Current ENSO Information
Technical ENSO Update
16 December 2002
Current Conditions
El Niño conditions strengthened somewhat during the period of late
November to early December as compared to the previous 30-day period. During
this time, equatorial SSTs from 175E to 90W longitude remained at or rose
to at least 1 degree C above normal, and anomalies of 2 degrees C or more
appeared both from 170W to 145W, as well as farther east from 130W to 95W.
This represents a slightly eastward-shifted distribution of the peak SST
anomalies as compared with one month earlier. In early December, SST anomalies
were near 2.5 to 3 degrees C locally, near 165-170W and farther east near
110-100W. This warmth, which now includes much of the NINO4, NINO3.4 and
NINO3 regions, is sustaining itself through the large-scale westerly wind
anomalies created by its own existence, through its resulting anomalous
convection near and east of the dateline. The heating caused by this convection
enables a coupling between the SST anomaly and the anomalous atmospheric
circulation patterns. In early December, some Madden-Julian Oscillation
(MJO) activity provided additional fuel to fortify the episode. Such MJO
activity had not played a major role in maintaining the El Nino over the
last several months. Consistent with the large-scale coupling, the Southern
Oscillation Index (SOI) continued to be below normal despite a short-lived
excursion into positive territory for a couple of weeks during November.
The continuation of these SST conditions, along with sufficient participation
of the atmosphere over the last several months, indicates that we are in
a mature basin-wide El Niño episode. Due to the eastward-shifted
convection and the associated zonal circulation changes, above normal SST
that had dominated the far western tropical Pacific through July has been
replaced by near normal SST over parts of Indonesia.
As a result of the westerly wind anomalies along the equator, the equatorial
thermocline has become flatter from west to east. In early December the
themocline remained deeper than normal in the central and eastern part
of the basin. Sub-surface positive temperature anomalies of 2 to 6 degrees
C in portions of the mixed layer accompany the deepened thermocline. These
temperature anomalies at depth will tend to reinforce the currently warm
SSTs for at least the next month, even with normally strong trades in the
eastern portion of the basin. In the far eastern tropical Pacific, from
80W to 95W, slightly to somewhat above average levels have been observed
since early November. The far eastern tropical Pacific is expected to warm
further as the episode enters the final portion of the mature stage, as
has been shown to occur in most previous El Niño episodes. This
is likely to produce periods of above normal rainfall over the region from
February to April when the climatological SST is peaking.
It is extremely unlikely that the above normal SST in the central equatorial
Pacific will substantially weaken to sub-El Niño levels (e.g., to
below 0.6 degrees above normal in the NINO3.4 region) over the next two
months. The continued presence of a large area of positive SST anomalies
of greater than 1.5 degrees C, the persistence of westerly wind anomalies
in the central Pacific, continued below normal SOI, eastward-shifted convection,
and the presence of positive subsurface temperature anomalies all favor
the continuation of the current SST conditions through the end of the year
and into the early part of 2003.
Expected ConditionsFor purposes of this discussion, neutral SST conditions
are defined as the sea surface temperatures (SSTs) in the NINO3.4 region
averaged over the Jan-Feb-Mar period (for which model forecasts are highlighted
here) being more than -0.6 and less than 0.6 degrees C, while SSTs of at
least this magnitude would be indicative of La Niña (if negative)
or El Niño (if positive). This definition allows about half of all
years to be classified as neutral. There is some variation among
ENSO model forecasts for the coming 7 months—particularly for the periods
following Apr-May-Jun when next year’s ENSO phase begins coming into question.
The number of models that are forecasting El Niño conditions to
continue through the remainder of 2002 and into early 2003 is similar to
that of one month ago. This month, 88% of the 16 models forecast a continuation
of El Niño conditions. At lead times of more than 4 months into
the future, statistical and dynamical models that incorporate information
about the ocean's observed sub-surface thermal structure generally exhibit
higher predictive skill than those that do not. Among models that do use
sub-surface temperature information, 100% are calling for SSTs to maintain
at least a minimum El Niño level between January and March 2003.
(Note
1) Overall, the ensemble of the available model forecasts implies a
high likelihood for El Niño conditions to continue into early 2003.
More models are calling for a moderate El Niño (anomalies between
1 and 2 degrees C, as exits in present observations) than a weak or a strong
one. Most of the models indicate that the current strength of the SST anomaly
in the NINO3.4 region is near the peak strength for this episode, to continue
only another month or two before beginning to slowly decline. During the
expected weakening of the event during the February to May period, El Niño
conditions may remain strong enough to produce climate effects in the usual
regions and seasons, such as rainfall deficits in northeastern Brazil.
ENSO forecast models have a relatively modest level of skill for forecasts
made during the February to April period. Skills are considerably higher
for forecasts made in December—at least for projections through March.
Caution is still advised, however, in interpreting the distribution of
model forecasts as the actual probabilities for the ENSO conditions expected
for the coming several months. The expected skill of one model versus another
has not been established using uniform validation procedures, which may
cause a difference in the above probability estimate from the true probability.
See also:
The Probability of an El Niño in 2002
IRI's assessment is that there is a virtually 100% probability of an El
Niño lasting into the first 2 to 4 months of 2003. This assessment
is based on the collective forecasts of many computer models of various
types, as well as on the experience of the several oceanographers and atmospheric
scientists familiar with ENSO. This is no change relative to the IRI's
November statement in which the probability was estimated at nearly 100%.
There is now greater certainty regarding the strength of this El Niño
as it goes through its current mature stage, as it is unlikely to change
significantly from its current moderate strength. Its peak strength is
likely to occur either during this month (December 2002) or during January
2003. The probability for a very strong El Niño (e.g. 1997-98
or 1982-83) is low, and of a low strength El Niño (e.g. 1977-78
or 1979-80) is also low, as it has already attained moderate strength for
several months. The peak magnitude of the NINO3.4 anomaly at maturity is
likely to be between 1.75 (as it is currently) and 2.2 degrees. It would
likely persist through March or April 2003 before declining to less than
0.5 degrees above normal.
Starting in January 2003, in addition to discussing the current ENSO
conditions, the IRI’s ENSO statement will assess the ENSO outlook for the
2003-2004 period.
Note 1 - Only models that produce a
new ENSO forecast every month are included in the above statement.
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