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

Abstract

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.