Tropical Climate Update

Monsoon arrives at Darwin

Friday 18 December saw a monsoon trough form across the northern Top End and an embedded weak tropical low develop in the Timor Sea. Monsoonal flow strengthened and extended south-west to the Kimberley the next day. The resulting two days of north-westerly winds from the surface to the mid-levels at Darwin satisfied the 2020-21 monsoon onset criteria, meaning that 19 December is the official monsoon onset date at Darwin for the 2020-21 northern wet season.

This season’s monsoon onset date at Darwin is more than a week earlier than average (29 December), and is a month earlier than last season’s onset date (18 January 2020). The last comparable early onset was in 2016-17

An early monsoon is typical of La Nina. This season, the onset was also aided by warmer than average ocean temperatures around Australia and the interaction of the MJO and other tropical waves to the Australian region.

A deepening tropical low (03U) crossed the Kimberley coast and tracked through the district’s south and is not expected to develop into a tropical cyclone.

A weak tropical low is currently located just outside the North-West Australia tropical cyclone region. There is a low risk for it to impact Cocos Islands for the coming week. However, international tropical cyclone forecast models, indicate there is a significantly increased risk for a tropical cyclone development in the central Indian Ocean to the eastern Madagascar at the end of December into early January 2021.

Read more about Australian Region tropical cyclone outlook and warnings.

Madden–Julian Oscillation becomes weak in the Western Pacific

The Madden–Julian Oscillation (MJO) has recently been active in the eastern Maritime Continent, assisting the development of the monsoon trough across the northern Top End and the Kimberley last week. The weak MJO pulse is currently meandering from the eastern Maritime Continent into the Western Pacific region.

At this time of the year, an active MJO in the Western Pacific is typically associated with above-average rainfall across far northern Australia and the northern Coral Sea. Climate models agree the MJO will be weak this week, with some predicting it will remain so into early January. However, the Bureau’s model indicates the MJO could strengthen over the western Indian Ocean in early January 2021.

Read more about the Madden–Julian Oscillation

Severe tropical cyclone Yasa devastated Fiji northern islands

Severe tropical cyclone Yasa reached category 5 strength to the west of Fiji on the 16th, and became the strongest cyclone of 2020 across the basins., as well as the earliest category 5 system in the South Pacific Region. Its 10-minute mean winds peaked at 250 km/h. Yasa crossed the northern island of Vanua Levu at its peak strength, causing widespread destructions to the island.