The asymmetric structure a monsoon depression and its evolution are investigated and their relationship to the motion of the depression are examined. Other aspects of the structure and evolution of the same storm are described by Hell and Smith (1998). The depression formed within the monsoon trough near the coast of northwestern Australia in February 1994 and was a prominent feature in that area for about a week. It remained quasi-stationary for the first three days and subsequently tracked south-westwards. This is the same as in the simplest barotropic model paradigm, which concerns the motion of an initially-symmetric vortex on a southern hemisphere beta-plane in the absence of an environmental flow. The subsequent motion in the model can be understood in terms of the advection of planetary vorticity by the vortex circulation, which leads to the evolution of a wavenumber-one asymmetry in the relative vorticity. The associated wavenumber-one streamfunction asymmetry consists of a pair of counter-rotating gyres (the so-called beta-gyres), which have a relatively uniform westward and poleward oriented flow between them, across the vortex core. This flow accounts for the westward and poleward drift of the vortex. Analogous idealized calculations using a three-layer baroclinic model by Dengler and Smith (1998) show a similar south-westwards drift as well as the development of asymmetries akin to the beta-gyres in the middle layer of the model.
The question arises: to what extent is barotropic dynamics relevant to understanding the motion of the monsoon depression? Or put another way, can we observe the beta gyres and does the flow between them account for the cyclone motion relative to the environmental flow as it does in the barotropic model? We show that the analysed data for the monsoon depression do not possess flow asymmetries akin to the beta-gyres, a fact that leads us to appraise the utility of "barotropic thinking" in "understanding" the motion of the depression.
References
Dengler, K. and R. K. Smith, 1998: A monsoon depression over northwestern Australia. Part II: A numerical model study. Aust. Met. Mag, 47, 135-144.
Hell, R. M. and R. K. Smith, 1998: A monsoon depression over northwestern Australia. Part I: A case study. Aust. Met. Mag. 47, 21-40.