monitoring the atlantic overturning circulation

Programme objectives

The programme's overall aim is:

to determine the variability of the Atlantic Meridional Overturning Circulation (AMOC), and its links to climate and to the ocean carbon sink, on interannual-to-decadal time scales.

This will be achieved through the continued support of the 26°N array and by supporting use of the data in three key areas:

  • Application of array data for improved ocean state estimation;
  • Use of array data to understand the role of the AMOC in climate variability and predictability;
  • Addition of biogeochemical sensors to the array and use to constrain biogeochemical fluxes.

About the RAPID-AMOC Programme

RAPID-AMOC (2014-2018) builds on the RAPID (2001-2007) and RAPID-WATCH (2008-2014) programmes and will extend the time series of the strength and structure of the Atlantic Meridional Overturning Circulation (AMOC) to 16 years.

Observing and understanding the changes in the AMOC is critically important for identifying the mechanisms of decadal climate variability and change, and for interannual-to-decadal climate prediction. Sustained observations are also necessary for assessing the possibility of future abrupt change in the AMOC similar to those seen in palaeoclimate records.

The RAPID array at 26°N is key to this effort. The array of instruments across the Atlantic from Morocco to Florida measure temperature, salinity and current velocities from the near surface to the sea floor. The array data is combined with observations from the Florida current and satellite measurements of surface winds to calculate the overturning circulation: northward flow in the upper layer, and southward flow in the deep ocean.

The 26°N array was first deployed in April 2004 as part of the RAPID Programme. Measurements continued during RAPID-WATCH to yield a 10-year time series of continuous AMOC measurements. Both RAPID and RAPID-WATCH also included other research projects, which used ocean model simulations and analysis of RAPID and other observations to increase scientific understanding of the AMOC and its role in Earth's climate.

Observations from the array have already revolutionised understanding of AMOC variability and documented its variability on seasonal to interannual timescales. The first few years of observations, demonstrated the feasibility of AMOC measurement, provided new insights into the seasonal cycle, and allowed apparent trends in previous historical 'snapshots' to be seen in the context of natural variability.

More recently, from autumn 2009 to spring 2010 the observations revealed a substantial weakening of the AMOC. A second weakening event occurred in late 2010. These events coincided with record low states of the North Atlantic Oscillation, and cold winter conditions over Europe. These anomalies would not have been detected without the presence of the array.

The possible role of the AMOC anomalies in the cold winter of 2009/10 and 2010/11 is now a topic of active research. Anomalies of this magnitude are not seen in state-of-the-art climate models, calling into question the reliability of these models to serve as guides for future behaviour of the AMOC.

The value of the array is likely to increase further as the length of the record increases, a more robust picture of the year-to-year variations of the AMOC develops, and perhaps further surprises emerge.