Synopsis: El Niño conditions are expected to develop during the next 3 months.

Sea surface temperature anomalies increased substantially in the central equatorial Pacific (Niño 3.4 region, Fig. 1) during July 2004, while anomalies greater than +0.5°C persisted in the Niño 4 region. The recent increase and eastward expansion of positive SST anomalies in the central equatorial Pacific indicate the possible early stages of a warm episode. SST anomalies greater than +0.5°C (~1°F) were found between 160°E and 120°W, with anomalies greater than +1°C extending from 180°W eastward to 125°W (Fig. 2). In spite of the anomalous warmth in the central equatorial Pacific during July, there appears to be little or no reflection of that warmth in the pattern of deep convection (precipitation) over the region (Fig. 3).

Considerable intraseasonal variability (MJO activity) in recent months has resulted in week-to-week and month-to-month variability in many atmospheric and oceanic indices. During mid-June through early July the easterlies weakened in many areas of the equatorial Pacific, as enhanced convection shifted eastward from the Indian Ocean to the western tropical Pacific. The greatest wind and convection anomalies occurred north of the equator in the western Pacific, associated with two typhoons. By mid-July the low-level winds and equatorial convection returned to near average in many areas of the equatorial Pacific. However, a strong oceanic Kelvin wave, initiated by the weaker-than-average easterly winds in June, has propagated eastward resulting in a substantial deepening of the oceanic thermocline and an increase in the subsurface temperature anomalies in the central and east-central equatorial Pacific (Fig. 4). This Kelvin wave is expected to reach the South American coast during August.

Based on the recent trends and observed oceanic and atmospheric patterns discussed above, there is about a 50% chance that the NOAA operational definition for El Niño [Oceanic Niño Index (ONI), a three-month running mean of the Niño 3.4 index, greater than or equal to +0.5°C] will be satisfied for the period June-August 2004. It seems most likely that SST anomalies in the Niño 3.4 region will remain positive, at or above +0.5°C, through the end of 2004. At this time it is not clear what, if any, impacts this event will have on ocean temperatures in the classical El Niño region (Niño 1+2) along the west coast of South America.

Approximately 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 end of 2004. The remaining forecasts indicate El Niño conditions (Niño 3.4 SST anomalies greater than or equal to +0.5°C) will develop within the next 3-6 months.

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: ncep.list.enso-update@noaa.gov or to Vernon.Kousky@noaa.gov