Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

The observed Atlantic Meridional Overturning Circulation (AMOC) at 26.5 degrees N shows interannual variability consisting of an increase from early 2004 to late 2005 and a following downtrend, which reaches a minimum in the winter of 2009/2010. These interannual AMOC fluctuations are dominated by changes in the upper mid-ocean geostrophic flow except during the winter of 2009/2010, when the anomalous wind-driven Ekman transport also has a significant contribution. The physical mechanisms for the interannual changes of the AMOC are proposed and evaluated in a two-layer model. While the Ekman transport is linked to the North Atlantic Oscillation (NAO), the anomalous geostrophic transport involves the oceanic adjustment to surface wind forcing. In particular, the intensification and weakening of the southward interior geostrophic flow is modulated by the internal Rossby wave adjustment to the surface wind forcing. The Gulf Stream, on the other hand, is controlled by both topographic waves along the US coast and westward propagating planetary waves. Our study suggests that a large part of the observed AMOC interannual variability at 26.5 degrees N can be explained by wind-driven dynamics. Key Points The AMOC at 26.5 degrees N has a downtrend between 2005 and 2010 These interannual fluctuations are dominated by changes in the geostrophic flow Most observed AMOC interannual changes can be explained by wind-driven dynamics