ENSO UPDATE 16 August 2002

Summary

The IRI's assessment is that there is a greater than 95% probability of El Niño conditions during the remainder of 2002 and into early 2003 with current conditions representing the early stage of this El Niño episode. This assessment is based on the collective forecasts of various ENSO prediction models, as well as on the experience of the several oceanographers and atmospheric scientists familiar with the El Niño phenomenon. Compared with the statement from one month ago, this probability has increased by more than 5%, since the substantially warmer than average sea surface temperatures that first emerged in late May are considered highly likely to continue for the next 6-9 months. The most likely strength of this El Niño will be weak to moderate, which is significantly less than the 1997-98 event. The associated climate effects in most regions are expected to be weaker than those associated with the 1997-98 El Niño, but may nonetheless be substantial in some regions.
 

General Discussion

Between mid-July and mid-August, sea surface temperatures (SSTs) in the east-central equatorial Pacific remained close to or above 1 degree C above normal. The strongest positive anomalies now exceed 1.5 degrees C over the central basin near and just east of the dateline. Somewhat below normal SST has developed over a sizeable area in the far eastern Pacific. Therefore, the strongest warm anomalies now cover the NINO4 and most of the NINO3.4 regions, but the area average in the NINO3 region has returned toward normal. However, this cool eastern SST is expected to become warm over the coming several months as the episode evolves. The atmosphere has continued to show features suggestive of El Niño, such as persistent westerly wind anomalies in the western and central tropical Pacific. A lower than average Southern Oscillation Index also has been observed during the last 5 months. During the last four weeks, anomalous convection has developed along the equator in the vicinity of the dateline, extending eastward to 160W longitude. Consistent with this eastward shift in convection has been some weakening of the positive SST anomalies that have dominated the western equatorial Pacific for the last several years. These recent developments are signs of further development of this El Niño episode.

The fact that we are very likely in the developing stage of an El Niño episode is reflected in the SST forecasts of a large set of dynamical and statistical forecast models. Relatively high skill is expected for the model forecasts at this time of the year. Depending on the type of forecast model, between 80% and 85% of the models predict El Niño conditions persisting into the October-November-December period and continuing through Northern Hemisphere winter. The remaining models call for near-neutral SST throughout 2002. (Note 1) Thus, overall, the models suggest that we are in the first half of an El Niño episode that will continue through the remainder of 2002 and into early 2003.

Note 1 - Only models that produce a new ENSO forecast every month are included in the above statement.
 

ENSO Quick Look
More Technical ENSO Comment
Summary of Model Predictions