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Dynamics of the Australian summer monsoon




Schubert, Wayne H., author
Ciesielski, Paul E., author
Harding, Kenneth W., author

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When a west-east line of deep convection forms over northern Australia, the potential vorticity field begins to change due to the latent heat release, with low level negative and upper level positive anomalies being induced. These potential vorticity patterns can be analytically derived by using a zonal balance model formulated in isentropic and potential latitude coordinates. The associated wind and mass fields can be found by solving an invertibility principle which is valid for these equatorial balanced flows. Since the convectively induced potential vorticity anomalies develop from an initial state which has potential vorticity increasing toward the north, reversed poleward gradients of potential vorticity are produced. The regions of potential vorticity gradient reversal are found on the poleward side of the ITCZ at low levels and on the equatorward side of the ITCZ at upper levels, just as in the observed fields during AMEX. For typical convective heating rates, significant potential vorticity gradient reversals occur quickly - on the order of a few days. This sets the stage for combined barotropic-baroclinic instability, the formation of tropical waves, and t he breakdown of the ITCZ. We can understand the barotropic aspects of this breakdown t rough a normal mode stability analysis of the nondivergent barotropic model with either a hyperbolic tangent shear layer basic state zonal wind or an idealized three region profile in which there is a central (ITCZ) region of anomalous absolute vorticity, surrounded by regions of undisturbed absolute vorticity. The latter model can be solved analytically, which allows direct interpretation of the breakdown in terms of the phase locking and growth of the counterpropagating vorticity anomalies (essentially Rossby waves) located on the two interfaces separating the three regions. In this sense the ITCZ is self-destructive and should not be viewed as a strictly steady state feature of the tropical circulation. In addition, according to this scenario, the potential vorticity dynamics of the Australian region are not unique, but are characterized by an ITCZ formation-breakdown cycle similar to that occurring in other tropical regions such as the tropical east Pacific and western Africa.


March 6, 1992.

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Monsoons -- Australia
Atmospheric circulation -- Australia
Ocean-atmosphere interaction
Convection (Meteorology)


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