La Niña conditions strengthened during September 2007, as negative SST anomalies along the equator expanded westward and now extend from 170oE to the South American coast (Fig. 1). The latest weekly analysis (Fig. 2) shows the largest SST departures (-2oC to -3oC) between 120oW and the coast, with departures of -0.5oC to -1oC centered near the date line. The magnitude of the negative SST anomalies increased in all of the Niño regions, with the Niño-3.4 index dropping to -1.2oC and the Niño-4 index dropping to -0.5oC by the end of the month (Fig. 3). The upper-ocean heat content (average temperatures in the upper 300 m of the ocean) in the central and east-central equatorial Pacific remained below average during September (Fig. 4), with temperatures ranging from 2oC to 4oC below average at thermocline depth (Fig. 5). Consistent with these conditions, the low-level easterly winds and upper-level westerly winds remained stronger than average across the central equatorial Pacific, convection remained suppressed throughout the central and eastern equatorial Pacific, and enhanced convection again covered parts of Indonesia and the far western Pacific. Collectively, these oceanic and atmospheric conditions reflect a strengthening La Niña.
The recent SST forecasts (dynamical and statistical models) for the Niño 3.4 region indicate a weak-to-moderate La Niña continuing into early 2008 (Fig. 6). Current atmospheric and oceanic conditions and recent trends indicate that La Niña will continue and may strengthen during the next 3 months.
Expected La Niña impacts during October – December include a continuation of above-average precipitation over Indonesia and below-average precipitation over the central equatorial Pacific. For the contiguous United States, potential impacts include above average precipitation in the Pacific Northwest, and continued below average precipitation in the Southwest.
This discussion is a consolidated effort of NOAA and its funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction Center web site (El Niño/La Niña Current Conditions and Expert Discussions). 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 8 November 2007. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message to: ncep.list.enso-update@noaa.gov.
