Weekly Tropical Climate Note

Fewer tropical cyclones likely this season for Australia

The Bureau of Meteorology has released its Australian tropical cyclone outlook for the season which runs from November 2019 to the end of April 2020. The outlook predicts that fewer than the long-term average of eleven cyclones will form in the Australian region this coming season. Northern Australia typically sees four tropical cyclones make landfall each season, so all coastal communities in Australia's tropics have a significant risk of being impacted by a tropical cyclone in any given year.

The dominant climate influence for Australian tropical cyclone activity is typically the El Niño-Southern Oscillation (ENSO); during El Niño years less than average cyclone numbers are typically observed, while in La Niña years, there are typically more than average. Despite sea surface temperatures in the tropical Pacific Ocean indicating a neutral (neither El Niño or La Niña) ENSO phase, the Southern Oscillation Index (SOI) has remained close to El Niño-like levels for much of the last three months. This is due to above-average pressure at Darwin, likely related to the ongoing strong positive Indian Ocean Dipole (IOD) event, and is a factor contributing to the below-average assessment for tropical cyclone activity in 2019-20.

Late withdrawal of the Indian monsoon

The Indian Meteorological Department recently stated that the retreat of the Indian southwest monsoon from northern India commenced on 9 October. This is the latest date on record; the previous latest date was 1 October 1961— also a positive IOD year— and much later than the average date of 1 September. The Indian monsoon was the strongest in recent years with above-average rainfall in 2019, in part due to the contribution of the positive IOD. Climate models currently indicate the positive IOD may persist longer than typical events do, and the delayed transition of the monsoon trough from the northern hemisphere to the southern hemisphere may be a related factor. A positive IOD which persists later than usual may also contribute to a delay of the onset of the Australian monsoon this season.

Typhoon Hagibis strikes Japan  

Typhoon Hagibis made landfall to the southwest of Tokyo last Saturday with sustained winds in excess of 150 km/h (equivalent to an Australian category 3 severe tropical cyclone) and generated extremely heavy rainfall which led to widespread flooding and landslides. Hagibis has been described by international media reports as Japan's biggest storm in decades. Hakone, a town east of Mount Fuji, had a 24-hour rainfall total of 942.5 mm, which is one of the highest daily rainfall totals ever recorded in Japan. There have been unofficial reports of daily totals in excess of 1000 mm which, if confirmed, would be the highest on record for Japan. Hagibis caused major damage in Japan, with reports of multiple fatalities, missing people, and significant damage to property. Hagibis was downgraded to below-tropical cyclone intensity during Sunday and is not expected to re-intensify.

Product code: IDCKGEW000

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