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Technical ENSO Update

16 July 2009


>	Current conditions
>	Expected conditions

Current Conditions

As of mid-July 2009, SSTs are above-average across much of the equatorial Pacific, indicative of weak El Niño conditions. Between mid-June and mid-July, SSTs in the east-central tropical Pacific have warmed from borderline El Niño conditions well into the weak El Niño category. Although slightly above-average SSTs and convection related to the weak La Niña earlier this year have been slow to decay in the western tropical Pacific, spells of westerly wind anomaly have been observed in this same region, initially in mid-June and for a second time currently. The equatorial Southern Oscillation Index (SOI) has remained slightly positive, but the traditional SOI became negative during late May and early June, but to return to positive territory recently. More importantly, equatorial heat content has been at above-average levels over the last several months, and SSTs have become above average in the eastern and east-central tropical Pacific. Although the zonal gradient in Pacific SST has not reached levels sufficient to induce zonal wind and convection anomalies indicative of full fledged El Nino, positive convection anomalies have emerged in the vicinity of the dateline since early July, while such anomalies observed in recent months in Indonesia and the western Pacific have been decreasing. The deepened thermocline in the eastern part of the basin, and associated positive subsurface sea temperature anomalies, are expected to continue fueling increases in SST in the eastern Pacific, or at the very least to ensure maintenance of the current positive SST anomalies. While this heat content anomaly is not particularly strong in magnitude, it is zonally widespread and vertically extensive to 100-150 meters depth from 150W longitude to well west of the dateline. Although heat content anomalies exist in the western part of the basin as well, the strongest sea temperature anomalies (currently just over 3 degrees C) reside in th eastern part.

For June 2009, the SST anomaly in the NINO3.4 region was 0.62 C, sufficient to be classified as indicative of weak El Nino conditions, and for the Apr-May-Jun season they were 0.24 degrees C above average. 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 Jul-Aug-Sep and the Sep-Oct-Nov seasons are approximately (-0.50C, 0.45) and (-0.75, 0.70), respectively.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is 0.8 C, indicating weak El Niño conditions in the tropical Pacific. What is the outlook for the ENSO status going forward? Wind anomalies along the equator in the western Pacific are currently westerly, but in the central and eastern portions of the basin winds remain more random. The thermocline, constituting the main element of the tropical Pacific system imparting ENSO predictability, is somewhat deeper than average throughout the entire equatorial Pacific basin, but is deeper to the greatest extent in the central and eastern parts.

July is largely past the so-called "predictability barrier" of boreal spring, typically making ENSO predictions easier than several months ago for forecasts going through boreal spring. The emergence of weak El Nino conditions during June and July is often, but not always, followed by continued El Nino conditions of at least weak, if not greater, magnitude for the remainder of the calendar year and often into the initial months of the next calendar year. The existence of positive sea temperature anomalies below the surface in the eastern tropical Pacific is conducive to El Niño development, as the warmer than average waters at depth (reflected by a positive thermocline depth anomaly), currently strongest at longitudes of 100-130W, are poised to rise to the surface in response to the easterly low-level Trade winds. This process has been maintaining SST anomalies of 1.0 C at roughly these same longitudes along the equator over the last 5 to 7 weeks. Coupling between the warmed central and eastern Pacific and the overlying atmosphere has so far been limited, but not absent, and includes an area of enhanced convection centered near the dateline. Westerly wind anomalies have weakened slightly during the last two weeks, likely related more to intraseasonal phenomena more than to the slower processes associated with the evolving El Nino. Coupling will likely increase as SSTs in the western Pacific, still slightly above average, return toward average to the west of the region currently experiencing westerly wind anomalies. Atmospheric aspects of El Niño typically are not strongly manifest until later into the ENSO seasonal cycle (e.g. late August) except during the onset of very strong El Niños. The currently developing El Nino does not appear likely to become such a very strong event, based on the timing of its development and the current subsurface heat content.

Presently, the models indicate probabilities of about 82% for maintaining El Niño conditions, and about 17% for dissipation to ENSO-neutral conditions for the Jul-Aug-Sep season in progress. Going forward, probabilities for El Niño stay near 80% for the remaining 3-month periods in 2009, decreasing to approximately 75% by early 2010, and falling below 50% by boreal spring 2010. Probabilities for La Niña conditions are predicted to be negligible, not rising to above 10% until the middle of boreal spring 2010.

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 somewhat varied in their ENSO forecasts through the 10-month forecast period. The statistical models, which were notably reluctant to predicting El Niño as recently as mid-May, are now predicting warmer conditions with the emergence of appreciably warmer initial conditions. Many of the dynamical models have continued predicting El Niño conditions of varying intensities, a majority showing somewhat greater warmth in current forecasts than those from last month. For the current Jul-Aug-Sep season, about 80% are predicting El Niño conditions, and 20% predict ENSO-neutral conditions. This 4:1 ratio continues until early in 2010, when the odds begins to decrease. 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, 12 of 15 (80%) indicate El Niño conditions for the Nov-Dec-Jan season, and 3 of 15 (20%) predict ENSO-neutral SSTs. (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 85-90% for Jul-Aug-Sep and Aug-Sep-Oct, decreasing into the 75-80% range throughout the rest of 2009, and to near 50% during boreal spring 2010. 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 a 82% probability for El Niño conditions in the Jul-Aug-Sep season in progress, remaining near 80% through Dec-Jan-Feb 2009/10 and decreasing rapidly after the first few months of 2010.