Basin-Mode Interactions and Selection by the Mean Flow in a Reduced-Gravity Quasigeostrophic Model

Abstract

The selection mechanisms of Rossby basin modes are investigated in the reduced-gravity quasigeostrophic framework. The linear solution of the wind-driven circulation is decomposed in a steady forced?dissipated solution and a time-dependent component. The steady solution consists in a classical Sverdrup flow dissipated in a thin western boundary layer. The time-dependent solution is a sum of Rossby basin modes with arbitrary amplitudes. The effect of the nonlinear term is handled through a weakly nonlinear analysis providing a set of evolution equations for the mode amplitudes. It is shown both analytically (infinite Burger number) and numerically (finite Burger number) that mode stability is related to the gyre configuration. For cyclonic or anticyclonic single gyres, all basin modes are neutral. In the traditional (reversed) double-gyre case, large-scale basin modes are damped (unstable). Pure basin-mode interactions yield triads with cycling energy and subharmonic instabilities. The latter provide a potential mechanism for spectral reddening.