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

Sea surface temperatures (SSTs) across the central Pacific Ocean remain close to average along the equator.

The latest values for the week ending 20 May are: NINO3 +0.1 °C, NINO3.4 +0.1 °C and NINO4 +0.3 °C. Persistent NINO3 or NINO3.4 values cooler than −0.8 °C typically indicate La Niña, while persistent values warmer than +0.8 °C are typical of El Niño.

Warmer than average SSTs (more than 1 °C above average) persist in areas of the South Pacific Ocean and the Tasman Sea, though sea surface temperatures between Australia and New Zealand have been cooling over recent weeks. The surface of the Tasman Sea has been persistently warmer than average since the second half of November 2017.

To the north of Australia, weak warm SST anomalies remain across the Maritime Continent, although SSTs off the northwest coast of Australia have moved closer to average values during the past two weeks.

The El Niño–Southern Oscillation (ENSO) remains neutral—neither in an El Niño nor La Niña phase. Climate models favour ongoing neutral conditions for the southern hemisphere winter, despite continued warming of the tropical Pacific Ocean.

Key indicators of ENSO are mostly near average. Sea surface temperatures (SSTs) are close to the long-term average across the tropical Pacific Ocean, with Nino3.4 positive for the first time since mid-August 2017. Waters beneath the surface are slightly warmer than average. In the atmosphere, the Southern Oscillation Index (SOI) and trade winds are both within the neutral range.

Models suggest further warming of the tropical Pacific Ocean is possible over the coming months, with two of eight models indicating an El Niño could develop after the southern winter. However other models fall well short of El Niño thresholds, resulting in a broad range of possible scenarios for spring.

When ENSO is neutral it has limited influence on Australian and global climate. During an El Niño, eastern Australia typically sees less rainfall than usual during winter and spring.

The Indian Ocean Dipole (IOD) is currently neutral, with most climate models indicating a neutral pattern is likely to persist through the southern winter. However, there is a lot of variation amongst models in their outlooks, with one (of six models) forecasting a positive IOD to develop in winter, and another predicting a negative IOD during spring. The accuracy of IOD event forecasts is lower at this time of the year.

Cloudiness near the Date Line has been generally below average since late February.

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

Trade winds for the 5 days ending 20 May are near average across the equatorial Pacific.

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.

All eight of the surveyed international climate models indicate that Pacific Ocean sea surface temperatures will remain within the ENSO-neutral range over the southern hemisphere winter.

Two models indicate an El Niño may develop during spring, with another approaching threshold values but remaining on the warm side of neutral. The remaining five models continue to indicate neutral conditions are the most likely outcome.

Sea surface temperature (SST) anomalies for April show SSTs were slightly cooler than average in the central and eastern tropical Pacific Ocean, although cool anomalies in this region have weakened compared to last month. Warm anomalies were present across most of the South Pacific around and south of 30°S, much of the Maritime Continent, to the west of Japan, and from the east of Mexico across the mid-latitudes of the central North Pacific Ocean.

The April value for NINO3 was −0.2 °C, NINO3.4 −0.3 °C, and NINO4 +0.1 °C.

The 30-day Southern Oscillation Index (SOI) to 20 May was −2.2, and the 90-day SOI was +7.4. The SOI has been within the neutral range since late April.

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 currently neutral. The weekly index value to 20 May was +0.15 °C.

One of the six international climate models surveyed by the Bureau anticipate that the IOD will become positive during the southern hemisphere winter, and remain so into spring. Meanwhile, one other model predicts a negative IOD event will form during spring.

It should be noted that outlook skill is lower at this time of year, as reflected in the spread of projected outcomes.

The four-month sequence of sub-surface temperature anomalies (to 17 May) shows the decay of shallow cool anomalies in the eastern equatorial Pacific Ocean over the past three months, and the eastward propagation of a pool of warmer than average water further below the surface. This area of warm anomalies now extends from 140°E to 120°W between about 100 m and 200 m depth.

Sub-surface temperatures for the 5 days ending 20 May show a pool of slightly warmer than average water between about 50 m and 150 m depth, extending across most of the equatorial Pacific, and rising into the shallow sub-surface in the far eastern equatorial Pacific. A small area of water more than 3 degrees warmer than average was present to the east of the Date Line at about 125 m depth.