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

For the week ending 24 September, sea surface temperatures (SSTs) were cooler than average in the eastern half of the equatorial Pacific Ocean, and close to average in the central to western equatorial Pacific. The strength of the cool anomalies has continued to increase compared to two weeks ago, continuing a cooling trend since mid-winter. 

Generally weak warm anomalies remain in the far western Pacific, and extend across most of the north of the Maritime Continent, along much of the east coast of Australia, and across much of the South Pacific and the mid-latitudes of the North Pacific.

Both NINO3 and NINO4 have cooled during the past fortnight. Latest weekly values for the week ending 24 September are: NINO3 −0.6 °C, NINO3.4 −0.3 °C, NINO4 +0.1 °C.

Persistent values below −0.8 °C are typically indicative of La Niña.

The El Niño–Southern Oscillation (ENSO) is neutral. However, sea surface temperatures (SSTs) have cooled in the central to eastern tropical Pacific since mid-winter. These SSTs are currently cooler than average but within the neutral range. Waters beneath the surface are also slightly cooler than average. Other indicators of ENSO, such as the Southern Oscillation Index (SOI) and trade winds, also remain at neutral levels.

All 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 four maintain these values for long enough to be classified as a La Niña event.

While unusual, it is not unheard of to see La Niña develop this late in the year—the Bureau will keep a close watch for further, or sustained, cooling of the equatorial Pacific. Of the late-developing La Niña events, their effect on summer rainfall has been mixed, with some leading to widespread wet conditions across eastern Australia, and others having minimal effect.

The Indian Ocean Dipole (IOD) is neutral and the model consensus suggests it will remain so. Three of the six climate models surveyed suggest positive IOD thresholds may be reached during spring, but it may now be too late to become an event. If a positive IOD eventuated it would be short-lived, as events typically decay by December.

Cloudiness near the Date Line has been somewhat below average (positive OLR anomalies) during the past two weeks. Cloudiness near the Date Line has fluctuated around average values since mid-April, but has generally been below average during September.

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 24 September were slightly stronger than average over the western tropical Pacific and near average in the central and eastern tropical Pacific. These stronger trades may be assisting the cooling of sea surface temperatures.

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 into summer. While conditions are currently ENSO-neutral, five of the models anticipate that La Niña thresholds will be reached during spring or early summer. However, there is less certainty about how long these values will be maintained, with only three indicating they will persist long enough to be classified as a La Niña.  

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 August show SSTs were close to average along the equator in the Pacific Ocean. Warm anomalies were located across most of the south Pacific with the strongest anomalies occurring off the east Australian coastline and to the east of New Zealand. Large parts of the North Pacific between Indonesia and Japan also had SSTs of more than 1 °C warmer than average.

The August values for the NINO3.4, NINO3 and NINO4 regions were 0.0 °C, 0.0 °C and +0.4 °C, respectively. All three August values cooled with respect to July, but remain firmly within the neutral range.

The 30-day Southern Oscillation Index (SOI) to 24 September is +4.1 (90-day value +5.2), within the ENSO neutral range.

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

The Indian Ocean Dipole (IOD) is neutral. The weekly index value to 24 September was +0.02 °C.

Most of the climate models surveyed by the Bureau indicate that the IOD will remain neutral during spring. Three of the six models suggest a positive IOD remains a possibility during spring. A positive IOD is typically associated with below average spring rainfall over southern and central Australia.

IOD events typically decay during spring, and the influence of the IOD on Australian climate is weak during the months 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 21 September) shows cool anomalies have continued to develop 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 more than 4 degrees cooler than average for September. An area of weak warm anomalies persist in the shallow sub-surface waters of the western equatorial Pacific Ocean, but has decreased in size compared to August.

The sub-surface temperature map for the 5 days ending 24 September shows water across much of the subsurface of the equatorial Pacific Ocean is slightly cooler than average. Water temperatures around 100 m depth in the eastern equatorial Pacific Ocean are more than 2 degrees cooler than average between 150°W and 120°W, reaching more than 3 degrees cooler than average in a small area. Sub-surface water temperatures were generally close to average in the top 100 m of the western equatorial Pacific Ocean.