Eigenfrequencies and horizontal structure of divergent barotropic instability originating in tropical latitudes

Abstract

The effect of horizontal divergence on barotropic instability has not been extensively studied. A systematic investigation of the eigenfunctions of the generalized Laplace tidal equation for monotonic mean zonal wind profiles having a single, narrow region of reversed vorticity gradient in tropical latitudes reveals that, in the limit of low planetary zonal wavenumber, the modes of barotropic instability bifurcate into weakly divergent modes of hemispheric scale, and strongly divergent, "internal' modes trapped about the source region, ie, equatorially trapped. Disturbances in the second category penetrate into the deep tropics - the side of the critical latitude with positive intrinsic frequency - as a Kelvin wave type of behavior not previously seen in this context. These results suggest, first, that hemispheric barotropic instability need not be purely nondivergent. In fact, the growth of weakly divergent modes is preferred. -from Author