Climate Driver Update history
Climate drivers in the Pacific, Indian and Southern oceans and the Tropics

The SST map for November shows below average SSTs extending along the equator in the central and eastern tropical Pacific Ocean, extending into the tropics south of the equator in the east of the basin. The strength of these anomalies has increased slightly along the equator in the central parts of the basin compared to October.

The strength of warmer than average SSTs anomalies in the Tasman Sea and to the northwest of Australia have also increased. 

The November values of the three key NINO indices were: NINO3 −0.8 °C, NINO3.4 −1.0 °C, and NINO4 −0.4 °C.

The 30-day Southern Oscillation Index (SOI) for the 30 days ending 20 December was +14.4. The MJO has been over the Australian longitudes for the past fortnight and is likely to have contributed to the recent increase in SOI values. The 90-day SOI value was +10.3.

Sustained negative values of the SOI below −7 typically indicate El Niño while sustained positive values above +7 typically indicate La Niña. Values between +7 and −7 generally indicate neutral conditions.

Trade winds for the 5 days ending 20 December were stronger than average over the central and western tropical Pacific. Trade winds strength is similar compared to two weeks ago.

During El Niño there is a sustained weakening, or even reversal, of the trade winds across much of the tropical Pacific. Conversely, during La Niña, there is a sustained strengthening of the trade winds.

The Madden–Julian Oscillation (MJO) is over the eastern parts of the Maritime Continent, to the north of Australia. It is weak in strength and is expected to remain weak as it continues to move eastwards across the western Pacific over this coming fortnight, before possibly strengthening again over the western Indian Ocean. Its current position is conducive to producing increased rainfall over northern Australia.

Sea surface temperatures north of 20°S in the Indian Ocean are average to warmer than average, while to the south, they are average to cooler than average. The Indian Ocean Dipole (IOD) is neutral. The latest weekly value of the IOD index to 20 December was +0.1 °C.

Compared to two weeks ago, there has been some cooling in the central Indian Ocean south of 20°S, as well as to the northwest of Australia.

All six surveyed climate models expect the IOD to remain neutral through summer. IOD events are typically unable to form between December and April as the monsoon trough shifts south over the tropical Indian Ocean.

Cloudiness near the Date Line was below average over the past fortnight and has generally been below average since early to mid-March.

Equatorial cloudiness near the Date Line typically increases during El Niño (negative OLR anomalies) and decreases during La Niña (positive OLR anomalies).

The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to 17 December) shows cooler than average water extending across the top 200 m of the sub-surface of the equatorial Pacific from around the Date Line and eastward across the basin.  The strength and extent of cooler than average water has remained similar since September with a slight weakening in November.

Weak warm anomalies persist across large parts of the column depth west of the Date Line, decreasing in depth towards the west of the basin. Compared to previous months, these warm anomalies are slightly stronger and larger in extent.

For the five days ending 20 December, sub-surface temperatures were above average in the western equatorial Pacific, reaching more than 2 degrees above average in a region between 170°W and 150°E at 100 to 150 m depth. These warm anomalies cover a larger region than they did two weeks ago. In the east, cool anomalies are weaker and cover a smaller region than a fortnight ago.

Despite some warming of sea surface temperatures in the central and eastern tropical Pacific Ocean over the past fortnight, La Niña remains at moderate levels. Model outlooks suggest the current La Niña is approaching its peak, with a likely return to neutral conditions during the late southern summer or early autumn.

The Southern Annular Mode (SAM) is strongly positive. While positive SAM often occurs during La Niña influence, the current event is also being driven by an exceptionally strong polar vortex over Antarctica. Positive values are expected until at least early 2021. A positive SAM at this time of year typically increases the chance of rainfall in eastern Australia. 

The Madden–Julian Oscillation (MJO) is weak, and currently located over the eastern parts of the Maritime Continent. It is forecast to move east and remain weak over the next fortnight, before possibly strengthening again in the western Indian Ocean in January.

Climate change is also influencing the Australian climate. Rainfall across northern Australia during its wet season (October–April) has increased since the late 1990s, with a greater proportion of high intensity short duration rainfall events.

Nationally averaged rainfall for December 2020 is likely to be in the ten wettest on record while climate outlooks for January to March 2021 indicate rainfall is likely to be above average for most of the eastern third of the country.

The Southern Annual Mode (SAM) is currently strongly positive. It is expected to remain positive into early 2021. The positive SAM event is being driven by La Niña and an exceptionally strong Antarctic polar vortex. Positive SAM during the spring to summer months typically enhances the La Niña wet signal in eastern Australia.

All international climate models surveyed by the Bureau indicate the current La Niña is at or near its peak and will likely persist until at least January 2021. All but two of the models indicate a return to neutral conditions (neither El Nino nor La Nina) by the end of the southern autumn in 2021. La Niña conditions are weaker now than they were at the same point in 2010.

La Niña increases the likelihood of above average rainfall across much of Australia during spring, and across much of northern and eastern Australia during summer. La Niña increases the chance of below average daytime temperatures for large parts of the country, particularly Queensland. It also increases the chance of tropical cyclones, and earlier first rains of the northern wet season.