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ENSO Alert System Status: La Niña Watch

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avatar ENSO Alert System Status: La Niña Watch
August 28, 2011 08:38PM
Synopsis: ENSO-neutral is expected to continue into the Northern Hemisphere fall 2011, with ENSO-neutral or La Niña equally likely thereafter.

During July 2011, ENSO-neutral was reflected in the overall pattern of small sea surface temperature (SST) anomalies across the equatorial Pacific Ocean (Fig. 1). All of the latest weekly Niño index values were generally near average (Fig. 2), ranging from –0.2oC (Niño-3.4) to 0.5oC (Niño-1+2). However, the subsurface oceanic heat content anomaly (average temperature anomalies in the upper 300m of the ocean, Fig. 3) continued to weaken and is currently near zero, which reflects the strengthening of the below-average temperatures at depth in the east-central Pacific Ocean (Fig. 4). The atmospheric circulation anomalies were more variable during the past month, but the monthly means still reflect aspects of La Niña. For example, convection continued to be enhanced over eastern Indonesia and Papua New Guinea, and generally suppressed over the central equatorial Pacific, mainly south of the equator (Fig. 5). Also, anomalous low-level easterly and upper-level westerly winds persisted over the central tropical Pacific. Thus, while tropical Pacific oceanic anomalies indicate ENSO-neutral, the atmospheric patterns continue to reflect La Niña-like conditions.

The majority of ENSO models, and all multi-model average forecasts (indicated by thicker lines, Fig. 6), indicate ENSO-neutral will continue into the Northern Hemisphere fall 2011 (three-month average in the Niño-3.4 index between –0.5oC and +0.5oC). Beyond the early fall, the forecasts are less certain with half of the models persisting ENSO-neutral conditions continuously through early 2012. Along with a few other models, the latest runs from the NCEP Climate Forecast System (CFS) models predict La Niña to re-develop during the fall (Fig. 7). This forecast is also supported by the ongoing La Niña-like tropical atmosphere, subsurface temperature trends, and the historical tendency for significant wintertime La Niña episodes to be followed by relatively weaker La Niña episodes the following winter. Therefore, ENSO-neutral is expected to continue into the Northern Hemisphere fall 2011, with ENSO-neutral or La Niña equally likely thereafter.

This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA's National Weather Service, and their 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 September 2011. 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.

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