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ENSO Update

18 March 2010

Summary

El Niño conditions remain moderate strength in the equatorial Pacific. For the March-May season in progress there is an approximately 85% probability of maintaining El Niño conditions. Probabilities for El Niño decrease to 50% for the April-June season, and to 25% for the June-August season. During the second half of 2010, the probability for La Niña conditions rises to 30%, which is slightly higher than the 25% climatological probability.

General Discussion

During mid-March, the NINO3.4 SST anomaly index continues at values indicative of a moderate El Niño category, having weakened somewhat from the stronger levels observed in December and early January. The event has maintanined a moderate to strong magnitude since mid-October, modulated by intermittently strong westerly wind anomalies in the western and central tropical Pacific. Strong westerly wind anomalies during late January through February allowed the subsurface heat content to maintain moderately strongly positive anomaly values. This heat content is slowing the decrease in the SST anomalies, permitting the El Niño episode to continue well into the northern spring season.

Out of a large set of dynamical and statistical forecast models, 90% indicate maintenance of El Niño conditions during the March-April-May season currently in progress. Overall, based on model forecasts and current observations of the ocean surface and subsurface, for the coming April-June season the probability for El Niño conditions is estimated at 50%, and for ENSO-neutral conditions approximately 45%. The most likely time for the NINO3.4 SST anomaly to decline to ENSO-neutral levels is during May. In the second half of 2010, while neutral conditions are most likely, there is a slightly greater probability for La Niña conditions to develop (30%) than for having a second year of El Niño conditions (20%).

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