Current ENSO Information
Technical ENSO Update
18 February 2003
Most of the positive SST anomalies across the equatorial Pacific have
weakened since mid-January, and as of mid February, SSTs off the west
coast of South America have cooled to slightly below
normal. However, the SST anomalies over much of the central equatorial
Pacific remain more than 1 degree C as of mid-February, and the
atmosphere continues to show many features indicative of
El Niño, including a slightly lower than average Southern Oscillation
Index, and above average tropical rainfall in the central
Pacific from about 170E to 155W longitude. These features are consistent
with the mature stage of El Niño.
For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4
region being in the warmest 25% 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 0.4 degrees C in Mar-Apr-May and as high as 0.75
degrees C in Oct-Nov-Dec. La Niña conditions are defined as NINO3.4 region SSTs being in the
coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of
the distribution. These definitions were developed such that the most commonly accepted El Niño
and La Niña 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 Niño conditions to continue for
the second quarter of 2003 (Apr-May-Jun) is 7 out of a total of 16, or 44%. 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, 6 out of 12 (50%) are calling for SSTs to
maintain at least a minimum El Niño level between April and June 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 longer lead forecast for Jun-July-Aug 2003, 3 of the 15
models (20%) forecast El Niño conditions, 10 models (67%) forecast neutral conditions, and 2
models (13%) forecasts La Niña conditions. If only the 11 models that use subsurface ocean
temperature data are included, these figures become 3 (27%), 6 (55%), and 2(18%). 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
into account factors in addition to this set of models, and indicates a slightly greater
likelihood of a La Niña than 18%--namely, 29% by Jun-Jul-Aug 2003. Some of the reasoning behind
this is mentioned in the following subsection.
Dynamics of El Niño Dissipation
One of the most important factors that will determine the rate of decay of the current
El Niño is the wind along the equator. While some 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-February), there is an area of anomalously
shallow thermocline perturbations in the western Pacific that have been moving
eastward and working to change the ENSO phase from warm to cool in the eastern part
of the basin. The majority of the equatorial thermocline is now slightly shallower
than normal. As the shallow anomalies from the west continue to move into the
eastern equatorial Pacific and the easterly trade winds return to the eastern Pacific,
the above normal temperatures left in the mixed layer of the central and eastern Pacific
should dissipate, leading the SST in the NINO3.4 region to 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 Niña without help from stochastic "weather" influences. On the other hand the
temperatures in central/eastern Pacific are steadily moving towards average conditions
(weaker temperature anomalies than in 1991/92 at this time, for
example), so the El Niño 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 Niño over
the coming two to four months is most likely, with a lesser likelihood of a reversal to a La
Niña or of a second El Niño year.
Note 1 - Only models that produce a
new ENSO forecast every month are included in the above statement.
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