Tropical Climate Update

Typical Australian cyclone season likely

The Bureau of Meteorology released its tropical cyclone outlook for 2017–18 on 9 October. The outlook suggests that near-average tropical cyclone numbers are likely across the Australian region during the official tropical cyclone season which runs from 1 November to 30 April. For the 1969/70–2016/17 period, the typical number of cyclones per season is between ten and thirteen. Tropical cyclone numbers are typically below average during El Niño and above average during La Niña. The El Niño–Southern Oscillation (ENSO) state is a significant determinant of cyclone numbers and the tropical cyclone outlook is based on the state of ENSO over the preceding July to September. For the 2017–18 tropical cyclone season, ENSO-neutral or weak La Niña-like conditions are most likely (see ENSO section below). During ENSO-neutral years, the first tropical cyclone to make landfall over Australia typically occurs in late December. In La Niña years, the first cyclone to make landfall over Australia typically occurs around the first week of December, a few weeks earlier than normal.

The northern Australian monsoon onset date is also strongly influenced by the ENSO state and is closely related to tropical cyclone formation. Onset of the monsoon for northern Australia (traditionally taken as the date monsoon conditions commence at Darwin) occurs when the monsoon trough over Australian longitudes has migrated southwards to Darwin's latitude. This heralds the start of the northwesterly monsoon winds and the associated heavy, widespread rainfall. The monsoon trough is often the favoured location for tropical low formation, which means whenever the monsoon trough is located over water near the Australian coastline, there is a heightened risk of tropical cyclone formation. An ENSO-neutral state typically sees a monsoon onset during the last week of December, while during La Niña the onset date is typically earlier, around the middle of December.  

Models predict Madden–Julian Oscillation will strengthen

The Madden–Julian Oscillation (MJO) signal was weak or indiscernible for the last several weeks, but in the last few days has strengthened over the western Maritime Continent. Most international climate models surveyed by the Bureau predict further strengthening will occur before the signal moves eastward over the Maritime Continent during the coming week. When the MJO is in this region at this time of the year, cloudiness and rainfall is typically enhanced across much of the Maritime Continent and South East Asia. The most significant effect for northern Australia associated in this situation is an increase in minimum temperatures over the northwest of the continent due to stronger than normal westerly winds. There is no significant effect on rainfall patterns over northern Australia with the MJO in the Maritime Continent at this time of the year.

See the Bureau's current MJO monitoring for more information.

ENSO-neutral conditions persist

Sea surface temperatures (SSTs) across the equatorial Pacific Ocean have warmed marginally in the last week as a result of weakened trade winds. The SST pattern across the equatorial Pacific Ocean is currently indicative of an ENSO-neutral state. Atmospheric indicators also depict an ENSO-neutral state.

A majority of international climate models surveyed by the Bureau suggest further cooling of the tropical Pacific Ocean is likely. Five of the eight models suggest SSTs will cool to La Niña thresholds by December 2017, but only three maintain these values for long enough to be classified as a La Niña event.

It is not unprecedented for La Niña to develop this late in the year, however it is unusual. Of the four late-developing La Niña events on record, three have coincided with above-average wet-season rainfall across northern Australia.

See the Bureau's current ENSO Wrap-Up for more information.