Current ENSO Information
Technical ENSO Update
18 April 2003
Current Conditions
SST anomalies are no longer of sufficient magnitude to constitute
El Nino conditions. The positive SST anomalies in the central
equatorial Pacific continued to weaken during the last month.
Subsurface indicators also confirm that this El Nino event is rapidly
coming to an end, as the equatorial thermocline is now shallower
than normal across the entire Pacific basin.
SSTs off the west coast of South America have cooled considerably
during the last several weeks due to the thermocline shoaling and
to increased trade winds which have strengthened the coastal upwelling
in the region. The cooler than normal SSTs extend westward from the
South American coast to about 140W in the equatorial eastern Pacific,
with typical anomalies of about 0.5 degrees C.
However, although the strength of the SST anomalies over most of
the central equatorial Pacific weakened to near-neutral conditions,
there is still a region of SST about 0.5 degrees C warmer than
normal in the central and western Pacific.
Consistent with these positive SST anomalies is an eastward
shift in the tropical convection, and a slightly negative SOI.
These features suggest that El Nino like impacts are still
being experienced by the tropical atmosphere.
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 19%. 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, 3 out
of 12 (25%) are calling for SSTs to maintain at least a minimum El Nino
level for the May-Jun-Jul period. (Note
1). Most of the models indicate that the remaining weak warm SST anomalies in the
NINO3.4 region will decrease further over the coming several months. For
the longer lead forecast for July-Aug-Sep 2003, 5 of the 16 models (31%)
forecast El Nino conditions, 9 models (56%) forecast neutral conditions,
and 2 models (13%) forecasts La Nina conditions. If only the 12 models that
use subsurface ocean temperature data are included, these figures become
4 (33%), 6 (50%), and 2(17%). 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 slightly different probability
distribution than that seen from the tally of model forecasts discussed here.
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 is determining 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 the dateline, the magnitude
and location of zonal wind anomalies across the whole basin is determining
the speed at which the event will dissipate. Presently (mid-April), the
equatorial thermocline is shallower than normal across the entire
Pacific basin. This will work to change the ENSO phase from toward cool
in the eastern part of the basin where mean upwelling is strongest.
As the easterly trade winds return to the eastern Pacific, the above normal
temperatures left in the mixed layer of the central Pacific
should dissipate further, and potentially even cool to temperatures
below-normal.
There are not, however, substantial off-equatorial shallow thermocline
anomalies to support what is happening on the equator.
Thus, 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 have moved
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 completion of the ongoing normal dissipation of
the El Nino over the coming season 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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