Influence of Mid-Latitudinal Planetary Waves on the Tropics under the Existence of Critical Latitude

Abstract

The influence of eastward-moving, mid-latitudinal planetary waves on the tropics is investigated, using a numerical model of primitive equations in spherical coordinates. Under the existence of a horizontal shear between the middle latitude westerly and the equatorial easterly, these waves encounter critical latitude where their phase speed is equal to the mean flow in the course of propagation.
The preliminary consideration using linearized equations shows that mid-latitudinal planetarywave with zonal wavenumber 1 under the mean flow of 35m/sec can affect the tropical atmosphere most intensely when it has a period of about 12 days, in case of no critical latitude. In the case with critical latitude, the wave becomes evanescent in the south of that latitude and cannot have any influence upon the equatorial disturbances.
However, in the model including non-linear interaction with zonal flow, this conclusion is greatly altered. The critical latitude can no longer be a barrier to the southward energy flow from the middle latitude and a new equatorial wave is instigated in the experiment. It is also revealed that the structure of the newly instigated wave is similar to the equatorial Kelvin wave.