Monitoring the MOC at 26.5°N

There is a northward transport of heat throughout the Atlantic, reaching a maximum of 1.3PW (25% of the global heat flux) around 24.5N. The heat transport is a balance of the northward flux of a warm Gulf Stream, and a southward flux of cooler thermocline and cold North Atlantic Deep Water that is known as the meridional overturning circulation (MOC). The heat transported by the MOC is given off to the atmosphere and much of it is carried eastward by westerly winds. This is an important contribution to northwestern Europe's mild climate. Numerical models suggest that the MOC is likely to weaken by about 30% in the coming century as a consequence of greenhouse gas emissions. Furthermore, paleoclimate records suggest that during the last Ice Age the MOC has undergone abrupt rearrangements that were responsible for a cooling of European climate of between 5-10C.

A principal objective of the RAPID programme is the development and maintenance of a pre-operational prototype system that will continuously observe the strength and structure of the MOC. An initiative has been formed to fulfill this objective and consists of three interlinked projects:
  1. Nineteen moorings were deployed in March 2004 across the Atlantic at 26.5N to measure the southward branch of the MOC (Hirschi et al., 2003, Baehr et al., 2004, Cunningham et al., 2007, Kanzow et al., 2007) .
  2. Additional moorings were deployed on the western boundary along 26.5N in the framework of the U.S. Meridional Overturning Circulation and Heatflux Array (MOCHA) project (Prof. Bill Johns, University of Miami) to resolve transport in the Deep Western Boundary Current (Johns et al., 2005, 2008, Bryden et al., 2005). These moorings allow surface-to-bottom density profiles along the western boundary, Mid-Atlantic Ridge, and eastern boundary to be observed. As a result, the transatlantic pressure gradient can be continuously measured.
  3. Dr Molly Baringer (NOAA/AOML) leads the monitoring of the northward branch of the MOC using submarine telephone cables in the Florida Straits (Baringer et al., 2001).
A more detailed description of the initiative is found in the initial project proposal (Marotzke et al., 2002).

Since 2004 the MOC monitoring array has been recovered and redeployed annually. The observations from the first year have shown that even on subannual timescales a large variability is found for the MOC (Cunningham et al., 2007, Kanzow et al., 2007). The transport timeseries have now been extended and cover the period from April 2004 to October 2007 (Figure 1). After its first phase from 2004 to 2008 the MOC monitoring project has now been funded until 2014 in the framework of RAPID-WATCH . This will provide a unique, decade long timeseries of the MOC.

Figure 1: Transport timeseries obtained from the first 10 years of observations at 26.5°N. The different curves show the MOC (red line) and its constituents, i.e. the transport through the Florida Straits (blue line), the Ekman transport (black line), and the density driven transport obtained from the mooring data (pink line). The transport units are Sverdrups (Sv, 1Sv = 106m3s-1). The mean and standard deviations for the different transports are 18.5 ±4.9Sv (MOC), 31.7 ±2.8Sv (Florida Straits), 3.5 ±3.4Sv (Ekman), and -16.6 ±3.2Sv (transport from mooring densities). Brief descriptions of how the transports are calculated are given below.


Monitoring the MOC at 26.5N
An estimate of the meridional flow relating to the MOC along 26.5N can be obtained by decomposing it into three components:
Transport through the Florida Strait
The northward flowing Florida Current (T) contains salty seawater that can conduct electricity. As charged particles in the seawater pass through the Earth's magnetic field (B), an electrical field is generated. This field induces a voltage (U) in submerged telephone lines crossing the Florida Strait (see Figure 2). After proper calibration, the induced voltage can be used as a continuous indicator of the strength of the ocean current through the strait. More details can be found at the project's website.

Figure 2: Monitoring currents in the Florida Straits using submerged
telephone cables between the US and Bahamas.

Ekman Transport
The zonal wind stress acting at the ocean surface generates a meridional(north-south) Ekman transport (see Figure 3). This mass transport is typically confined to the upper 50m of the ocean. At 26N, the Ekman contribution to the MOC is relatively small (2-4 Sv on average). However, it is responsible for the largest short-term (subannual) variability occurring in the MOC. In the Rapid MOC monitoring system initiative, the winds will be estimated using satellite and ship measurements.

Figure 3: Wind stress and Ekman transport.

Density Related Transport
If, for a particular ocean location, both the force of the wind acting on the ocean surface and the vertical change in sewater density is known, then vertical change in the meridional flow induced by the wind can be approximated. A goal of the RAPID MOC mooring array is to measure the vertical change in seawater density at a series of different longitudes between the Bahamas and the African continent. The differences in these measured density profiles can allow one to estimate the current velocities with respect to depth along 26.5N (see Figure 4).

Figure 4: Calculated velocities between pairs of adjacent vertical density profiles

MOC Estimate
The combination of the velocity fields from the three components (Florida Strait, Ekman, and Density-driven transport) forms a rough approximation of the meridional velocity across 26.5N. In general the corresponding computed net meridional mass transport is not zero and in order to obtain an MOC estimate a spatially (but not temporally) constant correction is added to the velocity field. Figure 5 illustrates the three components of the MOC: Florida Strait transport (red arrow), Ekman transport (green arrow), and density-induced transport (light blue arrows). The velocity correction is shown as the yellow arrows.

Figure 5: Estimate of the MOC.