The structure and development of heat waves in Southeastern Australia

It has been suggested that heat waves have been responsible for more fatalities in Australia than natural hazards such as tropical cyclones, floods, and bushfires. The underlying large– scale dynamical processes responsible for the development of heat waves in 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 data set. Composites of upper–level potential vorticity (PV) anomalies from reanalysis data 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. The northerly flow around the surface anticyclone advects hot air from the continental in- terior 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, destabilisation 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. The influence of tropical cyclones on the development of the upper level anticyclone is inves- tigated here. Tropical cyclones may affect heat waves indirectly, as the divergent outflow at upper levels perturbs the wave guide, leading to downstream development of the upper level ridge–trough couplet. The effect may also be direct, through the advection of anomalously anticyclonic PV from regions of deep convection in the vicinity of TCs into the upper level anticyclone. The results show that the direct reinforcement of the anticyclone is likely to be more important in the formation of severe heat waves in southeastern Australia. One of the most severe heat waves in Victoria occurred between 27 January and 8 February 2009. The synoptic and PV–Theta (Θ ) conditions during this heat wave illustrate the conditions which prevail during summer heat waves in Victoria. The influence of tropical convection on the intensity of the upper level anticyclone is also demonstrated for this case. Given the connection between tropical convection and heat waves, the relationship is established between heat waves, precipitation, and three modes of climate variability of importance for rainfall in Australia: the El Nino–Southern Oscillation (ENSO), the Madden–Julian Oscillation (MJO) and the Southern Annular Mode (SAM). Heat waves in Victoria in summer are more common during phases 3–6 of the MJO (when convection is enhanced over the east- ern Indian Ocean, Maritime Continent, and western Pacific Ocean) and La Nina phases of ENSO. The PV–Θ structure of the heat wave and pattern of convection varies with the phase of each mode of variability. Enhanced tropical convection results in a heat wave characterised by a monopole of anticyclonic PV, whereas suppressed convection is associated with a dipole of anticyclonic and cyclonic PV.