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

El Niño persists as positive IOD emerges in a warm Indian Ocean

Issued on 29 September 2015 |

The tropical Pacific ocean and atmosphere are reinforcing each other, maintaining a strong El Niño that is likely to persist into early 2016. Tropical Pacific sea surface temperatures are more than 2 °C above average, exceeding El Niño thresholds by well over 1 °C, and at levels not seen since the 1997–98 event. In the atmosphere, tropical cloudiness has shifted east, trade winds have been consistently weaker than normal, and the Southern Oscillation Index (SOI) is strongly negative.

Most international climate models surveyed by the Bureau of Meteorology indicate El Niño is likely to peak towards the end of 2015. Typically, El Niño is strongest during the late austral spring or early summer, and weakens during late summer to autumn.

The Indian Ocean Dipole (IOD) is in a positive phase, having exceeded the +0.4 °C threshold for the past 8 weeks. Recent values of the IOD index have been at levels not seen since the strong 2006 positive IOD event. Conversely, the Indian Ocean remains very warm on the broader scale.

Four out of five international models suggest the 2015 positive IOD event will persist until November, when it typically breaks down due to monsoon development.

El Niño is usually associated with below-average spring rainfall over eastern Australia, and increased spring and summer temperatures for southern and eastern Australia. A positive IOD typically reinforces the drying pattern, particularly in the southeast. However, sea surface temperatures across the whole Indian Ocean basin have been at record warm levels, and appear to be off-setting the influence of these two climate drivers in some areas.

Next update expected on 13 October 2015 | print version

Weekly sea surface temperatures

Warm anomalies persist along the equator from the South American coast to the Date Line and across most of the Pacific Ocean east of the Date Line in the northern hemisphere. Compared to two weeks ago, sea surface temperature (SST) anomalies have increased slightly in the eastern equatorial Pacific and remain similar in the central and western equatorial Pacific.

Anomalies for the week ending 27 September 2015 exceed +2 °C across most of the equatorial Pacific east of 170°W. Warm anomalies are also present in areas to Australia's west, and across the majority of the Indian Ocean.

All five NINO indices remain above +1 °C this week, and both NINO3 and NINO3.4 remain at or above +2 °C. NINO3 and NINO3.4 were last at these levels during the 1997–98 El Niño.

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 +2.1 +2.2 0.1 °C warmer
NINO3.4 +2.0 +2.0 no change
NINO4 +1.1 +1.1 no change
Monthly sea surface temperatures

The SST anomaly map for August 2015 shows positive anomalies extended from the South American coastline, across the equatorial Pacific, to just past the Date Line. Compared to July, the area covered by positive anomalies has increased north of the equator, covering most of the northern Pacific Basin east of the Date Line. Anomalies generally remained similar in strength to the previous month. Weak warm anomalies also persisted to Australia's east, and moderate to strong warm anomalies across much of the Indian Ocean.

NINO3 and NINO3.4 strengthened, reaching anomalies of +2.0 °C and +1.9 °C respectively for August 2015.

NINO3.4 has now exceeded the peak monthly anomaly of the values reached during the 2002 (+1.6 °C) and 2009 (+1.7 °C) El Niño. However, the current anomaly remains well behind the peak value during either 1982 or 1997 (+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 July August Temperature change
NINO3 +1.9 +2.0 0.1 °C warmer
NINO3.4 +1.5 +1.9 0.4 °C warmer
NINO4 +1.1 +1.1 no change
5-day sub-surface temperatures

The sub-surface temperature map for the 5 days ending 27 September 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 volume of water at around 75 m depth more than 7 °C warmer than average. Anomalies in this region remain similar to two weeks ago.

Cool anomalies in the western equatorial Pacific have strengthened when compared to two weeks ago, but a large volume of water at around 150 m depth remains more than 2 °C cooler than average.

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

In the mean 5-day values (upper panel), the thermocline is almost flat. The thermocline sits around the 20 °C region, and is considered the mid-point between the warmer surface waters, and cooler subsurface waters. An almost flat thermocline tends to only occur during strong El Niño events.

Monthly sub-surface temperatures

The four-month sequence of sub-surface temperature anomalies (to September) shows a generally consistent pattern of anomalies throughout the past four months. In September, warm anomalies have been present in the top 200 m of the equatorial Pacific sub-surface, extending between about 170°E and the South American coast. These anomalies in the eastern half of the equatorial Pacific exceed +4 °C in parts. Cool anomalies persist in the sub-surface of the western equatorial Pacific.

Southern Oscillation Index

During the past two weeks the Southern Oscillation Index (SOI) has remained strongly negative. The 30-day SOI value to 27 September is −18.1.

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

Trade winds

Trade winds for the 5 days ending 27 September show that westerly anomalies continue to persist across the western to central equatorial Pacific. Trade winds are reversed (i.e. winds are westerly) near the equator 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.

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 of the trade winds.

Cloudiness near the Date Line

Cloudiness near the Date Line has remained above-average through September, continuing the pattern generally observed since March.

Cloudiness along the equator, near the Date Line, is an important indicator of the El Niño–Southern Oscillation (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

Most of the eight international climate models surveyed by the Bureau indicate that the central Pacific Ocean is likely to reach its peak warm value towards the end of 2015. All surveyed models indicate that NINO3.4 will remain above the El Niño threshold through the first quarter of 2016. The surveyed models indicate values of NINO3.4 are likely to begin to decline during early to mid-summer, but remain above the threshold value until at least early autumn.

Click to see full-size map of NINIO3.4 SST plumes from POAMA forecasts, updated daily.
Indian Ocean Dipole
Timeseries graph of winter SST anomalies for the southern Indian Ocean
Timeseries graph of winter SST anomalies

Values of the Indian Ocean Dipole (IOD) index have been at or above the threshold level of +0.4 °C for eight weeks, indicating a positive IOD event. For the week ending 20 September, the IOD index measured +1.1 °C, the highest value since the strong 2006 event. The most recent weekly value is +0.8 °C for the week ending 27 September.

Sea surface temperatures (SSTs) in the Indian Ocean are warmer than average over much of the basin, and the southern Indian Ocean as a whole has been at record temperatures in recent months (see timeseries of winter anomalies). A positive IOD event is typically characterised by cooler-than-average waters off the coast of the Indonesian island of Sumatra (see About the Indian Ocean Dipole), and this can be seen in the Sea surface temperature section. Positive IOD events are often associated with lower rainfall in parts of central and southeastern Australia. Positive IOD events are more likely to occur during El Niño, which also is typically associated with a reduction in winter–spring rainfall in eastern Australia.

However, sea surface temperatures in the broader Indian Ocean basin also affect Australia's climate—it's likely that the widespread warm anomalies observed across the Indian Ocean have moderated the influence of these two climate drivers in some areas (see Climate Outlook).

Four of the five surveyed international climate models indicate this event is likely to persist through to the end of spring.

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

See also: IOD forecasts