Special Notice: Starting in May 2015, the ENSO Diagnostic Discussion will be released on the second Thursday of each month.
Synopsis: There is an approximately 70% chance that El Niño will continue through Northern Hemisphere summer 2015, and a greater than 60% chance it will last through autumn.
By the end of March 2015, weak El Niño conditions were reflected by above-average sea surface temperatures (SST) across the equatorial Pacific (Fig. 1), and by the expected tropical atmospheric response. The latest weekly Niño indices were +1.1oC in the Niño-4 region, +0.7oC in the Niño-3.4 region, and +0.6oC and +1.4oC in the Niño-3 and Niño-1+2 regions, respectively (Fig. 2). Subsurface temperature anomalies increased substantially during the month (Fig. 3) in response to a downwelling oceanic Kelvin wave, which resulted in strong positive subsurface anomalies across most of the Pacific (Fig. 4). Consistent with ocean-atmosphere coupling, enhanced convection shifted eastward to the central equatorial Pacific (Fig. 5), while low-level westerly wind anomalies continued over the western equatorial Pacific and upper-level easterly wind anomalies continued in the central Pacific. Also, both the traditional and the equatorial Southern Oscillation Index (EQSOI) remained negative during the month. Collectively, these features reflect weak El Niño conditions.
Compared to last month, more models predict El Niño (3-month values of the Niño-3.4 index equal to or greater than 0.5oC) to continue throughout 2015 (Fig. 6). These forecasts are supported by the increase in subsurface temperatures, enhanced convection over the Date Line, and the increased persistence of low-level westerly wind anomalies. However, model forecast skill tends to be lower during the Northern Hemisphere spring, which limits the forecast probabilities of El Niño through the year. At this time, there is also considerable uncertainty as to how strong this event may become. In summary, there is an approximately 70% chance that El Niño will continue through the Northern Hemisphere summer 2015, and a greater than 60% chance that it will last through autumn (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).
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