Mechanism of Interannual Variation of Meridional Overturning Circulation of the North Atlantic Ocean
Cecile Cabanes(1) , Tony Lee(1) , and Lee-Lueng Fu(1)
Jet Propulsion Laboratory,
4800 Oak Grove Dr.,
Pasadena, CA, 91109,
The low-frequency variability of the meridional overturning circulation (MOC) of the North Atlantic Ocean has important climate implications because it is the main carrier of the meridional heat transport in the ocean. Previous studies suggest that, on decadal time scales, the variability of the MOC is primarily caused by buoyancy forcing, and that sea level can be used as an indicator of the changes in MOC. In this study, we investigate the nature of the interannual variability of the MOC and its relation to sea level using an ECCO assimilation product in which TOPEX/Poseidon and JASON-1 altimeter data have been assimilated. The time series of the 1st Empirical Orthogonal Function (EOF) of the MOC are found to be coherent with the North Atlantic Oscillation (NAO) index. To decipher the processes associated with such interannual variation in the MOC, we decompose the MOC variation into the contributions by (1) the Ekman flow and its depth-independent compensation, (2) the vertical shear flow, and (3) the external mode associated with the barotropic gyre. The latter is caused by meridional barotropic currents flowing in different longitude bands over different depths and give rise to an apparent meridional overturning circulation upon zonal average. For example, when the Gulf Stream travels over shallower depths in the west and its interior return flow goes over much great depths in the east, the resulting meridional transport stream function is characterized by an apparent "overturning" with northward flow in the upper ocean and southward flow at depths. The vertical shear component is found to explain much of the variability of the MOC at mid-latitudes. The contributions by the Ekman component and external mode are important in the tropics and at high-latitudes, respectively. By thermal wind relation, the variation in geostrophic vertical shear is proportional to east-west density difference across the basin. This relation is used to diagnose how the local and remote wind forcing generate the east-west temperature difference across the basin to drive the anomalous MOC. The relation of the east-west density difference to sea level is also addressed.