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

For the week ending 22 October, sea surface temperatures (SSTs) were cooler than average along the equator in the central and eastern Pacific Ocean, and to the south of the equator along the coast of Peru in South America. There has been a significant cooling of SSTs in the central to eastern equatorial Pacific over the past two weeks, resuming the cooling trend seen since mid-winter. NINO3 and NINO3.4 have cooled by 0.6 °C and 0.5 °C respectively, compared to two weeks ago.

Weak warm anomalies persist across much of the western Pacific, extending well into both the north and south of the basin. Stronger warm anomalies continue to the south of Japan, and along large parts of the southeast coast of Australia.

All of NINO3, NINO3.4, and NINO4 have cooled during the past fortnight. Latest values for the week ending 22 October are: NINO3 −0.6 °C, NINO3.4 −0.5 °C, NINO4 −0.1 °C.

Persistent values below −0.8 °C are typically indicative of La Niña, while persistent values above +0.8 °C are typical of El Niño.

The El Niño Southern Oscillation (ENSO) is currently neutral. However, models suggest the tropical Pacific Ocean will continue to cool, making the chance of a La Niña forming in late 2017 at least 50%; around double the normal likelihood. While this means the Bureau's ENSO Outlook has shifted to La Niña WATCH, rainfall outlooks remain neutral due to competing climate drivers.

Following a brief period of warming, tropical Pacific surface waters cooled significantly in the past fortnight, and hence the central to eastern tropical Pacific Ocean is now generally cooler-than-average. Atmospheric indicators of ENSO, including the Southern Oscillation Index (SOI), trade winds and cloudiness near the Date Line, are also approaching La Niña levels.

Seven of the eight international climate models surveyed by the Bureau suggest that sea surface temperatures will reach or exceed La Niña thresholds by November 2017. However, indicators need to remain at La Niña levels for at least three months to be considered an event. This is forecast by six of the eight models. If a La Niña does occur this year it is likely to be short and weak, as sea surface temperatures are forecast to warm again in early 2018, as the austral autumn is the time when La Niña events normally decay.

The Indian Ocean Dipole (IOD) is currently neutral. All six international climate models indicate neutral conditions will continue through 2017. Indian Ocean Dipole events are typically unable to form between December and April due to the influence of the monsoon trough over the tropical Indian Ocean.

La Niña events typically bring above average rainfall to eastern Australia during late spring and summer. However, given the competing influence of other climate drivers (weakly warm waters to the north of Australia, and cooler waters in the eastern Indian Ocean), current climate outlooks do not favour widespread rainfall across Australia for November to January. Weak La Niña events in summer can also produce heatwaves in southeast Australia.

Cloudiness near the Date Line has been below average (positive OLR anomalies) during October. Cloudiness values have been generally below average since early August.

Equatorial cloudiness near the Date Line typically increases during El Niño (below average OLR) and decreases during La Niña (above average OLR).

Trade winds for the 5 days ending 22 October were slightly stronger than average over the central tropical Pacific and near average in both the far west and far east of the tropical Pacific. Strengthened trade winds have helped resume the cooling trend observed in SSTs over recent months.

This return to stronger trade winds is at least in part related to a strong Madden-Julian Oscillation event. The MJO is associated with strengthened easterly wind anomalies in the Pacific while it is active over the Maritime Continent, but as the event moves into the western Pacific Ocean it is likely to see trade wind strength decrease somewhat.

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

International climate models surveyed by the Bureau indicate that further cooling of equatorial Pacific Ocean sea surface temperatures is expected during spring and summer. Seven of the eight models anticipate that SSTs will reach or surpass La Niña thresholds during November. Six of these models indicate that these SSTs will persist long enough to be classified as a La Niña event; persistence for three months is generally considered the minimum length required to be considered an event.

Late forming La Niña are rare, but not unheard of. You can read about the evolution and effect of previous events in our La Niña Summaries.

Sea surface temperature (SST) anomalies for September show SSTs were cooler than average in the eastern equatorial Pacific Ocean. Generally weak warm anomalies were present across most of the South Pacific, the western Pacific, and the mid-latitudes of the North Pacific. Areas of stronger warm anomalies in excess of one degree above average were observed along parts of the east coast of Australia, across the Philippine Sea, and in areas of the mid- to high latitudes of the South Pacific, particularly to the east of New Zealand.

The September value for NINO3.4 was −0.3 °C, NINO3 −0.4 °C, and NINO4 +0.1 °C. All three September values cooled compared to August, but remained within the neutral range.

The 30-day Southern Oscillation Index (SOI) to 22 October is +13.0 (90-day value +7.1), well within the La Niña value range. 30-day SOI values have been above La Niña thresholds for three weeks.

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

The Indian Ocean Dipole (IOD) is neutral. The weekly index value to 22 October was −0.11 °C. All of the climate models surveyed by the Bureau indicate that the IOD will remain neutral into early 2018.

The influence of the IOD on Australian climate is weak during December to April. This is because the monsoon trough shifts south over the tropical Indian Ocean changing wind patterns, which prevents the IOD pattern from being able to form.

The four-month sequence of sub-surface temperature anomalies (to 19 October) shows cool anomalies persist across most of the equatorial Pacific Ocean down to a depth of 200 m. In some areas of the central equatorial Pacific sub-surface anomalies are up to 4 degrees cooler than average for October. In general terms, the extent and strength of sub-surface temperature anomalies is similar to those for September.

The sub-surface temperature map for the 5 days ending 22 October shows a large pool of slightly cooler than average water present in the subsurface of the eastern half of the equatorial Pacific Ocean. Water temperatures in an area around 50 to 100 m depth at around 120°W to 130°W were more than 4 degrees cooler than average. Sub-surface water temperatures were generally close to average across full column depth of the western half of the equatorial Pacific.