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

For the week ending 21 May, sea surface temperatures (SSTs) in the central to western equatorial Pacific Ocean were close to average. Weak warm SST anomalies persist across the central to eastern equatorial Pacific and much of the South Pacific, including areas immediately south of the equator. Warm anomalies greater than +1 °C were present in the far eastern equatorial Pacific.

The NINO3.4 SST anomaly has remained at around +0.5 °C since mid-April, with NINO3 remaining at or above +0.5 °C since mid-March. Both NINO3.4 and NINO3 are currently at +0.5 °C, while NINO4 is at +0.3 °C.

The El Niño–Southern Oscillation (ENSO) remains neutral. With the tropical Pacific Ocean warmer than average, and around half the international climate models reaching El Niño levels later in the year, development of El Niño in 2017 cannot be ruled out. The Bureau's ENSO Outlook remains at El Niño WATCH, meaning there is around a 50% chance—double the normal likelihood—of El Niño developing in 2017.

Sea surface temperatures across the central tropical Pacific remained half a degree warmer than average over the past month. This is below the El Niño threshold of +0.8 °C. Further warming in the coming fortnight is unlikely, with trade winds forecast to be stronger than average. All other ENSO indicators are also neutral.

Five of eight international climate models suggest the tropical Pacific Ocean is likely to warm above El Niño thresholds during the second half of 2017. However virtually all models now suggest less warming is likely to occur compared to their previous outlooks, indicating any event may be weak. Models have lower accuracy forecasting El Niño during the autumn months, though accuracy begins to improve from June.

El Niño is often, but not always, associated with a drier than average winter–spring over eastern Australia. Even if El Niño thresholds are not met, Australia may still see some El Niño-like effects if waters in the tropical Pacific Ocean remain warm.

The Indian Ocean Dipole (IOD) remains neutral. Four out of six climate models suggest a positive IOD is likely to develop during winter. A positive IOD is typically associated with a drier than average winter–spring for southern and central Australia.

Cloudiness near the Date Line is close to average. Values have been fluctuating around average over the past several weeks after having been persistently below average since August 2016.

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 21 May were close to average over most of the tropical Pacific, with trades slightly stronger than average over the far western equatorial Pacific. However models suggest they may strengthen during the next couple of weeks, suggesting further warming of the tropical Pacific Ocean waters is unlikely during the coming fortnight.

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.

Climate models surveyed by the Bureau indicate that El Niño remains possible for the second half of 2017. Five out of the eight surveyed models forecast SSTs in the central Pacific will reach or exceed the El Niño threshold at some point during winter or spring. However, some models show considerable spread across their projections, and three models favour neutral ENSO conditions.

It should be noted that model outlooks that span the southern hemisphere autumn tend to have lower accuracy than outlooks issued at other times of the year, and should be used with some caution. Accuracy begins to improve for outlooks generated in June.

SST anomalies for April show sea surface temperatures in the central to western equatorial Pacific Ocean were close to average. Weak warm anomalies were present in the eastern Pacific, across much of the Pacific south of the equator, and in the far western Pacific around eastern Australia including Tasmania and New Zealand.

The April values for the NINO3.4, NINO3 and NINO4 regions were +0.3 °C, +0.6 °C and 0.1 °C, respectively.

The 30-day Southern Oscillation Index (SOI) to 21 May was −3.7 (90-day value −0.9), within neutral territory.

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 21 May was +0.33 °C.

Four of the six surveyed models indicate a positive IOD is likely to form during winter. However, model skill is low at this time of year, so caution should be exercised when using these forecasts.

A positive IOD typically brings below average winter-spring rainfall to parts of southern and central Australia.

The four-month sequence of sub-surface temperature anomalies (to 18 May) shows water temperatures in the sub-surface of the equatorial Pacific Ocean are generally near average. Areas of weak warm anomalies persist in the top 150 m of the tropical Pacific west of 160°E, while in the eastern tropical Pacific an area of weak cool anomalies is present in the top 100 m between about 120°W and 100°W.

The sub-surface temperature map for the 5 days ending 21 May shows temperatures were generally close to average across the equatorial Pacific Ocean. In the eastern Pacific weak warm anomalies are apparent between the surface and a depth of 100 m. In the central Pacific there were weak cool anomalies at a depth of 100 to 200 m; these cool anomalies have strengthened over the past fortnight and are positioned further to the west than they were two weeks ago.