A climatology of Australian severe thunderstorm environments 1979-2011: inter-annual variability and ENSO influence

Severe thunderstorms present a significant threat to property and life in Australia during the warm season (September to April). However, these relatively infrequent events are poorly understood in terms of frequency and occurrence for much of the continent due to a lack of in-situ observations. With the spectre of a changing climate, there is an increasing need to understand thunderstorms and their impact on Australia, both in the past and for the future. To facilitate this, the relationship between severe thunderstorms and their associated environments is used as a probabilistic proxy for direct observations. To establish these conditions, a proximity climatology of environments was developed for observed severe thunderstorms in Australia during the period 2003-2010 using the ERA-Interim reanalysis. Proximity soundings from the reanalysis for observed severe thunderstorms were used to develop covariate discriminants that identify the increased probability of an environment to produce severe thunderstorms. The covariates use a combination of ingredients describing instability (mixed-layer convective available potential energy) and potential for organized severe convection (deep-layer wind shear). These discriminants have been extrapolated to produce a climatology of environments favourable to the development of severe thunderstorms over the period 1979-2011 from this reanalysis. The inter-annual variations in both the spatial and temporal distribution of convective environments over Australia were analysed, with particular focus on the influence of El Nino-Southern Oscillation (ENSO) on the occurrence of severe thunderstorm environments. These results suggest that while ENSO has a substantial impact on the spatial distribution of severe thunderstorm environments over the continent, the link to frequency is more uncertain.