A monsoon depression over north-western Australia. Part III: Motion

This paper examines the symmetric and asymmetric structure of a monsoon depression which formed in the large-scale monsoon trough over northwestern Australia in February 1994 and seeks to isolate the factors contributing to its motion. A day or two after formation, the depression moved in a generally southwestward direction, similar to the motion of an idealised barotropic vortex on a southern hemisphere beta-plane with no environmental flow. The extent to which the motion of the depression can be interpreted in terms of this simple paradigm is investigated. We identify three phases of quasi-steady motion, first towards the southwest and later towards the west. These are separated by periods when the centre remains almost stationary. We have investigated the deep-layer mean asymmetric vorticity and divergence structures of the depression and its environment to determine the extent to which the associated flow across the vortex centre can account for the depression’s motion. In Phase 1 the cross-vortex flow is influenced approximately equally by azimuthal wavenumber-one streamfunction asymmetries representing the flow of larger and equal or smaller horizontal scales compared with the scale of the monsoon depression. Furthermore, wavenumber-one velocity potential asymmetries of equal or smaller horizontal scales make a nonnegligible contribution to this flow. In Phase 2 the cross-vortex flow is dominated by streamfunction asymmetries of equal or smaller horizontal scales and in Phase 3 the large-scale streamfunction asymmetry dominates. In Phase 2, the streamfunction asymmetry closely resembles the beta-gyres in a barotropic model calculation. However, examination of the deep-layer mean vorticity analyses shows that there is at no time a clear-scale separation between the depression and its environment as is normally assumed in idealised model calculations.