Synopsis: ENSO-neutral conditions are expected to continue during the next three months.

Oceanic and atmospheric conditions in the Pacific basin continued to reflect the neutral phase of the ENSO cycle during May 2004. Sea surface temperatures were warmer-than-average in the western and central equatorial Pacific (Niño 3.4 and 4 regions) and cooler-than-average in the eastern equatorial Pacific (Niño 3 and 1+2 regions) during the month (Fig. 1).

Positive SST anomalies greater than +0.5°C (~1°F) were found between 130°E and 170°W, and in portions of the region between 170°W and 120°W, while negative SST anomalies less than -0.5°C were found between 120°W and the South American coast (Fig. 2).

Over the past several months equatorial Pacific SST anomalies have been largest in the western Pacific, resulting in an enhanced east-west gradient of SST that has been associated with stronger-than-average easterly winds over the central equatorial Pacific (Fig. 3, top) and enhanced precipitation over the western equatorial Pacific (Fig. 3, bottom). These conditions are consistent with a steeper-than-average thermocline slope in the central equatorial Pacific, with positive (negative) subsurface temperature departures in the western (eastern) portion of the basin (Fig. 4).

Slightly more than half of the statistical and coupled model forecasts indicate near neutral conditions in the tropical Pacific (Niño 3.4 SST anomalies between -0.5°C and +0.5°C) through the northern summer and fall 2004. The remaining forecasts indicate that El Niño conditions will develop within the next 3-6 months. Given the recent trends and observed oceanic and atmospheric patterns discussed above, it is more likely that ENSO-neutral conditions will continue for the next 3 months (through August 2004). There is considerable uncertainty about what will happen after August 2004.

This discussion is a consolidated effort of NOAA and its funded institutions. Weekly updates for SST, 850-hPa wind, OLR and features of the equatorial subsurface thermal structure are available on the Climate Prediction Center web page at http://www.cpc.ncep.noaa.gov (Weekly Update). Forecasts for the evolution of El Niño/La Niña are updated monthly in the Forecast Forum section of CPC's Climate Diagnostics Bulletin. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send your e-mail address to: