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 a large area of the central to eastern tropical Pacific for the week ending 25 February. However, both the strength and spatial extent of cool anomalies have been declining, resulting in neutral values of NINO indices in the central and eastern Pacific in recent weeks.

Latest values for the week ending 11 March are: NINO3 −0.6 °C, NINO3.4 −0.6 °C, NINO4 −0.1 °C. 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.

Strong positive SST anomalies persist across the south Pacific and the Tasman Sea. The strength of the warm anomalies has generally decreased compared to two weeks ago, but surface waters remain more than 1 degree warmer than average from the eastern side of the Great Australian Bight, across the Tasman Sea, to well east of New Zealand. 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, or slightly warmer than average, although weak warm anomalies persist across northern parts of the Maritime Continent.

The 2017–18 La Niña has ended. El Niño–Southern Oscillation (ENSO) indicators have eased back to neutral levels over the past several weeks. This means the ENSO Outlook has shifted from LA NIÑA to INACTIVE.

The end of the La Niña is clear in oceanic and atmospheric indices. Sea surface temperatures have warmed steadily since December, and are now in the neutral range. Waters beneath the surface have also warmed. In the atmosphere, cloudiness near the Date Line has returned to near-average levels, and trade winds are generally near average across the equatorial Pacific. Likewise, the Southern Oscillation Index (SOI) is well within the neutral range.

Most models indicate that ENSO-neutral is the most likely scenario through the southern hemisphere autumn and into winter. However, model accuracy during autumn is lower than at other times of year. A neutral ENSO pattern does not necessarily signify average rainfall and temperature for Australia. Rather, it indicates a reduced chance of prolonged very wet or dry, or very hot or cold conditions, and that other climate drivers may have greater influence over the coming months.

The weak and short–lived La Niña had relatively little effect on Australian rainfall patterns over the 2017–18 summer. However, it may have kept temperatures higher than average in southern parts of the country due to weather patterns being slower moving, and further south than normal.

The Indian Ocean Dipole (IOD) is currently neutral. IOD events are unable to form between December and April. Current outlooks suggest a neutral IOD for autumn and early winter.

Cloudiness near the Date Line has been below average for March to date, but was above average for much of 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 11 March were 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 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 February show SSTs were cooler than average in the central and eastern tropical Pacific Ocean. Warm anomalies were present across most of the South Pacific south of around 30°S, around the Maritime Continent and across the Philippine Sea, and in parts of the mid-latitudes of the North Pacific.

The February value for NINO3 was −0.7 °C, NINO3.4 −0.6 °C, and NINO4 −0.2 °C.

The 30-day Southern Oscillation Index (SOI) to 11 March was +3.2 (90-day value +0.9), 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 11 March was +0.23 °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 February) shows the cool anomalies in the central and eastern equatorial Pacific Ocean weakened over the past three months. Meanwhile, a pool of warmer than average water 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 degrees.

Sub-surface temperatures for the 5 days ending 11 March show a pool of warmer than average water in the western to central  equatorial Pacific around and east of the Date Line and between 100 and 200 m below the surface. This region has shifted further east compared to two weeks ago, and is expected to continue moving east over the coming weeks.

The development of warm sub-surface waters in the western equatorial Pacific is typical during the breakdown of a La Niña event.

Temperatures in the sub-surface of the eastern tropical Pacific were near average for the 5 days ending 11 March.