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

Sea surface temperatures (SSTs) remain cooler than average in much of the central to eastern tropical Pacific for the week ending 25 February. However, these cool anomalies have been declining in strength over recent weeks, and continued to move towards near-average temperatures over the last fortnight.

Latest values for the week ending 25 February are: NINO3 −0.5 °C, NINO3.4 −0.6 °C, NINO4 −0.1 °C.

Strong positive SST anomalies are present across the south Pacific and the Tasman Sea. The strongest warm anomalies are now located on the eastern side of New Zealand, but waters around Tasmania also remain more than 1 degree warmer than average. Strong warm anomalies have persisted across the surface waters of the Tasman Sea since the second half of November 2017.

To the north of Australia SSTs are generally close to average, although weak warm anomalies persist across northern parts of the Maritime Continent.

Persistent NINO3 or NINO3.4 values cooler than −0.8 °C are typically indicative of La Niña, while persistent values warmer than +0.8 °C are typical of El Niño.

La Niña continues its decline, with the central tropical Pacific Ocean warming over the past fortnight. Most models indicate a return to neutral conditions is likely early in the southern autumn.

The decline of this La Niña is evident in oceanic and atmospheric patterns, with several indicators recently returning to levels more consistent with a neutral ENSO phase. Sea surface temperatures are very close to neutral levels, cloudiness near the Date Line has increased, and trade winds are generally near normal across the equatorial Pacific. However, the current pulse of the Madden-Julian Oscillation (MJO) has been strong, and the effects of wind variations associated with it are likely to have amplified the decline. As the MJO progresses east, its effect will reverse, meaning some La Niña indicators are likely to strengthen briefly.

Four of eight international climate models surveyed by the Bureau maintain La Niña values through March. By May, only one model still exceeds La Niña thresholds. For July, all eight are within the neutral range. This ENSO event has had relatively little effect on Australian rainfall patterns over the 2017–18 summer.

The Indian Ocean Dipole (IOD) is currently neutral. IOD events are unable to form between December and April.

Cloudiness near the Date Line has fluctuated around average since the start of February. Cloudiness in this region has been slightly above average for February as a whole, consistent with the decline of La Niña.

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 25 February were near average across the equatorial Pacific.

The passage of a very strong pulse of the Madden-Julian Oscillation (MJO) across the Maritime Continent was behind the recently westerly wind burst, which has now abated. The MJO is currently passing from Africa into the Indian Ocean, and may result in a temporary strengthening of La Niña indicators in the next fortnight, before they resume their return to neutral values.

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 international climate models surveyed by the Bureau indicate that the current event is likely near its end.

Four of the eight surveyed models maintain values close to La Niña thresholds for March, but all models indicate equatorial Pacific sea surface temperatures are likely to continue to warm over the coming months. Only one model anticipates NINO3.4 will meet La Niña thresholds for May, and all models predict ENSO will be in a neutral phase during the southern hemisphere winter.

Sea surface temperature (SST) anomalies for January show SSTs were cooler than average in the central and eastern tropical Pacific Ocean and across large areas of the Pacific south of the equator and east of 110°W (i.e. the area to the west of South America). Generally weak warm anomalies were present across most of the remainder of the South Pacific, most of the western Pacific, and parts of the mid-latitudes of the North Pacific. Areas of stronger warm anomalies in excess of two degrees above average occurred in the Tasman Sea between southeastern Australia and New Zealand.

The January value for NINO3 was −0.8 °C, NINO3.4 −0.6 °C, and NINO4 −0.2 °C. NINO3 and NINO3.4 warmed slightly compared to December values, while NINO4 held steady.

The 30-day Southern Oscillation Index (SOI) to 25 February is −5.0 (90-day value +1.1), which is within the neutral range. While the SOI fluctuates more during the southern hemisphere summer due to movement of tropical systems, it has spent most of 2018 to date within the 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 +7 and −7 generally indicate neutral conditions.

The Indian Ocean Dipole (IOD) is neutral. The weekly index value to 25 February was +0.20 °C. All six of the climate models surveyed by the Bureau indicate that the IOD will remain neutral into the southern hemisphere winter of 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 22 February) shows the cool anomalies in the central and eastern equatorial Pacific Ocean have weakened over the past three months. Meanwhile, a pool of warmer than average water has developed below the surface of the western equatorial Pacific Ocean, a typical feature of a decaying La Niña.

The areal extent and strength of cool anomalies in the east remains similar to those for January. Warm anomalies in the west have increased in strength, reaching a peak around 160°E and 150 m depth where they exceed +3.5 degrees in a small area.

Sub-surface temperatures for the 5 days ending 25 February show a pool of warmer than average water in the western half of the equatorial Pacific between 100 and 200 m below the surface. This region has warmed and expanded further east compared to two weeks ago.

The development of warm sub-surface waters in the western equatorial Pacific is a typical precursor to the breakdown of a La Niña event, as these warmer temperatures can migrate east and erode the cool anomalies that support the event.

Temperatures in the sub-surface of the eastern tropical Pacific continue near average to slightly cooler than average. Cool anomalies in this region have strengthened slightly compared to two weeks ago.