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ENSO Update
15 April 2010
Summary
El Niño conditions have decreased to a weak level in the equatorial Pacific,
and are expected to dissipate during early May. For the April-June season
in progress, there is an approximately 38% probability of maintaining El
Niño conditions. Probabilities for El Niño decrease to 20%
for the June-August season and to 10% during the final quarter of 2010, while probabilities
for La Niña conditions rise to 35% in late 2010, which is slightly higher than
the 25% climatological probability.
General Discussion
During mid-April, the NINO3.4 SST anomaly index has values
indicative of the weak El Niño category, having decreased
from the moderate(+) levels observed from November through January.
Strong westerly wind anomalies during late January through February allowed the
existing subsurface heat content to maintain moderately strongly positive anomaly
values. Since early March zonal wind anomalies in the equatorial Pacific have
returned to near-average, and the heat content, while still above-average, is
in the process of also returning toward average. The El Niño episode
is continuing well into the northern spring season. However, it is now
weakening and may dissipate to neutral by the end of April or early May.
Out of a large set of dynamical and statistical forecast models,
45% indicate maintenance of El Niño conditions during
the April-May-June season currently in progress. Overall, based on model
forecasts and current observations of the ocean surface and subsurface,
for the coming May-July season the probability for El Niño
conditions is estimated at only 20%, and for ENSO-neutral conditions approximately
70%. The most likely time for the NINO3.4 SST anomaly to decline to ENSO-neutral
levels is during early May. In the second half of 2010, while neutral
conditions are most likely, there is a somewhat greater probability for La
Niña conditions to develop (35%) than for having a second year of El
Niño conditions (10%).
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