On Wind‐Driven Energetics of Subtropical Gyres

The flow of energy in the wind‐driven circulation is examined in a combined theoretical and numerical study. Based on a multiple‐scale analysis, we find the mesoscale field in the ocean interior is strongly affected by, but does not feed back onto, the ventilated thermocline. In the western boundary region, the associated currents first appear as coastal jets, conserving mean energy, and later as separated jet extensions where the mesoscale is energized by the mean field. It is in the separated jet zone where the primary loss of general circulation energy to the mesoscale occurs. These ideas are tested by an analysis of a regional 1/12° primitive equation numerical model of the North Atlantic. The predictions of the theory are generally supported by the numerical results. The one exception is that topographic irregularities in the coastal jet spawn eddies, although these eddies contribute modestly to the energy budget. We therefore conclude the primary sink of wind input into the mean circulation is in the separated jet, and not the interior. The analysis also shows wind energy input to be much smaller than the interior energy fluxes; thus, the general circulation largely recirculates energy. Plain Language Summary Atmospheric winds provide energy to the ocean general circulation through surface stress, forcing the so‐called wind‐driven oceanic gyres. Although the primary energy sink of this large‐scale circulation is usually recognized to be energy transfers toward smaller scales, details remain unclear. In this paper, we argue that the ocean receives energy over the broader interior from the wind and recirculates that energy to the open ocean Gulf Stream, where it is lost to ocean eddies. We test these ideas by analyzing a 1/12°, primitive equations numerical simulation of the North Atlantic. The predictions of the theory are generally supported by the numerical results. Lastly, we note the energy moved in the general circulation greatly exceeds that added by the wind, implying the circulation acts like a flywheel. This very inertial character of the circulation resembles a classical model first recognized by Fofonoff in 1954. Key Points Multiple‐scale analysis of the wind‐driven circulation argues the primary sink of energy occurs in the separated boundary current jet Analyses of a 1/12° ocean simulation supports the theory; coastal topographic features play a secondary role Subtropical gyre dynamics are largely inertial, recirculating large amount of e