Seasonal Climate Outlook

Recent climate

August rainfall was generally below average in the SWLD, but above average along the south coast from a cut off low in early August. Winter (June to August) rainfal was below average. August maximum temperatures were above average. August minimum temperatures were average to above average. Maximum temperatures for winter was the highest on record for Western Australia. Decile 2-3 rainfall from 1 April to date for the majority of the SWLD.

Rainfall deciles for 1 April to 6 September 2020 in the South West Land Division. Indicating below average rainfall for the majority of the SWLD.
Rainfall deciles for 1 April to 6 September 2020 in the South West Land Division.

In August, the atmospheric pressure was lower than normal over the SWLD.

Sea surface temperatures over large parts of the Indian Ocean are warmer than average. The September to November 2020, SST forecast by the Bureau of Meteorology indicates SSTs are likely to remain warm, north of WA.

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the southern hemisphere. SAM is currently negative, but is expected to become  positive for the remainder of September.  In a La Niña event, SAM is usually in the positive phase. A positive SAM at this time of the year means less rainfall for SWWA. The Bureau now forecasts SAM in their Climate Driver Update.

The Indian Ocean Dipole (IOD) index passed the threshold level three weeks ago and three of the six surveyed climate models indicate these levels are likely to be sustained at negative IOD values during September, and four of six for October and November. For this to be considered an official negative IOD event, the IOD index remain at or cooler than −0.4 °C for eight weeks. Models indicate the IOD returning to neutral by December.

The El Niño–Southern Oscillation (ENSO) is currently neutral. The Bureau have increased their La Niña WATCH from Active to Alert. There is currently a 70% chance or triple the normal risk of a La Niña developing in the Pacific Ocean.  A La Niña is often associated with increased chance of widespread rains and flooding over eastern, central and northern Australia, more tropical cyclones than average and prolonged very warm periods in the south.

If a La Niña forms this usually means above average number of tropical systems (cyclones and lows) across northern Australia. The first Australian landfall typically occurs in early December, which is about 3 weeks earlier than usual. For further information, see the Bureau of Meteorology’s Climate Driver Update and the Northern Rainfall Onset.

The table below gives a summary of past month and three-month South West Land Division (SWLD) climate conditions, and can indicate what is likely to occur in the near future if climate conditions follow the current pattern.

Climate Indicator Past month Past Three months
SWLD Rainfall Mixed Below average
SWLD Mean Temperature Above average Very much above average
SWLD atmospheric pressure Lower Higher
Indian Ocean Sea surface temperature Warmer Warmer
El Niño/Southern Oscillation (ENSO) Neutral Neutral
Indian Ocean Dipole (IOD) Negative Neutral
Southern Annular Mode (SAM) Negative Negative