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

Product Code: IDCKGEWW00

CURRENT STATUS as at 14^th August 2008
Next update expected by 3^rd September 2008 (three weeks after this update).

Summary: Neutral conditions in the Tropical Pacific

The tropical Pacific Ocean remains ENSO-neutral, with little change in the state of the ocean and atmosphere across the tropical Pacific over the past three weeks. Trade winds are close to normal strength except in the far west where they are slightly stronger that normal. Sea surface temperatures remain warmer than normal in the eastern Pacific while they are near-normal west of 150°W. The SOI remains close to zero and cloudiness near the dateline has also been near-normal. The most significant change is in the ocean subsurface where cooling has occurred across most of the Pacific basin, indicating a shift toward more neutral conditions.

With these neutral conditions and near-normal ocean heat content there is little potential for the warm waters in the eastern Pacific to develop into an El Niño event in 2008. This is supported by climate model forecasts that show neutral conditions should persist until the end of the year. As winter is a period of relatively high predictability the persistence of neutral conditions is considered very likely in the coming months. However, the Pacific will continue to be monitored closely for any signs of El Niño growth. Indian Ocean conditions have moderated with the weak positive Indian Ocean Dipole forecast to dissipate over the coming months by most models. About IOD.

From the NOAA/PMEL/TAO website.

In Brief

• Sea surface temperatures are now near normal across the central and western Pacific Basin.
• Sea surface temperatures are warmer than normal in the eastern Pacific with warm anomalies now extending along the equator from the South American coast to around 150°W.
• Warm sub-surface anomalies have now moderated across the Pacific basin.
• The 30-day SOI has remained neutral with a value of +1.
• Trade Winds are close to average across much of the equatorial Pacific, stronger than average in the far western Pacific.
• Cloudiness near the date-line is near-normal.
• Dynamic computer models predict neutral conditions to continue in the coming months.

Details

Sea surface temperature data for July show that remnants of the La Niña pattern of cool anomalies dissipated in the central to western Pacific and the eastern Pacific remained slightly warmer than normal. Warm anomalies persisted near the South American coast, extending west along the equator to 150°W. The monthly NINO indices reflected this, with all indices near normal in July. The NINO3, NINO3.4 and NINO4 indices warmed by around 0.3°C during July and were +0.6°C, +0.1°C and −0.3°C respectively.

In terms of weekly data, the NINO3, NINO3.4 and NINO4 indices have warmed to +0.81°C, +0.27°C and −0.16°C respectively, a rise of about 0.1°C over the past three weeks. When compared with three weeks ago, the 7-day SST anomaly map shows an increase of the positive anomalies in the far eastern equatorial Pacific. Positive anomalies above +0.5°C now extend west from the South American coast to around 140°W. Negative anomalies in the western to central Pacific have continued to dissapate, with no cool anomalies greater than −0.5°C near the equator. An animation of recent SST changes is available.

Sub-surface waters have cooled since late July. Weak warm anomalies now exist close to the surface in the eastern Pacific and at around 150m depth in the west, while slightly cooler-than-normal water sits below 100m depth in the central Pacific. The greatest anomalies are slightly greater than +2.0°C but cover only very small areas. A recent map for the 5 days ending 12th August shows the weak cooling that has occurred in the sub-surface across the Pacific. An animation of recent sub-surface changes is available.

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

Trade Winds across the Pacific have fluctuated around the long-term average during the past month and are now close to normal strength in the east and central parts of the basin. A region of slightly stronger than normal Trade Winds remains in the far western equatorial Pacific. The latest weekly results shown in the TAO/TRITON map (small image above), for the five days ending 12th August, are consistent with this.

The Southern Oscillation Index (SOI) had not changes significantly in the past 3 weeks (see SOI graph, SOI table). The July value was +2 and the 30-day SOI as of 12th August remained neutral at +1.6 (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. Over the past few months it has returned to near normal conditions, in keeping with the near average SST values observed in the region.

The latest survey (late July) of six international dynamic computer models, shows they all predict neutral temperatures across the Pacific until the end of 2008. They show central and western Pacific temperatures warming slightly so that by late spring temperatures across the basin will be slightly warmer than normal but still neutral. 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 next March.

THE NEXT UPDATE OF THE DETAILED SECTION ABOVE IS EXPECTED BY 3RD SEPTEMBER

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.