Weather systems and their contribution to seasonal rainfall in Australia

Abstract This study takes a weather object approach to understand the variability of seasonal rainfall across the Australian continent, with a focus on cyclones, anticyclones, fronts, warm conveyor belts, potential vorticity streamers, and cut‐off lows. The contribution of these weather objects to seasonal rainfall is quantified. Up to 60% of summer rainfall in northern Australia can be attributed to cyclones, whereas potential vorticity streamers contribute up to 50% of winter rainfall in the states of Victoria and southern South Australia. Clustering of monthly rainfall anomalies identifies four distinct rainfall patterns in both summer and winter that are linked to changes in the frequency of weather objects. In particular, two summer clusters characterise wetter northern Australia and reflect the synoptic patterns of the active monsoon over northern Australia, with enhanced moisture transport from surrounding oceans, a moister atmosphere, and increased frequency of rainfall‐producing weather systems. One winter cluster characterises wetter eastern Australia, associated with a dipole of lows in southern Australia and highs over the Tasman Sea at upper levels, and a significantly increased frequency of rainfall‐producing systems accompanied by enhanced upward motion and anomalously northerly moisture transport in eastern Australia. Between the 2000–2019 and 1980–1999 periods there is a pronounced increase (decrease) in summer (winter) rainfall over the northern (southern) part of Australia. In the north, the increased summer rainfall is mostly associated with increased rainfall from cyclones and fronts, whereas winter rainfall reductions over the south are linked mainly to decreased rainfall from potential vorticity streamers, cyclones, and warm conveyor belts.