Using large-scale diagnostic quantities to investigate change in frequency of East Coast Low events.

The Eastern Seaboard of Australia (ESB) is a different climatological entity to the rest of eastern Australia. Major climate drivers such as the EL Nino – Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) have substantially weaker correlations with rainfall in this region than elsewhere in eastern Australia. Recent research suggested that east coast lower (ECL) is the dominant driver of climate, which strongly influenced extreme rainfall along the ESB. ECLs are intense low-pressure systems which occur on average several times each year off the eastern coast of Australia, in particular, southern Queensland, New South Wales (NSW), and eastern Victoria. ECLs often intensify rapidly overnight making them one of the most dangerous weather systems to affect and damage the eastern coast of Australia each year. They are also a major source of water for the reservoirs serving coastal communities since ECL events were identified as being responsible for most of the high inflow in the NSW coastal catchments.
In this study, two large-scale diagnostic quantities, Isentropic Potential Vorticity (IPV) and geostrophic vorticity (GV), were taken as indicators of ECLs, which were calculated using outputs of historical and future WRF simulations from the NSW/ACT Regional Climate Modelling (NARCliM) project. NARCliM uses regional climate model WRF to perform an ensemble of simulations for the present and the projected future climate. WRF was run in three different physics combinations (R1, R2 and R3) that have been shown to perform well in the South-East Australia and were chosen based on performance and independence. These three RCMs were used to simulate three time periods (1990-2009, 2020-2040 and 2060-2080). Four selected GCMs (MIROC-medres 3.2, ECHAM5, CGCM 3.1 and CSIRO mk3.0) from CMIP3 are used as initial and boundary conditions for the WRF simulations. Outputs from six simulations (MIROC and ECHAM5 driven) are available by now and used in this study.
The monthly thresholds for IPV/GV were quantified for matching the indicated monthly ECLs to subjectively analysed monthly ECLs for the first time period (1990-2010). The same monthly thresholds were applied on the other two future time periods to get the monthly distribution of indicated ECLs. The diagnostic results for the future period were compared with those for the historical period to identify the change in frequency and seasonal shift in distribution.
All six simulations showed the same trend of decrease in frequency for IPV indicated ECLs. Only five in six simulations showed the same trend for GV indicated ECLs. The mean magnitude of decrease in frequency was smaller for MIROC driven simulations than ECHAM5 driven simulations, R3 simulation than other two (R1 and R2) simulations. Changes in monthly distribution showed that the decreases in frequency were mostly in cold season, and increases in frequency were dominantly in winter season, especially for the period of 2060-2080. This suggests that the total number of ECLs is likely to decrease in the future and there can be a shift in monthly distribution from cold season to warm season for future ECL events.