Weekly Tropical Climate Note

Northern rainfall onset likely to be earlier than average

Last Thursday, 30 July, the third issue of the Bureau’s Northern Rainfall Onset (NRO) outlook for season 2020-21 was released. The chance of the first rains for northern Australia arriving early in 2020-21 is higher than average over much of inland northern Australia, with the highest likelihood across southwestern, central and some coastal parts of Queensland, and southern Northern Territory. Other parts of northern Australia have roughly equal chances of an early or late rainfall onset.

'Rainfall onset' relates to the timing of the first significant rains observed after 1 September. The measure of 50 mm is used for this time of the year, as it is considered to be sufficient rainfall to stimulate pasture growth after the dry months of the northern dry season (May to September).

Earlier than usual rainfall onset is linked to La Niña forming during spring, which would increase the likelihood of above-average rainfall across much of Australia. Expected warmer-than-average temperatures across the waters around northern Australia are also conducive to above-average rainfall.

The next update will be released Thursday 13 August, with fortnightly updates continuing until the end of August.

Read more about the northern rainfall onset

Madden–Julian Oscillation moving across Maritime Continent

A pulse of the Madden–Julian Oscillation (MJO) has continued to track eastwards during the past week and is now located to Australia's north, in the western Maritime Continent. Most climate models indicate the pulse will slow down in the coming days before weakening, although some indicate it will strengthen further and remain discernible for at least another week.

At this time of the year, an MJO pulse over the Maritime Continent has only a small influence on rainfall patterns across northern Australia. Further north, parts of the Maritime Continent, including Papua New Guinea, and South-East Asia typically observe above-average rainfall.

Read more about the Madden–Julian Oscillation

Tropical cyclone activity over the western North Pacific Ocean

For the first time since at least 1949, no named storms (i.e. equivalent to a category 1 or stronger Australian tropical cyclone) developed in the western North Pacific basin in July 2020. In a typical year, this region sees between 3 and 4 tropical cyclones in July. This continues the well below-average tropical cyclone activity in the western North Pacific Ocean this year. Only one typhoon (equivalent to a category 3 Australian tropical cyclone) had been observed to the end of July 2020, compared to the long-term average of about 5.

In the past week, 2 named storms developed over the western North Pacific Ocean, bringing the 2020 total to 4 tropical storms, compared to an average of between 8 and 9.

Ex-tropical storm Sinlaku passed over Hainan Island and the east coast of northern Vietnam on 2 August as a weak tropical storm (equivalent to category 1 Australian tropical cyclone). Sinlaku generated strong winds and heavy rain as it made landfall. As it tracks further inland as a tropical depression, it is expected to produce heavy rains across parts of mainland China.

Typhoon Hagupit (Dindo) formed to the east of the Philippines and has recently tracked towards the north. It passed close to Taiwan in the last 24 hours and more recently made landfall on mainland China's east coast with mean winds estimated at more than 125 km/h.

The active MJO pulse over the Maritime Continent, along with another tropical atmospheric wave—an equatorial Rossy wave—contributed to the development of tropical cyclones Sinlaku and Hagupit. 

Further tropical cyclone information and warnings for this region available from the Japan Meteorological Agency