The Influence of Meridional Shear on Planetary Waves. Part 2: Critical Latitudes

Abstract

With the simplifying assumption that the mean zonal wind is a function of latitude only, numerical and analytical methods are applied to study the effects of critical latitudes (where the Doppler-shifted frequency is 0) on planetary waves. On the midlatitude beta-plane, it is shown that the modes divide into two limiting classes. The low-order, vertically propagating modes are confined to that side of the critical latitude where the mean winds are westerly as found by Dickinson (1968b). The high-order modes, although vertically trapped, are indifferent to the singularity and oscillate sinusoidally on both sides of the critical latitude as if it were not present. On the sphere, there is also a third class of low-order modes which are latitudinally trapped near the pole where the winds are easterly and also are unaffected by the critical latitude. Numerical studies show that it is the location of the critical latitude far more than the intensity or shape of the winds that controls the dynamics of the low-order, vertically propagating modes. The most striking conclusion on the equatorial beta-plane is that sufficiently strong linear shear, although stable by conventional criterion, makes the Kelvin wave unstable. Together with the transparency of the high-order global modes, this shows that strong baroclinity may drastically alter the behavior of waves with critical latitudes, from that predicted by the barotropic or near-barotropic models so widely applied to critical latitudes in the past.