The structure and evolution of heatwaves in SE Australia

September 27, 2014 Leave a comment

source: ARCCSS Heat waves in Australia are thought to have been responsible for more fatalities in Australia over the past two centuries (around 4000) than any other natural hazard.

Heat waves are thought to have been responsible for more fatalities in Australia over the past two centuries (around 4000) than other natural hazards such as tropical cyclones, floods, and lightning strikes. Heat waves in southeastern Australia are also frequently accompanied by heavy rainfall over the northeast of the continent and adjacent oceans.

Heat waves in Victoria in summer (December – February) are associated with a strong, slow-moving surface high pressure system over the Tasman Sea, which advects warm air from the hot continental interior in a northerly or northwesterly flow over Victoria. This synoptic pattern is well known, and is also seen on days of high fire danger in the region.

New research published in the Journal of Climate reveals the fundamental dynamical atmospheric processes that lead to the development of heat waves in Victoria. Long wavelength waves (Rossby waves) in the atmosphere propagate along the jet stream, the band of strong westerly upper-level winds that circles the globe at mid-latitudes in both hemispheres. These waves can grow in amplitude and overturn (or break) to the southeast of Australia. When this occurs, the resultant stirring of the atmosphere brings upper-level air from the tropics to the mid-latitudes, and polar air to the subtropics; this leads to the formation an upper-level ridge over the southeast of the continent and a trough over the northeast (Fig.1).

Figure 1: Mean 350 K PV anomaly and wind field for all Victorian heat wave days. PV anomalies (PVU) are shaded according to the colour bar (anticyclonic/cyclonic shaded blue/red respectively). The reference wind vector (20 ms^-1) is at the bottom right corner of the plot. The thick black 2-PVU contour indicates the dynamical tropopause.

The upper-level ridge is associated with a surface high-pressure system, which brings the hot northerly winds to Victoria. The trough over the northeast of the continent may destabilise the atmosphere, enhancing convection over the region (Fig. 2) and leading to increased rainfall and sometimes flooding. This pattern of extreme temperatures in Victoria and heavy rain in Queensland was evident during the prolonged heat wave prior to the Black Saturday bushfires in January-February 2009.

Figure 2: Mean positive rainfall anomaly (mm) shaded as per the colour bar for (a) the day prior to heat wave onset, (b) day 1 of the heat wave, and (c) the day after the heat wave ends, with 350 K cyclonic PV anomalies contoured in grey at 0.25 PVU intervals.

An improved understanding of the dynamical processes that lead to the formation of heat waves in Victoria should improve the predictability of these extreme events, which are one of the foremost natural hazards in Victoria.