The Structure and Evolution of Heat Waves in Southeastern Australia

The underlying large-scale dynamical processes responsible for the development of heat waves in Victoria, southeastern Australia, in summer are presented here. Heat waves are defined as periods of at least three days and two nights for which daily maximum and minimum temperatures exceed the 90th percentile for a particular location and month, using a station daily temperature dataset. Composites of upper-level potential vorticity anomalies from the Interim ECMWF Re-Analysis (ERA-Interim) reveal that heat waves in southeastern Australia are associated with propagating Rossby waves, which grow in amplitude and eventually overturn. The process of overturning generates an upper-level anticyclone over southern Australia and an upper-level trough to the northeast, with maximum amplitudes near the tropopause. The northerly flow associated with the circulation around the surface anticyclone advects hot air from the continental interior over the southeast of Australia, leading to extreme,surface temperatures. Composite rainfall shows that precipitation is enhanced in the vicinity of the upper-level trough over northeastern Australia, consistent with adiabatically forced vertical motion, destabilization of the atmosphere, and modified moisture fluxes. Heat waves in the southeast are frequently accompanied by heavy rainfall over the northeast of the continent and adjacent ocean.