A new index for the Atlantic Meridional Overturning Circulation at 26°N

The Atlantic Meridional Overturning Circulation (AMOC) has received considerable attention, motivated by its major role in the global climate system. Observations of AMOC strength at 26°N made by RAPID-WATCH, provide our best current estimate of the state of the AMOC. The period 2004-2011 when RAPID AMOC is available is too short to assess decadal variability of the AMOC. In this modelling study, we define a new AMOC index (called AMOCSV ) at 26°N that combines the Florida Straits transport, the Ekman transport and the southward geostrophic Sverdrup transport. Our main hypothesis in this study is that the Upper Mid-Ocean geostrophic transport calculated using the RAPID array, is also wind-driven and can be approximated by the geostrophic Sverdrup transport at interannual and longer timescales. This index is expected to reflect variations in the AMOC at interannual / decadal time scales. This estimate of the surface branch of the AMOC can be constructed as long as reliable measurements are available for the Gulf Stream and for wind stress. To test the reliability of the AMOCSV on interannual and longer timescales, two different NEMO simulations are used: a forced and a coupled simulation. Using these simulations the AMOCSV captures a substantial fraction of the AMOC variability and is in good agreement with the AMOC transport at 26°N on both interannual and decadal timescales. These results indicate that it might be possible to extend the observation-based AMOC at 26°N back to the 1980s.