Current ENSO Information
Technical ENSO Update
17 January 2003
Current Conditions
The SST anomalies over most of the eastern and central equatorial Pacific remain more than 1
degree C as of mid-January. Since mid-December 2002, most warm SST anomalies across the
equatorial Pacific have weakened. Off the west coast of South America SST anomalies cooled to
less than one-half degree C above normal during early January, although they may increase in
the next couple months, as these far eastern SSTs sometimes warm at the end of El Niņo
episodes. The atmosphere continues to show many features indicative of El Niņo, including
reduced easterly trade winds in the western and central tropical Pacific, a lower than average
Southern Oscillation Index, and above average tropical rainfall in the central Pacific, from
170E to 155W longitude. These features are consistent with the mature stage of El Niņo.
As a result of the weaker trade winds, or westerly wind anomalies along the equator, the
equatorial thermocline has become flatter - deeper in the east and shallower in the western
equatorial Pacific. In December sub-surface positive temperature anomalies of 2 to 4 degrees C
accompanied the deepened thermocline in the east. The deep thermocline anomalies are slowly
dissipating via equatorial ocean wave dynamics, and the associated temperature anomalies at
depth have weakened somewhat as compared with mid-November to mid-December.
Expected Conditions
For purposes of this discussion, El Niņo SST conditions are defined as SSTs in the NINO3.4
region being in the warmest 30% of their climatological distribution for the 3-month period in
question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST
anomaly varies seasonally, being close to 0.5 degrees C in Mar-Apr-May and as high as 0.75
degrees C in Oct-Nov-Dec. La Niņa conditions are defined as NINO3.4 region SSTs being in the
coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 45% of
the distribution. These definitions were developed such that the most commonly accepted El Niņo
and La Niņa episodes are reproduced. There is some variation among ENSO model forecasts for the
coming 7 months-particularly for the periods following Apr-May-Jun when the ENSO phase becomes
most uncertain. The number of models that are forecasting El Niņo conditions to continue for
the first quarter of 2003 (Jan-Feb-Mar) is 14 out of the total of 15 (93%). This is similar to
the forecasts of one month ago. 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 Niņo level between January and March 2003.
(Note 1). Most of the
models indicate that the warm SST anomalies in the NINO3.4 region will steadily decrease over
the coming several months. For the much longer lead forecast for Jun-July-Aug 2003, 3 of the 14
models (21%) forecast El Niņo conditions, 10 models (71%) forecast neutral conditions, and 1
model (7%) forecasts La Niņa conditions. If only the 10 models that use subsurface ocean
temperature data are included, these figures become 3 (30%), 6 (60%), and 1(10%). Caution is
advised in interpreting the distribution of model forecasts as the actual probabilities for
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. The IRI's probabilistic ENSO
forecast takes
into account factors in addition to this set of models, and indicates a somewhat greater
likelihood of a La Niņa than 10%--namely, 26% by Jun-Jul-Aug 2003. Some of the reasoning behind
this is mentioned in the following subsection.
See also:
Dynamics of El Niņo Dissipation
One of the most important factors that will determine the rate of decay of the current El Niņo
is the wind along the equator. While anomalous westerly winds will probably continue near the
dateline as a response to the above normal SST near and to the east of the dateline, the
magnitude and location of zonal wind anomalies across the whole basin will determine the speed
at which the event will dissipate. Presently (mid-January), there is an area of anomalously
shallow thermocline in the western Pacific that will move eastward and work to change the ENSO
phase from warm to cool in the eastern part of the basin. It is possible that these shallow
anomalies could occupy the eastern equatorial Pacific as early as 2 months from now and decay
the warmth left in the mixed layer; in that case the SST in the NINO3.4 region would decline to
the neutral category by late April or early May.
Although there are shallow anomalies to the north of equator to support what is happening on
the equator in the western basin, the region of shallow thermocline anomalies does not have
great strength or extent. There may not be enough strength in the shallow themocline anomalies
to spawn a La Nina without help from stochastic "weather" influences. On the other hand there
is not much warmth left in central/eastern Pacific (less than in 1991/92 at this time, for
example), so the El Niņo is also unlikely to continue much longer without help from "weather"
events such as strong westerly wind bursts in conjunction with the Madden-Julian Oscillation
(MJO). What the above factors suggest is that a rather normal dissipation of the El Niņo over
the coming two to four months is most likely, with a lesser likelihood of a reversal to a La
Niņa and a still lower likelihood of a second El Niņo year.
Note 1 - Only models that produce a
new ENSO forecast every month are included in the above statement.
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