Cluster of rare storms causing flooding indicates risk overlap

A Risk Frontiers article says the cluster of storms causing extreme rainfall and flooding in Queensland and New South Wales is the result of a rare combination of influences, while warning of serious risks if climate change brings a greater overlap between tropical cyclones and east coast depressions.

The paper says the recent cluster of storms should be seen in the context of back-to-back La Nina events, a Positive South Annular Mode (SAM), and a warming atmosphere and ocean.

The critical ingredients affecting the group of storms were high lockdown and an outbreak of cold air early in the season coupled with the strongly positive SAM outage, he says.

The paper states that a statistically significant trend toward more positive SAM summers is attributed to the combined anthropogenic influence of ozone layer depletion and increased greenhouse gas concentrations.

“Could outbreaks of cold air be more frequent in the summer, fueling warm, humid surface flow to cause explosive hybrid subtropical storms along the eastern seaboard?” asks the newspaper.

“This would create an overlapping climate risk from both TC [tropical cyclone] and ECL [east coast lows]. The migration of the peak ECL season into late summer and early fall can increase the likelihood of severe weather hazard.

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

“In Australian longitudes, these temperature anomalies extended from the tropical north of Queensland to the central eastern Antarctic plateau. It was definitely an outlier in instrumental recording of the last century,” says ClimaLab lead scientist Ian Goodwin.

The backgrounder examines previous 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 flooding in late summer in eastern Australia is generally associated with the southward passage of tropical cyclones and depressions, but the weather of the first two storms was very different from that associated with their landfall or to their decomposition. Transitions from inland troughs to the formation of east coast lows are also typical in fall and early winter.

The document states that the synoptic nature and the group of three storms causing extreme rainfall, flooding and impacts such as landslides are rare and require analysis to place them in historical and future climate context.

Storm 1, an unusual hybrid system with built-in thunderstorms that brought severe weather from February 23-28, hit southeastern Queensland hard, in part due to a blockage south of Australia.

“If that hadn’t happened, the tropical/subtropical trough would have deteriorated much faster,” says Dr Goodwin.

The second storm from February 28 to March 9 was a deep east coast low, which developed into an inland trough over Queensland and moved parallel to the east coast, producing an airflow at land and resulting precipitation.

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

“Unfortunately, the target of the maximum rainfall anomaly was located over the Northern Rivers region of New South Wales, resulting in compound flooding,” the paper said.

According to Risk Frontiers, Storm 3 evolved from a weak tropical depression and formed over the Coral Sea in a distinct way from its predecessor, but similarities led to its intensification and it has also been influenced by Indian Ocean hurricane activity and southern blocking highs.

While storm clusters aren’t unusual, “Storm 1’s synoptic development and precipitation intensities for all storms were anomalous and record-breaking,” says Dr Goodwin.

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