More Technical ENSO Comment
Current Conditions
The basic ENSO indices are approximately
neutral, tending toward slightly warm, in February 2002. For purposes of
this discussion, neutral ENSO conditions are defined as the sea surface
temperatures (SSTs) in the Niño 3.4 region averaged over the Jun-Jul-Aug
period (for which model forecasts are highlighted here) being between -0.6
and 0.6 degrees C, while less than -0.6 would be indicative of La Niña
and more than 0.6 indicative of El Niño. This definition allows
about half of all years to be classified as neutral. February's neutral
condition includes not only the SSTs in the central and eastern tropical
Pacific Ocean (with Niño 3.4 SST anomaly of 0.2 degree C), but also
the low level winds across the equatorial Pacific and the Southern Oscillation
Index (SOI). A more detailed look at the SSTs shows that in the vicinity
of the date line in the central tropical Pacific they have been above normal
by over 1 degree C during February 2002. The waters of the far eastern
equatorial Pacific have recently warmed from slightly below normal in December,
through near normal in January, to approximately 1 degree C above normal
in February (with locally stronger anomalies immediately off the coasts
of Ecuador northern Peru). The sub-surface sea temperature across the tropical
Pacific has been above normal (i.e. the thermocline deeper than normal)
in the western and central parts of the basin during much of year 2001.
In December 2001 and early January 2002 strong westerly winds in the far
western Pacific created a strong additional anomaly of deepened thermocline
in the western and central Pacific. This anomaly propagated eastward beneath
the ocean surface along the equator to where the warmer than normal temperatures
were more easily translated to the surface through upwelling currents in
the eastern equatorial Pacific. Through this process, the SST increased
substantially in early February in the far eastern tropical Pacific. Additional
westerly wind bursts over the course of the next two months would likely
impact this year's ENSO condition.
Expected Conditions - See summary of model predictions
There is considerable variation among ENSO
model forecasts for the coming 9 months. Compared to one month ago, a slightly
larger proportion of the models has begun to forecast a shift toward El
Niño conditions. At long lead times of 6 to 9 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, slightly more than half of them are calling for
warming to at least the minimum El Niño level to take place between
March and July of 2002. Of the models not indicating appreciable warming,
all indicate a continuation of near-neutral conditions; none indicate a
return to cold phase conditions. (Only models that produce a new ENSO forecast
every month are included in the above statement.) Overall, the ensemble
of the available model forecasts implies a moderate likelihood for El Niño
conditions of weak to moderate strength by mid-2002. If El Niño
conditions do develop by that time, a continuation of such conditions would
be expected through at least March of 2003, as El Niño events tend
to reach their peak magnitude near the end of the year. Weighting equally
each of the available models to arrive at a multi-model ensemble for the
ENSO forecasts, the probabilities for cold, neutral and warm conditions
for the second half of 2002 would be 0%, 53% and 47%, respectively. However,
if we omit the models that do not explicitly use subsurface sea temperature
data, these probabilities become 0%, 45% and 55%. Because of the relatively
modest level of forecast skill of any ENSO model at this uncertain time
of the year, caution is advised in interpreting the distribution of model
forecasts as the actual probabilities for the ENSO conditions expected
for middle and later parts of this year. In addition to overall modest
skill, the expected skill of one model versus another has not been established
using uniform validation procedures; this may also cause a difference of
the above probability estimate from the true probability.
Requirements for El Niño Formation
A transition toward warm ENSO conditions
would occur if the equatorial SST between the dateline and the South American
coast moved toward becoming generally above normal by amounts that would
rank in the top 25% of the historical climatological distribution for the
time of the year, and if this took place between April and June and continued
into July. Two major atmospheric changes would occur in parallel with this
SST behavior: the easterly trade winds in the central and eastern tropical
Pacific would weaken and possibly even reverse direction, and deep convection
that is normally limited to the western and central tropical Pacific would
migrate eastward over the warmed SST in the east-central Pacific. Such
growth in area and intensity of the warm water, and the associated atmospheric
changes, will occur only if the ocean/atmosphere dynamical feedbacks are
mutually enhancing, in the following manner: The convection associated
with regions of positive SST anomalies, whether they be off the coast of
South America or farther west near the dateline, would lead to a pervasive
weakening of the easterly trade winds. The convection would add heat to
the atmospheric, resulting in a weaker east-west temperature gradient along
the equator. The weaker trade winds could then allow additional warm water
from the western Pacific to move eastward, further weakening the temperature
gradient, leading to further reduction in the winds, and so on. The uncertainty
lies in whether such a sequence of coupling events will take place between
the tropical Pacific ocean and atmosphere to a sufficient extent. The significant
variation among the forecasts of the different models is a very real reflection
of the present uncertainty. The next two to three months are the time of
year in which the climate system is most amenable to the growth of El Niño
events through the positive feedback of ocean and atmosphere dynamics just
described. If such feedback occurs, there would come a "point of no return",
normally in late April, May or June, in which an El Niño would definitely
be in progress and would last for nearly one year. With each passing month
between now and May, the certainty will increase as to whether the events
leading to a positive ocean-atmosphere feedback are taking place. This
will enable us to state more definitively whether a sustained warming in
the central and eastern half of the tropical Pacific basin and a widespread
pattern of westerly wind anomalies and eastward-expanded tropical Pacific
convection -the hallmarks of El Niño -are developing.
The Probability of an El Niño in 2002
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 the ENSO phenomenon,
the IRI's assessment of the probability of an El Niño by mid-2002,
lasting into early 2003, is 65%. This is substantially above the climatological
probability of about 20-25% (i.e. one El Niño per 4 to 5 years),
and is also an increase relative to the IRI's February statement in which
the probability was estimated at 55%. Nonetheless, it still leaves some
uncertainty. If an El Niño does occur, its likely strength is uncertain
at this time, but on the basis of the current forecasts of dynamical and
statistical ocean/atmosphere models it would appear weak to moderate rather
than very strong. It would likely form during the April through June period,
and last until February to May of 2003.
ENSO Update Statement
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