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

Product Code: IDCKGEWW00

CURRENT STATUS as at 11^th November 2009
Next update expected by 25^th November 2009 (two weeks after this update).

Summary: Tropical Pacific Ocean warmest since 2002 El Niño

Central equatorial Pacific Ocean temperatures have continued to warm over the past two weeks, and are now at their highest levels since at least the El Niño event of 2002. Similarly, the 30-day Southern Oscillation Index is lower than at any time since 2005. Leading climate models suggest tropical ocean temperatures will remain above El Niño thresholds into the first quarter of 2010.

A sustained weakening of the Trade Winds during October and early November enabled central Pacific equatorial temperatures to rise up to 2°C above normal. However, average to stronger than average Trade Winds currently over the western Pacific may curtail any further warming during the next fortnight. The distribution of tropical cloud has similarities to the patterns observed in the 2002 and 2006 El Niño events, while recent rainfall patterns over Australia are typical of mature El Niño conditions.

Despite a warming of the oceans to Australia's northwest over the past week, the Indian Ocean Dipole (IOD), as measured by the Dipole Mode Index (DMI), is neutral. The Bureau's POAMA model suggests neutral IOD conditions will persist over the coming months.

See IOD forecasts, DMI values.

From the NOAA/PMEL/TAO website.

In Brief

• The tropical Pacific Ocean sea surface has continued to warm in central and eastern areas, with a typical El Niño pattern clearly present. The central Pacific has now warmed to a level that has not been observed since the 2002 El Niño.
• The sub-surface water of the tropical Pacific has also continued to warm, with temperatures as much as 6°C above normal in some regions.
• The latest approximate 30-day SOI value is −15; the monthly value for October was also −15. The SOI has recently stabilised after a rapid fall in value through October.
• Following a sustained weakening of the Trade Winds during October, a pulse of average or stronger than average Trades is developing in the western Pacific.
• Cloudiness near the date-line has been slightly below average in recent weeks. However, cloudiness to the west of the date-line has been consistently above average, as also occurred during the 2006 El Niño and to a lesser extent in the 2002 event.
• Most leading international computer models surveyed by the Bureau predict that El Niño conditions will persist throughout the southern hemisphere summer.

Details

Ocean surface temperatures in the central equatorial Pacific have continued to warm and now exceed levels typical of an El Niño event by their greatest margin since the start of the year. The SST anomaly map for October is available here; the map shows warm anomalies in excess of +1°C covering most of the tropical Pacific east of 160°E, with anomalies exceeding +2°C in parts of the central Pacific. While the central Pacific warmed in October, the far western Pacific west of about 160°E cooled steadily. Until recently, warm SST anomalies had persisted in this region, which was atypical for an El Niño. The map shows near-normal SSTs covering most of the western Pacific and northern waters around Australia. The monthly NINO indices for October were +0.9°C, +1.1°C and +1.3°C for NINO3, NINO3.4 and NINO4 respectively. When compared with September values, NINO4 warmed by approximately +0.5°C and NINO3.4 by approximately +0.2°C. NINO3 remained similar in magnitude.

In terms of weekly data, the most recent NINO indices are +1.4°C, +1.7°C and +1.5°C for NINO3, NINO3.4, and NINO4 respectively. When compared with two weeks ago, all NINO indices have warmed; NINO3 by approximately 0.4°C, NINO3.4 by approximately 0.5°C and NINO4 by approximately 0.1°C. All three NINO indices currently have their highest weekly value for 2009. The 7-day SST anomaly map shows warm anomalies in excess of +1°C covering most of the tropical Pacific east of 160°E, with anomalies exceeding +2°C on a weekly scale across most of the central tropical Pacific. When compared with anomalies observed two weeks ago, the ocean surface has warmed in both the central and eastern Pacific. An animation of recent recent SST changes is available.

The sub-surface of the equatorial Pacific Ocean has also continued to warm in the last two weeks. A large volume of warmer than normal water is now well established beneath the surface of the tropical Pacific. A four-month sequence of Pacific Ocean equatorial temperature anomaly is available here. The sequence shows cooling of the sub-surface from July through September followed by a relatively rapid warming during October. This warming has continued into November. A recent map for the 5 days ending 9 November shows a large volume of sub-surface water more than 3°C warmer than normal for this time of the year extending across much of the central to eastern equatorial Pacific. Warm anomalies in excess of 6°C are now evident between 150°W and 120°W on a weekly scale. When compared with two weeks ago, the sub-surface has warmed strongly in the central Pacific, related to a strong westerly wind burst in the western Pacific during October. This warming has propagated eastwards during recent weeks. An animation of recent sub-surface changes is available.

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

Trade winds were weaker than the long-term mean across most of the tropical Pacific during October, with a strong westerly wind burst in the western Pacific. The westerly wind burst in the western Pacific was linked to the warming of the surface and sub-surface of the tropical ocean in October and early November as well as to a strong drop in the SOI. A pulse of strengthened Trade Winds has developed over the western Pacific during early November, in association with an approaching Madden-Julian Oscillation (MJO). On a weekly scale weak easterly anomalies are evident in the western equatorial Pacific, while Trade winds are close to the long-term mean in the eastern Pacific. The latest weekly wind anomalies are shown in the TAO/TRITON map (small image above) for the five days ending 9 November.

The SOI has recently stabilised after a rapid fall in October. The latest approximate 30-day SOI value is −15; the monthly value for October was also −15. The SOI is now at values typical of an El Niño event.(SOI graph, SOI table).

Cloudiness near the date-line over the equatorial Pacific is another important indicator of El Niño conditions, as it typically increases near and to the east of the dateline during these episodes. Cloudiness near the date-line has generally been slightly below average in recent weeks. However, there has been significant enhanced cloudiness to the west of the date-line, a pattern which is unusual when compared with typical El Niño events, but not unprecedented as such conditions were also observed during the recent 2006 El Niño event and to a lesser extent in the 2002 event. Cloudiness over Indonesia and much of northern and eastern Australia has been below average over the last few months, which is consistent with typical El Niño conditions.

Most international computer models are currently predicting that El Niño conditions will persist throughout the southern hemisphere summer. Five of six international models surveyed by the Bureau of Meteorology forecast SSTs to remain above threshold levels into early 2010. Most models are predicting that Pacific Ocean SSTs will start to cool by March next year, which is the typical timing for the decay of El Niño events. Recent forecasts from the POAMA model, run daily at the Bureau of Meteorology, show a continuation of warming with SST remaining above El Niño thresholds into 2010, peaking over the summer months.

THE NEXT UPDATE OF THE DETAILED SECTION ABOVE IS EXPECTED BY 25TH NOVEMBER 2009

Archive of previous ENSO Wrap-Ups

Other Useful Links

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

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.