Midlatitude Rossby Wave Forcing of Equatorial Kelvin Waves

Observations strongly suggest that convectively coupled Kelvin waves can be generated by extratropical wave activity. This mechanism appears to be particularly efficient over the Australian region, where wave activity appears immediately after the extratropical Rossby waves propagate into the region during the Austral winter. This interaction occurs where the zonal wind is strongly sheared both in the meridional and vertical directions. In order to understand this phenomenon the authors study the linear primitive equations in the presence of barotropic and baroclinic shear.

The sheared analogs of the Matsuno modes are calculated as well as their dispersion characteristics. The resulting waves show a broad range of instabilities and can be categorized into three groups. First there are the classical Matsuno modes modified by shear. Second there are extratropical "free" Rossby waves. Third, there are Rossby waves which are meridionally confined to the shear layer resulting from the instability of that layer.

We find that a shear unstable equatorial Rossby wave, when transported by strong equatorial westerlies, acquires a pressure and zonal velocity profile above the equator which is extremely similar to an equatorial Kelvin wave. Although this seems like a coupled Rossby/Kelvin instability (in the vein of Sakai's Rossby-Kelvin instability), it is not the same phenomenon. The Rossby-Kelvin instability requires the frequency of a stable equatorial Rossby mode to coincide with the stable Kelvin wave frequency in order for the two modes to create a stable/unstable pair. Our results show that unstable Rossby waves need only have their frequencies Doppler shifted to that of the Kelvin wave frequency (by the underlying shear) in order that they acquire a Kelvin-like component - the Matsuno Kelvin wave remains stable for these values of shear.