Changes in Weather Systems during Anomalously Wet and Dry Years in Southeastern Australia

This study focuses on the rainfall-producing weather systems in the southern Murray-Darling Basin (MDB), Australia. These weather systems are divided into objects: cyclones, fronts, anticyclones, warm conveyor belt (WCB) inflows, WCB ascents, potential vorticity (PV) streamers, and cutoff lows. We investigate the changes in the frequency, amplitude, and relative position of these objects as the daily and seasonal rainfall change. Days on which the rainfall is heavy, especially in winter, are characterized by more PV streamers, cutoff lows, cyclones, fronts, and WCBs in the region. In contrast, dry days are characterized by more anticyclones over southeastern Australia in winter and summer. The effect of upper-level weather objects (PV streamers and cutoff lows) on lower-level objects, and their importance in producing rainfall, is quantified using the quasigeostrophic v equation and separating the vertical motion into that induced by the upper and lower levels. On heavy rainfall days in winter, PV streamers and cutoff lows force strong upward motion in the lower troposphere, promoting cyclogenesis at lower levels, forcing ascent in the WCBs, and producing rain downstream of the southern MDB. Lower-level ascent forced by upper-level objects is important for the development of heavy rainfall in both seasons, although particularly in winter. Rainfall is attributed to individual objects. PV streamers and WCBs contribute most to the winter and summer rainfall, respectively. The difference in rainfall between anomalously wet and dry years can be explained in winter by the changes in the rainfall associated with PV streamers, whereas in summer it is mostly due to a reduction in the rainfall associated with WCBs.