Dynamics of Wind-driven Atlantic Meridional Overturning Circulation Variability

The dynamical processes governing the wind-driven Atlantic Meridional Overtuning Circulation (AMOC) variability are studied using observations and a variety of models, ranging from a simple forced Rossby wave model to an eddy-resolving Ocean General Circulation Model(OGCM). To better identify the mechansims for the AMOC variability at different time scales, the AMOC is decomposed into Ekman and geostrophic transport components. On seasonal time scale, the AMOC varability is determined by both components in the extratropics but dominated by the Ekman transport in the tropics. While the Ekman transport is directly related to zonal wind stress seasonality, the comparison between different numerical models shows that the geostrophic transport involves a complex oceanic adjustment to the wind forcing. The oceanic adjustment is further evaluated by separating the zonally integrated geostrophic transport into eastern and western boundary currents and interior flows. Our results indicate that the seasonal AMOC cycle in the extratropics is controlled mainly by local boundary effects, where either the western or eastern boundary can be dominant at different latitudes, while in the northern tropics it is the interior flow and its lagged compensation by the western boundary current that determines the seasonal AMOC variability. The AMOC interannual variability is quantified in both in-situ observations at 26.5°N and numerical models. The observed AMOC interannual anomalies consist 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°N can be explained by wind-driven dynamics. In addition, the basinwide AMOC responses to the interannual wind forcing are investigated in an eddy-resolving OGCM. The diagnostic analysis suggests that topographic waves and interior baroclinic Rossby waves play essential roles in modulating the AMOC interannual variability throughout the Atlantic basin. The proposed mechanisms are evaluated in a simple two-layer model. The high-latitude anomalies are communicated into the lower latitudes by topographic waves and account for about 50% of the AMOC interannual variability in the subtropics. The topographic waves and the large scale Rossby waves excited by wind forcing set up coherent AMOC interannual variability across the tropics and subtropics. The comparisons between simple model and OGCM results suggest that a large fraction of interannual AMOC variations in the OGCM can be explained by wind-driven dynamics