Unstable waves on oceanic fronts: large amplitude behavior and mean flow generation.

Abstract

A primitive equation numerical model is used to study the large amplitude behaviour of unstable waves on an oceanic density front, concentrating on a single wave model corresponding to the fastest growing linear solution. At small amplitude the model results agree well with linear theory, and at large amplitude 'backward breaking' occurs and vortex pairs are formed, as have been observed in laboratory experiments. Vortex stretching due to advection across layer depth contours favours formation of the vortex pairs, with the result that the BETA effect is not necessary for vortex detachment, as it was in a previous quasigeostrophic study by Ikeda. Examination of the energetics allows a life cycle to be identified for the waves, and shows that kinetic energy is fed into the mean flow through Reynolds stress. It is shown that the BETA effect is important in determining the precise form of the mean flow generated, and this is interpreted in terms of the deep potential vorticity fluxes. For realistic parameters the mean flows generated agree well with observations of deep mean flows near the Gulf Stream, in particular there is a counterflow (westward) directly below the original position of the front and a positive (eastward) flow displaced to the south. This pattern is not found in the results of eddy resolving general circulation models and is qualitatively different from the three jet structure found in Ikeda's study of a symmetric, quasi geostrophic jet.