Instability and Nonlinear Equilibrations of the Large Scale Wind-driven Flow

While it has been known for several decades that the variability of the ocean circulation in the gyre interiors is much stronger than the mean flow, the sources of these strong eddies are still not well understood. Baroclinic instability of the large scale circulation is a likely source, but it has been shown that zonal flows with weak shear typical of the gyre interiors can not generate very strong variability. I have recently addressed the question of whether the meridional component of the wind-driven gyres might generate strong mesoscale eddies more effectively than can zonal flows. Scale analaysis and and nonlinear numerical modelling studies show that the potential energy stored in the meridional component of the large-scale wind-driven gyres may be effectively converted to eddy kinetic energy even for very weakly sheared mean flows. For parameters typical of the interior of the mid-latitude subtropical gyres the eddy kinetic energy at equilibration is two orders of magnitude greater than the mean kinetic energy.