A DIAGNOSTIC STUDY OF THE WIND-DRIVEN AND BUOYANCY-DRIVEN NORTH-ATLANTIC CIRCULATION

A two-moving-layer diagnostic model with a variable depth mixed layer on top is applied to the annual mean climatology of the North Atlantic. Estimates of the annual mean mass flux from the mixed layer into the upper thermocline (called mixed layer pumping) and subsurface heating rates are obtained on the basin scale from hydrographic data alone. No independent measures of wind stress or other surface forcings are required. In addition, it is shown that the nonlinear terms in the vorticity equation are critical to balance the subsurface heat flux in the Gulf Stream and over most of the subtropical gyre. Evidence is presented for the existence of interior, southeastern, northeastern, and recirculation regimes in the North Atlantic. These regimes are distinguished by the behavior of the characteristic trajectories of the system and by whether the flow is a direct cell (interfacial flux and vertical velocity have the same sign) or indirect cell (interfacial flux and vertical velocity have the opposite sign). It is demonstrated here that one can make use of analytic approaches in the analysis of historical data sets to yield relatively simple solutions that give a direct link between theory and observations.