Australian Summer Monsoon Bursts: A Moist Static Energy Budget Perspective

Abstract

The Australian monsoon's wet season is associated with sequences of wet and dry conditions known as bursts and breaks, which usually have timescales of a week or two. There are several hypotheses for the physical processes involved in monsoon bursts, ranging from the effects of the Madden-Julian Oscillation to extratropical influences.We analyse rainfall bursts in Northern Australia using a moist static energy (MSE) budget framework. First, we separate the bursts into pre-monsoon, monsoon, and post-monsoon based on simple monsoon onset and retreat criteria. We then apply ERA5 data to calculate the MSE budget for each burst and construct composite bursts for each of the three types.We find that the horizontal advection of MSE over the tropical northern Australian convergence zone is the most critical term in the budget for the day-to-day precipitation variation. An analysis of the MSE-related gross moist stability (GMS) reveals that the GMS framework is able to predict periods of convective growth and decay before and after monsoon bursts, with the exception of the pre-monsoon bursts which do not follow the characteristic evolution of tropical convective systems. We hypothesise that this is because pre-monsoon bursts have a stronger extratropical influence. We find that the growth phase of convection in monsoon and post-monsoon bursts is associated with a notable reduction of the advection of dry air into the monsoon region. We show that this is likely the result of a rearrangement of the circulation ahead of the burst.