Current ENSO Information
Technical ENSO Update
15 November 2002
Current Conditions
El Niño conditions strengthened somewhat during the period of late
October to early November. During this time, equatorial SSTs from 175E
to 95W longitude remained at or rose to at least 1 degree C above normal,
and anomalies of 2 degrees C or more appeared both from 175W to 145W, as
well as farther east from 120W to 100W. In early November, SST anomalies
reached near 3 degrees C locally, near 160-165W. This stronger warmth,
which now includes much of the NINO4, NINO3.4 and all but the easternmost
part of 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. Episodes of westerly wind anomalies in association
with Madden-Julian Oscillation (MJO) have not played a major role in maintaining
the El Nino over the last two months. Consistent with the large-scale coupling,
the Southern Oscillation Index (SOI) continued to be below normal. The
continuation of these SST conditions, along with the full participation
of the atmosphere for several months now, indicates that we are in a basin-wide
El Niño episode that is near maturity. 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that had dominated the far western tropical Pacific through July has been
replaced by near normal or below normal SST over parts of Indonesia.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 November the
themocline remained deeper than normal in the central and east-central
part of the basin. Sub-surface positive temperature anomalies of 2 to 7
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, near normal SSTs have risen to somewhat above average
levels during the last month, and as a result, SSTs along the immediate
coasts of Ecuador and northern Peru have risen to somewhat above normal.
The far eastern tropical Pacific is expected to warm further as the episode
continues, as has been shown to occur in nearly all previous El Niño
episodes. This is likely to produce periods of above normal rainfall over
the region from January to April when the climatological SST is peaking.
There is virtually no possibility that the above normal SST in the central
equatorial Pacific could substantially weaken 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 Conditions
For purposes of this discussion, neutral SST conditions are defined as
the sea surface temperatures (SSTs) in the NINO3.4 region averaged over
the Dec-Jan-Feb period (for which model forecasts are highlighted here)
being more than -0.7 and less than 0.7 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. Although there is some variation among
ENSO model forecasts for the coming 7 months, the spread among the forecasts
is not large. Compared to one month ago, there has been no change in the
high number of models that are forecasting El Niño conditions to
continue through the remainder of 2002 and into early 2003. This month,
93% of the 15 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 December 2002 and February 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) than a weak or a strong one. If El Niño conditions
are present during the December to February period, a continuation of such
conditions would be expected through at least March or April of 2003, as
El Niño events tend to reach their peak magnitude near the end of
the year and often persist into several months of the following year. Out
of the 14 ENSO forecast models examined, 13 of them forecast at least a
0.6 degree C positive anomaly in the NINO3.4 region for early 2003 (Feb-Mar-Apr),
while the remaining 1 predicts neutral SST anomalies. If we omit the models
that do not explicitly use subsurface sea temperature data, the counts
become 10 out of 10 warm, versus 0 out of 10 neutral. Because of the persistence
of the warm SST observed in the NINO3.4 region since late May, our assessment
of the probability of an El Niño continuing into early 2003 is the
same as that reflected collectively in the models that use subsurface sea
temperature data—i.e., near 100%. The continuation of complete participation
of the atmosphere is indicative of a maturing of the El Niño. It
appears nearly certain that the El Niño SST conditions seen over
the last 3 to 4 months will remain in place for the next 4 to 6 months.
Confidence in the forecasts is high, as the statistics of historical ENSO
data indicate that the observations during late northern summer usually
set the stage for what is to be expected for the remainder of the year
and into the beginning of the following year.
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 November. 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 this year, very likely lasting into early 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
September statement in which the probability was estimated at greater than
99%. There is still some uncertainty regarding the strength of this El
Niño as it fully matures over the coming couple of months. It now
appears most likely to be in the upper half of the moderate range. 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. The magnitude of the NINO3.4 anomaly at maturity is likely
to be between 1.4 and 2.4 degrees C, placing it most likely into the upper
portion of the moderate range or, less likely, in the lower portion of
the strong (>2 degrees) category. It would likely persist through March
or April 2003.
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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