ENSO Update  16 April 2002

Summary

The overall tropical Pacific has moved closer to El Niño conditions, with sea surface temperatures increasing over most of the equatorial eastern Pacific. However, the atmosphere has not to date assumed a large-scale anomaly pattern characteristic of ENSO warm events. An unbroken band of  anomalously warm sea surface temperatures now stretches from the coast of South America westward to beyond the dateline. There is therefore an enhanced likelihood of an El Niño in 2002. However, some uncertainty still exists, since in order for an El Niño sea surface temperature scenario to occur, the atmosphere must play its required part in the ocean-atmosphere coupling. While the current state of the sea surface temperature will encourage the initiation of atmospheric El Niño behavior, it is not guaranteed. The evolution of the system during the remainder of April and during May is expected to lead to greater certainty in the forecasts in the next one to two months. If an El Niño develops, past events suggest it would continue for at least the remainder of the year and likely through March of 2003.

General Discussion

Equatorial Pacific SSTs of close to or greater than one-half degree C above average exist from the coast of South America to west of the dateline. One month ago there had been above average SST in the central Pacific and near the South American coast, with a long stretch of generally neutral SST in between. This intermediate region includes the El Niño-related index areas of Nino 3 and Nino 3.4. The warmer than average water that had emerged in early February in the eastern tropical Pacific caused heavy rainfall in some coastal locations in Ecuador and northern Peru. This situation is continuing. Ocean conditions seen so far in 2002 have been observed prior to the onset of many past El Niño events. While the current SST state will encourage the initiation of El Niño behavior in the atmosphere, we cannot be sure that an El Niño has commenced until both the SST and the atmosphere are engaged. The evolution of the system during the remainder of April and May is expected to lead to greater certainty in the forecasts by the end of May.

The above interpretation is reflected in the difference among forecast models as to whether an El Niño will occur. These differences are consistent with the fact that the forecast models are known to have only moderate skill in predicting the onset of El Niño events during the current time of year (February through April).  The expected skill this month is greater than it was in March, and it will be still greater next month. With this in mind, depending on the type of forecast model, between 50% and 70% of these models predict El Niño conditions existing by the July-August-September period.  All of the remaining models call for a continuation of near-neutral conditions throughout 2002; none indicates the development of La Nina conditions.  (Only models that produce a new ENSO forecast every month are included in the above statements.)  Overall, the models suggest an enhanced likelihood that the temperatures in the central and eastern tropical Pacific will reach (or continue at) the level of at least a minimal El Niño by the middle of 2002. 

More Technical ENSO Comment

Summary of Model Predictions