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ENSO Update

Current oceanic observations show El Niño/Southern Oscillation (ENSO) characteristics in the tropical Pacific region which are not coupled with the atmosphere

In July 2004, the tropical Pacific showed mixed warming and cooling signals, and Southern Oscillation Index (SOI) fluctuations occurred from April 2004. The equatorial sea surface temperature (SST) anomalies were below average near the South American coast but above average over the remainder of the equatorial tropical Pacific east of the Date Line. These were more than 1°C above average near the Date Line. The NINO3 SST anomaly was +0.0°C while NINO4 was +1.0°C. The subsurface temperatures at 150 m depth just east of the Date Line showed a strong positive anomaly (exceeding +3.0°C), which developed following a strong westerly wind burst at 160°E in June 2004. There was also reduced convection in the Indonesian and Australian regions and along the equatorial Pacific.

During August 2004, the SOI became more negative with the 3-month mean reaching -1.1 (June-August), the lowest since spring 2002. Equatorial SST anomalies were above normal across much of the Pacific with NINO3 and NINO4 SST anomalies being +0.3°C and +0.9°C respectively. Subsurface temperatures showed a positive anomaly (exceeding +2.0°C) centred near 110°W and propagating eastwards. There was some anomalous convection about and west of the Date Line north of the Equator and the westerly zonal wind anomalies were evident across the equatorial Pacific.

September 2004 had mixed signals, with the sea surface and subsurface temperatures showing El Niño diagnostics, but with the atmosphere yet to couple and respond to the oceanic signals. The SOI weakened during September to -0.4 while the equatorial SST anomalies increased compared with August. The NINO3.4 anomaly was +0.9°C (August was +0.6°C) and the subsurface temperatures were +2.0°C or more in the eastern Pacific at ~100 m depth. The westerly wind anomalies seen in August abated during September.

Overall, the oceanic observations show a weak El Niño state, but the atmosphere is yet to become coupled to the oceans. It is therefore difficult to categorise the current El Niño/Southern Oscillation (ENSO) state in the Pacific. However, there are other observations which are diagnostic of weak El Niño impacts such as the reduced convection over the Indonesian region and cooler SSTs in the Southwest Pacific. Several climate models (5 out of 12) are forecasting a weak El Niño event based on warming of the SSTs in the NINO3.4 region through to early 2005. Hence, the current incoherence in oceanic and atmospheric signals in the tropical Pacific means that there is a continuing need for close monitoring of the weak ENSO situation and its climate impacts.

Acknowledgements

The Australian Bureau of Meteorology, the International Research Institute for Climate Prediction (US), US National Center for Environmental Prediction, World Meteorological Organization, US Climate Prediction Center and the Pacific National Meteorological Services.