IRI/CPC ENSO Quick Look | Download PDF version
Technical ENSO Update
15 March 2012
Recent and Current Conditions
Weak La Niña conditions emerged in early August 2011, became weak/moderate strength during northern autumn 2011, stayed at that level through most of northern winter, and returned to a weak level beginning in mid-February 2012. As of mid-March 2012, SST anomalies remain in weak La Niña territory in the central and east-central equatorial Pacific, but have become warmer than average in the eastern part of the basin. For February the SST anomaly in the NINO3.4 region was -0.69 C, indicative of weak La Niña conditions, and for the December-February season the anomaly was -0.94 C. Starting in December 2011, the IRI's definition of El Niño conditions began to follow that of NOAA/Climate Prediction Center, in which the SST anomaly in the NINO3.4 region (5S-5N; 170W-120W) exceeds 0.45 C. Similarly, for La Niña, the anomaly must be -0.45 C or less. The climatological probabilities of La Niña, neutral, and El Niño conditions vary seasonally, and are shown in a table at the bottom of this page for each 3-month season. The most recent weekly SST anomaly in the NINO3.4 region is -0.7 C, indicating weak La Niña conditions in the tropical Pacific; this is consistent with the -0.69 C level observed in February.
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. Now, in the middle of March, 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 have continued to be below average and at a level indicative of La Niña, but have been warming toward the -0.45 threshold that would mark reentry to neutral conditions. Subsurface temperatures across the equatorial Pacific have weakened to near-average during late February and early March, both because of weakening of the cold pool beneath the surface in the east-central tropical Pacific and the development of above-average water temperatures near and at the surface in the eastern portion. The currently weak La Niña condition is expected to dissipate during late March or early April as the coupled system returns to neutral and the west-to-east SST anomaly gradient from just west of the dateline to near the South American coast weakens due to the expected dissipation of enhanced low level easterlies west of the dateline and weakening of the current westerly anomalies associated with the above-average SST in the eastern part of the basin.
As of mid-March, most of the dynamical and statistical models predict below average, but ENSO-neutral, conditions for the Mar-May season, while only a few continue to show weak La Niña condition. All models indicate warming from their starting anomaly values. For the Mar-May season, 21% of the models indicate La Niña conditions, and 79% indicate neutral conditions. For Apr-Jun, these figures become 8% and 92%, respectively. Jumping ahead to Jul-Aug-Sep, 73% indicate neutral conditions and 27% predict El Niño conditions. At lead times of 4 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. Among models that do use subsurface temperature information, 76% predict ENSO-neutral SSTs for the Jul-Sep 2012 season, none predict La Niña conditions, and 24% predict El Niño conditions. See 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 26% for Mar-May, decreasing to 17% for Apr-Jun, 13% for May-Jul, and down to 11% by Jun-Aug. Model probabilities for ENSO-neutral conditions are 74% for Mar-May, rising to 78% by Apr-Jun and 67% for May-Jul. Probabilities for El Niño are near 0% for Mar-May, 5% for Apr-Jun, 20% for May-Jul, and 32% by Jun-Aug. 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 20 models on the IRI/CPC plume describe, on average, a dissipation of the La Niña during the March-April period of 2012, with highest liklihood of dissipation near the end of March or beginning of April. Past experience has shown that some models are biased in the direction of prolonging ENSO episodes for somewhat too long a period at the end of the typical ENSO cycle. The current model predictions do not appear to be materially affected by this bias, as most of them show neutral ENSO conditions (even if still slightly below average) for March-May, when current conditions in mid-March are still indicative of weak La Niña conditions. Thus, most of the models are indicating imminent dissipation. Currently, for the second half of the year, neutral ENSO appears most likely, and development of El Niño has a higher predicted likelihood than a return to La Niña. However, factors such as known specific model biases and recent changes that the models may have missed will be taken into account in the official outlook, which includes human judgement in combination with the model guidance, generated by CPC and IRI and to be issued at the beginning of April.
Using the 0.45 C thresholds, the climatological probabilities of La Nina, neutral, and El Nino conditions for each 3-month season are as follows:
See also: Summary of model predictions