Tropical low formation during the Australian monsoon: the events of January 2013

Abstract

An analysis of tropical low formation in the Australian monsoon during January 2013 is presented based on European Centre for Medium Range Weather Forecast (ECMWF) analyses. The focus is on three lows that developed on the monsoon shear line, two of which eventually became tropical cyclones. One of the disturbances intensified as it made a loop over land. Interpretations of the formation are given in terms of vorticity dynamics. The roll up of low-level absolute vorticity associated with the shear line is a prominent feature of the early development of the lows, a mechanism akin to that believed to operate in the formation of hurricanes from Atlantic easterly waves. A notable feature of the two lows that eventually became tropical cyclones was the occurrence of persistent bouts of deep convection near the circulation centre. The development of the low that didn’t achieve tropical cyclone status was apparently thwarted by strong vertical shear and drying of the middle troposphere.