Weekly Tropical Climate Note

Late-season tropical cyclone in Australian Region

Ex-tropical cyclone Mangga interacted with a cold front to bring severe weather to much of Western Australia's south west coast from 24-26 May. A wind gust of 131 km/h was recorded at Cape Leeuwin on the southwest coast on the morning of 25 May, while many other southern locations experienced wind gusts in excess of 90 km/h. Sea level was also well in excess of normal along the coastline, with some areas of the southwest having sea levels around a metre higher than normal. Mangga formed over the southern Indian Ocean on 21 May, near Sumatra, and was briefly a category 1 tropical cyclone in the far west of Australia's tropical cyclone area of responsibility. Soon after entering the Australian region, Mangga lost its tropical characteristics, and was designated as an ex-tropical cyclone. However, the system maintained a low central pressure region which, combined with the cold front, generated severe winds.

While Australian tropical cyclones are uncommon in May, they are not unprecedented. Tropical cyclones have formed in the Australian Region as late as July. Early May tropical cyclones are relatively common, with three such storms in the last five years. The most recent tropical cyclone occurring in the second half of May was the category 1 storm, Pierre, in 2007.

With Mangga included, the total number of tropical cyclones during the 2019-20 Australian tropical cyclone season stands at eight.

Rare category 5 tropical cyclone in the Bay of Bengal

The broadscale climate drivers which aided the development of Mangga, also contributed to the development of tropical cyclone Amphan in the northern Indian Ocean. Both storms were associated with a strong burst of westerly winds over the equatorial Indian Ocean, related to Madden–Julian Oscillation and Kelvin wave activity. Amphan was designated a Super Cyclonic Storm on 19 May by the India Meteorological Department, equivalent to an Australian category 5 tropical cyclone. Storms of this intensity are relatively rare in the Bay of Bengal, with roughly one per decade observed.

Amphan weakened somewhat before reaching land and made landfall on 21 May, just west of the India/Bangladesh border at the head of the Bay of Bengal, with sustained winds to around 150 km/h (comparable to an Australian category 3 tropical cylcone). A storm surge, heavy rainfall and localised flooding was also reported with Kolkata, on India's east coast, seeing close to 200 mm of rain.

Madden–Julian Oscillation rapidly moved through Australian region

Recent analysis indicates a pulse of the Madden–Julian Oscillation (MJO) which assisted the formation of Amphan and Mangga, rapidly tracked eastwards through the Maritime Continent, to Australia's north, in recent days, and is now in the western Pacific region. However, the area of enhanced convection associated with the MJO remains over the eastern Indian Ocean, suggesting the rapid movement of the MJO is mixed up with a Kelvin wave, a faster-moving tropical atmospheric wave.

Most climate models indicate the MJO will slow down in the coming days while continuing to track eastwards across the Pacific Ocean. The combination of fast eastward propagation and a forecast location to the east of the Australian region, means that any significant influence on rainfall in the Australian region is unlikely in the next fortnight.

Read more about the Madden–Julian Oscillation