![]() ![]() During positive phases of SAM (i.e., when the Southern Hemisphere midlatitude jet is shifted poleward), heat waves over southeastern Australia tend to occur more frequently and tend to last longer ( Perkins et al. Heat waves in southeastern Australia occur more frequently during episodes of enhanced convection over the Maritime Continent and over northern Australia for example, during La Niña phases of ENSO, during MJO phases 3–6 of the Wheeler and Hendon (2004) Real-time Multivariate MJO (RMM) index, or during active periods of the Australian monsoon (e.g., Parker et al. Numerous studies have investigated how heat waves in southeastern Australia relate to sea surface temperature anomalies and to the dominant intraseasonal-to-interannual modes of climate variability such as the southern annular mode (SAM Rogers and van Loon 1982), the Madden–Julian oscillation (MJO Madden and Julian 1972), and El Niño–Southern Oscillation (ENSO Bjerknes 1969). This picture, however, is an essential part of understanding the mechanisms controlling the location and strength of heat waves, including the changes anticipated in a warmer world. Thus, a comprehensive physical picture of how the processes on various time scales interact is still missing. Although statistical investigations of the links between subseasonal and seasonal modes of climate variability and southeastern Australian heat waves have received much attention, few studies have focused on the dynamical mechanisms acting on shorter time scales. And future projections suggest an increase in the intensity, frequency, and duration of heat waves in a warmer climate (e.g., Alexander and Arblaster 2009 Cowan et al. Since the middle of the nineteenth century, heat waves in Australia have killed more than 5000 people, making these extreme weather situations the deadliest natural hazard of the continent ( Coates et al. Summer heat waves in Australia have a major effect on many sectors of the community, economy, and natural environment. Likewise, the role of the local surface sensible heat fluxes is deemphasized. 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. From eastern Australia, the air parcels are advected southward as they become incorporated into the near-surface anticyclone that defines the heat wave. 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. This descent is accompanied by a strong adiabatic warming. ![]() ![]() 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. ![]() These trajectories account for 44% of all trajectories forming the anticyclonic upper-level potential vorticity anomalies that characterize heat waves in the region. 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. 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. 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. ![]()
0 Comments
Leave a Reply. |