The RAPID project: Monitoring the atlantic meridional overturning circulation (AMOC)

Hallo MOOCers,

This blog is brought to you by Dr. Aurélie. Duchez, a physical oceanographer at the National Oceanography Centre, Southampton. The topic of this blog is the Atlantic meridional overturning circulation (AMOC), which the RAPID project is aimed to monitor.

RAPID data available are freely available at: http://www.rapid.ac.uk/rapidmoc/

Observed decline of the Atlantic Meridional Overturning Circulation as observed by the RAPID array at 26°N

Global ocean circulation is an important factor in climate variability and change. In particular, changes in the strength of the Atlantic meridional overturning circulation (AMOC) have been implicated in ancient climate events, as well as in recent climate anomalies such as the rapid warming of the North Atlantic Ocean in the mid-1990s.

Fig. 1: Schematic diagram illustrating the component parts of the AMOC and the 26°N observing system. Black arrows represent the Ekman transport (predominantly northward). Red arrows illustrate the circulation of warm waters in the upper 1100 m, and blue arrows indicate the main southward flow of colder deep waters. The array of moorings used to measure the interior geostrophic transport is illustrated too.
Fig. 1: Schematic diagram illustrating the component parts of the AMOC and the 26°N observing system. Black arrows represent the Ekman transport (predominantly northward). Red arrows illustrate the circulation of warm waters in the upper 1100 m, and blue arrows indicate the main southward flow of colder deep waters. The array of moorings used to measure the interior geostrophic transport is illustrated too.

A mooring array known as the RAPID-MOCHA array, has been deployed at 26°N between the Bahamas and the Canary Islands and has provided continuous measurements of the strength and variability of this circulation since 2004. The AMOC and its component parts are monitored by combining a transatlantic array of moored instruments with submarine-cable based measurements of the Gulf Stream and satellite derived Ekman transport (Fig. 1 and 2).

Fig. 2: Schematic of one of the tallest moorings of the RAPID array.
Fig. 2: Schematic of one of the tallest moorings of the RAPID array.

The time series has recently been extended to October 2012 (Fig. 3) and the results show a downward trend since 2004. From April 2008 to March 2012 the AMOC was an average of 2.7 Sv weaker than in the first four years of observation (95% confidence that the reduction is 0.3 Sv or more). Ekman transport reduced by about 0.2 Sv and the Gulf Stream by 0.5 Sv but most of the change (2.0 Sv) is due to the mid-ocean geostrophic flow. The change of the mid-ocean geostrophic flow represents a strengthening of the subtropical gyre above the thermocline.

Fig. 3: Ten-day (colors) and three month low-pass (black) time series of Gulf Stream transport
(blue), Ekman transport (green), upper mid-ocean transport (magenta), and overturning transport
(red) for the period 1 April 2004 to 1 October 2012. A dashed black line shows the mean
annual cycle for the AMOC. Positive transports correspond to northward flow.

The increased southward flow of warm waters is balanced by a decrease in the southward flow of Lower North Atlantic Deep Water below 3000 m. The transport of Lower North Atlantic Deep Water slowed by 7% per year (95% confidence that the rate of slowing is greater than 2.5% per year).

For more information about this work, you can find in this paper:
Smeed, D. A., McCarthy, G., Cunningham, S. A., Frajka-Williams, E., Rayner, D., Johns, W. E., Meinen, C. S., Baringer, M. O., Moat, B. I., Duchez, A., and Bryden, H. L.: Observed decline of the Atlantic Meridional Overturning Circulation 2004 to 2012, Ocean Sci., 10, 29-38, doi:10.5194/os-10-29-2014, 2014.

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