The influence of topography on midlatitude cyclones on Australia’s east coast.

The east coast of Australia has a relatively high frequency of midlatitude cyclones, locally known as East Coast Lows (ECLs), which can cause severe weather including widespread flooding and coastal erosion. The elevated topography close to the east coast has been hypothesized to play a role in both the genesis and impacts of cyclones in this region, but existing studies have been limited to case studies of individual events. In this paper we present the results from two 20 year simulations over the Australian region using the Weather Research and Forecasting Model and assess the results from removing all topography in the region on both mean atmospheric circulation and ECL frequency. Removing topography results in an increase in sea level pressure to the south of Australia and an increase in moisture flux convergence and rainfall near the east coast, as well as a decrease in potential vorticity to the north of the ECL region. This results in a change in the spatial distribution of cyclones, with a 37% decrease in the frequency of cyclones that develop to the south of the ECL region but a 20% increase in cyclones near the east coast. This results in little overall change in the frequency of ECLs and suggests that coarse topography is unlikely to be responsible for the difficulties in simulating coastal cyclones in global climate models.

Figure 3. Changes in average circulation patterns in 1990–2009 when topography was removed: (a) MSLP (hPa), (b) 200 hPa geopotential height (m), (c) average 500–1000 hPa horizontal moisture flux (vectors) and divergence (contours), (d) 800 hPa and 250 hPa winds (m/s), (e) average precipitation (%), and (f) 500 hPa potential vorticity (PVU). The dashed lines indicate statistical significance at the 5% level using a t test on annual data, while a grey line in the bottom figures shows the eastern seaboard region used for calculating rainfall anomalies.