Rare flood-causing storm cluster points to overlap risks

A Risk Frontiers paper says the cluster of storms causing extreme rainfall and flooding in Queensland and NSW arose from a rare combination of influences, while warning of severe risks if changing climate patterns lead to greater overlap between tropical cyclones and east coast lows.

The paper says the recent storm cluster needs to be viewed in the context of back-to-back La Nina events, a positive Southern Annular Mode (SAM) and warming of the atmosphere and ocean.

Critical ingredients affecting the storm cluster were a blocking high and a very early season cold air outbreak associated with the breakdown of the strong positive SAM, it says.

The paper says a statistically significant trend towards more positive SAM summers is attributed to the combined anthropogenic influence of ozone depletion and increasing greenhouse gas concentrations.

“Could cold air outbreaks be more frequent in summer, fuelling warm moist surface flow to drive explosive hybrid subtropical storms along the eastern seaboard?” the paper asks.

“This would create an overlapping climate risk from both TC [tropical cyclone] and ECLs [east coast lows]. The migration of the peak ECL season into late summer-early autumn may increase the probability of severe climate risk.”

The catastrophic storms reflected some typical climate driver patterns, but the “quasi-stationarity of the atmospheric circulation” was unusual and there were “significant records in warm and cold surface air and sea surface temperature anomalies”.

“In the Australian longitudes, these temperature anomalies spanned from tropical north Queensland to the central east Antarctic plateau. This was definitely an outlier in the instrumental record of the past century,” ClimaLab Principal Scientist Ian Goodwin says.

The briefing paper examines preceding conditions, the first two storms that produced extreme rainfall and flooding from February 23 to March 9 as well as a third storm from March 28 to April 2.

Moderate to severe late summer flooding in eastern Australia is usually associated with the southward passage of tropical cyclones and lows, but the weather from the first two storms was very different to that associated with their landfall or decay. Transitions from inland troughs to the formation of east coast lows are also typical during autumn and early winter.

The paper says the synoptic nature and the cluster of the three storms causing extreme rainfall, flooding and impacts such as landslides is rare and requires analysis to place them in the historical and future climate context.

Storm 1, an unusual hybrid system with embedded thunderstorms that brought severe weather from February 23-28, hit southeast Queensland hard partly due to a blocking high south of Australia.

“If this had not occurred, then the tropical dip/subtropical low would have decayed much more quickly,” Dr Goodwin says.

The second February 28-March 9 storm was a deep east coast low, which developed in an inland trough over Queensland, and tracked parallel to the eastern seaboard, producing onshore airflow and resulting rainfall.

The third storm, which formed in late March, was also an east coast low. Although not unusual for early autumn, it delivered another extreme weather event with high rainfall intensity and daily totals.

“Unfortunately, the bullseye for maximum rainfall anomaly was situated over the Northern Rivers of NSW region resulting in a compound flood event,” the paper says.

Risk Frontiers says Storm 3 evolved from a weak tropical low and formed over the Coral Sea in a way distinct from its predecessor, but similarities drove its intensification and it was also influenced by Indian Ocean cyclone activity and southern blocking highs.

While storm clusters are not unusual, “the synoptic development of Storm 1 and the rainfall intensity of all storms were anomalous and record-breaking”, Dr Goodwin says.