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HOME > Expert Assessments > ENSO Diagnostic Discussion
issued by
October 7, 2004

Synopsis: Warm-episode conditions are expected to continue into early 2005.

Positive sea surface temperature (SST) anomalies greater than +0.5°C persisted in the central and western equatorial Pacific (Niņo 4 and Niņo 3.4 regions), and expanded eastward into the eastern equatorial Pacific (Niņo 3 region) during September 2004 (Fig. 1). By the end of the month, positive SST anomalies greater than +0.5°C (~1°F) were found between 155°E and 110°W, with anomalies greater than +1°C extending from 160°E eastward to 120°W (Fig. 2). The increase and eastward expansion of the area of anomalous warmth in the central equatorial Pacific during July-September indicate the early stages of a warm (El Niņo) episode. Through the end of September conditions were not yet indicative of a basin-wide El Niņo, particularly due to the presence of below-average SSTs in the far eastern equatorial Pacific between 95°W and the South American coast.

Through most of 2004 MJO activity has resulted in week-to-week and month-to-month variability in many atmospheric and oceanic indices. In the past few months the warmth in the central equatorial Pacific has supported eastward shifts of enhanced convection associated with the convectively active phase of the Madden-Julian Oscillation (MJO) across the western equatorial Pacific. This activity has been associated with periods of weaker-than-average easterlies that initiated eastward-propagating oceanic Kelvin waves, which contributed to a deeper-than-average oceanic thermocline and an increase in surface and subsurface temperature anomalies in the central and eastern equatorial Pacific (Fig. 3).

The NOAA operational definition for El Niņo [Oceanic Niņo Index (ONI), a three-month running mean of the Niņo 3.4 index, greater than or equal to +0.5°C] was satisfied for the period June-August 2004, with an ONI value of +0.7°C. Based on the recent evolution of oceanic and atmospheric conditions and on a majority of the statistical and coupled model forecasts, it seems most likely that SST anomalies in the Niņo 3.4 region will remain positive, at or above +0.5°C, through early 2005.

Expected global impacts include drier-than-average conditions over Indonesia(through early 2005), northern and northeastern Australia (November 2004-February 2005), and southeastern Africa (November 2004-March 2005). If the warming in the tropical Pacific should strengthen and spread eastward to the South American coast, then wetter-than-average conditions can be expected in coastal sections of Ecuador and northern Peru during the first few months of 2005, and drier-than-average conditions can be expected to develop in the eastern Amazon late this year and spread to Northeast Brazil during February through April 2005.

El Niņo wintertime impacts over the United States vary considerably depending on the character (distribution and intensity) of the warming in the tropical Pacific. Composite impacts for selected ranges of the ONI for El Niņo episodes since 1950 (Fig. 4) show that the areal extent of warmer-than-average (wetter-than-average) conditions increases across the northern (southern) United States, as the strength of El Niņo increases. The current warming in the tropical Pacific is expected to continue through the upcoming winter, with models indicating an ONI in the range of +0.5ēC to +1.4ēC. Thus, the winter outlook for the United States (Fig. 5) is a blend of the composite impacts for comparable historical El Niņo episodes and the effects of long-term trends. Warmer-than-average conditions are expected in the West and in the northern Plains, while cooler and wetter-than-average conditions are expected for portions of the South and Southeast.

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 (Weekly Update). Forecasts for the evolution of El Niņo/La Niņa are updated monthly in the Forecast Forum section of CPC's Climate Diagnostics Bulletin. The next ENSO Diagnostics Discussion is scheduled for 10 November 2004.  To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send your e-mail address to: or to

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Page last modified: July 10, 2003
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