ENSO Wrap-Up
Current state of the Pacific and Indian Ocean


Mature El Niño in the tropical Pacific region

Issued on 18 August 2015 |

The 2015 El Niño has continued to strengthen over the past fortnight. The ocean and atmosphere are reinforcing each other, with tropical Pacific Ocean temperatures well above El Niño thresholds, consistently weakened trade winds, and a strongly negative Southern Oscillation Index. Strong coupling of the tropical Pacific Ocean and atmosphere is typical of a mature El Niño, and suggests only a small chance of the event finishing before the end of the year.

All international climate models surveyed by the Bureau of Meteorology indicate the tropical Pacific is likely to warm further, peaking later in the year. Typically, El Niño peaks during the late austral spring or early summer, and weakens during late summer to autumn.

El Niño is usually associated with below-average winter–spring rainfall over eastern Australia and above-average daytime temperatures over the southern half of the country. However, El Niño is not the only influence on rainfall and temperature; other factors, such as sea surface temperatures to the north of Australia and in the Indian Ocean, also affect Australia's climate.

The Indian Ocean Dipole (IOD) continues to remain neutral. A positive IOD event remains possible, with three of the five international models indicating a positive IOD is likely during spring. The other two models remain neutral.

Next update expected on 1 September 2015 | print version


Weekly sea surface temperatures

The spatial distribution of sea surface temperature (SST) anomalies remains generally similar to that of two weeks ago—warm anomalies persist along the equator from the South American coastline to about 170°E. Anomalies for the week ending 16 August exceeded +2 °C across most of the eastern equatorial Pacific and smaller areas of the central Pacific.

All five NINO indices remain above +1 °C this week.

Warm anomalies were present across most of the northern half of the Pacific basin, except for parts of the northwest, and have strengthened in the tropics compared to two weeks ago. The eastern half of the northern Pacific was more than one degree warmer than average, with large areas more than two degrees warmer than average. Warm anomalies were also present along parts of the east coast of Australia, in areas to Australia's west, and across much of the Indian Ocean.

Click to see full-size map showing temperature anomalies in the tropical Pacific
Baseline period 1961–1990.
Index Previous Current Temperature change
(2 weeks)
NINO3 +1.9 +1.9 no change
NINO3.4 +1.7 +1.8 0.1 °C warmer
NINO4 +1.0 +1.0 no change
Monthly sea surface temperatures

The SST anomaly map for July 2015 shows positive anomalies extended from the South American coastline, across the equatorial Pacific, past the Date Line to around 160°E. Compared to June, the strength of these anomalies have increased in the eastern Pacific. Strong warm anomalies also persisted across much of the northeast of the Pacific Basin, with weak warm anomalies to Australia's east, and moderate to strong warm anomalies across much of the Indian Ocean.

NINO3 measured its warmest monthly anomaly since the 1997–98 El Niño, with an anomaly of +1.9 °C for July 2015, ahead of +1.7 °C in June 2015 and +1.6 °C December 2009. The July 2015 value of NINO3.4 was +1.5 °C.

In the coming weeks, the NINO3.4 region may exceed the peak anomaly values reached during the 2002 (+1.6 °C) and 2009 (+1.7 °C) El Niño. The current El Niño has already exceeded the 2006 peak of +1.2 °C, but current anomalies still remain well short of the 1982 and 1997 peaks (+2.8 °C and +2.7 °C respectively). Note: peak values are typically recorded late in the year.

Click to see full-size map showing temperature anomalies in the tropical Pacific
Baseline period 1961–1990.
Index June July Temperature change
NINO3 +1.6 +1.9 0.3 °C warmer
NINO3.4 +1.3 +1.5 0.2 °C warmer
NINO4 +1.1 +1.1 no change
5-day sub-surface temperatures

The sub-surface temperature map for the 5 days ending 16 August shows temperatures were warmer than average in the top 150 m of the central to eastern equatorial Pacific and cooler than average below the surface of the ocean in the western equatorial Pacific. Water in far eastern Pacific sub-surface was very much warmer than average, with a large area of anomalies around 75 m depth more than 6 °C warmer than average. This area of +6 °C anomalies has increased compared to two weeks ago.

Cool anomalies in the western equatorial Pacific have also increased compared to two weeks ago, but slightly. A broad area of anomalies more than 2 °C cooler than average persists between around 100 m and 200 m depth.

The pattern of warm anomalies in the eastern sub-surface and cool anomalies in the west is consistent with a well-established El Niño.

Monthly sub-surface temperatures

The four-month sequence of sub-surface temperature anomalies (to July) shows a generally consistent pattern of anomalies throughout the past four months. Warm anomalies were evident for July across the top 200 m of the equatorial Pacific sub-surface between about 160°E and the South American coast. Monthly anomalies across large areas of the eastern half of the equatorial Pacific reached more than +4 °C. An area of cool anomalies persisted in the sub-surface of the western equatorial Pacific.

Southern Oscillation Index

The Southern Oscillation Index (SOI) has remained relatively steady over the past two weeks, at a value slightly more negative than two weeks ago. The 30-day SOI value to 16 August was −20.3.

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

Trade winds

Trade winds for the 5 days ending 16 August show westerly anomalies were present over the western to central equatorial Pacific. Trade winds were reversed (i.e. westerly winds) to the west of the Date Line.

Trade winds have been consistently weaker than average, and on occasion reversed in direction (i.e. westerly rather than easterly), since the start of 2015.

Cloudiness near the Date Line

Cloudiness near the Date Line was above average during most of August to date, but has recently been close to average.

Cloudiness along the equator, near the Date Line, is an important indicator of ENSO, as it typically increases (negative OLR anomalies) near and to the east of the Date Line during El Niño and decreases (positive OLR anomalies) during La Niña.

Model outlooks

All eight of the surveyed international climate models indicate the central Pacific Ocean will warm further during the coming months. All surveyed models indicate that NINO3.4 will remain above El Niño thresholds until at least the start of 2016.

Click to see full-size map of NINIO3.4 SST plumes from POAMA forecasts, updated daily.
Indian Ocean Dipole

The Indian Ocean Dipole (IOD) is currently neutral. The weekly value of the IOD index to 16 August was +0.38 °C. Sea surface temperatures in the Indian Ocean are warmer than average over much of the basin.

Three of the five surveyed international climate models indicate a positive IOD event will occur during the southern hemisphere spring.

Positive IOD events, often associated with lower rainfall in central and southeastern Australia, are more likely to occur during El Niño. Between 50% and 60% of all historical El Niño events have seen a positive IOD develop at the same time. Positive IOD events are often associated with lower rainfall in parts of central and southeastern Australia. Conditions will be monitored closely.

Click to see full-size map of IOD SST plumes from POAMA forecasts, updated daily.

See also: IOD forecasts