path in the upper ocean were present at Line W during 5 of the 18 sections. For these 5 sections the LADCP velocity sections suggest the upper ocean trough is accompanied by a large cyclone in the deep ocean in the DSOW density layer. The occurrence of deep cyclones in conjunction with Gulf Stream troughs as inferred from the LADCP sections along Line W is consistent with previous observations (from 1988 to 1990) in the region from a moored array in the Synoptic Ocean Prediction (SYNOP) experiment. The SYNOP array suggested deep cyclones are present here about 35% of the time. The composite velocity section produced from the 5 Line W transects sampling through a Gulf Stream trough suggests that a typical cyclone reaches swirl speeds of greater than 30 cm/s at 3400 m depth and has a radius (distance between the center and the maximum velocity) of ~75 km. The tracer data suggest that these cyclones affect not only the deep velocity structure along Line W, but also provide a mechanism for water exchange between the DWBC and the interior.

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Bell, Michael J.: Meridional overturning circulations driven by surface wind and buoyancy forcing.  Poster   View abstract

 
Meridional overturning circulations driven by surface wind and buoyancy forcing
Michael J. Bell1*

* Presenting author

1) Met Office, Climate Science, UK

A conceptual picture of the Meridional Overturning Circulation (MOC) has been developed using 2- and 3-layer models governed by the planetary geostrophic equations and simple global geometries. The picture has four main elements. First cold water driven to the surface in the South Atlantic north of Drake passage by Ekman upwelling is transformed into warmer water by surface heating. Second the model’s boundary conditions constrain the depths of the isopycnal layers to be almost flat along the eastern boundaries of the ocean. This results in warm water reaching high latitudes in the northern hemisphere where it is transformed into cold water by surface heat loss (the third element). The final element of the picture is the assumption that western boundary currents are able to close the circulations. The results from a set of numerical experiments for the upwelling limb in the Southern Hemisphere are summarised in a simple conceptual schematic. Analytical solutions have been found for the down-welling limb assuming the wind stress in the Northern Hemisphere is negligible. The expression for the depth of the isopycnal interface on the eastern boundary obtained by combining these solutions in a 2-layer model is similar to that obtained for the depth of the pycnocline by Gnandesikan (1999).

Bell, M. J. 2014 Water mass transformations driven by Ekman upwelling and surface warming in sub-polar gyres. Submitted to J. Phys. Oceanogr.

Bell, M. J. 2014 Meridional overturning circulations driven by surface wind and buoyancy forcing. Submitted to J. Phys. Oceanogr.

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Blaker, Adam et al.: Historical analogues to the recently observed minima in the Atlantic meridional overturning circulation.  Slides   View abstract

 
Historical analogues to the recently observed minima in the Atlantic meridional overturning circulation
Adam Blaker1*, Joel Hirschi1, Gerard McCarthy1, Bablu Sinha1, Sarah Taws2, Robert Marsh2, Andrew Coward1, Beverly de Cuevas1

* Presenting author

1) NOC, Marine Systems Modelling, UK
2) University of Southampton, Ocean Earth Sciences, UK

Observations of the Atlantic meridional overturning circulation (AMOC) by the RAPID 26°N array show a pronounced minimum in the northward transport over the winter of 2009/10, substantially lower than any observed since the initial deployment in April 2004. It was followed by a second minimum in the winter of 2010/2011. We demonstrate that ocean models forced with observed surface fluxes reproduce the observed minima. Examining output from five ocean model simulations we identify several historical events which exhibit similar characteristics to those observed in the winter of 2009/10, including instances of individual events, and two clear examples of pairs of events which happened in consecutive years, one in 1969/70 and another in 1978/79. In all cases the absolute minimum, associated with a short, sharp reduction in the Ekman component, occurs in winter. AMOC anomalies are coherent between the Equator and 50°N and in some cases propagation attributable to the poleward movement of the anomaly in the wind field is observed. We also observe a low frequency (decadal) mode of variability in the anomalies, associated with the North Atlantic Oscillation (NAO). Where pairs of events have occurred in consecutive years we find that atmospheric conditions during the first winter correspond to a strongly negative Arctic Oscillation (AO) index. Atmospheric conditions during the second winter are indicative of a more regional negative NAO phase, and we suggest that this persistence is linked to re-emergence of sea surface temperature anomalies in the North Atlantic for the events of 1969/70 and 2009/10. The events of 1978/79 do not exhibit re- emergence, indicating that the atmospheric memory for this pair of events originates elsewhere. Observation of AO patterns associated with cold winters over northwest Europe may be indicative for the occurrence of a second extreme winter over northwest Europe.

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Born, Andreas et al.: Multiple equilibria as a possible mechanism for decadal variability in the North Atlantic Ocean.  Poster   View abstract

 
Multiple equilibria as a possible mechanism for decadal variability in the North Atlantic Ocean
Andreas Born1,2*, Juliette Mignot1,2,3, Thomas Stocker1,2

* Presenting author

1) Climate and Environmental Physics, Physics Institute, University of Bern
2) Oeschger Center for Climate Change Research, University of Bern, Switzerland
3) LOCEAN-IPSL, UPMC-CNRS-IRD-MNHN, France

Decadal climate variability in the North Atlantic has received increased attention in recent years, because modeling results suggest predictability of several years. However, the applicability of these results in the real world is challenged by an incomplete understanding of the underlying mechanisms. Here, we show that recent attempts to reconstruct the decadal variations in one of the dominant circulation systems of the region, the subpolar gyre (SPG) are not always consistent. A coherent picture is partly recovered by a simple conceptual model solely forced by reanalyzed surface air temperatures. This confirms that surface heat flux indeed plays a leading role for this type of variability as has been suggested in previous studies. Performance of this conceptual model is tested against a statistical stochastic model. Results suggests that large variations in the SPG correspond to the crossing of a bifurcation point that is predicted from idealized experiments and an analytical solution of our model. Hysteresis and the existence of two stable modes of the SPG circulation shape its response to forcing by atmospheric temperatures. The identification of the essential dynamics and the reduction to a minimal model of SPG variability provides a quantifiable basis and a framework for future studies on decadal climate variability and predictability.

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Childers, Katelin et al.: Directly Measured Currents and Estimated Transport Pathways of Atlantic Water between 59.5N and the Iceland-Faroes-Scotland Ridge.  Poster   View abstract

 
Directly Measured Currents and Estimated Transport Pathways of Atlantic Water between 59.5N and the Iceland-Faroes-Scotland Ridge
Katelin Childers1*, Charles Flagg1, Thomas Rossby2, Corinna Schrum3,4

* Presenting author

1) Stony Brook University, School of Marine and Atmospheric Science, USA
2) University of Rhode Island, Graduate School of Oceanography, USA
3) University of Bergen, Geophysical Institute, Norway
4) Bjerknes Center for Climate Research, Norway

Using vessel-mounted acoustic Doppler current profiler (ADCP) data from four different routes between Scotland, Iceland and Greenland, we map out the mean flow of water in the top 400 m of the northeastern North Atlantic. The poleward transport east of the Reykjanes Ridge decreases from ~8.5 to 10 Sv (1 Sverdrup = 10^6 m3/s) at 59.5 to 61°N to 6 Sv crossing the Iceland-Faroe-Scotland Ridge (IFSR). The two longest ~1200 km transport integrals have ~1.4 to 0.94 Sv uncertainty, respectively. The overall decrease in transport can in large measure be accounted for by a ~1.5 Sv flow across the Reykjanes Ridge into the Irminger Sea north of 59.5°N and by a ~0.5 Sv overflow of dense water along the Iceland Faroe Ridge. A remaining 0.5 Sv flux divergence is at the edge of detectability, but if real could be accounted for through wintertime convection to greater than 400 m and densification of upper ocean water.

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Condron, Alan: High resolution ocean model simulations with interactive icebergs to estimate AMOC sensitivity to increased Greenland ice sheet melt.  Poster   View abstract

 
High resolution ocean model simulations with interactive icebergs to estimate AMOC sensitivity to increased Greenland ice sheet melt
Alan Condron1*

* Presenting author

1) University of Massachusetts Amherst, Geoscience, USA

Results from several high-resolution model simulations assessing the impact of increased Greenland ice sheet (GrIS) melt on AMOC strength will be presented. A global ocean model (MITgcm) is integrated at an eddy permitting (1/6 deg.; 18-km) resolution and coupled to a new dynamic-thermodynamic iceberg model (MITberg) to more accurately simulate ocean-cryosphere interactions. MITberg uses a multilevel keel model to realistically simulate ocean drag below the waterline, solves all relevant melt terms, and includes a parameterization to simulate the fracturing of overhanging ice caused by wave erosion. The model was spun-up for 25 years with 405 Gt/yr (~0.013 Sv) of ice calved from 33 major ice streams along the edge of the GrIS. At any one time ~6700±430 icebergs are present in the North Atlantic and are mainly confined to narrow coastal boundary currents (Labrador, EGC, WGC). In the Control simulation, an average of 458 icebergs drift south of 48°N each year in the Labrador Current with peak fluxes occurring in late-spring/early summer. These results are in remarkable agreement (r=0.5) with historical observations (annual mean 474) collected by the International Ice Patrol at this latitude since 1900. Ocean circulation changes resulting from increased GrIS freshwater forcing are assessed by instantaneously increasing GrIS discharge to 3160 Gt/yr (~0.1 sv) (Exp.1) and linearly increasing ice discharge by 27.5 Gt/yr^2 to reproduce several recently observed increases in GrIS melt (Exp.2). In Exp.1 there is a 10-fold increase in the total number of icebergs in the North Atlantic (~50,000-60,000 per yr) and a ~8-fold increase (up to ~3700) in the number of icebergs drifting south of 48N each year. An increase in iceberg activity of this magnitude would present a considerable hazard to commercial shipping and oil platforms operating near the Grand Banks of Newfoundland and in the northwest Atlantic. After 10 years the Labrador Sea is ~1.5 psu fresher and the eastern subpolar North Atlantic 0.3 psu fresher, while AMOC shows a 3.5Sv (18.6%) reduction. Results from a longer (30-year) integration of Exp.1, as well as Exp.2, will be presented at the meeting.

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Danabasoglu, Gokhan et al.: North Atlantic Simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II): Inter-Annual to Decadal Variability and Trends.  Slides   View abstract

 
North Atlantic Simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II): Inter-Annual to Decadal Variability and Trends
Gokhan Danabasoglu1*, Stephen G. Yeager1, Who M. Kim2

* Presenting author

1) National Center for Atmospheric Research, USA
2) Texas A&M University, USA

Simulated inter-annual to decadal variability and trends in the North Atlantic for the 1958-2007 period from twenty global ocean – sea-ice coupled models are presented. These simulations are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The present study represents a continuation of our previous work which documented the mean states in the North Atlantic from the same models. A major focus here is the representation of Atlantic meridional overturning circulation (AMOC) variability and trends in the participating models. Relationships between AMOC variability and those of some other related variables, such as subpolar mixed layer depths, the North Atlantic Oscillation (NAO), and the Labrador Sea upper-ocean hydrographic properties, are also investigated. In general, AMOC variability shows three distinct stages. During the first stage that lasts until mid- to late-1970s, AMOC remains lower than its long-term (1958-2007) mean. Thereafter, AMOC intensifies with maximum transports achieved in mid- to late-1990s. This enhancement is then followed by a weakening trend until the end of our integration period. This sequence of low frequency AMOC variability is consistent with previous studies. Regarding strengthening of AMOC between about mid-1970s and mid-1990s, our results support a previously identified variability mechanism where AMOC intensification is connected to increased deep water formation in the subpolar North Atlantic, driven by NAO-related surface fluxes. The simulations tend to show general agreement in their representations of, for example, AMOC, sea surface temperature (SST), and subpolar mixed layer depth variabilities. In particular, the observed variability of the North Atlantic SSTs is captured well by all models. These findings indicate that simulated variability and trends are primarily dictated by the atmospheric data sets which include the effects of ocean dynamics. Despite these general agreements, there are many differences among the model solutions, particularly in the spatial structures of variability patterns. For example, the location of the maximum AMOC variability differs among the models between Northern and Southern Hemispheres.

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Delworth, Thomas L. et al.: The impact of the North Atlantic Oscillation on 20th century climate through its influence on the Atlantic Meridional Overturning Circulation.  Slides   View abstract

 
The impact of the North Atlantic Oscillation on 20th century climate through its influence on the Atlantic Meridional Overturning Circulation
Thomas L. Delworth1*, Fanrong Zeng1, Liping Zhang1,2

* Presenting author

1) Geophysical Fluid Dynamics Laboratory/NOAA, USA
2) Princeton University, USA

The impact of the North Atlantic Oscillation (NAO) on the Atlantic Meridional Overturning Circulation (AMOC) and large-scale climate is assessed using simulations with three different climate models. Perturbation experiments are conducted in which patterns of anomalous fluxes corresponding to the NAO are added to the model ocean; in companion experiments no such fluxes are added. Differences between the experiments illustrate how the model ocean and climate system respond to the NAO. A positive phase of the NAO tends to strengthen the AMOC by extracting heat from the subpolar gyre, thereby increasing deepwater formation, horizontal density gradients, and the AMOC.

The flux forcings have the spatial structure of the observed NAO, but the amplitude of the forcing varies in time. The temporal variation of the imposed fluxes is one of the following types: (a) sudden switch on of the flux forcing, (b) vary the amplitude of the flux forcing sinusoidally in time with distinct periods varying from 2 to 200 years, (c) vary the flux forcing to match the observed time sequence of the NAO over the 20th and early 21st centuries. The first two types of experiments are idealized, while the third attempts an assessment of the impact of NAO-induced AMOC anomalies in the observed record. In the idealized experiments we show that the response of the AMOC to NAO variations is small at short time scales, but increases up to the dominant time scale of internal AMOC variability (20-30 years for the models used). The amplitude of the response of the AMOC, and associated oceanic heat transport, is approximately constant as the timescale of the forcing is increased further. In contrast, the response of other properties, such as hemispheric surface air temperature or Arctic sea ice, continues to increase as the time scale of the forcing becomes progressively longer. The larger response of temperature and sea ice is associated with an increased impact of radiative feedback processes at progressively longer time scales. The impact of the NAO on the AMOC and climate is a function of the dominant timescale of internal AMOC variability, as well as the background mean state. In the experiments using the observed sequence of the NAO we estimate the contribution of NAO-induced AMOC anomalies to hemispheric temperature variations in the 20th and early 21st centuries. We show that NAO-induced AMOC variations contributed substantially to multidecadal warming and cooling of the Northern Hemisphere, including cooling from the 1960s through the 1980s, and warming from the 1980s through the 2000s.

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Doddridge, Edward W. et al.: Meridional propagation of meridional volume transport anomalies in an idealised model.  Poster   View abstract

 
Meridional propagation of meridional volume transport anomalies in an idealised model
Edward W. Doddridge1*, David P. Marshall1

* Presenting author

1) University of Oxford, Physics, UK

An idealised reduced-gravity model is used to investigate the meridional coherence of meridional overturning circulation anomalies in the Atlantic ocean. The model is run at both eddy-resolving and non-eddying resolutions, the latter comparable with ocean circulation models currently used for climate prediction. Our idealized model produced meridionally coherent volume transport anomalies. A time-lagged autocorrelation of meridional volume transport anomalies shows southwards propagation at all latitudes.

Anomalies in the western boundary current of the subpolar gyre propagate advectively, but are compensated by changes in the gyre circulation. The zonally integrated transport propagates at the speed of a boundary wave, showing that previous idealised results are robust in the presence of a mean flow.

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Eriksen, Charles C.: Progress toward AMOC Estimation using Deepgliders.  Slides   View abstract

 
Progress toward AMOC Estimation using Deepgliders
Charles C. Eriksen1*

* Presenting author

1) University of Washington, School of Oceanography, USA

In preparation for use in estimating the Atlantic Meridional Overturning Circulation (AMOC), long-range autonomous underwater Deepgliders are being used to continuously survey the full open deep ocean water column near the Bermuda Atlantic Time Series (BATS) site 90 km southeast of Bermuda through winter and spring 2014 & 2015.

Deepgliders have carried out 3-month missions in the vicinity of the BATS site from March to June 2014 and again from January through present, 2015. These missions surveyed an 80 km corner-to-corner bow-tie pattern centered on the BATS site, repeating the pattern fortnightly. So far, more than 100 dive cycles to depths between 4000 and 4850 m in the repeat survey have been carried out. These have produced more than 200 profiles of temperature, salinity, and dissolved oxygen at 0.5-3 m resolution from the top meter of the water column to within 30 m of the ocean bottom, all transmitted in near-real time via satellite telemetry. Collocated monthly shipboard CTD casts have been carried out simultaneously with Deepglider dives to evaluate Deepglider data quality. The current mission is using battery energy at a rate that implies endurance of more than one year.

In addition to gathering hydrographic sections, Deepgliders estimate barotropic (full ocean depth averaged) current. Such currents have been persistently to the southwest at speeds in the range 0.05-0.10 m/s in the BATS region, except during the passage of submesoscale anticyclones, when they are stronger. The eddies, distinguished by cool, fresh water nearly saturated in dissolved oxygen between ~1000 m and ~2000 m depth, are of presumed subpolar origin. Two such eddies have been observed passing through the BATS region in 6 months of observation so far.

Additional Deepglider deployments at Bermuda are planned to include transects to and from the continental slope offshore the U.S., crossing the western boundary current system (Gulf Stream and Deep Western Boundary Current). A pair of Deepgliders is planned to be deployed offshore Great Abaco Island, Bahamas, as part of the RAPID-MOCHA array. Preliminary results from these missions will be presented at the meeting, should they have become available.

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Garzoli, Silvia L. et al.: Monitoring the MOC in the South Atlantic: A 'SAMOC Initiative' update.  Poster   View abstract

 
Monitoring the MOC in the South Atlantic: A 'SAMOC Initiative' update
Silvia L. Garzoli1,2*, Alberto Piola3, Sabrina Speich4, Edmo Campos5, Mike Roberts6, Renellys C. Perez1,2, Thierry Terre7, Christopher S. Meinen2

* Presenting author

1) University of Miami/Cooperative Institute for Marine and Atmospheric Studies, USA
2) NOAA/Atlantic Oceanographic and Meteorological Laboratory, USA
3) Departamento de Oceanografía, Servicio de Hidrografía Naval, and Departamento de Ciencias de la Atmósfera y los Océanos
4) Laboratoire de Météorologie Dynamique, École Normale Supérieure,
5) Oceanographic Institute, University of São Paulo,
6) Oceans and Coasts Research, Department of Environmental Affairs,
7) Laboratoire de Physique des Océans, Ifremer,

Variations in the Meridional Overturning Circulation (MOC) are known to have global implications to the climate system, however until recently most MOC observing programs have been focused in the North Atlantic. Recent model and data analyses have suggested that critical water mass changes to the upper and lower limbs of the MOC occur in the South Atlantic, and only limited latitudinal coherence has been found to date between the MOC observations made by the North Atlantic observing systems at different latitudes. As a result, a priority for the USAMOC Science Team has been the establishment of a MOC observing system in the South Atlantic, and recently the International CLIVAR panel endorsed a South Atlantic MOC (“SAMOC”) Initiative to both strengthen existing programs seeking to study the MOC in the South Atlantic and to encourage further expansion of the MOC observing system in the region. This presentation will summarize the present status of the international SAMOC observing system and present recent observation and modeling results developed through coordination by the international SAMOC Initiative.

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Gastineau, Guillaume: Atmospheric influence of the Atlantic Meridional Overturning Circulation.  Slides   View abstract

 
Atmospheric influence of the Atlantic Meridional Overturning Circulation
Guillaume Gastineau1*

* Presenting author

1) Sorbonne Universités, UPMC/CNRS/IRD/MNHN, LOCEAN/IPSL

The North Atlantic Oscillation (NAO) dominates the European and North American climate variability during the cold season, for time scales ranging from 10 days to several decades. At decadal time scale, the SST also has an influence, as a warming in the subpolar Atlantic Ocean leads to a negative NAO in winter. Such SST influence might involve the ocean dynamics, as climate models show that the Altantic meridional overturning circulation (AMOC) is followed by a supolar Atlantic basin warming accompanied by a similar atmospheric response. The atmospheric changes seem to be driven by the diabatic heat flux in the main eddy development region, over the Gulf Stream/North Atlantic current region. But the stratosphere or the tropical Atlantic SST forcing may also play an important role.
To further establish the causality links between the ocean and the atmosphere, ensembles of atmosphere-only simulations are designed. The atmospheric model simulations use prescribed SST and sea-ice anomalies that follow an intensification of the AMOC in the coupled model IPSL-CM5A-LR. We confirm that the main influence is due to warm subpolar Atlantic SST anomalies north of 30°N and the associated upward heat flux which are responsible for a decrease of the lower-tropospheric baroclinicity in the region of maximum eddy growth. But we also found that the positive sea-ice anomalies over the Arctic associated with a larger AMOC further amplify the atmospheric circulation anomalies, as they act to warm the stratosphere one month before the tropospheric circulation anomalies.

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Hermanson, Leon et al.: The role of the AMOC in multi-annual predictions of the Atlantic subpolar gyre in Met Office models.  Slides   View abstract

 
The role of the AMOC in multi-annual predictions of the Atlantic subpolar gyre in Met Office models
Leon Hermanson1*, Nick Dunstone1, Rosie Eade1, Niall Robinson1, Adam Scaife1, Doug Smith1

* Presenting author

1) Met Office, Hadley Centre, UK

Decadal variability in the North Atlantic and its subpolar gyre (SPG) has been shown to be predictable in climate models initialized with the concurrent ocean state. Numerous impacts over ocean and land have also been identified. We use three versions of the Met Office Decadal Prediction System to provide a multimodel ensemble forecast of the Atlantic Meridional Overturning Circulation (AMOC) and related impacts. We present evidence that the ensemble forecast is able to skilfully predict these quantities over recent decades. The recent cooling trend in the SPG is predicted to continue until at least the end of 2017 due to a decrease in the SPG heat convergence related to a slowdown of the AMOC. We also consider the role of the AMOC in predictability of the SPG in a new relatively high resolution decadal prediction system (approximately 0.25 degree in the ocean, approximately 60km in the atmosphere). The overflows over the Greenland-Scotland ridge are too weak in this model leading to deep Labrador Sea waters that are too light and a high-latitude AMOC that is shallower than previous models. Consequently, predictions initialized with observed deep water densities do not predict the evolution of the AMOC as well as in previous versions of the model with smaller biases. This highlights the importance of a realistic model background state and variability for making decadal predictions.

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Hodson, Dan et al.: North Atlantic Cooling: Anatomy, Causes and Impacts.  Poster   View abstract

 
North Atlantic Cooling: Anatomy, Causes and Impacts
Dan Hodson1*, Jon Robson1, Rowan Sutton1

* Presenting author

1) University of Reading, NCAS-Climate, Department of Meteorology

North Atlantic Sea Surface Temperatures underwent a rapid cooling during the 1960s, a period when the SSTs outside the North Atlantic showed a long-term warming trend. Several hypotheses for the origin of this observed cooling exist - changes in anthropogenic aerosol forcing or changes in heat convergence due to changes in the Atlantic Meridional Overturning Circulation, being leading contenders. It is clear that the Atlantic did not cool uniformly over this period. We therefore ask - how did the cooling evolve, and does this reveal anything about the likely causes?

Through analysis of multiple observational datasets, it is demonstrated that the cooling proceeded in several distinct stages. Cool anomalies initially appeared in the mid-1960s in the Nordic Seas and Gulf Stream extension, before spreading to cover most of the subpolar gyre. Subsequently, cool anomalies spread into the tropical North Atlantic before retreating, in the late 1970s, back to the subpolar gyre. There is strong evidence that changes in atmospheric circulation, linked to a southward shift of the Atlantic ITCZ, played an important role in the event, particularly in the period 1972–76. Theories for the cooling event must account for its distinctive space–time evolution. The authors’ analysis suggests that the most likely drivers were 1) the ‘‘Great Salinity Anomaly’’ of the late 1960s; 2) an earlier warming of the subpolar North Atlantic, which may have led to a slowdown in the Atlantic meridional overturning circulation; and 3) an increase in anthropogenic sulphur dioxide emissions.

The climate impacts of such a cooling event cannot be determined from observations alone. Atmosphere models driven by fixed SSTs provide one route to examining the impacts, however such experiments cannot capture any potential atmosphere-ocean coupled feedbacks that could amplify and persist the atmospheric response. Such a coupled response can be examined using an atmosphere model coupled to a mixed layer ocean model. Here we present results from experiments using such a model to explore the regional and global climate impacts of North Atlantic cooling. These experiments have relevance to the current climate system, in view of the evidence of a possible slowdown in the AMOC happening now.

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Holliday, N. Penny et al.: Observations of subpolar North Atlantic variability and overturning circulation from the Extended Ellett Line.  Slides   View abstract

 
Observations of subpolar North Atlantic variability and overturning circulation from the Extended Ellett Line
N. Penny Holliday1*, Stuart A. Cunningham2, Stefan F. Gary2, Clare Johnson2, Matthew P. Humphreys3

* Presenting author

1) National Oceanography Centre, UK
2) Scottish Association for Marine Science, UK
3) University of Southampton, UK

Since 1996 the GO-SHIP section known as the "Extended Ellett Line" has been measuring the properties (temperature, salinity, carbonate chemistry) and overturning circulation between Iceland and Scotland. The section monitors the upper limb of the AMOC in the subpolar North Atlantic (SPNA): the northward flow of warm water into the Nordic Seas and Arctic, and eastern subpolar mode waters that travel cyclonically around the subpolar gyre to the Labrador Sea. Four decades of high quality ship-based measurements in the easternmost basin of the SPNA, the Rockall Trough, enhanced by data in the wider Iceland Basin, reveal long term salinity and carbonate chemistry variability that represents conditions across the SPNA and Nordic Seas. We will describe the observed variability in properties and discuss the uncertainties associated with the measurements. We compute the mean and variance of the overturning circulation at the section and consider the implications for present and future observing networks including OSNAP.

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Jackson, Laura C et al.: Climate impacts of an AMOC shutdown in a high resolution GCM.  Slides   View abstract

 
Climate impacts of an AMOC shutdown in a high resolution GCM
Laura C Jackson1*, Ron Kahana1, Tim Graham1, Mark Ringer1, Tim Woollings2, Jennifer Mecking3, Richard Wood1

* Presenting author

1) Met Office, Hadley Centre, UK
2) University of Oxford, Atmospheric Physics, UK
3) University of Southampton, Ocean and Earth Science, UK

We present the impacts from a hypothetical slowdown in the Atlantic Meridional Overturning Circulation (AMOC) in a state-of-the-art global climate model (HadGEM3), with particular emphasis on Europe. This is the highest resolution coupled global climate model to be used to study the impacts of an AMOC slowdown so far. Many results found are consistent with previous studies and can be considered robust impacts from a large reduction or collapse of the AMOC. These include: widespread cooling throughout the North Atlantic and northern hemisphere in general; less precipitation in the northern hemisphere midlatitudes; large changes in precipitation in the tropics and a strengthening of the North Atlantic storm tracks.

The focus on Europe, aided by the increase in resolution, has revealed previously undiscussed impacts, particularly those associated with changing atmospheric circulation patterns. Summer precipitation decreases (increases) in northern (southern) Europe and is associated with a negative summer North Atlantic Oscillation (NAO) signal. Winter precipitation is also affected by the changing atmospheric circulation, with localised increases in precipitation associated with more winter storms and a strengthened winter storm track. Stronger westerly winds in winter increase the warming maritime effect while weaker westerlies in summer decrease the cooling maritime effect. In the absence of these circulation changes the cooling over Europe's landmass would be even larger in both seasons.

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Jackson, Laura C et al.: Mechanisms of MOC hysteresis in a GCM and the importance of hosing scenarios.  Poster   View abstract

 
Mechanisms of MOC hysteresis in a GCM and the importance of hosing scenarios
Laura C Jackson1*, Robin S Smith2, Richard Wood1

* Presenting author

1) Met Office, Hadley Centre, UK
2) Reading University, UK

A previous paper (Hawkins et al, 2011) described the first time in which a hysteresis of the Meridional Overturning Circulation has been found in a general circulation model (FAMOUS). They showed that, for a given input of fresh water into the north Atlantic, there existed two possible states: one with a strong overturning in the north Atlantic and the other with a reverse Atlantic cell.

In this study we investigate the mechanisms behind the hysteresis. We assess the changes in surface fluxes and advection that lead to nonlinear MOC changes and the connection between the Atlantic and Pacific overturning circulations. The formulation of the hosing scenario, and in particular how the input of fresh water into the Atlantic is compensated, is found to be very important.

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Johns, William et al.: Perspectives on ocean heat transport in the North Atlantic from the first decade of the RAPID-MOCHA array.  Slides   View abstract

 
Perspectives on ocean heat transport in the North Atlantic from the first decade of the RAPID-MOCHA array
William Johns1*, Jian Zhao1, Gerard McCarthy2, David Smeed2, Christopher Meinen3, Molly Baringer3, Eleanor Frajka-Williams2, Elaine McDonagh2, Brian King2, Darren Rayner2

* Presenting author

1) Rosenstiel School of Marine and Atmospheric Science, University of Miami, USA
2) National Oceanography Centre, UK
3) NOAA Atlantic Oceanographic and Meteorological Laboratory, USA

Since April 2004, continuous estimates of the oceanic meridional heat transport in the Atlantic have been derived from the RAPID-MOCHA-WBTS observing system along 26.5°N. Several improvements in the methodology for both the AMOC and heat transport calculation have been implemented recently, which have been applied retrospectively to the entire time series. These include improvements in the surface extrapolation of interior geostrophic velocities, adoption of the TEOS-10 equation of state, updated Gulf Stream temperature transport calibration, and weekly optimal interpolation of Argo and RAPID mooring data to estimate the interior temperature transport.

The mean values for the AMOC strength and northward heat transport from the 10-year time series (2004-2014) are 17.0 Sv and 1.24 PW, respectively. Both the AMOC strength and the heat transport have decreased in recent years compared to values observed prior to 2009; the 5-year means for the pentad 2009-2013 were 15.6 Sv (1.14 PW) compared to values of 18.7 Sv (1.34 PW) for the pentad 2004-2008. The decline in the heat transport of 0.2 PW between these periods is significant: it is equivalent to a net decrease in surface heat flux of ~7 W m-2 over the entire North Atlantic, and represents a net deficit in heat delivery to the North Atlantic of 1.0 PW during the last 5 years, nearly equivalent to one year's worth of the typical heat transport. Observations of ocean heat content (OHC) from Argo data show that the North Atlantic OHC reached a decadal peak in about 2007 and has since declined, consistent with the lower recent heat transport values recorded by the 26.5°N array.

More than 90% of the interannual variability that has occurred in the meridional heat transport is contained in the overturning component of the heat transport, while the gyre component has maintained a stable mean value. Both Ekman and Gulf Stream variability contribute to large short-term changes in the AMOC and heat transport, including occasional heat transport reversals, while the interannual variability of the heat transport is dominated by the geostrophic circulation and mostly by the mid-ocean heat transport. Analysis of GCMs and simpler forced dynamical models suggests that most of the interannual AMOC and heat transport variability can be explained by wind-forced changes in mid-ocean circulation associated with first baroclinic mode Rossby waves, that are excited by interannual wind stress curl anomalies in the central and western part of the basin.

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Johnson, Helen L. et al.: Reconstructing recent Atlantic overturning from surface wind and buoyancy forcing.  Slides   View abstract

 
Reconstructing recent Atlantic overturning from surface wind and buoyancy forcing
Helen R. Pillar1,2, Patrick Heimbach3,4,5, Helen L. Johnson1*, David P. Marshall6

* Presenting author

1) University of Oxford, Earth Sciences, UK
2) University of Copenhagen, Niels Bohr Institute, Denmark
3) Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Sciences
4) University of Texas at Austin, Institute for Computational Engineering and Sciences, USA
5) University of Texas at Austin, Jackson School of Geosciences, USA
6) University of Oxford, Atmospheric, Oceanic and Planetary Physics

The Atlantic Meridional Overturning Circulation (AMOC) carries a substantial amount of heat poleward in the North Atlantic and is projected to weaken over the next century in response to greenhouse gas emissions, with implications for the North Atlantic storm track, hurricane frequency, European climate, regional sea level, and global terrestrial and marine ecosystems. The AMOC is believed to be a key driver of multidecadal variations in North Atlantic sea surface temperatures and a potential source of regional climate predicability. The strength of the AMOC at 26°N has been continuously monitored since 2004 and exhibits large variability on all time scales. Here we investigate how much of the observed AMOC variability can be reconstructed by projecting observed atmospheric variability onto model-based estimates of AMOC sensitivity to surface wind, thermal and freshwater forcing over the preceding 15 years. We find that local, instantaneous wind forcing dominates the AMOC variability on short time scales, whereas subpolar heat fluxes dominate on interannual to decadal time scales. The reconstructed AMOC is able to reproduce most of the interannual variability observed by the RAPID-MOCHA array at 26°N, but not the apparent decadal trend, requiring the integrated response to subpolar heat fluxes over at least the past two decades.

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King, Brian A. et al.: Regional variability of freshwater in the North Atlantic in the RAPID/Argo era.  Slides   View abstract

 
Regional variability of freshwater in the North Atlantic in the RAPID/Argo era
Brian A. King1*, Elaine L. McDonagh1, Damien Desbruyeres1, N. Penny Holliday1

* Presenting author

1) National Oceanography Centre, Southampton, Marine Physics and Ocean Climate

The most striking interannual variability in the strength of the Atlantic Meridional Overturning Circulation since moored measurements began in 2004, was a reduction for a period of 18 to 24 months in 2009/10. This event, as measured by the 26.5N array, reduced the amount of heat and salt transported northwards by the AMOC, and was associated with a deficit of heat and salt in the region of the subtropical Atlantic north of the monitoring array. Our analysis of Argo data in the region north of 26.5N showed that the heat deficit recovered more quickly than the salt deficit. We have constructed time series of heat and freshwater flux across 26.5N, which are a synthesis of Argo measurements and the basin endpoint measurements made by the 26.5N moorings. By combining the spatial inventories determined from Argo data only, with the horizontal fluxes due to the AMOC, we identify the AMOC contribution to the freshwater budget north of 26.5N, and its persistence in time and spatial signature. We will show the vertical and horizontal distribution of changes in inventory, and the impact of air-sea exchange on the separate evolution of temperature and salinity. Finally, we will show the extent to which the subtropical and subpolar variability in freshwater are either correlated to each other or to variability in the AMOC measured at 26.5N.

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Kravtsov, Sergey et al.: Attribution of multidecadal climate trends in observations and models.  Poster   View abstract

 
Attribution of multidecadal climate trends in observations and models
Sergey Kravtsov1*, Marcia Wyatt2, Judith Curry3, Anastasios Tsonis1

* Presenting author

1) University of Wisconsin-Milwaukee, Mathematical Sciences, USA
2) University of Colorado - Boulder, Geosciences, USA
3) Georgia Tech., Earth and Atmospheric Sciences, USA

We analyzed long-term trends and multidecadal variability in a network of well-known climate indices based on the sea-surface temperature and sea-level pressure fields, in observations and CMIP5 model simulations. Various ensemble-mean estimates of the forced variability were derived from 18 independent ensembles of these simulations, each using a single model with fixed physics package and an identical forcing history. It was shown that the residual intrinsic variability time series over the total of 116 simulations considered are statistically independent. The 18 estimates of the forced signal were further used for semi-empirical attribution of the observed climate variability. The model uncertainty results in a fairly broad range of the “intrinsic” multidecadal variability so estimated. Furthermore, the observed surface temperature “intrinsic” residuals exhibit the levels of multidecadal variance overwhelmingly exceeding those of the simulated intrinsic multidecadal variability. This may reflect the climate models’ consistently underestimating the true amplitude of the forced multidecadal variability, and/or be indicative of the lack of multidecadal intrinsic dynamics in these models.

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Kröger, Jürgen et al.: Spurious initialization in near term AMOC predictions.  Poster   View abstract

 
Spurious initialization in near term AMOC predictions
Jürgen Kröger1*, Jin-Song von Storch1

* Presenting author

1) Max-Planck-Institut für Meteorologie, Ozean im Erdsystem, Germany

A new assimilation procedure is proposed aiming at suppressing spurious initialization in the coupled forecast system for decadal predictions developed in the framework of the German BMBF (Bundesministerium für Bildung und Forschung) project MiKlip (Mittelfristige Klimaprognosen).

Ocean reanalysis products are commonly used for initialization in climate predictions. Inherent to the state estimates of these products is an imprint of their underlying dynamical models which may have a negative impact on the skill of predictions. Applying the state estimates only in regions where the estimates are better constrained by real observations may lead to better skill.

To test this hypothesis, two sets of ensemble hindcasts are performed where the common full area initialization is compared to a reduced area initialization with the latter set applying a masking in the assimilation procedure that mimics the recent world ocean's data coverage (including the network of ARGO observations). Except for the tropical South Atlantic, multi-year AMOC hindcasts with reduced area initialization outperform those with full area initialization almost everywhere.

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Le Bras, Isabela A. et al.: A decade of Line W mooring observations of the Deep Western Boundary Current.  Slides   View abstract

 
A decade of Line W mooring observations of the Deep Western Boundary Current
Isabela A. Le Bras1,2*, Ruth Curry2, John M. Toole2

* Presenting author

1) MIT-WHOI Joint Program, Physical Oceanography, USA
2) Woods Hole Oceanographic Institution, Department of Physical Oceanography, USA

The Line W moored array, on the continental slope southeast of New England, measured the North Atlantic's Deep Western Boundary Current (DWBC) properties and velocity from 2004 to 2014. The DWBC is the primary branch of the Atlantic Meridional Overturning Circulation's (AMOC) cold limb, bringing North Atlantic Deep Water (NADW) equatorward along the continental slope. We separate NADW into neutral density classes based on their origin: Upper Labrador Sea Water (ULSW), Classical Labrador Sea Water (CLSW) from the Labrador Sea, and Overflow Waters (OW) from the Nordic Seas.

Building on the work of Pena-Molino et al. 2011, we analyze intermediate water properties at the central Line W mooring, for which there are observations starting in 2001. We find a continued trend of increasing planetary potential vorticity (PPV) in the CLSW range, reflecting a decrease in CLSW production in the Labrador Sea. The CLSW also warms (+0.01 degrees C per year) and becomes saltier (+0.001 per year) over the course of the record. These results are consistent with measurements in the Labrador Sea (Kieke et al. 2014) and indicate an approximate 10 year travel time from the Labrador Sea to Line W. Future work includes assessing the connectivity with measurements of the DWBC at 53N (Fischer et al. 2010).

We also present extended DWBC transport estimates calculated as in Toole et al. 2010, from the full moored array. The time mean transport of all NADW in the DWBC is 23.5 Sv with a standard deviation of 12.59 Sv. The CLSW portion of the DWBC has a mean transport of 7.73 Sv with a standard deviation of 4.07. The CLSW transport is decreasing at a rate of 4% a year, consistent with our water property findings from the single mooring analysis.

Transport estimates differ when data from all 6 moorings (available post 2008) are used instead of the original 5, raising questions about interpreting DWBC measurements from an array that ends in mid-ocean. Some of this difference is likely due to deep cyclones associated with Gulf Stream warm core rings. Andres et al. have recently shown that these cyclones alter the deep velocity structure along Line W in hydrographic sections, and may provide a mechanism for exchange between the DWBC and the interior. We plan to investigate this further using the mooring data.

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Loder, John W. et al.: Recent Variability in Water Mass Properties in the Labrador Sea and Scotian Rise Regions.  Slides   View abstract

 
Recent Variability in Water Mass Properties in the Labrador Sea and Scotian Rise Regions
John W. Loder1*, Igor Yashayaev1, Miguel A. Morales Maqueda0,2

* Presenting author

1) Fisheries and Oceans Canada, Bedford Institute of Oceanography, Canada
2) National Oceanography Centre, United Kingdom

Historical and recent moored, survey/vessel and Argo datasets are used to describe long-term variability in water mass properties of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) in the Labrador Sea (LS) and Scotian Rise (SR) regions. Focus is on the occurrence of moderate-to-deep convection and variability in intermediate, deep and abyssal waters in the LS, and on the properties of these components of the Deep Western Boundary Current (DWBC) on the SR. Variability in the upper and intermediate layers of the LS over the past century has been dominated by a multi-decadal variation with temperature and salinity peaks in the 1960s-1970s and early 2000s, apparently related to a combination of atmospheric forcing and larger-scale oceanographic variability in the subpolar North Atlantic. In recent years there has been weak cooling and freshening of the upper layer, possibly related to an increase in Greenland and Arctic meltwater, while during the past two decades, the intermediate layer has been intermittently (e.g., 2008, 2014) ventilated by deep convection, resulting in temporary cooling and freshening followed by slow recovery. In the LS’s abyssal layer, there has been an unprecedented increase in temperature and salinity between 2000 and 2010, related to changes in Denmark Strait Overflow Water (DSOW). On the SR, there has been overall warming at intermediate depths over the past half century, as well decadal-scale variability related in part to the changes in water mass properties (e.g. Labrador Sea Water) of the DWBC. There is also an indication of decadal-variability signals in the DSOW reaching the SR.

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Lozier, Susan et al.: Introduction to OSNAP: Overturning in the Subpolar North Atlantic.  Poster   View abstract

 
Introduction to OSNAP: Overturning in the Subpolar North Atlantic
Susan Lozier1*, Feili Li1

* Presenting author

1) Duke University, Earth and Ocean Sciences, United States

OSNAP (Overturning in the Subpolar North Atlantic) is an international program designed to provide a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic. The OSNAP observing system, deployed in the summer of 2014, consists of moored instruments and gliders along a line extending from southern Labrador to the southwestern tip of Greenland across the mouth of the Labrador Sea (OSNAP West), and from the southeastern tip of Greenland to Scotland (OSNAP East). The observing system also includes subsurface floats in order to trace the pathways of overflow waters in the basin and to assess the connectivity of currents crossing the line. In this poster we will describe the OSNAP objectives, and the observation and modelling components of the program.

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MacGilchrist, Graeme A. et al.: Variability of the North Atlantic ocean ventilation.  Slides   View abstract

 
Variability of the North Atlantic ocean ventilation
Graeme A. MacGilchrist1*, David P. Marshall2, Helen L. Johnson1, Camille Lique1, Laura Jackson3, Richard A. Wood3, Matthew Thomas4

* Presenting author

1) University of Oxford, Earth Sciences, U.K.
2) University of Oxford, Atmospheric, Oceanic and Planetary Physics
3) Met Office, Hadley Centre, U.K.
4) Yale University, Geology and Geophysics, U.S.

In the subtropical gyres, only water subducted in late winter will remain in the ocean interior for longer than a seasonal cycle. The fast seasonal migration of outcropping density surfaces relative to the slow southward advection of water in the ocean interior means that water subducted before late winter has insufficient time to escape entrainment back into the mixed layer as the outcrop advances southward, a phenomenon denoted Stommel’s demon. We investigate whether a similar process operates on inter-annual timescales in the North Atlantic. Lagrangian analysis of a 1/4 degree ocean model reveals the ventilation age on interior density surfaces. We find that significant inter-annual variability exists and investigate its link to changes in the location of the late winter outcrop from year to year. Our results suggest that Stommel’s demon is a multi-year process that impacts ventilation on timescales longer than a seasonal cycle. This carries implications for the structure and properties of the main thermocline, nutrient cycling and the transport of atmospheric tracers into the ocean interior.

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Marshall, David et al.: A relation between the volume transports of the Atlantic Meridional Overturning Circulation and Antarctic Circumpolar Current.  Poster   View abstract

 
A relation between the volume transports of the Atlantic Meridional Overturning Circulation and Antarctic Circumpolar Current
David Marshall1*, Helen Johnson2

* Presenting author

1) University of Oxford, Department of Physics, United Kingdom
2) University of Oxford, Department of Earth Sciences, United Kingdom

A simple heuristic model is developed to explain the relative volume transports of the Atlantic Meridional Overturning Circulation (17.5 Sv) and Antarctic Circumpolar Current (137 Sv) in terms of three depth scales: the e-folding depth of the global stratification, the depth of maximum overturning streamfunction and the depth of Drake Passage. For realistic values, the model is able to explain the factor 8 difference in the magnitudes of these currents.

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Marzocchi, Alice et al.: The North Atlantic subpolar circulation in an eddy-resolving global ocean model.  Poster   View abstract

 
The North Atlantic subpolar circulation in an eddy-resolving global ocean model
Alice Marzocchi1,2*, Joel J.-M. Hirschi1, N. Penny Holliday1, Stuart A. Cunningham1,3, Adam T. Blaker1, Andrew C. Coward1

* Presenting author

1) National Oceanography Centre Southampton , UK
2) University of Bristol, School of Geographical Sciences , UK
3) Scottish Association for Marine Science, UK

The subpolar North Atlantic represents a key region for global climate, but most numerical models still have well described limitations in correctly simulating the local circulation patterns. Here, we present the analysis of a 30-year run with a global eddy-resolving (1/12°) version of the NEMO ocean model. Compared to the 1° and 1/4° equivalent versions, this simulation more realistically represents the shape of the Subpolar Gyre, the position of the North Atlantic Current, and the Gulf Stream separation. Other key improvements are found in the representation of boundary currents, multi-year variability of temperature and depth of winter mixing in the Labrador Sea, and the transport of overflows at the Greenland–Scotland Ridge. However, the salinity, stratification and mean depth of winter mixing in the Labrador Sea, and the density and depth of overflow water south of the sill, still present challenges to the model. This simulation also provides further insight into the spatio-temporal development of the warming event observed in the Subpolar Gyre in the mid 1990s, which appears to coincide with a phase of increased eddy activity in the southernmost part of the gyre. This may have provided a gateway through which heat would have propagated into the gyre's interior.

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Mavilia, Irene et al.: Atlantic Multidecadal Variability in a multi-model ensemble of CMIP5 simulations: an assessment of its spectral characteristics and its non-stationary behaviour.  Poster   View abstract

 
Atlantic Multidecadal Variability in a multi-model ensemble of CMIP5 simulations: an assessment of its spectral characteristics and its non-stationary behaviour
Irene Mavilia1,2*, Alessio Bellucci1, Panos Athanasiadis1, Silvio Gualdi1,3, Rym Msadek4, Yohan Ruprich-Robert4

* Presenting author

1) Euro-Mediterranean Center on Climate Change (CMCC), Italy
2) Ca’ Foscari University, Italy
3) Istituto Nazionale di Geofisica e Vulcanologia (INGV), Italy
4) NOAA/Geophysical Fluid Dynamics Laboratory, USA

The Atlantic Multidecadal Variability (AMV) is a coherent pattern of variability of the North Atlantic Sea Surface Temperature field affecting several components of the climate system in the Atlantic region and the surrounding areas. AMV is thought to be the surface signature of the Atlantic meridional overturning circulation (AMOC) variability. Our current knowledge of the AMV is based on a relatively short observational record, which severely limits our understanding of the mechanisms involved, as well as the characterisation of the low-frequency tail of the variability spectrum. In order to quantify accurately the contribution of anthropogenic forcings to the observed climatic changes, it is essential to understand better the natural climate variability occurring at long time scales. Here, the behaviour of the AMV is examined in a set of multi-century CMIP5 pre-industrial climate simulations performed with different Coupled General Circulation Models (CGCMs). Only the longest simulations (minimum 500-year long) of the CMIP5 archive have been used. In these simulations the AMV exhibits a non-stationary behaviour, which is objectively assessed. In some of the models a shorter time scale mode (∼20 years) seems to alternate with a longer time scale mode (∼60 years) with a gradual shift from one to another across different epochs, involving also their co-existence. A multi-model analysis of the AMV allows us to investigate similarities and differences across an ensemble of state-of-the-art climate models and to identify the prominent simulated mechanisms of air-sea interaction at mid-latitude and over decadal and longer time scales. The relationship between the detected AMV behaviour and the variability of the AMOC is also examined. The non-stationary behaviour identified in most models suggests that the character of the observed AMV may undergo significant changes in the future. This ongoing analysis will provide further insight into the dynamics of the AMV variability.

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McCarthy, Gerard D. et al.: Insights and impacts: the first 10 years of continuous observations of the Atlantic overturning circulation.  Slides   View abstract

 
Insights and impacts: the first 10 years of continuous observations of the Atlantic overturning circulation
Gerard D. McCarthy1*, William E. Johns2, Chris S. Meinen 3, Molly O. Baringer3, Darren Rayner1, Bengamin I. Moat1, Eleanor Frajka-Williams4, David A. Smeed1

* Presenting author

1) National Oceanography Centre, Southampton, UK
2) RSMAS, University of Miami, USA
3) AOML, NOAA, USA
4) University of Southampton, National Oceanography Centre, UK

The RAPID/MOCHA/WBTS is a joint UK-US project that has been measuring the Atlantic Overturning circulation (AMOC) at 26.5 N in the North Atlantic since 2004. Here we present some of the key results from the first 10 years of the program.

The first year’s measurements revealed a highly variable AMOC that encompassed all previous ship-based, hydrographic estimates of the AMOC, thus showing that a perceived decline could be encompassed in short-term variability. Seasonal variability in the AMOC was larger than expected with a 6 Sv range, with the largest single component derived from density fluctuations at the eastern boundary.

Interannual variability, far larger than that in the present state of the art climate models, was seen in 2009/10. A 30% reduction lasted 18 months, cooling the subtropical North Atlantic significantly and elevating sea levels in New York by 13 cm. The existence of continuous heat transport measurements enabled us to show that the main cause of the cooling was a reduction in ocean heat convergence rather than air-sea fluxes.

The winter of 2010/11 revealed a second consecutive winter of low AMOC: a double dip. Whether ocean re-emergence or the change in AMOC circulation was the cause of the SST tripole pattern pattern that emerged in the winter of 2010/11 is a topic of ongoing research. Nonetheless, this SST pattern was shown to be sufficient to push the atmosphere into a second consecutive negative wintertime North Atlantic Oscillation (NAO) and increased predictability of this negative NAO.

Most recently a multi-year decline in the AMOC has been observed. This 0.5 Sv/year decline is much larger than the long-term decline predicted due to anthropogenic climate change. The decline first reported on the 8.5-year timeseries has continued in the 10-year timeseries. The magnitude of the decline is so large as to suggest it may be decadal variability.

The Atlantic is a region with large multi-decadal climate signals. A decline in the AMOC is consistent with a declining phase of the Atlantic Multi-decadal oscillation of sea-surface temperatures that is predicted by a number of authors. Circulation proxies based on tide gauges have supported the hypothesis that ocean heat transport dominates the changes in ocean heat content on decadal timescales and eventually sea surface temperature variations. With continuous measurements of heat transport from the RAPID array we will be able to quantify this dynamic mechanism. On even longer timescales, continuous AMOC measurements will show whether the predicted decline in the AMOC due to anthropogenic climate change is occurring.

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McCarthy, Gerard D. et al.: Sea-level fluctuations show Ocean Circulation impact on Atlantic Multidecadal Variability.  Poster   View abstract

 
Sea-level fluctuations show Ocean Circulation impact on Atlantic Multidecadal Variability
Gerard D. McCarthy1*, Ivan D. Haigh2, Joel J.-M. Hirschi1, Jeremy P Grist1, David A. Smeed1

* Presenting author

1) National Oceanography Centre, Southampton, UK
2) University of Southampton, National Oceanography Centre, UK

We present observational evidence that ocean circulation controls the decadal evolution of heat content and consequently sea-surface temperatures (SST) in the North Atlantic. Positive (negative) phases of the Atlantic multidecadal oscillation (AMO) are associated with warmer (cooler) SSTs. Positive phases of the AMO have been linked with decadal climate fluctuations including increased summer precipitation in Europe; increased northern hemisphere land temperatures, fewer droughts in the Sahel region of Africa and increased Atlantic hurricane activity. It is widely believed that the Atlantic circulation controls the phases of the AMO by controlling the decadal changes in heat content in the North Atlantic. However, due to the lack of ocean circulation observations, this link has not been previously proven. 
We present a new interpretation of the sea-level gradient along to the east coast of the United States to derive a measure of ocean circulation spanning decadal timescales. We use this to estimate heat content changes that we validate against direct estimates of heat content. We use the longevity of the tide gauge record to show that circulation, as interpreted in sea-level gradient changes, drives the major transitions in the AMO since the 1920’s. 
We show that the North Atlantic Oscillation is highly correlated with this sea-level gradient, indicating that the atmosphere drives the circulation changes. The circulation changes are essentially integrated by the ocean in the form of ocean heat content and returned to the atmosphere as the AMO. 
An additional consequence of our interpretation is that recently reported accelerations in sea-level rise along the US east coast are consistent with a declining AMO that has been predicted by a number of authors.

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Megann, Alex et al.: Remedying excessive numerical diapycnal mixing in the GO5.0 NEMO configuration.  Poster   View abstract

 
Remedying excessive numerical diapycnal mixing in the GO5.0 NEMO configuration
Alex Megann1*, Dave Storkey2

* Presenting author

1) Marine Systems Modelling, National Oceanography Centre, Southampton
2) Met Office, Exeter, UK

If numerical ocean models are to simulate faithfully the upwelling branches of the global overturning circulation, they need to have a good representation of the diapycnal mixing processes which contribute to conversion of the bottom and deep waters produced in high latitudes into less dense watermasses. It is known that the default class of depth-coordinate ocean models such as NEMO and MOM5, as used in many state-of-the art coupled climate models and Earth System Models, have excessive numerical diapycnal mixing, resulting from advection across coordinate surfaces.

The GO5.0 configuration of the NEMO ocean model, on an “eddy-permitting” 0.25° global grid, is used in the current UK GC1 and GC2 coupled models. Megann and Nurser (2015) have shown, using the isopycnal watermass analysis of Lee et al (2002), that spurious numerical mixing is substantially larger than the explicit mixing prescribed by the mixing scheme used by the model. It will be shown that increasing the biharmonic viscosity by a factor of three tends to suppress small-scale noise in the vertical velocity in the model. This leads to changes in the overturning circulation, especially when expressed as a function of density: these are associated with a significant reduction in the numerical mixing in GO5.0, and we shall show that this leads to large-scale improvements in model biases.

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Meinen, Christopher S.: Monitoring Florida Current transport at 27°N using pressure gauges.  Poster   View abstract

 
Monitoring Florida Current transport at 27°N using pressure gauges
Christopher S. Meinen1*

* Presenting author

1) NOAA/Atlantic Oceanographic and Meteorological Laboratory, Physical Oceanography Division, United States of America

The continuous daily estimates of the Florida Current/Gulf Stream volume transport made via submarine cable between Florida and Grand Bahama Island are a critical component of the Meridional Overturning Circulation (MOC) observing array at 26.5°N. Furthermore, with more than 30 years of daily estimates, the Florida Current transport record is one of the longest ocean transport records in existence and it represents an important time series for validating and testing ocean and coupled ocean-atmosphere numerical models. While the NOAA program that funds the cable observations, the Western Boundary Time Series project, is fairly securely funded, the project is dependent upon an out-of-service telephone cable that will, in all likelihood, break at some unknown time in the future. Results will be presented on tests of a possible backup system for the cable – pressure/tide gauges maintained on either side of the Straits of Florida near the cable termini. Six years of data from 2008-2014 suggest that a pair of pressure gauges captures some of the key variations in the Florida Current transport, although the overall correlation is not as good as one might hope (r = 0.76). Correlations do not improve significantly after the removal of high frequency (periods less than 90 days) variations, which is consistent with the barotropic and baroclinic components of the flow in the Straits of Florida varying independently of one another at seasonal and shorter time scales. Some implications for future observing of the Florida Current will be discussed.

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Menary, Matthew B. et al.: Testing the impact of CMIP5 model biases on the simulation of North Atlantic decadal variability.  Poster   View abstract

 
Testing the impact of CMIP5 model biases on the simulation of North Atlantic decadal variability
Matthew B. Menary1,2*, Daniel L. R. Hodson2, Jon I. Robson2, Rowan T. Sutton2, Richard A. Wood1, Jonathan Hunt3

* Presenting author

1) Met Office Hadley Centre, UK
2) NCAS-Climate, University of Reading, UK
3) University of Oxford, UK

Instrumental observations, palaeo-proxies, and climate models suggest significant decadal variability within the North Atlantic subpolar gyre (NA SPG). However, a combination of a poorly sampled observational record and a diverse range of model behaviours have left the precise nature of this variability unclear. Here, we analyse an exceptionally large ensemble of 42 present-generation climate models to test whether NA SPG mean state temperature and salinity biases systematically affect the representation of decadal variability. We find that biases in the Labrador Sea co-vary and influence whether density variability is controlled by temperature or salinity changes. Ocean horizontal resolution is a good predictor of the biases and affects whether models choose northern or southern feedbacks within the NA SPG. Despite these relationships, we find no link to the spectral characteristics of the variability. Our results suggest that the mean state and evolution of anomalies within the NA SPG are not independent.

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Mignot, Juliette et al.: Decadal prediction skill with surface nudging.  Poster   View abstract

 
Decadal prediction skill with surface nudging
Juliette Mignot1,2,3*, Javier Garcia-Serrano1, Didier Swingedouw4, Agathe Germe1, Sébastien Nguyen1, Pablo Ortega1,5, Eric Guilyardi1,5, Sulagna Ray1

* Presenting author

1) LOCEAN/IPSL, UPMC-CNRS-IRD-MNHN, France
2) Climate and Environmental Physics, Physics Institute, University of Bern
3) Oeschger Center for Climate Change Research, University of Bern, Switzerland
4) EPOC-CNRS, Université de Bordeaux, France
5) NCAS-Climate, University of Reading, UK

Predictability of the Atlantic meridional circulation and related temperature variations is investigated in the IPSL system, initialized through surface nudging. Two decadal prediction ensembles, based on the same climate model (IPSL-CM5A- LR) and the same surface nudging initialization strategy are analyzed and compared with a focus on upper-ocean variables in different regions of the globe. One ensemble consists of 3-member hindcasts launched every year since 1961 while the other ensemble benefits from 9 members but with start dates only every 5 years. Analysis includes anomaly correlation coefficients and root mean square errors computed against several reanalysis and gridded observational fields, as well as against the nudged simulation used to produce the hindcasts initial conditions. The last skill measure gives an upper limit of the predictability horizon one can expect in the forecast system, while the comparison with different datasets highlights uncertainty when assessing the actual skill. Results provide a potential predictability skill (verification against the nudged simulation) beyond the linear trend of the order of 10 years ahead at the global scale, but essentially due to the non-linear response to external forcing (i.e. volcanoes). At regional scale, we obtain 1 year in the tropical band, 10 years at extratropical latitudes in the North Atlantic and 5 years at tropical to subtropical latitudes in the North Atlantic, for both sea surface temperature (SST) and upper-ocean heat content. Actual prediction skill (verification against observational or reanalyzed data) is overall more limited and less robust. Even so, large actual skill is found in the extratropical North Atlantic for SST. The interplay between initialization and internal modes of variability limits the actual predictability skill of the AMOC.

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Msadek, Rym et al.: Exploring the influence of increased atmospheric resolution on AMOC-related predictability.  Slides   View abstract

 
Exploring the influence of increased atmospheric resolution on AMOC-related predictability
Rym Msadek1*, Yohan Ruprich-Robert2, Tom Delworth3

* Presenting author

1) NOAA/GFDL, USA
2) Princeton University, USA
3) NOAA/GFDL, USA

Several models have shown that climate anomalies like those observed following the mid-1990s North Atlantic subpolar gyre warming could be predicted one to few years in advance when the ocean and more specifically the AMOC is initialized from observational estimates (Yeager et al. 2013, Robson et al. 2013, Msadek et al. 2014). These initialized decadal prediction experiments were all based on rather coarse-resolution coupled models, both for their ocean and atmospheric components. The atmosphere plays an important role in the decadal variability of the AMOC through changes in wind and surface fluxes. The quality of the simulation of the near-surface oceanic and the atmospheric climate can be considerably improved as the atmospheric resolution is increased from ˜2º like in the GFDL CM2.1 model to 0.5º like in the GFDL-FLOR model (Jia et al. 2014, Wittenberg et al. 2014). Here we explore the impact of increased atmospheric resolution on the decadal predictability associated with the AMOC by comparing two suites of initialized experiments based on the GFDL CM2.1 and FLOR coupled models, which share the same ocean and sea ice models but differ in their atmospheric component. The AMOC in the CM2.1 and FLOR models exhibits the same mechanism of decadal variability but has a slightly longer time scale in FLOR. We explore the extent to which increased atmospheric resolution impacts the predictive skill associated with the AMOC in the GFDL models when the models are initialized from the same ocean-sea ice initial conditions. We focus on specific periods during which the AMOC was shown to play a role in driving North Atlantic changes and remote climate anomalies like during the mid-1990s or during the recent 2004-2012 AMOC decline (Smeed et al. 2013).

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Oliver, Kevin I. C. et al.: Reconstructing Atlantic and global overturning from meridional density gradients.  Slides   View abstract

 
Reconstructing Atlantic and global overturning from meridional density gradients
Edward D. Butler1, Kevin I. C. Oliver1*, Joel J.-M. Hirschi2, Jennifer V. Mecking1

* Presenting author

1) University of Southampton, National Oceanography Centre Southampton, UK
2) National Oceanography Centre, UK

Numerous attempts have been made to scale the strength of the meridional overturning circulation (MOC), principally in the North Atlantic, with large-scale, basin-wide hydrographic properties. In particular, various approaches to scaling the MOC with meridional density gradients have been proposed, but the success of these has only been demonstrated under limited conditions. Here we present a scaling relationship linking overturning to twice vertically-integrated meridional density gradients via the hydrostatic equation and a “rotated” form of the geostrophic equation. This provides a meridional overturning streamfunction as a function of depth for each basin. Using a series of periodically forced experiments in a global, coarse resolution configuration of the general circulation model NEMO, we explore the timescales over which this scaling is temporally valid. We find that the scaling holds well in the upper Atlantic cell for multi-decadal and longer timescales, accurately reconstructing the relative magnitude of the response for different frequencies and explaining over 85% of overturning variance on timescales of 64 to 2048 years. Despite the highly nonlinear response of the Antarctic cell in the abyssal Atlantic, between 76% and 94% of the observed variability at 4000m is reconstructed on timescales of 32 years (and longer). The scaling law is also successfully applied in the Indo-Pacific. These results indicate that meridional density gradients and overturning are linked via meridional pressure gradients, and that both the strength and structure of the MOC can be reconstructed from hydrography on multi-decadal and longer timescales provided that the link is made in this way. This has gives us the potential to reconstruct the Atlantic MOC over much of the 20th Century, and can contribute to long-term monitoring efforts.

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Ortega, Pablo et al.: Reconciling two alternative mechanisms behind bi-decadal AMOC variability.  Poster   View abstract

 
Reconciling two alternative mechanisms behind bi-decadal AMOC variability
Pablo Ortega1,2*, Juliette Mignot2,3, Didier Swingedouw4, Florian Sévellec1,2, Eric Guilyardi1,2

* Presenting author

1) LOCEAN Laboratory-IPSL, Université Pierre et Marie Curie, France
2) NCAS Climate/Department of Meteorology, University of Reading, United Kingdom
3) Climate and Environmental Physics and Oeschger Centre for Climate Change Research, University of Bern, Switzerland
4) EPOC, Université de Bordeaux, France

Understanding the preferential timescales of variability in the North Atlantic, usually associated with the Atlantic meridional overturning circulation (AMOC), is essential for the prospects of decadal prediction. However, the wide variety of mechanisms proposed from the analysis of climate simulations, potentially dependent on the models themselves, has stimulated the debate of which processes take place in reality. One mechanism receiving increasing attention, identified both in idealised models and observations, is a westward propagation of subsurface buoyancy anomalies, that impact the AMOC through a basin-scale intensification of the zonal density gradient, enhancing the northward transport via thermal wind balance. In this study, we revisit a control simulation from IPSL-CM5A-LR, characterised by a strong AMOC periodicity at 20 years, previously explained by an upper ocean-atmosphere-sea ice coupled mode with a direct effect on convection activity South of Iceland. Our study shows that this mechanism interacts constructively with the basin-scale mode in the subsurface. This constructive feedback may explain why bi-decadal variability is so intense in this coupled model as compared to others.

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Ortega, Pablo et al.: Reconstructing extreme AMOC events through nudging of the ocean surface.  Slides   View abstract

 
Reconstructing extreme AMOC events through nudging of the ocean surface
Pablo Ortega1,2*, Eric Guilyardi1,2, Didier Swingedouw3, Juliette Mignot1,4, Sebastien Nguyen1

* Presenting author

1) LOCEAN Laboratory-IPSL, Université Pierre et Marie Curie, France
2) NCAS Climate/Department of Meteorology, University of Reading, United Kingdom
3) EPOC, Université de Bordeaux, France
4) Climate and Environmental Physics and Oeschger Centre for Climate Change Research, University of Bern, Switzerland

Rapid and large changes in the Atlantic meridional overturning circulation (AMOC) can strongly impact climate at the global scale. These rapid fluctuations emerge frequently in models as a result of internal climate variability, but in the real world, the lack of long-enough continuous observations has prevented their identification. Indirect estimates of past AMOC variability can be obtained through the use of climate models, following different assimilation techniques. However, the fidelity of these products can only be partially validated with the limited in situ measurements available. To overcome some of these limitations, we here follow a perfect model approach with the IPSL-CM5A-LR model to assess the performance of several nudging techniques towards different sets of surface variables (i.e. sea surface temperature and salinity relaxation, wind stress restoring) in reconstructing the simulated AMOC variability. The motivation to use only surface nudging comes from the longer well-observed surface ocean variables when compared to the sub-surface. We first use the standard 2-months relaxation time scale for surface restoring, classically used for ocean-only simulations. A specific focus is made on the representation of an extreme positive peak in the target control simulation used as “surrogate reality”. Our analysis highlights the sensitivity to the initial conditions, and recommends the use of an ensemble of nudged simulations to guarantee a correct estimate of uncertainty. All the standard nudging approaches used here succeed in reproducing the timing of the extreme AMOC peak, but underestimate its amplitude. A careful analysis of the AMOC precursors reveals that this underestimation comes from a deficit in the formation of the dense water masses in the main regions of convection. This issue is largely corrected in an improved nudged simulation that uses a varying relaxation term, proportional to the mixed layer depth. This development improves the restoring of surface temperature and salinity in the regions of convection, and eventually the representation of AMOC variability, preventing unphysical restoring fluxes elsewhere. This is therefore a promising nudging strategy that applied to the real world can help to better constrain the recent AMOC variability over the last few decades.

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Perez, Renellys et al.: Circulation and Water Mass Variability in the South Atlantic.  Slides   View abstract

 
Circulation and Water Mass Variability in the South Atlantic
Renellys Perez1,2*, Rym Msadek3, Silvia Garzoli1,2, Ricardo Matano4, Christopher Meinen2

* Presenting author

1) University of Miami, CIMAS, USA
2) NOAA, AOML, USA
3) NOAA, GFDL, USA
4) Oregon State University, CEOAS, USA

Most observational and modeling efforts on the Atlantic meridional overturning circulation (AMOC) have been focused on the North Atlantic and the Southern Oceans, which are the preferential sites for deep-water formation. There are, in contrast, fewer studies on the South Atlantic Ocean, which actively transforms AMOC-relevant water masses as they transit the basin. In this presentation, we characterize the natural modes of variability associated with the AMOC in the South Atlantic. We examine the variability of sea level anomalies, water mass properties, and meridional volume transport of the regional boundary currents (Brazil Current, North Brazil Current, Deep Western Boundary Current, and Benguela Current) and determine whether the primary mechanisms responsible for the variability of each of those fields are related to the mechanisms that govern the AMOC variability. Our analysis is based on models and observations. The model results include state-of-the-art eddy-permitting to eddy-resolving NOAA/GFDL climate simulations, ocean-only model simulations forced with CORE interannual forcing, and process-oriented numerical experiments using the Regional Ocean Modeling System. The observations include time series measurements of water mass properties and velocities inferred from moorings along 34.5°S, hydrographic transects, gridded temperature and salinity data sets, gridded sea level anomalies, sea surface temperature, surface currents, and winds obtained from satellite and satellite-in situ blended products. We will examine the extent to which a better representation of meso-scale features, which are key contributors to the variability of the South Atlantic circulation, can affect the representation of the AMOC in global climate models and hence yields a better comparison with observations.

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Polo, Irene et al.: Understanding boundary density variability at 26N and its relation to the geostrophic transports in the North Atlantic.  Poster   View abstract

 
Understanding boundary density variability at 26N and its relation to the geostrophic transports in the North Atlantic
Irene Polo1*, Keith Haines1, Jon Robson1, Rowan Sutton1, Magdalen Balmaseda2, Chris Roberts3

* Presenting author

1) University of Reading, Department of Meteorology, UK
2) European Centre for Medium-range Weather Forecasts, UK
3) Met Office Hadley Centre, UK

We have investigated the density variability at 26N because the AMOC has been monitored since 2004 in the framework of RAPID. Ocean-only NEMO at both, 1 degree and 0.25 degree horizontal resolutions has been used for the period 1958-2010. 1degree resolution uses the model version and forcing fluxes are those used for the ECMWF ORAS4 while 0.25 degree resolution uses the model version from Met office and CORE2 forcing. For understanding important drivers of the zonal density gradient, the NEMO experiments are defined in such a way that only one of either the wind stress or the buoyancy forcing has inter-annual variability, while the other has a seasonally varying climatology. We then seek to distinguish the main forcing-related signals which are imprinted in the density vertical profiles at the eastern and western boundaries at 26N. We present the results of the variability modes from this set of experiments and the relationship between boundaries. Buoyancy-forced density variability has vertical structure and time-scales substantially different from wind-forced variability. On sub-annual to inter-annual (<3 years) time- scales, variations in the density gradient are driven largely by local wind-forced coastal upwelling at both the western and eastern boundaries. These density variations are felt in the upper 1500m at the western and eastern boundaries simultaneously. Inter-annual variations (3-7 years) related to winds occur due to excitation of Rossby waves in the central Atlantic, which propagate westward to interact with the western boundary. On decadal time scales (7-13 yrs), buoyancy forcing related to the North Atlantic Oscillation dominates variability in the AMOC. This decadal density variability is related to freshwater changes over the Labrador Sea. Anomalies propagate along the coast and are felt at both boundaries up to 3500m, with the eastern boundary lagging from 7 months to 3 years. Two speeds of propagation are found at different levels: one fast response at ~1300m and a slow response at ~3000m. Similar propagating features are found in the control experiment when truncation at 800m is applied to the calculation of density variability modes at western boundary. The fact that the boundaries are linked when the buoyancy is the main forcing could be a useful tool to assimilate low frequency AMOC signals. We plan to use these modes of variability to condition a data assimilation experiment. Additional results using observations from the RAPID-array at 26N for the period 2004-2014 are also presented. Density variability at the western boundary show two main modes at inter-annual time-scales associated with different wind forcing patterns: The leading mode has anomalous density around 1000m and is related to the geostrophic transport, while the second mode shows deeper density anomalies and is related to Eastern boundary Ekman pumping. Results from observations are discussed with caution considering the short-period of study.

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Rayner, Darren et al.: Developing a telemetry system for the RAPID 26N moorings..  Poster   View abstract

 
Developing a telemetry system for the RAPID 26N moorings.
Darren Rayner1*, Miguel Charos-Llorens2, Stephen Mack2, Jonathan Campbell3

* Presenting author

1) National Oceanography Centre, Marine Physics and Ocean Climate, UK
2) National Oceanography Centre, Ocean Technology and Engineering, UK
3) Campbell Ocean Data (formerly of NOC), UK

In 2013 the RAPID-WATCH programme commissioned development of a new system for returning data from instruments on a mooring. Previous attempts of developing a system at the start of the RAPID programme proved unviable for the RAPID 26M array due to difficulties experienced with maintaining a surface expression on the tall (up to 5000m deep) moorings.
The new system does not have a permanent surface expression and instead uses data pods that are released at pre-defined times to relay the data stored on them to shore. A data pod system has previously been developed at NOC in the form of the MYRTLE-X lander. For this development we have built on this existing capability and our experience gained through inductive mooring telemetry by linking a mooring to the lander via an acoustic modem.
Data are collected from the instruments by a central hub (referred to as the buoy controller) on the mooring via inductive telemetry. The buoy controller then compresses the data and transmits it through the acoustic modem to the lander. The lander writes these data to the data pods which are intended to surface and transmit the data to shore using the Iridium satellite network.
Here we present details of the development along with the results of field trials in both the sheltered waters of a Scottish loch and in deeper water offshore of Gran Canaria.

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Roussenov, Vassil M. et al.: The role of the MOC in forming gyre-scale heat content anomalies.  Poster   View abstract

 
The role of the MOC in forming gyre-scale heat content anomalies
Vassil M. Roussenov1*, Richard G. Williams1, M. Susan Lozier2, Doug Smith3

* Presenting author

1) University of Liverpool, School of Environmental Sciences, UK
2) Duke University, Nicholas School of the Environment, USA
3) Met Office, Hadley Centre, UK

North Atlantic climate variability on decadal time scales is often characterised by basin-scale changes in sea surface temperature, which are generally attributed to coherent changes in the meridional overturning circulation (MOC). However, this view is inconsistent with striking gyre-scale contrasts in ocean heat content over the basin: in the periods of warmer subtropics the subpolar gyre is cool, and vice versa. We explore how the gyre-scale changes in heat content are mechanistically controlled using dynamical assimilations of historical temperature and salinity data over the last 60 years. The heat content anomalies are usually associated with thermocline and overturning anomalies, a warmer heat content associated with a deeper thermocline and often with a weaker MOC anomaly. The tendency of the subtropical heat content is primarily controlled by the heat transport convergence, dominated by the Ekman component of the MOC. Stronger Trade winds enhance the influx of heat from the tropics, augmented by the air-sea heat fluxes, leading to a deeper thermocline. The tendency of the subpolar heat content does not though directly link to the air-sea fluxes, but instead is controlled by the convergence in heat transport, dominated by the geostrophic component. Density increases in the Labrador Sea induced by atmospheric forcing are followed by a density increase along the western boundary leading to enhanced overturning at the subtropical/subpolar interface, which then drive a warming of the subpolar gyre. Hence, the heat content anomalies for each gyre are formed via different mechanisms involving the MOC, either directly via the winds and air-sea heat fluxes over the subtropical gyre or indirectly via atmospheric-induced changes in boundary density over the subpolar gyre.

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Ruprich-Robert, Yohan: Interdecadal modulation of ENSO by the Atlantic Multidecadal Variability.  Slides   View abstract

 
Interdecadal modulation of ENSO by the Atlantic Multidecadal Variability
Yohan Ruprich-Robert1*

* Presenting author

1) Princeton University, AOS, USA

In this study, the climate impacts of the AMOC decadal variability have been investigated through its hypothesized Sea Surface Temperature (SST) fingerprint: the so-called Atlantic Multidecadal Variability (or AMV). We performed experiments based on the GFDL CM2.1 model in which SSTs in the North Atlantic sector are restored to the observed AMV pattern, while other basins are left fully coupled. In order to explore and isolate the potential AMV impacts and maximize the signal-to-noise ratio, we use an ensemble simulation of 100 members that are run for 11 years. We find that a positive AMV leads to the set-up of a negative Pacific Decadal Variability pattern that is characterized by cold anomalies in the tropical Pacific and warm anomalies in the western Pacific mid-latitudes. The response of the tropical Pacific ocean to a positive phase of the AMV is similar to that observed during a development of La Niña event. However, the AMV-induced anomalies are not stationary and our results suggest that the AMV can modulate ENSO on interdecadal timescale. We investigate the frequency of occurrence of El Niño and La Niña events and find that the probability of La Niña events increase by about 15% during the first 3 years of the simulation and is followed by an increase of El Niño events for the following 2 years. We propose a physical mechanism to explain this modulation of ENSO by the North Atlantic variability.

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Saenko, Oleg et al.: Subpolar Atlantic convection and heat budget in observations and models.  Poster   View abstract

 
Subpolar Atlantic convection and heat budget in observations and models
Oleg Saenko1*, Igor Yashayaev2, Paul Myers3, Gregory Smith4

* Presenting author

1) Environment Canada, Canadian Centre for Climate Modelling and Analysis, Canada
2) Fisheries and Oceans Canada, Bedford Institute of Oceanography, Canada
3) University of Alberta, Department of Earth and Atmospheric Sciences, Canada
4) Environment Canada, Meteorological Research Division, Canada

Deep convection in the subpolar Atlantic Ocean, such as in the Labrador Sea and Greenland Sea, is an important component of the Atlantic meridional overturning circulation. It is preconditioned by weak stratification, cyclonic circulation and surface buoyancy loss. The loss of heat to the atmosphere is thought to be resupplied by narrow boundary jets, although the details of this process are not well understood. Limited observations and high-resolution model simulations indicate that eddies play a key role in the subpolar Atlantic heat budget, by removing heat from the boundary regions and supplying it to the areas of deep convective mixing – a process not represented in low resolution ocean-climate models. Furthermore, in some places, such as off the west coast of Greenland, the boundary jets themselves appear to be partly maintained by the eddies, through inverse barotropic instability. Sensitivity experiments, based on a high-resolution global model, show that under warmer surface climate conditions the cyclonic circulation, heat transport convergence and convection all weaken in the Labrador Sea, whereas they intensify in the Greenland Sea.

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Schmid, Claudia et al.: An observations and model based analysis of meridional transports in the South Atlantic.  Poster   View abstract

 
An observations and model based analysis of meridional transports in the South Atlantic
Claudia Schmid1*, Sudip Majumder2,1, George R. Halliwell1

* Presenting author

1) NOAA/AOML, Physical Oceanography Division, USA
2) CIMAS/University Miami, USA

An analysis of transports of the Atlantic Meridional Overturning Circulation (AMOC) in the South Atlantic Ocean based on three-dimensional velocity fields derived from Argo data and AVISO sea surface heights collected in the years 2000-2014 reveals its spatial and temporal variability. Because these velocity fields end at 2000m the deeper layers need to be padded with climatology. The uncertainty of the results is quantified by sub-sampling the output fields from several models with data assimilation at the resolution of the observation-based fields. Models used for this purpose include the high-resolution HYCOM (HYbrid Coordinate Ocean Mode) and NCODA (Navy Coupled Ocean Data Assimilation) global reanalysis model, the Simple Ocean Data Assimilation (SODA) model and the NCEP (National Center for Environmental Prediction) GODAS (Global Ocean Data Assimilation System) model. Sensitivity to the wind field used in the analysis will be assessed using different wind fields. The relative importance of the eastern boundary, western boundary and interior transports to total variability will be studied.

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Schmittner, Andreas: Using carbon isotopes to reconstruct AMOC changes during the last deglaciation.  Slides   View abstract

 
Using carbon isotopes to reconstruct AMOC changes during the last deglaciation
Andreas Schmittner1*

* Presenting author

1) Oregon State University, College of Earth, Ocean

A synthesis of carbon isotope data (d13C) from deep sea sediments is used together with model simulations to estimate AMOC changes during the initial phase of the last deglaciation. This phase was characterized by ice rafting and cold conditions in the North Atlantic (Heinrich Stadial Event 1, HS1). The reconstructions show decreases in d13C in the North Atlantic with largest values at intermediate depths at high latitudes. The amplitude decreases further south and deeper in the water column. This pattern agrees well with model simulations in which the AMOC was collapsed, but the simulations starting from pre-industrial conditions overestimate the amplitude of the d13C changes. In the model atmospheric CO2 increases and d13C of atmospheric CO2 declines as a response to the AMOC shutdown, consistent in amplitude and rate with ice core measurements. Sensitivity of these results to different initial conditions will be explored.

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Sherwin, Toby et al.: Insights into the Mesoscale Circulation of the Rockall Trough.  Poster   View abstract

 
Insights into the Mesoscale Circulation of the Rockall Trough
Toby Sherwin1*, Dima Aleynik1, Mark Inall1

* Presenting author

1) Scottish Association for Marine Science, Physics and Technology, UK

The Rockall Trough is the nearest region of truly deep oceanic water west of the British Isles, and it is one of the main conduits for warm salty water that is transported to high latitudes as well as the source of oceanic water feeding onto the shallow European Shelf. A full picture of decadal variations of the circulation in the AMOC, and of the Sub-Polar Gyre in particular, requires a proper understanding of the mesoscale variability of this eastern boundary region.
Annual scientific cruises have monitored the temperature and salinity of the Trough since 1975. Since 1995 satellite altimeters have measured small fluctuations in level of the sea surface to determine variations in the European slope current that is an important northward transport path on the eastern side of the Trough, and to quantify the intensity of large horizontal eddies that mix its ocean waters. In recent years remotely operated underwater gliders have begun to supplement these observations with regular crossings of the Trough to report detailed in situ measurements of currents, temperature and salinity from the surface to 1000 m.
This talk describes the results of the first such glider mission which involved eight crossings over the winter of 2009/2010. Its principal findings are that:
i. Much of the surface and deeper meandering current field in the central Rockall Trough is driven by deep eddies that have migrated into the Trough from both its northern and southern entrances.
ii. Surface currents appear to be much stronger during the autumnal period of seasonal surface stratification than in late winter when the upper Trough is mixed to a depth of 600 m.
iii. In late 2009, during a period of unusually large eddy activity, a chance arrangement of some deep circulations caused a westward deflection of the slope current that resulted in a large quantity of slope water being carried to, and thereby warming, the upper 500 m of the western side of the Trough.
iv. Limitations to the altimeter observations the sea surface are identified. By combining them with glider measurements it is shown that they do not pick out the mean flow in narrow slope currents either side of the Rockall Trough.
Novel measurement techniques invariably result in new scientific understanding. The insights derived from this first underwater glider mission in the Rockall Trough confirm that it is a dynamically active region but that much of this activity is driven by deep hidden processes and results in significant fluctuations in the observed mean conditions. There are important implications here for OSNAP in the interpretation of satellite altimeter observations, the understanding of the causes of variability in the results of regular ocean monitoring programmes, and in the establishment and interpretation of the results of ocean modelling exercises.

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Sonechkin, Dmitry M.: Statement of the problem on the mutual adjustment of the atmosphere and ocean in 'fast' and 'slow' time.  Poster   View abstract

 
Statement of the problem on the mutual adjustment of the atmosphere and ocean in 'fast' and 'slow' time
Dmitry M. Sonechkin1*

* Presenting author

1) P.P. Shirshov Oceanology Institute, Russian Academy of Sciences, Russia

It is generally accepted to believe that atmosphere drives the ocean dynamics in the fast timescales of days and decades, and vice versa, the ocean significantly affects long-term (seasonal) and super long-term (interannual and longer) variations of the atmosphere. The evident reason of this believing consists of an enormous difference in the heat capacity of these two subsystems of the global climate system. However, according to my knowledge, nobody gave a rigorous mathematical justification of this believing. The aim of my report is to pose the problem of the atmosphere and ocean mutual adjustment with using the ratio of the atmosphere and ocean heat capacities as “the small parameter”. It turns out that the adjustment of the ocean to the atmospheric influences in the “fast” timescales is a certain “regular” problem. It means that the respective oceanic responses to the atmospheric influences are stable, and so predictable in principle. Instead, the adjustment of the atmosphere to the oceanic influences in the “slow” timescales is a kind of the so-called “singular” problem. The zero-order guess of this problem solution (when the value of the above “small parameter” is assumed to equal to zero) consists of a steady state of the atmospheric subsystem (pseudo) randomly chosen from a number of such steady states existing. According to the well-known results for the energy-balance climate models, it is almost for certain that the steady state being chosen turns out to be unstable to small disturbances of the initial atmospheric state. Therefore, this zero-order guess can not be observed in reality, and instead, significant deviations of the atmosphere from this steady state should take place. The signs of these deviations can be arbitrary. In practice, it means that the first-order guess of the problem (when the value of the “small parameter” is taken into consideration in a linear sense) has to be used. The essence of this guess is determined by free atmospheric variations unadjusted to the oceanic state. The moral of this result is that the use of long-lived oceanic anomalies for very long-term predictions of the atmospheric behavior is an ill-posed problem.

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Sparrow, Sarah et al.: The Impact of North Atlantic Sea Surface Temperatures on Attributing Human Influence for the Winter 2013/14 Southern England Floods.  Poster   View abstract

 
The Impact of North Atlantic Sea Surface Temperatures on Attributing Human Influence for the Winter 2013/14 Southern England Floods
Sarah Sparrow1,3*, Nathalie Schaller2, Karsten Haustein3, Andy Bowery1, Myles Allen2,3

* Presenting author

1) University of Oxford, Oxford e-research centre, United Kingdom
2) University of Oxford, Physics, United Kingdom
3) University of Oxford, Environmental Change Institute, United Kingdom

The succession of storms reaching Southern England in the winter of 2013/2014 resulted in severe floods that generated £451 million insured losses. A large attribution study of 134,354 simulations has been performed as part of the weather@home project for this winter to identify the role of human induced climate change on the resulting floods. Two groups of simulations are performed one labeled ‘Actual’ representing the 2013/2014 winter as observed and the other labeled ‘Natural’ representing conditions as they might have been without human influences. For the Natural simulations, observed sea surface temperature (SST) patterns have an anthropogenic component removed, which is derived from differences between the “Historical” and “HistoricalNat” runs of 11 different CMIP5 models. We show that simulations otherwise identical apart from the naturalized SST pattern used, can exhibit very different dynamical responses. Consequently, for some of the 11 CMIP5 patterns, human influences had an attributable effect on southern England precipitation, whereas for SST patterns based on other CMIP5 models, no attributable human impact was detected. This study highlights the importance of North Atlantic SSTs in driving European weather.

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Tett, Simon et al.: The Day Before Tomorrow: How might the Atlantic Meridional Overturning Circulation have changed?  Poster   View abstract

 
The Day Before Tomorrow: How might the Atlantic Meridional Overturning Circulation have changed?
Simon Tett1*, Toby Sherwin2, Gabi Hegerl1

* Presenting author

1) University of Edinburgh, School of Geosciences, UK
2) Scottish Marine Institute, Scottish Association for Marine Science, UK

Mass transports from seven different ocean reanalysis (DePreSys, MOVE-CORE, SODA-2.2.4, GFDL-CM2.1-ECDA, K7ODA, GECCO2 and ORA-S4) were computed across three lines across the North Atlantic (Greenland-Scotland Ridge (GSR), 41N and 26N) and these compared with observational estimates. GSR transport was computed by computing total transport with respect to temperature and finding the maximum transport across the ridge. Using uncertainty from observational error and random variability in the reanalysis Bayes weights were used to compute multi-reanalysis mean transports. Uncertainties in the multi-reanalysis mean were computed through bootstrapping. As the Bayes weights are dominated by only two ocean reanlayis (ORA-S4 and SODA-2.2.4) these uncertainties need to be interpreted cautiously. A similar analysis was done for many CMIP5 simulations. Neither reanalyses nor simulations show significant trends relative to the uncertainty in the multi-reanalysis or multi-model mean respectively. However, uncertainty in the reanalysis is small enough on decadal scales to examine decadal variability in the AMOC. The ocean reanalysis have decadal variability with a magnitude of about 1 Sv that is coherent across much of the North Atlantic with maximum northward transports circa 1970, 1985 and 1995.

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Thornalley, David J.R. et al.: A Holocene and last 1000 yr perspective on variability in the deep currents of the AMOC: An exceptional twentieth century slowdown of the AMOC?  Slides   View abstract

 
A Holocene and last 1000 yr perspective on variability in the deep currents of the AMOC: An exceptional twentieth century slowdown of the AMOC?
David J.R. Thornalley1,2*, Delia Oppo2, Paola Moffa-Sanchez3,4, Ian R. Hall4, Lloyd Keigwin2

* Presenting author

1) University College London, Department of Geography, UK
2) Woods Hole Oceanographic Institution, Department of Geology and Geophysics, USA
3) Rutgers, Department of Marine and Coastal Sciences, USA
4) Cardiff University, School of Earth and Ocean Sciences, UK

Several proxy and modelling studies suggest that there may have been considerable change in the operation the AMOC during the present interglacial. Yet despite its importance for regional and global climate, the history of the AMOC is poorly constrained. Improving our knowledge of past longer term AMOC variability will contribute to our general understanding of the dynamics of ocean circulation and the role it may play in causing or amplifying climate variability on multi-decadal to millennial timescales. It also enables a longer term perspective on the current state of, and recent trends in the AMOC.

We present Holocene grain-size records in depth transects from Blake Outer Ridge and Cape Hatteras, sampling the full-depth range of the Deep Western Boundary Current (DWBC), part of the lower limb of the AMOC. These records complement a depth-transect of grain-size records sampling the Iceland-Scotland (I-S) overflow, showing Holocene variations that reflect deglacial meltwater forcing in the early Holocene and insolation-forced trends from the middle-to-late Holocene (Thornalley et al., 2013, Climate of the Past). We will also present detailed grain-size records for the last 1,000 years, both in a depth transect of cores off Cape Hatteras, and from cores in the Iceland Basin, sampling the I-S overflow. Our extensive datasets enable us to provide a coherent synthesis of changes in the flow strength of key components of the AMOC on multi-decadal to millennial timescales.

Specific questions to be addressed include: How well coupled are Holocene trends in Iceland-Scotland overflow and the DWBC? How did I-S overflow and the AMOC vary during the last millennium, including the last ~150 years since the end of the Little Ice Age? Initial results suggest a long-term anti-phasing of the Nordic overflows, wherein mid-late Holocene weakening of the I-S overflow has been compensated for by a strengthening of Denmark Strait overflow. We will also report on pronounced centennial-millennial scale reductions in the inferred flow strength at sites bathed by Labrador Sea Water (LSW). Emerging results for the last millennium also indicate an exceptional slowdown in the flow of ISOW and LSW during the twentieth century, providing paleoceanographic support for the findings of Rahmstorf et al (2015, Nature Climate Change)

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Van Sebille, Erik et al.: The fate of Mediterranean Outflow Water in the North Atlantic and its mixing with Labrador Sea Water.  Poster   View abstract

 
The fate of Mediterranean Outflow Water in the North Atlantic and its mixing with Labrador Sea Water
Erik Van Sebille1*, Robert Marsh2, Gerard McCarthy3, Jesus Peña-Izquierdo4, Josep Pelegrí4

* Presenting author

1) Imperial College London, Grantham Institute & Department of Physics, United Kingdom
2) University of Southampton, United Kingdom
3) National Oceanographic Centre, United Kingdom
4) Institut de Ciències del Mar, Spain

Mediterranean Outflow Water spreads into the North Atlantic Ocean below the thermocline, forming a tongue that slowly moves westward until it reaches the North American coast. Meanwhile, Labrador Sea Water formed at high latitudes in the North Atlantic flows southward along the American coast in the Deep Western Boundary Current, and flows on similar density levels as the Mediterranean Outflow Water.

The two water masses mix at some point, to jointly cross the Equator in the Deep Western Boundary Current. The question is how and where this mixing happens. In particularly, it is unclear how variability in the strength of formation the two water mass relates and impacts on the southward export of mid-depth water. This is important because, while the two water masses have the same density, they have vastly different temperature and salinity properties. Better knowledge of the mixing processes will allow for an improved understanding of AMOC dynamics.

Here, we show results from eddy-resolving models where we track the water flowing from both the southern tip of the Labrador Sea and the Strait of Gibraltar, using virtual Lagrangian particles. We analyse the trajectories of these particles to see where and how the two water masses mix, and reconcile this with information on the modeled and observed salinity field in the North Atlantic. We show what the role of meso-scale eddies is, and how the interaction of the two water masses ultimately sets the thermohaline structure of the mid-depth North Atlantic Ocean.

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Williams, Ric et al.: Climate sensitivity to ocean sequestration of heat and carbon.  Poster   View abstract

 
Climate sensitivity to ocean sequestration of heat and carbon
Ric Williams1*, Philip Goodwin2, Andy Ridgwell3

* Presenting author

1) University of Liverpool, School of Environmental Sciences, UK
2) University of Southampton, Department of Ocean and Earth Sciences, UK
3) University of Bristol, School of Geographical Science, UK

Ocean ventilation is a crucial process leading to heat and anthropogenic carbon being sequestered from the atmosphere. The rate by which the global ocean sequesters heat and carbon has a profound effect on the transient global warming. Based on theory for an idealised atmosphere and ocean (Goodwin et al., 2015), the relative rates by which heat and carbon are sequestered affects whether transient global warming depends nearly linearly on the cumulative carbon emissions; this dependence is referred to as the Transient Climate Response to Emissions, the TCRE. As the ocean drawdown of heat declines in time, the sensitivity of surface warming to radiative forcing increases in time. On the other hand, as the ocean drawdown of carbon increases in time, the sensitivity of radiative forcing to carbon emissions decreases in time. The sensitivity of surface warming to cumulative carbon emissions (the TCRE) depends on the product of the sensitivity of surface warming to radiative forcing and the sensitivity of radiative forcing to cumulative carbon emissions. Thus, the sensitivity of surface warming to cumulative carbon emissions (the TCRE) only weakly varies with time. This response is illustrated with model integrations with an Earth System Model (GENIE), configured as a coupled atmosphere and ocean with an active meridional overturning: there are partly opposing changes in time of the sensitivity of surface warming to radiative forcing and sensitivity of radiative forcing to carbon emissions. The response is likely to be modified in climate models with additional climate forcing from non-CO2 greenhouse gases and aerosols, and possibly by a more realistic representation of ocean circulation.

Goodwin, P., R.G. Williams and A. Ridgwell, 2015. Sensitivity of climate to cumulative carbon emissions due to compensation of ocean heat and carbon uptake. Nature Geoscience, 8, 29-34, doi:10.1038/ngeo2304.

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Yeager, Stephen G. et al.: Predicted growth of Atlantic sea-ice in the coming decade.  Slides   View abstract

 
Predicted growth of Atlantic sea-ice in the coming decade
Stephen G. Yeager1*, Haiyan Teng1, Gokhan Danabasoglu1

* Presenting author

1) NCAR, Climate and Global Dynamics, USA

There is little doubt that we will see a decline in Arctic sea-ice cover in this century in response to anthropogenic warming, and yet internal climate variations are expected to generate considerable spread in the multi-year trends in Arctic sea-ice extent for many more years into the future. Variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC), in particular, would appear to play an important role in modulating rates of sea-ice loss because of the associated variations in heat transport into the high latitude North Atlantic. We present evidence that the extreme negative trends in Arctic winter sea-ice extent in the late 1990s were a predictable consequence of the preceding decade of persistent positive winter North Atlantic Oscillation (NAO) conditions and associated spin-up of the thermohaline circulation (THC). Initialized forecasts made with the Community Earth System Model decadal prediction system indicate that relatively low rates of North Atlantic Deep Water formation in recent years will result in a continuation of a THC spin-down that began more than a decade ago. Consequently, projected 10-year trends in winter Arctic winter sea-ice extent seem likely to be much more positive than has recently been observed, with the possibility of actual decadal growth in Atlantic sea-ice in the near future.

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Zhai, Xiaoming et al.: A simple model of the response of the Atlantic to the North Atlantic Oscillation.  Poster   View abstract

 
A simple model of the response of the Atlantic to the North Atlantic Oscillation
Xiaoming Zhai1*, Helen L. Johnson2, David P. Marshall3

* Presenting author

1) University of East Anglia, School of Environmental Sciences, UK
2) University of Oxford, Department of Earth Sciences, UK
3) University of Oxford, Department of Physics, UK

The response of an idealised Atlantic ocean to wind and thermohaline forcing associated with the North Atlantic Oscillation (NAO) is investigated both analytically and numerically in the framework of a reduced-gravity model. The NAO-related wind forcing is found to drive a time-dependent "leaky" gyre circulation that integrates basin-wide stochastic wind Ekman pumping and initiates low-frequency variability along the western boundary. This is subsequently communicated, together with the stochastic variability induced by thermohaline forcing at high latitudes, to the remainder of the Atlantic via boundary and Rossby waves. At low frequencies, the basin-wide ocean heat content changes owing to NAO wind forcing and thermohaline forcing are found to oppose each other. The model further suggests that the recently reported opposing changes of the meridional overturning circulation in the Atlantic subtropical and subpolar gyres between 1950-1970 and 1980-2000 may be a generic feature caused by an interplay between the NAO wind and thermohaline forcing.

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Zhang, Rong: Impact of AMOC on the Low Frequency Variability of Summer Arctic Sea Ice Extent.  Slides   View abstract

 
Impact of AMOC on the Low Frequency Variability of Summer Arctic Sea Ice Extent
Rong Zhang1*

* Presenting author

1) NOAA, GFDL, USA

Satellite observations reveal a substantial decline in September Arctic sea ice extent (SIE) since 1979, which has played a leading role in the observed recent Arctic surface warming and has often been attributed, in large part, to the increase in greenhouse gases. The observed decline trend and future projections of ice-free summer by some climate models forced with increasing anthropogenic greenhouse gases bring up the potential for trans-Arctic shipping in the near future. However, the detail mechanisms causing the low frequency variability of summer Arctic SIE are still unclear. The most rapid decline in summer Arctic sea ice actually occurred during the recent global warming hiatus period. The CMIP5 multi-model mean response to changes in anthropogenic radiative forcings exhibits much less decline in September Arctic SIE but stronger warming in global mean surface temperature than that observed over the recent hiatus period, implying that natural variability might have played an important role in the observed recent decline in September Arctic SIE.

In this study, it is shown that AMOC and the associated Atlantic heat transport into the Arctic have played a significant role in the low frequency variability of summer Arctic SIE using the GFDL coupled climate model. At low frequency the March Barents Sea SIE anomaly is dominated by the anti-correlated anomaly in the Atlantic heat transport into the Arctic, thus is also significantly correlated with September Arctic SIE anomaly. The observed March Barents Sea SIE has a very similar normalized decline trends as the observed September Arctic SIE since 1979, consistent with an increasing trend in the Atlantic heat transport into the Arctic. The estimated increase in the Atlantic heat transport into the Arctic since 1979 is consistent with the strengthening of AMOC over this period as implied by AMOC fingerprints, and could have contributed substantially to the observed summer Arctic SIE decline. If the AMOC and the associated Atlantic heat transport into the Arctic were to weaken in the near future due to internal variability, there might be a hiatus in the decline of September Arctic SIE, and a delay in attaining a summer ice-free Arctic. This plausible scenario with enormous social and economical impacts cannot be ignored.

This study also shows that at multidecadal/centennial time scales, changes in poleward atmosphere heat transport across the entire Arctic Circle are compensating to and dominated by AMOC induced Atlantic heat transport anomalies into the Arctic, i.e. a stronger AMOC and associated enhanced Atlantic heat transport into the Arctic ocean leads to both reduced summer Arctic SIE and reduced poleward atmosphere heat transport into the Arctic. Hence variations in the atmosphere heat transport across the Arctic Circle provide a negative feedback to September Arctic SIE variations at multidecadal/centennial time scales.

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Zika, Jan D.: How overturning circulation sets ocean heat uptake and why this matters for transient climate change.  Slides   View abstract

 
How overturning circulation sets ocean heat uptake and why this matters for transient climate change
Jan D. Zika1*

* Presenting author

1) University of Southampton, National Oceanography Centre

Heat transport between the surface and deep-ocean strongly influences transient climate change. Mechanisms setting this transport are investigated using coupled climate models and by projecting ocean circulation into the temperature-depth diagram. In this diagram, a `cold cell' linked to Antarctic Bottom Water cools the deep ocean and is balanced by a `warm cell' linked to the Atlantic Meridional Overturning Circulation which warms the deep ocean. With anthropogenic warming, the cold cell collapses while the warm cell continues to warm the deep ocean. Changes in the advective vertical heat flux dominate over changes in the diffusive vertical heat flux. Simulations with increasingly strong warm cells, set by their mean Southern Hemisphere winds, exhibit increasing deep-ocean warming and hence weaker transient surface warming in response to the same anthropogenic forcing. It is argued that the accurate description of the mean overturning circulation – including its strength and the temperature of its upwelling and downwelling branches - is key to simulating transient climate change.

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Zou, Sijia et al.: Breaking the linkage between the Labrador Sea Water production and its export to the subtropical gyre.  Poster   View abstract

 
Breaking the linkage between the Labrador Sea Water production and its export to the subtropical gyre
Sijia Zou1*, Susan Lozier1

* Presenting author

1) Duke Unviersity, Earth and Ocean Sciences, United States

The linkage between the Labrador Sea Water (LSW) production and its export into the subtropical gyre primarily derives from Eulerian-based studies that show LSW property anomaly in the Labrador Basin leading the property anomaly at 26°N by about 10 years. However, few studies have shown a direct relationship between the LSW productivity and its export variability to the subtropical basin. In the present study, we launch Lagrangian floats in the LSW with an eddy-permitting ocean circulation model (ORCA025) per year (1961-2004) across AR7W and track them for 40 years. No significant correlation is found between the number of floats launched in the Labrador Basin (LSW production) and the number of floats exported to the subtropical gyre (LSW export). Moreover, the simulated floats take an unexpectedly long time to be exported: 22 ± 10 years to the subtropical gyre and 30 ±8 years to 30°N. This is in contrast to the 10 years’ transit time of LSW properties in the Eulerian frame, indicating different mechanisms that control the LSW volume export and property propagation.

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