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EL NIŅO/SOUTHERN OSCILLATION (ENSO)

DIAGNOSTIC ADVISORY 2000/5

Cold Phase

May 10, 2000

The large-scale oceanic and atmospheric circulation patterns continued to reflect cold episode (La Niņa) conditions in the tropical Pacific during April. Consistent cold episode-related oceanic features include: below normal SSTs (negative anomalies greater than -1°C) in the western and central Pacific (Fig. 1), and above- (below-) normal subsurface temperatures in the western (eastern) equatorial Pacific (Fig. 2). Related atmospheric features include: stronger-than-normal low-level easterly winds over the western tropical Pacific, above-normal precipitation over Indonesia and the eastern Indian Ocean, suppressed rainfall over the western equatorial Pacific, and a strongly positive value of the SOI (+1.2). These features have been highly persistent since late 1998, with the patterns of anomalous SST and tropical precipitation being quite similar during March-April 1999 and March-April 2000 (Fig. 3 and Fig. 4).

Despite the persistence of many cold-episode related features, SST anomalies have been increasing during recent weeks throughout most of the eastern equatorial Pacific, which has resulted in substantial increases in the Niņo 3.4, Niņo 3, and Niņo 1+2 indices since mid-February 2000 and in the appearance of positive SST anomalies between 80°W and 120°W (Fig. 1). This local warming of the SSTs is likely to be short-lived, as the large-scale subsurface thermal structure does not favor the development of a warm episode at this time.

The most recent NCEP coupled model forecasts and statistical model forecasts, as well as other available forecasts, exhibit considerable spread in the evolution of the SSTs over the next 3-9 months. The NCEP coupled model forecast and the latest LDEO forecast indicate that cold episode conditions will weaken during the next 3 months, followed by near-normal conditions during August-October and by slightly warmer-than-normal conditions later in the year. The NCEP statistical model forecast shows a similar evolution through October, but indicates near-normal conditions remaining through April 2001. Other available coupled model and statistical predictions indicate the continuation of cold episode conditions through the end of 2000. The lack of any rapid evolution in the subsurface thermal structure and the persistence of low-level easterly anomalies over the central and western equatorial Pacific continues to support a slower decay of the cold episode conditions than is shown by either the NCEP coupled model or the NCEP statistical model. Thus, it is likely that cold episode conditions will gradually weaken over the next 6 months and that near-normal or slightly cooler than normal conditions will be present in the tropical Pacific at the end of the year.

Weekly updates for SST, 850-hPa wind, and OLR are available on the Climate Prediction Center homepage at: http://www.cpc.ncep.noaa.gov (Weekly Update).

 

Climate Prediction Center
National Centers for Environmental Prediction
NOAA/National Weather Service
W/NP52, RM 605, WWB
5200 Auth Road
Camp Springs, MD 20746-4304
e-mail: vkousky@ncep.noaa.gov