Two Synoptic Routes to Subtropical Heat Waves as Illustrated in the Brisbane Region of Australia

The physical mechanisms by which heat waves in the Brisbane region of Australia develop are elucidated through trajectory and composite analyses. Trajectories are started close to the surface during heat waves and integrated backward. Those trajectories for which the net diabatic heating lies in the uppermost pentile are called strongly diabatic, while those in the lowermost pentile are called weakly diabatic. In the weakly diabatic case, air parcels originate eastward of the continent over the ocean and warm almost entirely by adiabatic compression in an anticyclonic environment. Surprisingly, strongly diabatic heat waves in this region are characterized by an upper‐tropospheric cyclonic anomaly off the southeastern corner of Australia. The air parcels originate far to the southwest of the heat wave, and their temperature increases roughly in equal parts by adiabatic compression as air parcels subside on the rear flank of this cyclonic anomaly and surface sensible heating as air parcels are advected horizontally. Plain Language Summary Heat waves are the deadliest natural hazard in Australia. Motivated by the projection that the number of extremely hot days in subtropical Australia will increase in a warmer climate, this study aims to develop a comprehensive physical picture of the processes leading to the extreme temperatures. The approach taken is to compare heat waves dominated by land surface‐atmosphere interactions to those that are due to atmospheric dynamical processes using the Brisbane region as a specific example. The key finding is that unlike heat waves in other parts of the world, heat waves are not necessarily associated with a high pressure system but with a low pressure system off the southeastern corner of Australia. Hence, it seems likely that when considering how heat waves might change in the future, not only are the location and intensity of subtropical high pressure systems important but also are the location, intensity, and variability of the midlatitude storm tracks. Key Points Heat waves around Brisbane can form through adiabatic compression in an anticyclonic environment Heat waves in the region can also occur in the wake of a midlatitude trough In this case they form through a combination of adiabatic compression and surface sensible heating