Synopsis: A transition to ENSO-neutral conditions is expected by June 2011.
La Niña weakened for the third consecutive month, as reflected by increasing surface and subsurface ocean temperatures across the equatorial Pacific Ocean. All four Niño indices ranged between –0.3oC and –0.8oC at the end of March 2011 (Fig. 1). Subsurface oceanic heat content anomalies (average temperatures in the upper 300m of the ocean, Fig. 2) became weakly positive in response to the continued eastward progression of a strong oceanic Kelvin wave, which has begun to shoal in the eastern Pacific (Fig. 3). However, the basin wide extent of negative SST anomalies remained considerable throughout the month (Fig. 4). Also, La Niña impacts on the atmospheric circulation remained strong over the tropical and subtropical Pacific. Convection remained enhanced over much of Indonesia and suppressed over the western and central equatorial Pacific (Fig. 5). Also, anomalous low-level easterly and upper-level westerly winds have persisted in this region. Collectively, these oceanic and atmospheric anomalies reflect a weakening La Niña, but with ongoing global impacts.
Nearly all of the ENSO models predict La Niña to continue weakening in the coming months, and the majority of models indicate a return to ENSO-neutral by May-June-July 2011 (three month average in the Niño-3.4 index between –0.5oC and +0.5oC; Fig. 6). While there is confidence in ENSO-neutral conditions by June 2011, the forecasts for the late summer and beyond remain highly uncertain. At this time, all of the multi-model forecasts (shown by the thick lines) suggest ENSO-neutral conditions will persist from June through the rest of the year. However, the spread of individual model forecasts and overall model skill at these lead times leaves the door open for either El Niño or La Niña conditions by the end of 2011.
La Niña will continue to have global impacts even as the episode weakens through the Northern Hemisphere spring. Expected La Niña impacts during April-June 2011 include suppressed convection over the west-central tropical Pacific Ocean, and enhanced convection over Indonesia. Potential impacts in the United States include an enhanced chance for below-average precipitation across much of the South, while above-average precipitation is favored for the northern Plains. An increased chance of below-average temperatures is predicted across the northern tier of the country (excluding New England). A higher possibility of above-average temperatures is favored for much of the southern half of the contiguous U.S. (see 3-month seasonal outlook released on March 17th, 2011).
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