Variability of sea surface height and circulation in the North Atlantic: Forcing mechanisms and linkages

• SSH variations are mostly caused by surface heat flux changes. For the depth-integrated circulation, the model results suggest that the contribution of the surface momentum flux is dominant.
• In the subpolar region, variations of gyre strength, SSH at Bravo and the NAO index are highly correlated.
• In the subtropical region, the surface momentum flux plays a dominant role in driving variations of the gyre circulation and AMOC.

Simulations with a coarse-resolution global ocean model during 1958–2004 are analyzed to understand the inter-annual and decadal variability of the North Atlantic. Analyses of Empirical Orthogonal Functions (EOFs) suggest relationships among basin-scale variations of sea surface height (SSH) and depth-integrated circulation, and the winter North Atlantic Oscillation (NAO) or the East Atlantic Pattern (EAP) indices. The linkages between the atmospheric indices and ocean variables are shown to be related to the different roles played by surface momentum and heat fluxes in driving ocean variability. In the subpolar region, variations of the gyre strength, SSH in the central Labrador Sea and the NAO index are highly correlated. Surface heat flux is important in driving variations of SSH and circulation in the upper ocean and decadal variations of the Atlantic Meridional Overturning Circulation (AMOC). Surface momentum flux drives a significant barotropic component of flow and makes a noticeable contribution to the AMOC. In the subtropical region, momentum flux plays a dominant role in driving variations of the gyre circulation and AMOC; there is a strong correlation between gyre strength and SSH at Bermuda.