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

Product Code: IDCKGEWW00

CURRENT STATUS as at 1^st July 2009
Next update expected by 8^th July 2009 (one week after this update).

Summary: El Niño event likely

More evidence of a developing El Niño event has emerged during the past fortnight, and computer forecasts show there's very little chance of the development stalling or reversing.

Responding to continued weak Trade Winds, equatorial sea-surface temperatures are now more than 1°C above normal in the eastern half of the Pacific. Although it has risen in the past fortnight, the Southern Oscillation Index (SOI) remains below zero at around −2.

A more complete picture of the situation in Pacific will be available next week when the final June indices are calculated.

El Niño events are usually (but not always) associated with below normal rainfall in the second half of the year across large parts of southern and inland eastern Australia.

Another adverse sign for southeastern Australian rainfall is the recent trend to positive values in the Indian Ocean Dipole (IOD), as measured by the Dipole Mode Index (DMI).

See IOD forecasts, DMI values.

From the NOAA/PMEL/TAO website.

In Brief

• The Pacific Ocean sea surface is currently significantly warmer than the long-term average across most of the tropical Pacific, especially central to eastern areas.
• A large amount of the sub-surface water of the tropical Pacific is also warmer than the long-term average, particularly in the east.
• The latest 30-day SOI value is −2, while the monthly value for May was −5.
• Trade winds remain weaker than normal across the central equatorial Pacific.
• Cloudiness near the date-line is near-normal, and is yet to show a consistent trend towards El Niño conditions.
• All international climate models predict the tropical Pacific to continue to warm and to be above El Niño thresholds throughout most of the second half of 2009.

Details

The central and eastern equatorial Pacific sea surface warmed through June. This warming was a continuation of a steady warming trend that has been observed since February 2009. Weak warm SST anomalies are now well established across most of the equatorial Pacific. The monthly indices for May were +0.6°C, +0.4°C and +0.4°C for NINO3, NINO3.4 and NINO4 respectively. The monthly indices for June will be calculated early next week.

In terms of weekly data, the most recent NINO indices are +1.0°C, +1.0°C and +0.7°C for NINO3, NINO3.4 and NINO4 respectively. Over the past two weeks the SST has warmed in all three regions. NINO3 has warmed by approximately 0.2°C, NINO3.4 by 0.5°C and NINO4 by 0.3°C. The 7-day SST anomaly map shows the persistence and further development of positive anomalies across most of the central and eastern equatorial Pacific when compared with two weeks ago. Anomalies of greater than +1.0°C are now evident across most of the central and eastern equatorial Pacific on a weekly scale. An animation of recent SST changes is available.

The sub-surface of the equatorial Pacific has also continued to steadily warm through June. A large volume of warmer than normal sub-surface water is evident across the entire tropical Pacific. A recent map for the 5 days ending 28 June shows the warmer than normal sub-surface water extending across the equatorial Pacific. Sub-surface anomalies exceed +2.0°C across most of the central and eastern tropical Pacific, with a small area of water where anomalies exceed +4.0°C evident between 130°W and 100°W on a weekly scale. An animation of recent sub-surface changes is available.

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

The Trade winds were weaker than normal across much of the equatorial Pacific during June, especially in central and eastern areas. During the past week or so, easterly anomalies have become established in the western tropical Pacific; this is likely to be assoiciated with the recent rise in the SOI. Trade flow is generally slightly stronger than it was two weeks ago across the central to eastern Pacific as well. The latest weekly wind anomalies are shown in the TAO/TRITON map (small image above) for the five days ending 28 June.

The Southern Oscillation Index (SOI) fell sharply through the first three weeks of May. The SOI dropped to an approximate 30 day value of −11 on the 22 May after an April value of +9. Recently the SOI has increased to a current (29 June) approximate 30 day value of −2. (SOI graph, SOI table). If model predictions of a developing El Niño are correct, the SOI would be expected to fall again, that is, become more strongly negative.

Cloudiness near the date-line over 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 near the dateline has increased recently, from below average to near-normal. Although cloudiness near the date-line has been increasing, it is yet to show a clear trend towards El Niño conditions.

All international dynamic computer models surveyed by the Bureau of Meteorology predict further warming of the Pacific Ocean SST in coming months. All models predict SST to be above El Niño thresholds throughout most of the second half of 2009. One of the surveyed models has slower weaker warming than the other five, but all six models predict El Niño conditions to be established by the southern spring at the latest. As all models agree in the development of El Niño conditions and predictability of El Niño conditions is high at this time of the year, the probability of an El Niño event occurring in 2009 is high; a definite increase from a month ago. Recent forecasts from the POAMA model, run daily at the Bureau of Meteorology, show a steady warming with El Niño conditions developing in July. Pacific conditions and model predictions will continue to be monitored closely.

THE NEXT UPDATE OF THE DETAILED SECTION ABOVE IS EXPECTED BY 8TH JULY 2009

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.