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Technical ENSO Update

18 March 2010


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-March 2010, SSTs are still well above-average throughout the central and most of the eastern equatorial Pacific, indicative of moderate El Niño conditions. Weak El Niño conditions emerged in mid-June and lasted until October, when they increased to moderate to strong levels due to the accumulated effects of intermittently strong westerly wind anomalies in the western and/or central Pacific. The traditional Southern Oscillation Index (SOI) became negative during October, and has remained negative since then. The equatorial SOI also became negative in October, returned to near-average during November and December, then becomame strongly negative in January and February. Positive convection anomalies were observed intermittently near and just west of the dateline between June and September, and became somewhat stronger and more persistent since October. Since late January, the convection anomalies became stronger and have been located near and just east of the dateline so as to efficiently influence the zonal and meridional atmospheric circulation patterns in the manner observed during previous moderate to strong El Nino episodes. Equatorial Pacific oceanic heat content had been above-average since early in 2009, and became more strongly so after October. During January and early February 2010 the heat content anomaly somewhat decreased, but since mid-February has not decreased further, remaining well above average. This pause in the depletion of anomalous sub-surface heat portends a likely continuation of positive SST anomalies during the remainder of March and possibly through much of April.

For February 2009, the SST anomaly in the NINO3.4 region was 1.23 C, sufficient to be classified as moderate El Niño conditions for this time of year. For the Dec-Jan-Feb season the anomaly was 1.54 degrees C. 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 distribution. The NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Mar-Apr-May and the Apr-May-Jun seasons are approximately (-0.40C, 0.40) and (-0.45, 0.45), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 1.2 C, indicating moderate El Niño conditions in the tropical Pacific, essentially unchanged from the 1.23 C level observed in February. What is the outlook for the ENSO status going forward? Although the spatial pattern of SST anomalies and subsurface temperature anomalies became similar to those reminiscent of El Niño events somewhat late in this ENSO cycle (January and February), they have done so with considerable anomaly magnitude, representing greater late-cycle endurance of the event than observed in some past events of similar strength. Postitive convection anomalies continue presently near and just east of the dateline. Owing to the air-sea coupling associated with this canonical configuration, the atmosphere has been acting as previously observed during moderate to strong El Nino events in terms of teleconnection patterns of climate both near and remote from the tropical Pacific.

March is the time of year when existing ENSO events are often in their waning phase, and typically either dissipate or persist for up to two subsequent months. For this event, it seems most likely that El Niño conditions will persist at least through April 2010, and, given the still moderately strong subsurface anomalies resulting from the strong westerly wind anomalies from late January through February, may endure through early or middle May. While persistence of El Niño conditions through a good portion of northern spring seems likely, a double-year event (such as what occurred in 1986-87-88) does not appear likely, as negative subsurface sea temperature anomalies reside in the western tropical Pacific, and the downwelling Kelvin wave associated with the above-mentioned zonal wind anomalies will have reached the South American coast by the end of May (still within the northern spring transition period), with no additional wind-induced pulses in view at present.

Presently, the models and observations taken together indicate probabilities of about 85% for maintaining El Niño conditions and about 14% for dissipation to ENSO-neutral conditions for the Mar-Apr-May season in progress. Going forward, probabilities for El Niño decrease to approximately 50% by Apr-May-Jun, falling to climatological probabilities of 25% by Jun-Jul-Aug. Probabilities for La Niña conditions are predicted to be negligible until May-Jun-Jul, when they rise to 10%. By Jul-Aug-Sep, the probability for La Niña begins exceeding that for El Niño, and during the later portion of 2010 the La Niña and El Niño probabilities become 30% and 20%, respectively.

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 0.40 degrees C in boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal 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.

The models are in rough agreement in their ENSO forecasts through the first few seasons of the 10-month forecast period, but show large differences beginning in northern summer. The statistical and dynamical models agree in predicting weakening El Niño conditions during the next 1 to 3 months. However, the details of the timing and the rate of dissipation differ among models, and there is great disagreement in the outlook for the coming ENSO cycle from northern summer onward: Some models predict neutral ENSO, some La Nina, and some a second year of El Nino. For the current Mar-Apr-May season, 90% of the models are predicting El Niño conditions; none predict ENSO-neutral conditions. At lead times of 4 or more 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 of 13 (23%) indicate El Niño conditions for the Jul-Aug-Sep season, 9 of 13 (69%) predict ENSO-neutral SSTs, and 1 of 13 (8%) predict La Niña conditions. (Note 1). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.

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. This method shows probabilities for El Niño at about 92% and 50% for Mar-Apr-May and Apr-May-Jun, respectively, declining to 22% by Jun-Jul-Aug and 18% by Jul-Aug-Sep. Probabilities for La Niña increase to 20-30% from Jul-Aug-Sep onward through Nov-Dec-Jan 2010/11. 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. In particular, this approach considers only the mean of the predictions, and not the range across the models, nor the ensemble range within individual models.

The IRI's probabilistic ENSO forecast takes into account the indications of this set of models, the outcome of the standard error approach described above, and additional factors such as the very latest observations that may have developed after the initialization times of some of the models. It indicates an 85% probability for El Niño conditions in the Mar-Apr-May season in progress, decreasing to near 32% for May-Jun-Jul 2010 and to 20% by Aug-Sep-Oct and the seasons to follow, while La Niña probabilities rise to 30% beginning Sep-Oct-Nov, remaining there through the rest of 2010. Neutral ENSO conditions have at least 50% likelihood from May-Jun-Jul onward.