IRI ENSO Update / Forecast
Technical ENSO Update
Summary of Model Forecasts
Technical ENSO Update
19 January 2006
Current ConditionsFor the month of December, SST conditions within the equatorial Pacific showed approximately 1.0C below-average anomalies in the eastern Pacific and 0.5C above-average anomalies in the western Pacific. During early September 2005 cold anomalies began to develop off the west coast of South America and extended to approximately 110W. The cause of that localized anomaly appears to have been an enhanced high pressure region over western South America and the SE tropical Pacific, just south of the equator. This high pressure anomaly led to enhancement of the SE trades and cross-equatorial flow, thus enhancing coastal and equatorial upwelling in the region. In mid-November, easterly wind anomalies developed along the equator in response to the anomalous zonal temperature gradient. These wind anomalies strengthened in the central Pacific during December, with another strong and persistent appearance of easterly anomalies in mid-January. The enhanced easterlies both advected the cold anomalies westward and shoaled the local thermocline, leading to an increase in the magnitude and spatial extent of the cold anomalies in the eastern equatorial Pacific from mid-November to mid-January 2006. Currently, the NINO3.4 index of ENSO has reached La Nina conditions, but it must maintain that strength over at least 3 consecutive months in order to be classified as a La Nina event. The existence of a La Nina event has become increasingly likely since December as the ocean-atmosphere coupling occuring over the last couple months has created large scale shoaling of the thermocline in the eastern equatorial Pacific, with maximum anomalies exceeding 30m.
For December 2005, the SSTs in the NINO3.4 region were 0.58 degree C below average, and for the Oct-Nov-Dec season were 0.20 degrees C below average. This evolution in central/eastern Pacific SST, and the attendant changes in the winds and sub-surface structure, are indicatative of developing La Nina conditions. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical disctibution. The magnitude of NINO3.4 anomaly necessary to qualify as El Niño (La Niña) conditions in the coming seasons (Jan-Feb-Mar and Feb-Mar-Apr) is approximately 0.5C (-0.5C).
With the continuing development of below-average SSTs in the
eastern equatorial Pacific, due to air-sea coupling, the development of
a La Niña event has become the most likely outcome for the
coming 3-month season. There is an estimated 50% likelihood, or a
doubling of the climatological odd, for La
Niña to develop and an approximate 1%
probability for El Niño, leaving 49% probability that neutral
conditions will prevail over the JFM 2006 season.
Although La Niña conditions have been observed over the
last couple weeks, there is still uncertainty surrounding the
maintenance of those cold SST anomalies. There are shallow thermocline
anomalies in the central/eastern Pacific that can perpetuate, and even
amplify, the current cold anomalies; however, the models do not
indicate much potential for growth of the anomalies. The eastern
Pacific is also transitioning toward its warmest climatological
temperatures, which occur March;
this is a very unusual time of year for a La Niña event to
develop. Furthermore, as the projected magnitude of the La Niña,
were it to occur, is weak, detructive interference from westerly wind
bursts associated with the MJO could weaken the oceanic cooling enough
to place the anomalies back in the neutral category. Such MJO activity
could potentially be triggered by the persistent warm anomalies of the
western Pacific, but this type of high frequency variability is not
well captured by ENSO prediction models.
The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models as well as the observed conditions. 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 0.45 degrees C in northern spring season and as high as 0.75 degrees C in late northern autumn. 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, mainly for the longer-lead seasons. No models are forecasting El Niño conditions to occur in the Feb-Mar-Apr period, but 10 of the 19 models (53%) are suggesting the occurence of La Niña SST conditions to develop within that season. 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, no models call for El Niño conditions for the Feb-Mar-Apr period; 7 of 14 models (50%) indicate SSTs exceeding the threshold for La Niña (Note 1). Half of the models indicate that the SST conditions in the NINO3.4 region will continue to be near-normal over the next couple seasons. For the longer lead forecast for May-Jun-Jul 2006, 1 of the 17 models (6%) forecasts El Niño conditions, 14 models (82%) forecast neutral conditions, and 2 models (12%) forecast La Niña conditions. If only the 12 models that use subsurface ocean temperature data are included, these figures become 0 (0%), 11 (92%), and 1 (8%). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities for the 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.
An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the forecasts of all models, and to construct a standard error function centered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. When this method is applied to the current model forecasts, results indicate a probability of El Niño that is much lower than the climatological value (25%)--beginning at about 1% for Jan-Feb-Mar, rising to 25% by mid-2006. For La Niña the probabilities begin at 50% for Jan-Feb-Mar and decrease to 25% in mid-2006. The same cautions mentioned above for the distribution of model forecasts apply to this alternative method of inferring probabilities, due to differing model biases and skills.
probabilistic ENSO forecast takes into account the indications of
this set of models, the outcome of the standard error approach
above, and additional factors such as the very latest observations that
have developed after the initialization times of some of the models. It
indicates much higher than average probabilities for La Niña
conditions, and much lower than climatological probabilities for both
through the next season. Those probabilities move towards
climatological values at longer leads.
In terms of climate impacts, it must be noted that the
atmosphere responds to changing patterns of SST anomalies. The fact
that the western tropical Pacific has been so anomalously warm means
that the zonal gradient of SST along the equator in the Pacific has
been notably strengthened for several months even though the SST
anomalies in the eastern Pacific have been weak until just recently.
The result of the enhanced SST gradient has been a La
Niña-like precipitation response of the the tropical
Indo-Pacific region, and thus La Niña-like modification
of the tropical heating of the atmosphere. As a result the recent
monthly, and even seasonal, precipitation anomalies have appeared La
Niña-like, particularly in tropical regions.
Note 1 - Only models that
produce a new
ENSO forecast every month are included in the above statement.