Synopsis: Weak warm-episode (El Niño) conditions are expected to continue into early 2005.

Positive sea surface temperature (SST) anomalies greater than +0.5°C (~1°F) persisted across most of the central and western equatorial Pacific during November 2004 (Fig. 1). By early December, positive equatorial SST anomalies greater than +1°C (~2°F) were found from 160°E eastward to 155°W and locally in areas between 155°W and the South American coast (Fig. 2). The pattern of anomalous warmth in the equatorial Pacific in recent months and the most recent 5-month running mean value of the Southern Oscillation Index (-0.6) indicate that a weak warm (El Niño) episode has developed.

Since late 2003 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. This intraseasonal variability has been superposed on 1) an upward trend in SST anomalies east of the date line (Fig. 3) and 2) a gradual increase and eastward shift in the upper-ocean heat content anomalies during the last year (Fig. 4). 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 weak warm episode (El Niño) conditions will persist through early 2005. However, there is considerable uncertainty concerning future developments in the extreme eastern equatorial Pacific (the classical El Niño region).

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

El Niño 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 Niño episodes since 1950 (Fig. 5) show that the spatial extent of warmer-than-average (wetter-than-average) conditions increases across the northern (southern) United States, as the strength of El Niño 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 +0.5ºC to +1.2ºC. Thus, the winter outlook for the United States (Fig. 6) is a blend of the composite impacts for comparable historical El Niño 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 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. The next ENSO Diagnostics Discussion is scheduled for 6 January 2005.  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.