A View From Me on ’23: James Cosgrove, RMS

Will Summer 2022-23 result in a season of tropical cyclones and floods for Australia?

October to April represents the peak period for extreme weather in Australia, with the risk of flooding, tropical cyclones, bushfires, and severe thunderstorms typically higher than at any other time of the year. And looking at this season’s latest outlook it indicates a potentially wet and windy summer for parts of Australia, with an increased risk of tropical cyclones and flooding.

With tropical cyclone activity in the Northern Hemisphere now winding down for the year, meteorologists now turn their attention down under, as the 2022–23 Australian tropical cyclone season gets underway, a season that started on November 1 and runs for six months to April 30.

An average season sees 11 tropical cyclones form within the Australian region. This region encompasses the seas and oceans that surround Australia including the southeast Indian Ocean, Timor Sea, Banda Sea, Arafura Sea, Gulf of Carpentaria, Coral Sea, and the Tasman Sea.

During an average season, four tropical cyclones make landfall in Australia, and at least one has crossed the Australian coastline in every season since reliable records began in the 1970s.

For the 2022-23 season, the Australian Bureau of Meteorology estimates an above-average number of tropical cyclones (see Figure 1), with a 73 percent chance of more tropical cyclones than average, and a 27 percent chance of fewer than average.

The forecasts for an above-average season reflect the influence of several key oceanic and meteorological factors, including the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Madden-Julian Oscillation (MJO).

The La Niña phase of the ENSO officially developed in September 2022 and sees cooler-than-average sea surface temperatures in the central tropical Pacific. Most seasonal forecasts favor La Niña conditions prevailing until early 2023, meaning that warmer-than-average sea surface temperatures are expected to remain in the Tropics surrounding Australia throughout the season.

Warmer sea surface temperatures typically enhance tropical cyclone activity by providing increased energy and moisture to the environment, and this would mark the third consecutive Australian summer where La Niña conditions have prevailed and influenced the region’s tropical cyclone season.

While tropical cyclone activity in the Australian region has large inter-seasonal variability due to the influence of naturally occurring climate factors, including ENSO, the number of tropical cyclones in the Australian region is generally higher during La Niña seasons.

Further, the first landfalling tropical cyclone on the Australian coast typically occurs earlier than normal in La Niña years, around the middle of December, whereas during non-La Niña years, the date of the first tropical cyclone Australia landfall is typically early January.

Sea surface temperatures in the Indian Ocean may also impact the Australian region’s tropical cyclone season. The Indian Ocean Dipole (IOD) measures the difference between sea surface temperatures in the tropical western and eastern Indian Oceans.

During its negative phase, as is currently observed, there are warmer than average temperatures in the eastern Indian Ocean and cooler than average temperatures in the western Indian Ocean. This typically results in stronger westerly winds across the Indian Ocean and greater convection near Australia.

This oceanic-meteorological setup, combined with the La Niña conditions in the central tropical Pacific, can often result in an increased probability of cyclogenesis in the Australian region.

Like the North Atlantic and Western North Pacific Basins, tropical cyclone formation in the Australian region is rarely spread consistently throughout the season. Quiet periods are followed by bursts of activity typically associated progression of the Madden-Julian Oscillation (MJO).

The MJO is an intra-seasonal, eastward-propagating area of enhanced and suppressed tropical convection that provides a large-scale environment that is either favourable or unfavourable for tropical cyclone development. The MJO is the major driver of both fluctuations in tropical weather on weekly to monthly timescales and of the intensity of the North Australian Monsoon, which in turn can promote cyclogenesis.

The impact of the MJO and its relative timings cannot be skilfully predicted at seasonal timescales and therefore its impact on the 2022–23 Australian tropical cyclone season remains uncertain.

East-West Rainfall and Flood Risk Split

Alongside its forecast of an above-average tropical cyclone season, the Bureau has also warned of a generally wetter summer for Australia’s eastern states. We have already seen widespread flooding in four states during mid-November after extensive rainfall.

For the summer period between December to February, above median rainfall is likely for the Cape York Peninsula, the majority of eastern Queensland, areas east of the Great Dividing Range of New South Wales, and most of Victoria and eastern Tasmania.

Some areas have a greater than 80 percent chance of exceeding the median total. Per the Bureau’s seasonal outlook, northern Queensland has at least twice the average chance of unusually high rainfall over the next three months. Conversely, below-median rainfall is expected for large parts of Western Australia and the western Top End, with a heightened risk of bushfires in these regions.

While localised flooding can occur in any Australian wet season (October to April), there is an increased risk of widespread and prolonged riverine flooding across northern and eastern Australia this summer. Rivers are already at high levels, dams are full, and catchments are largely saturated across much of eastern Australia, meaning that any significant rainfall accumulation over the next three months has the potential to result in flash- and river flooding.