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The El Nino Southern Oscillation (ENSO) plays a major role in the variability of Australia's climate. Periodic droughts and wet conditions follow the ENSO phases. Understanding ENSO and its effects on Australia is never more important than in the cases of extreme events such as severe droughts or flooding. In this paper, we examine changes in the basic climate state over Australia during the two extreme flooding periods of January 1974 and January 2011. 1973-1976 and 2010-2012 are considered the two most severe La Nina periods in terms of precipitation and flooding in Australia since 1900. The largest floods occurred during January 1974 and late December 2010 through January 2011, in both cases flooding the Queensland and New South Wales coastal areas, and in the latter case causing floods inland and south down to Victoria. We employ complementary avenues of analysis, looking at both the synoptics and atmospheric dynamics of the southern hemisphere climate during these events. Synoptically, the circulation over Australia often changes significantly during La Nina phases. In strong La Nina periods, often there are persistent low pressure systems over Australia during flooding periods, replacing or displacing the typical high pressure system seen in neutral or El Nino conditions. In 2011, higher sea surface temperatures (SSTs) in the western Pacific, due to an unusually strong negative Indian Ocean Dipole (IOD), caused lower pressure over the Indian Ocean, Australia, Indonesia and the western Pacific and above-average amounts of atmospheric moisture. These low pressure systems persisted over Australia with the aid of blocking high pressure systems in the Tasman Sea and easterly onshore winds. These events, coupled with anomalously strong westerly winds bringing down the large amounts of monsoonal tropical moisture, led to intense flooding. January 1974 was a similar case, albeit with a weaker negative IOD and smaller SST anomalies. To distinguish the classes of disturbances active during these events, their initial growth, growth rates, and interactions, we examine the leading dynamical modes in both cases. Dynamical modes associated with these flooding events are determined using a primitive equation instability model. We find that heavy rainfall is associated with increased growth rates of Kelvin waves, intraseasonal oscillations (ISO), monsoon disturbances, and associated blocking over the Tasman Sea, as well as some changes in the extratropical storm track modes. In particular, we find that the Kelvin wave grows explosively and is strongly convectively coupled in the Australian monsoonal region, enhancing convection over Australia. In addition, intraseasonal oscillations grow strongly in the Australian region and contribute to the convection and rainfall, at the same time. We find similar Kelvin waves and monsoonal modes in both events, although in 1974 both were slightly stronger than in 2011. Our dynamical analyses agree with the synoptic observations and provide a more complete description of the reasons for the severity of both events. It appears necessary for several factors to be both present and timed accordingly for flooding impacts to be maximal. In both 1974 and 2011, active monsoonal modes and intraseasonal oscillations met unusually rapidly growing Kelvin waves. In 2011, a strongly negative IOD with high SSTs made more moisture available. Understanding the nature of these extreme events over Australia is a fundamental step in the pursuit of accurate predictions of future ENSO impacts, Australia's future climate and climate extremes, and water resources planning.

Synoptic and dynamical analyses of ENSO extreme events over Australia