Synopsis: La Niña conditions are expected to continue during the next 3-6 months.

The patterns of anomalous ocean temperatures, atmospheric circulation and precipitation are consistent in indicating La Niña conditions in the tropical Pacific. During January negative equatorial SST anomalies less than –0.5ºC were observed at most locations between the date line and the South American coast, while anomalies greater than +0.5ºC were restricted to the region between Indonesia and 160ºE (Fig. 1). Negative SST departures increased in magnitude in the Niño 4 and Niño 3.4 regions (Fig. 2), as the oceanic cold tongue strengthened in the central equatorial Pacific.

During January above-average precipitation (negative OLR anomalies, Fig. 3, top panel) was observed over Indonesia, the Philippines and northern Australia, while below-average precipitation (positive OLR anomalies) was observed over the central equatorial Pacific. Stronger-than-average low-level (850-hPa) easterly winds (Fig. 3, middle panel) persisted over the central equatorial Pacific, and anomalous upper-level (200-hPa) cyclonic circulation centers were observed in both hemispheres (Fig. 3, bottom panel). These patterns are similar to those observed during previous La Niña episodes.

Over the past several months most of the statistical and coupled model forecasts have trended towards cooler conditions in the tropical Pacific through mid-2006. The spread of the most recent statistical and coupled model forecasts (weak La Niña to ENSO-neutral) indicates some uncertainty in the outlooks. However, current conditions (stronger-than-average easterly winds over the central equatorial Pacific) and recent cooling trends in observed oceanic conditions support continuation of La Nina conditions in the tropical Pacific during the next 3-6 months.

Based on current conditions in the tropical Pacific, the most recent SST predictions, and on results from historical studies on the effects of cold episodes, we expect wetter-than-normal (drier-than-normal) conditions to prevail over Indonesia/Philippines (central equatorial Pacific) during the remainder of the NH winter. That pattern of tropical precipitation favors a northward shift in the position of the jet stream over the eastern North Pacific during winter, which is usually accompanied by drier-than-normal conditions over southern California and Arizona. The recent patterns of anomalous temperature and precipitation for the United States (Fig. 4) are similar to wintertime patterns observed during previous La Niña episodes (Fig. 5), except for temperature over the northern Plains and in the Pacific Northwest, which are normally colder than average.

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 9 March 2006. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message: ncep.list.enso-update@noaa.gov.