A theoretical model of Australian northwest cloudband disturbances and Southern Hemisphere storm tracks: The role of SST anomalies

Abstract

The authors examine the effects of an enhanced sea surface temperature gradient, between the central Indian Ocean and the Indonesian archipelago, on the structure of the monthly averaged three-dimensional July global circulation and particularly on its consequent instability properties. This study has been conducted using a two-level primitive-equation model including a wave-CISK cumulus heating parameterization and using basic states taken from two AGCM simulations of the July circulation, with and without an enhanced SST gradient. The growing disturbances have been analyzed for various strengths of the cumulus heating. The study has focused on circulation changes associated with the development of Australian northwest cloudband disturbances and changes in Southern Hemisphere storm tracks. With an enhanced SST gradient, the authors have found a new group of modes that can be associated with circulation changes surrounding the onset of the simulated northwest cloudband events. Such modes are shown to compare favorably with simulated perturbations in the circulation and the results of an EOF analysis. With an enhanced gradient, there is also a discernible equatorward shift of the storm track instability modes over the Australian region, and the modes have much larger amplitude on the subtropical jet. The upper-level divergence is also more concentrated in the Australian region.