The physical mechanisms by which heat waves in the Sydney and Brisbane regions develop are elucidated through trajectory and composite analyses.
In Sydney, back trajectories starting from the upper-troposphere anticyclonic potential vorticity (PV) anomaly southeast of Australia fall into two groups, based on either diabatic cooling or heating in the final 72 h. Parcels that are cooled come from the upstream middle troposphere over the Indian Ocean, while those that are heated originate in the lower half of the troposphere over the southwest of the continent. Parcels that are heated ascend and specific humidity rapidly decreases and water is precipitated, indicating that the upper-tropospheric anticyclone which is an essential dynamical feature of Sydney heat waves may be amplified when the lower troposphere to the southwest is relatively moist. The motion of parcels that are cooled is essentially adiabatic, and their PV anomaly becomes marked as the Rossby wave packet amplifies. Trajectories from the near-surface air mass originate offshore to the east and south. These parcels are initially heated through adiabatic compression, followed by surface sensible heating.
In Brisbane, two synoptic routes to subtropical heat waves are identified. The physical processes resulting in elevated near-surface temperatures are adiabatic compression as air parcels subside and remote surface sensible heating as air parcels are advected horizontally. Trajectories started near the surface in the Brisbane region fall into two categories: strongly and weakly diabatically heated. In the strongly diabatic case, parcels originate to the southwest of Australia and cross the southeastern continent, where they undergo heating through surface sensible heating in the wake of a midlatitude low. In the weakly diabatic case, parcels originate over the Tasman Sea to the east before turning anticylonically to reach Brisbane from the west, and undergo heating through adiabatic compression.