Tropical Climate Update

Out-of-season tropical cyclone activity

Two tropical cyclones have been observed in the Australian region during the past week, outside of the standard Australian tropical cyclone season which runs from the start of November to the end of April.

A system which developed in the Indonesian area of responsibility, tropical cyclone Lili, passed through Australia's Northern region on 10 May. Lili was a short-lived storm which formed on 9 May and dissipated the following day without making landfall as a tropical cyclone. Lili generated heavy rain which led to flooding in the Indonesian province of Malaku and Timor Leste. Lili is now the latest system on record in Australia's Northern region, surpassing tropical cyclone Verna which was observed in the region on 3 May 1977.

Tropical cyclone Ann, which formed over the Coral Sea on the morning of 12 May, peaked at category 2 intensity on 13 May as it tracked westwards towards the Queensland northeast coast. It has been re-classified as a tropical low but is still forecast to make landfall tomorrow (15 May). Despite recent weakening, ex-tropical cyclone Ann is still expected to generate strong to gale-force winds and heavy rain along its track. The last time a tropical cyclone was observed in Australia's Eastern region this late in the season was tropical cyclone Pierre in 2007.

However, since 2007 two tropical cyclones have been observed in the Western region during May: Greg in 2017 and Quang in 2015. In the historical record of tropical cyclones in Australia since 1970, roughly equal numbers of May cyclones were observed in Australia's Eastern and Western regions. It is rare to observe a storm in Australia's Northern region in May. Two tropical cyclones in the Australian region during May is unusual and has previously only been observed four other times since 1970 (2002, 1989, 1986 and 1972). There has not been three tropical cyclones recorded in any year in May in the Australian region.

With the inclusion of Lili and Ann, the number of tropical cyclones in the Australian region for the 2018-19 season is eleven, equal to the long-term average. Each of Australia's three tropical cyclone regions had five storms during the season (noting that a single tropical cyclone can pass over multiple regions). This is above average for the Northern and Eastern regions and below average for the Western region.

Madden–Julian Oscillation tracks across Pacific Ocean

A moderate strength pulse of the Madden–Julian Oscillation (MJO) continues to tracked eastwards over the Pacific Ocean, and is currently approaching American longitudes. The influence of the MJO was likely an important factor in the formation of tropical cyclones Lili and Ann. At this time of the year, as the MJO reaches the Americas and further east, rainfall across the Maritime Continent and northern Australia is typically suppressed. Tropical cyclone activity, as well as non-MJO wave activity (in the form of equatorial Rossby wave activity) continues to maintain moisture in the Maritime Continent region, but as these influences diminish in the coming week, a return to more typical dry season weather is expected across northern Australia. Further north over the Maritime Continent, the region continues to be influenced by tropical waves such as the MJO and equatorial Rossby waves.

Read more about the Bureau's current MJO monitoring.