Eddy-Driven Recirculations from a Localized Transient Forcing

The generation of time-mean recirculation gyres from the nonlinear rectification of an oscillatory, spatially localized vorticity forcing is examined analytically and numerically. Insights into the rectification mechanism are presented and the influence of the variations of forcing parameters, stratification, and mean background flow are explored. This exploration shows that the efficiency of the rectification depends on the properties of the energy radiation from the forcing, which in turn depends on the waves that participate in the rectification process. The particular waves are selected by the relation of the forcing parameters to the available free Rossby wave spectrum. An enhanced response is achieved if the parameters are such to select meridionally propagating waves, and a resonant response results if the forcing selects the Rossby wave with zero zonal group velocity and maximum meridional group velocity, which is optimal for producing rectified flows. Although formulated in a weakly nonlinear wave limit, simulations in a more realistic turbulent system suggest that this understanding of the mechanism remains useful in a strongly nonlinear regime with consideration of mean flow effects and wave–mean flow interaction now needing to be taken into account. The problem presented here is idealized but has general application in the understanding of eddy–eddy and eddy–mean flow interactions as the contrasting limit to that of spatially broad (basinwide) forcing and is relevant given that many sources of oceanic eddies are localized in space.