Current ENSO Information
ENSO Update
16 May 2003
Summary
Following the dissipation of the 2002-03 El Niño in April, sea surface
temperatures across the central and eastern tropical Pacific have continued
to decrease and are currently below average. There is now a significant
possibility that a La Niña may develop. Based on the behavior of
past La Niña event onsets, and recent surface and subsurface observations
and model forecasts, there is an estimated likelihood of 55% that La Niña
will develop by June. This is approximately double the historical odds.
If a La Niña does not develop, it is most likely that the tropical
Pacific will remain in near-neutral conditions. If a La Niña does
develop, associated climate effects could be experienced as early as June
or July.
General Discussion
Between mid-April and mid-May, sea surface temperatures (SSTs) have decreased
from near neutral to below normal levels in much of the eastern equatorial
Pacific. The weak El Niño that existed into early April has now
completely dissipated, and SSTs have become similar to what is seen in
the very early stages of a La Niña. The atmosphere has also exhibited
trends towards conditions associated with cooler than average SST conditions,
with a continued increase in the Southern Oscillation Index, somewhat stronger
than average trade winds, and below average tropical rainfall in the central
and eastern tropical Pacific.
The skill of SST forecast models is still relatively low at this time
of year. Forecasts from a large set of dynamical and statistical models
are currently divided between those showing near to slightly below average
SSTs, and some indicating the development of La Niña conditions
A small number of models indicates above average SSTs. (Note
1) Based on past events, current observations of the ocean surface
and subsurface, the chances for La Niña are suggested to be greater
than what is indicated collectively by the models. In summary, current
observations and model forecasts suggest that the SST will likely continue
to cool, remaining slightly below average with an estimated 55% probability
of attaining La Niña status by June. If a La Niña does develop,
the associated climate effects could begin being experienced by June or
July.
Note 1 - Only models that produce a
new ENSO forecast every month are included in the above statement.
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