Current ENSO Information
Technical ENSO Update
18 March 2003
Current Conditions
Most of the positive SST anomalies across the equatorial Pacific have continued
to weaken since mid-February, 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-March, and the atmosphere continues to show some basic
features indicative of El Nino, 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 Nino.
Expected Conditions
For purposes of this discussion, El Nino 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 Nina 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 Nino and La Nina 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 Nino conditions
to continue for the May-Jun-Jul period, 2003, is 6 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 Nino
level between for the May-Jun-Jul period. (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 July-Aug-Sep 2003, 4 of the 15 models (27%)
forecast El Nino conditions, 10 models (67%) forecast neutral conditions,
and 1 model (7%) forecasts La Nina conditions. If only the 11 models that
use subsurface ocean temperature data are included, these figures become
4 (36%), 6 (55%), and 1(9%). 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 Nina than 9%--namely, 25% by Jul-Aug-Sep 2003. Some of the reasoning
behind this is mentioned in the following subsection.
See also:
Dynamics of El Nino Dissipation
One of the most important factors that will determine the rate of decay
of the current El Nino 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-March), 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 thermocline 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 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 Nino 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
Nino over the coming two to four months is most likely, with a lesser likelihood
of a reversal to a La Nina or of a second El Nino year.
Note 1 - Only models that produce a
new ENSO forecast every month are included in the above statement.
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