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 degrees N made by the Rapid Climate Change (RAPID) array provide the best current estimate of the state of the AMOC. The period 2004-11 when RAPID AMOC is available is too short to assess decadal variability of the AMOC. This modeling study introduces a new AMOC index (called AMOC(sv)) at 26 degrees N that combines the Florida Straits transport, the Ekman transport, and the southward geostrophic Sverdrup transport. The main hypothesis in this study is that the upper midocean geostrophic transport calculated using the RAPID array is also wind-driven and can be approximated by the geostrophic Sverdrup transport at interannual and longer time scales. This index is expected to reflect variations in the AMOC at interannual to 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 time scales, two different numerical 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 degrees N on both interannual and decadal time scales. These results indicate that it might be possible to extend the observation-based AMOC at 26 degrees N back to the 1980s.