Current Information
ENSO Update
16 September 2002
Summary
The IRI's assessment is that there is a nearly 100% probability that
El Niño conditions will continue for the remainder of 2002 and into
early 2003. This assessment is based on the collective forecasts of various
ENSO prediction models, as well as on the experience of the several oceanographers
and atmospheric scientists familiar with the El Niño phenomenon.
This probability has hardly changed from that given one month ago. Substantially
warmer than average sea surface temperatures that first emerged in late
May are highly likely to continue for the next two to three seasons. It
is most likely that this will be a moderate El Niño, which, while
slightly stronger than what was expected one month ago, is still significantly
less strong than the 1997-98 event. The associated climate effects in most
regions are expected to be weaker than those associated with the 1997-98
El Niño, but may nonetheless be substantial in some regions.
General Discussion
Between mid-August
and mid-September, sea surface temperatures (SSTs) in most of the east-central
equatorial Pacific remained above 1 degree C above normal, with strongest
positive anomalies between 1.5 and 2 degrees C over the central basin just
east of the dateline. Somewhat below normal SST continues in the far eastern
Pacific. The warm anomalies in the NINO4 and NINO3.4 regions for August
averaged near or just above 1 degree, while in the NINO3 region it averaged
about one-half degree. The relatively cooler eastern SST is expected to
warm over the coming several months as the El Nino episode matures. The
atmosphere continues to show all features indicative of El Niño,
such as persistent westerly wind anomalies in the western and central tropical
Pacific, a lower than average Southern Oscillation Index, and anomalous
equatorial convection near the dateline and extending eastward to 160W
longitude. Consistent with this eastward shift in convection has been the
recent appearance of negative SST anomalies and negative precipitation
anomalies in parts of the far western Pacific and Indonesia. All of the
above features indicate that the El Niño is progressing toward maturation.
The fact that we are very certainly in the first half of an El Niño
episode is reflected in the SST forecasts of a large set of dynamical and
statistical forecast models. Relatively high skill is expected for the
model forecasts at this time of the year. Depending on the type of forecast
model, between 85 and 100% of the models predict El Niño conditions
persisting into the November-December-January period and continuing into
early Northern Hemisphere spring of 2003. The remaining models call for
near-neutral SST throughout 2002. (Note
1) Thus, overall, the models suggest that we are in the first half
of an El Niño episode that will mature in the next 2-4 months and
continue into early 2003.
Note 1 - Only models that produce
a new ENSO forecast every month are included in the above statement.
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