Strong warm episode (El Niño) conditions continued throughout the equatorial central and eastern Pacific during August. Equatorial sea surface temperature (SST) anomalies (departures from normal) were greater than +1C everywhere from 180W east to the South American coast. Anomalies exceeding +4C were observed along the equator east of 120W. In many areas of the eastern equatorial Pacific, August SST anomalies were the largest observed during the last 50 years.

The equatorial oceanic thermocline during August continued to be much deeper than normal across the eastern Pacific and shallower than normal in the western Pacific. Consistent with this structure, subsurface ocean temperatures were much above normal in the east-central and eastern Pacific, with anomalies exceeding +8C to the east of 120W at thermocline depth.

Tropical rainfall was enhanced over the central and eastern tropical Pacific, and suppressed over all of Indonesia during August. Low-level (850-hPa) winds continued to be much weaker than normal over the tropical Pacific. By early September the easterlies were absent over most of the equatorial Pacific and westerlies were observed between New Guinea and 180W. Weaker than normal easterlies have been observed across most of the equatorial Pacific since March.

Over the past few seasons the NCEP statistical (CCA) and coupled model predictions have consistently indicated the development and persistence of a strong warm episode. The latest NCEP forecasts indicate that strong warm episode conditions will continue through early 1998. In interpreting the SST anomalies from these forecasts, it is important to keep in mind that the smaller anomalies predicted during March-May 1998 may have a larger impact on the global atmospheric circulation than the very large anomalies currently observed, because they occur near the peak of the SST annual cycle in the tropical eastern Pacific. In fact, the areal extent of SSTs greater than 28C (the threshold for deep convection) increased with time through March-May 1998.

Based on the NCEP SST predictions and the results from historical studies on the effects of warm episodes, we expect drier than normal conditions to continue over Indonesia and eastern Australia during the next several months. Drier than normal conditions are also likely over most of Central America and the Caribbean Sea. Rainfall should be heavier than normal over the central and eastern equatorial Pacific and from central Chile eastward across northeastern Argentina, Uruguay and southern Brazil. During the northern winter season we expect wetter than normal conditions to prevail over most of the extreme southern United States, and warmer than normal conditions to develop over the northern United States from the Rocky Mountains eastward to the Great Lakes.

Weekly updates for SST, 850-hPa wind, and OLR are available on the Climate Prediction Center homepage at: http://nic.fb4.noaa.gov (ENSO Update).