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 scales 
analysis of the ocean interior, we find the mesoscale field is strongly 
affected by the ventilated thermocline, but no feed back from the eddies 
to the large scale is found.  We then analyze the western boundary 
region arguing that the associated currents divide between coastal jets, 
which conserve mean energy, and open ocean, separated jet extensions
where the mesoscale is energized by the mean field.   It is the 
separated jet zone where the primary loss of general circulation energy 
to the mesoscale occurs.  Connections to the `Thickness Weighted 
Average' form of the primitive equations are made which support the 
differing roles of the eddies in these regions.  These ideas are then 
tested by an analysis of a regional primitive equation 1/12-degree 
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 
they 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 forcing is much smaller than the interior energy fluxes. Thus, the 
general circulation is characterized as recirculating energy in the 
manner of a Fofonoff gyre.