Synoptic-Scale Modulation of Convection during the Australian Summer Monsoon

Abstract

Time-longitude representations of Japanese Geostationary Meteorology Satellite (GMS) infrared imagery indicate the existence of major synoptic-scale banding of convection within the Australian summer monsoon. This bands can be interpreted as active and break phases of major convective activity within the monsoon. This study relates the occurrence of convection and its organization into synoptic-scale bands to observable flow features. GMS Digital Equivalent Blackbody Temperature (TBB) data and wind fields from the Australian Numerical Meteorology Research Centre (ANMRC) tropical analysis scheme for the 1978/79 and 1983/84 seasons were composited relative to wind field surges and bands of enhanced and suppressed convective activity. Some low-level wind surges in the South China Sea produced a modulation in the convective activity within the preexisting bands but did not seem to be associated with their formation. Surges in the Southern Hemisphere trade-wind easterlies and the southerly jet off the west coast of Australia were not associated with any major change in convective activity. The organization of the convection into synoptic-scale bands was associated with the Southern Hemisphere 200 mb flow. Areas of enhanced convective banding were cast of upper-level tropospheric troughs. The troughs and associated subtropical jet streaks had amplified from the south, interacting and enhancing the monsoonal convection. West of the trough, in the region of subsiding air, the convection was suppressed. Independent studies taken from the 1984/85 season showed that this type of interaction was discernable for individual cases.