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

For the Pacific basin as a whole, oceanic and atmospheric conditions continue to reflect the neutral phase of the ENSO cycle. However, sea surface temperature (SST) anomalies in the equatorial Pacific increased during April 2004 in the Niño 3.4 and 4 regions, and decreased in the eastern Pacific (Niño 3 and 1+2 regions, Fig. 1), as the equatorial cold tongue strengthened. By the end of the month, positive SST anomalies greater than +0.5°C (~1°F) were observed in the region between Indonesia and 180°W, and negative anomalies (less than -2°C in some places) were observed between 120°W and the South American coast (Fig. 2). Since January 2004 equatorial Pacific SST anomalies have been largest in the western portion of the basin. This has resulted in an enhanced east-west gradient of SST, which has been associated with stronger-than-average easterly winds over the central equatorial Pacific (Fig. 3, top), enhanced precipitation over the western equatorial Pacific (Fig. 3, bottom) and a steeper-than-average thermocline slope in the central equatorial Pacific, as represented by positive (negative) subsurface temperature departures in the western (eastern) portion of the basin (Fig. 4).

Slightly more than half of the 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 early fall 2004. The remaining forecasts indicate that El Niño will develop within the next three-six months and intensify through the end of the year. Many oceanic and atmospheric indices have displayed considerable intraseasonal variability related to Madden-Julian Oscillation (MJO) activity during the last several months, which together with the time of year, may be contributing to the diversity of statistical and coupled model forecasts for the tropical Pacific. 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 July 2004). There is considerable uncertainty about what will happen after July 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: