Technical ENSO Update

19 April 2012

Recent and Current Conditions

Expected Conditions

What is the outlook for the ENSO status going forward? The official diagnosis and outlook was issued earlier this month in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI. It stated that La Niña conditions were still in force at that time. Now, in the middle of April, a new set of model ENSO predictions is available as shown in the IRI/CPC ENSO prediction plume, discussed below. The current east-central tropical Pacific SSTs are still below average, but only very slightly, and are no longer at the -0.45 threshold required to be indicative of La Niña. Subsurface temperatures across the equatorial Pacific not only weakened to near-average during March, but now in April feature more of a heat surplus than a deficit in the upper part of the ocean from the date line eastward to 100W. The thermocline depth along the equator is now above average in the western and central tropical Pacific as well as the far eastern portion, with only weakly shallower than average depth from about 100W to 150W. Acknowledgement of the dissipation of the weak La Niña condition that existed at the beginning of April is expected in the official consensus ENSO forecast and discussion to come in early May, as the warming that has occurred in the last two weeks is not anticipated to be a short-term fluctuation that will return to La Niña levels later in April or May.

As of mid-April, most of the dynamical and statistical models predict ENSO-neutral conditions for the Apr-Jun season, while only one or two continue to show a weak La Niña condition. All models then indicate warming from their starting anomaly values. For the Apr-Jun season, 8% of the models indicate La Niña conditions, while 92% indicate neutral conditions. Jumping forward to Jul-Sep, 62% indicate neutral conditions and 38% predict El Niño conditions. At lead times of 3 or more months into the future, statistical and dynamical models that incorporate information about the ocean's observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. For the Jul-Sep season, among models that do use subsurface temperature information, 55%  predict ENSO-neutral SSTs, and 45% predict El Niño conditions. (Note 1). Caution is advised in interpreting the distribution of model predictions 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 more quantitatively precise and less vulnerable to sampling errors than the categorical tallying method used above. This alternative method uses the mean of the predictions of all models on the plume, equally weighted, and constructs a standard error function centered on that mean. The standard error is Gaussian in shape, and has its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill results in a relatively narrower error distribution, while low skill results in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities for La Niña at 10% for Apr-Jun, decreasing to single digit probabilities between 5% and 10% through the remainder of 2012. Model probabilities for ENSO-neutral conditions are 88% for Apr-Jun, settling to near 50% from Jul-Sep onward. Probabilities for El Niño are near 1% for Apr-Jun, 20% for May-Jul, 35% for Jun-Aug, and 44% for Jul-Sep, remaining approximately in the 42-46% range for most of the remainder of 2012. A plot of the probabilities generated from this most recent IRI/CPC ENSO prediction plume using the multi-model mean and the Gaussian standard error method summarizes the model consensus out to about 10 months into the future. The same cautions mentioned above for the distributional count of model predictions apply to this Gaussian standard error 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 total range across the models, nor the ensemble range within individual models.

The probabilities derived from the more than 25 models on the IRI/CPC plume describe, on average, dissipation of the La Niña around initialization time (early April). This dissipation is confirmed by recent data showing the Nino3.4 index at negative levels weaker than the -0.45 C threshold for La Niña. Currently, for the middle and second half of the year, while neutral ENSO appears most likely, development of El Niño also has a reasonable chance (but still falling short of 50%) to occur. Redevelopment of La Niña appears very unlikely, according to the models, despite that a few models indicate cool-neutral conditions through the calendar year. Factors such as known specific model biases and recent changes that the models may have missed will be taken into account in the next official outlook, which includes some human judgement in combination with the model guidance, generated by CPC and IRI, to be issued early in May.

----- LN N EN -----Climatological Probabilities

DJF 37 28 35

JFM 34 37 29

FMA 30 48 22

MAM 26 54 20

AMJ 24 54 22

MJJ 25 51 24

JJA 25 50 25

JAS 27 46 27

ASO 29 40 31

SON 32 34 34

OND 34 31 35

NDJ 37 27 36

See also: 

• Summary of model predictions 

Note 1 - Only models that produce a new ENSO prediction every month are included in the above statement.
 
 

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