Extratropical cyclones are the main cause of extreme wind, wave and rainfall events in the subtropical eastern seaboard of Australia. Current Global Climate Models (GCMs) lack sufficient resolution to adequately resolve the severity of extreme events associated with these storms, leading to considerable uncertainties in future climate projections for this region. Most major extratropical cyclones in the vicinity of the east coast of Australia, known as East Coast Lows, are associated with a high amplitude upper-tropospheric trough. This observation led to the development of a diagnostic method based on strong cyclonic geostrophic vorticity in the upper-troposphere, to indicate the likelihood of East Coast Low occurrence, with previous studies showing that the diagnostic method yields meaningful results when applied to GCMs. Here we apply the diagnostic method to 18 different CMIP5 GCMs using two different emission pathways (RCP4.5 and RCP8.5). The results indicate fewer East Coast Lows should be expected in response to increasing atmospheric greenhouse gas concentrations, with projected reductions in the frequency of occurrence of severe wind, wave and rainfall impacts associated with these storms. This consistency allows the importance of the magnitude of the external forcings (i.e. emission pathways) to be clearly shown. The projections produced for eastern Australia from this diagnostic method are useful for informing initiatives such as the Natural Resource Management (NRM) projections currently being prepared for Australia, as a complementary method to projections based on direct output from GCMs and other forms of downscaling. In contrast to some other downscaling methods (including both dynamical and statistical methods), the results produced by the diagnostic method are remarkably consistent between different GCMs, indicating a considerable degree of confidence in the projected changes.

References:

Mills, G. A., R. Webb, N. E. Davidson, J. Kepert, A. Seed and D. Abbs, 2010: The Pasha Bulker east coast low of 8 June 2007. Centre for Australian Weather and Climate Research, Australia, CAWCR Technical Report 23.

Dowdy, A. J., G. A. Mills and B. Timbal, 2012: Large-scale diagnostics of extratropical cyclogenesis in eastern Australia. International Journal of Climatology, DOI: 10.1002/joc.3599.

Dowdy, A. J., G. A. Mills, B. Timbal and Y. Wang, 2013: Changes in the risk of extratropical cyclones in eastern Australia. Journal of Climate, 26, 1403-1417.

Dowdy, A. J., G. A. Mills, B. Timbal and Y. Wang, 2013 (in press): Understanding rainfall projections in relation to extratropical cyclones in eastern Australia. Australasian Meteorological and Oceanographic Journal.