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posted on 19.06.2019by Julian F. Quinting, Michael J. Reeder
Although heat waves account for more premature deaths in the Australian region than any other natural disaster, an understanding of their dynamics is still incomplete. The present study identifies the dynamical mechanisms responsible for heat waves in southeastern Australia using 10-day backward trajectories computed from the ERA-Interim reanalyses. Prior to the formation of a heat wave, trajectories located over the south Indian Ocean and over Australia in the lower and midtroposphere ascend diabatically ahead of an upper-level trough and over a baroclinic zone to the south of the continent. These trajectories account for 44% of all trajectories forming the anticyclonic upper-level potential vorticity anomalies that characterize heat waves in the region. At the same time, trajectories located over the south Indian Ocean in the lower part of the troposphere descend and aggregate over the Tasman Sea. This descent is accompanied by a strong adiabatic warming. A key finding is that the temperatures are raised further through diabatic heating in the boundary layer over eastern Australia but not over the inner Australian continent. From eastern Australia, the air parcels are advected southward as they become incorporated into the near-surface anticyclone that defines the heat wave. In contrast to past studies, the importance of cloud-diabatic processes in the evolution of the midlatitude large-scale flow and the role of adiabatic compression in elevating the near-surface temperatures is emphasized. Likewise, the role of the local surface sensible heat fluxes is deemphasized.