ENSO Wrap-Up
A regular commentary on the El Niño-Southern Oscillation

Product Code: IDCKGEWW00

CURRENT STATUS as at 2^nd July 2008
Next update expected by 23^rd July 2008 (three weeks after this update).

Summary: Neutral ENSO conditions likely to persist

Neutral ENSO conditions are now firmly established in the tropical Pacific. Over the past two weeks most indicators of ENSO have weakened further. Sea surface temperatures are now close to normal (generally within 1°C) across the entire basin, including close to the dateline where the last remnants of the La Niña had persisted. The Trade Winds have moderated further, cloudiness near the dateline has been near-normal and the SOI has also been in the neutral range since May. Temperatures in the sub-surface of the tropical Pacific Ocean have also moderated in the centre and west of the basin but weak warming has occurred in the east. All ENSO indicators are now neutral.

Climate model forecasts of ENSO show that neutral conditions are likely to continue thoughout 2008. Most models show some warming in the coming season. None suggest a redevelopment of a La Niña and only a minority predict an El Niño developing. Those forecasts that do were based on conditions in early June when Trade Winds had weakened, a situation that has now eased. More recent forecasts favour neutral conditions. As winter is a period of relatively high predictability the persistence of neutral conditions is considered the most likely scenario in the coming months. However, there is still a small chance of an El Niño in 2008, as events have been known to evolve late in the year. The Pacific will continue to be monitored closely for any signs of El Niño growth.

The Indian Ocean is currently in a positive phase of the Indian Ocean Dipole (IOD), but the index used to measure the IOD has weakened considerably since its peak in early June. Those models that forecast the IOD show it persisting but moderating further throughout the rest of the year. As a positive IOD is known to increase the chance of below normal winter-spring rainfall in southeast Australia, its evolution will also be closely monitored. About IOD.

From the NOAA/PMEL/TAO website.

In Brief

• Sea surface temperatures are now near normal across the entire Pacific Basin.
• Warm sub-surface anomalies have now moderated in the central and western Pacific with only weak warming in the east.
• The 30-day SOI has steadily climbed to a value of +4.
• Trade Winds are close to average across the equatorial Pacific.
• Cloudiness near the date-line is near normal.
• The majority of dynamic computer models predict neutral conditions to continue in the eastern to central Pacific in the coming months, but two predict Pacific temperatures above the El Niño threshold by September.

Details

Compared with May, sea surface temperature data for June showed a further warming across the equatorial Pacific. Remnants of the La Niña pattern of cool anomalies weakened further in the central to western Pacific, as the eastern Pacific continued to slowly warm. Warm anomalies persisted near the South American coast, with an area of positive anomalies greater than +1°C remaining evident at the monthly timescale. The monthly NINO indices reflected this, with all indices now near normal in June. The NINO3, NINO3.4 and NINO4 indices warmed by around 0.2 to 0.3°C during June and were +0.2°C, −0.5°C and −0.7°C respectively.

In terms of weekly data, the NINO3.4 and NINO4 indices have steadily warmed over the past fortnight at 0°C and −0.3°C respectively. SST anomalies in the NINO3 area have remained steady over the past fortnight, which is reflected in the index value of +0.4°C, a slight rise of 0.1°C. When compared with two weeks ago, the 7-day SST anomaly map shows a weakening of positive anomalies in the eastern equatorial Pacific, with positive anomalies above +1°C now only present in one small area in the far east. Negative anomalies in the western to central Pacific have continued to weaken further, with cool anomalies now above −0.5°C and only covering a small area of the central Pacific. An animation of recent SST changes is available.

The La Niña signal in the sub-surface weakened further in June, with positive anomalies present along the entire thermocline. Only a small area of weak negative anomalies persisted in the mixed layer near the date-line. The large area of positive anomalies present in the western Pacific weakened further since May. There has also been a continued decrease in June of the strength of the negative sub-surface anomalies in the central Pacific mixed layer, with all anomalies now above −1°C. A recent map for the 5 days ending 30th June shows a weakening of positive anomalies in the sub-surface of the central Pacific, with no anomalies greater than +1°C, and only a small area of anomalies greater than +2°C in the western Pacific. Anomalies greater then +3°C remain in the sub-surface in the eastern Pacific. An animation of recent sub-surface changes is available.

An archive of past SST and sub-surface temperature charts is available.

Trade Winds in the central and western Pacific fluctuated around the long-term average during June. A region of weakened Trade Winds remains across the central to eastern equatorial Pacific. The latest weekly results shown in the TAO/TRITON map (small image above), for the five days ending 30th June, are consistent with this, showing close to average Trade Winds across much of the equatorial Pacific, with weakened tradewinds in the east.

The Southern Oscillation Index (SOI) rose steadily during June (see SOI graph, SOI table). After a May value of −4.3, the 30-day SOI as of 30th June returned to positive values, at +4.2 (see graph above).

Cloudiness near the date-line in the central to western Pacific is another important indicator of warm/cool ENSO conditions, as it normally increases/decreases (negative OLR/positive OLR anomalies) during these episodes. Cloudiness around the equatorial date-line had mostly been below average since late May 2007, particularly so from December. It has recently returned to near normal conditions, in keeping with the near average SST values observed in the region.

The latest survey (late June) of six international dynamic computer models, shows the majority predict neutral temperatures in the central to eastern Pacific continuing over the southern late winter to spring. However, two models do predict Pacific temperatures to rise a little over the El Niño threshold, with the remainder predicting conditions on the warm side of neutral. Of the five that predict beyond four months, four indicated neutral conditions to continue through to the end of 2008, with one predicting warm conditions (El Niño). Recent forecasts from the POAMA model, run daily at the Bureau of Meteorology, predict neutral conditions to remain in the eastern to central Pacific through to January.

THE NEXT UPDATE OF THE DETAILED SECTION ABOVE IS EXPECTED BY 23RD JULY

The links below can be used to keep track of important developments across the Pacific Basin.

Other Useful Links

The Weekly Tropical Climate Note issued by the Darwin office of the Bureau of Meteorology discusses the main features of the tropical atmosphere and ocean, including the intra-seasonal oscillation or 30-60 day wave which is thought to sometimes impact on the development of El Niño events.

The Bureau of Meteorology Research Centre (BMRC) has recently developed maps of Out-going Longwave Radiation (OLR), a useful El Niño monitoring tool. Negative anomalies show areas which, in general, have been cloudier (and potentially wetter) than normal.

The TAO / TRITON data display page is excellent for creating your own plots of numerous variables that are relevant to El Niño.

Note however that information coming from other countries is likely to describe timing and impacts relevant to those countries, which will not be the same as those in Australia.