More Technical ENSO Comment

The basic ENSO indices were neutral to slightly warmer than neutral in March 2002. However, anomalously warm SST conditions emerged in the equatorial Pacific by the end of the second week of April, with a band of SSTs of 0.5 degrees C or more above average stretching from the coast of South America to west of the dateline.  For purposes of this discussion, neutral SST conditions are defined as the sea surface temperatures (SSTs) in the Nino 3.4 region averaged over the Jul-Aug-Sep period (for which model forecasts are highlighted here) being between -0.6 and 0.6 degrees C, while less than -0.6 would be indicative of La Nina and more than 0.6 indicative of El Niño.  This definition allows about half of all years to be classified as neutral.  March's neutral condition is reflected not only in the SSTs in the central and eastern tropical Pacific Ocean (with Nino 3.4 SST anomaly of 0.2 degree C), but also in the low level winds across the equatorial Pacific and the Southern Oscillation Index (SOI).  A more detailed look at the SSTs shows that in the vicinity of the date line in the central tropical Pacific they have been above average by over 1 degree C during March, having continued from earlier months.  The waters of the far eastern equatorial Pacific had warmed to more than 1.5 degrees C above average in February (with locally stronger anomalies immediately off the coasts of Ecuador and northern Peru).  The substantially warmer than average sub-surface sea temperature across the tropical Pacific that has been seen throughout most of the year weakened somewhat in March as some of it came to the surface.  This warm subsurface water had been further enhanced by strong westerly winds in the far western Pacific in December, resulting in substantially increased SST in early February in the far eastern tropical Pacific. Significant westerly wind bursts in the western and central equatorial Pacific have not been observed since the above-mentioned major wind event. However, additional westerly wind bursts over the course of the next one month could further enhance the likelihood and strength of an El Niño in 2002.

Expected Conditions - See summary of model predictions 

There is some variation among ENSO model forecasts for the coming 9 months. Compared to one month ago, a larger proportion of the models has begun forecasting at least weak El Niño conditions. At long lead times of 6 to 9 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, between half and two-thirds of them are calling for warming to at least the minimum El Niño level to take place (or continue) between May and July of 2002.  Of the models not indicating appreciable warming, all indicate a continuation of near-neutral conditions; none indicate cold conditions.  (Only models that produce a new ENSO forecast every month are included in the above statement.)  Overall, the ensemble of the available model forecasts implies a moderately high likelihood for El Niño conditions of weak to moderate strength by mid-2002.  If El Niño conditions do develop by that time, a continuation of such conditions would be expected through at least March of 2003, as El Niño events tend to reach their peak magnitude near the end of the year.  Out of the 14 ENSO forecast models examined,  9 of them forecast at least a 0.7 degree C positive anomaly in the Nino 3.4 regions for the second half of 2002 (e.g. Sep-Oct-Nov), while the remaining 5 predict neutral SST levels. If we omit the models that do not explicitly use subsurface sea temperature data, the counts become 8 out of 10 versus 2 out of 10.  Because of the relatively modest level of forecast skill ENSO forecast models at this time of the year, caution is advised in interpreting the distribution of model forecasts as the actual probabilities for the ENSO conditions expected for the later parts of this year. In addition to the overall skill, the expected skill of one model versus another has not been established using uniform validation procedures; this may also cause a difference of the above probability estimate from the true probability. Confidence in the model forecasts has increased somewhat compared with what they were one month ago, and will increase further in the May forecasts.

Requirements for El Niño Formation

A transition toward warm ENSO conditions appears to be occurring, as the equatorial SSTs between the dateline and the South American coast have generally attained levels that rank in the top 25% of the historical climatological distribution for this time of the year, and this is taking place between April and June when the Pacific ocean-atmosphere system is most conducive to ENSO phase changes.  Two major atmospheric changes are still required in parallel with this SST behavior: the easterly trade winds in the central and eastern tropical Pacific must weaken or even reverse direction, and deep convection that is normally limited to the western and central tropical Pacific must migrate eastward over the warmed SST in the east-central Pacific.  Such growth in area and intensity of the warm water, and the associated atmospheric changes, occur if the ocean/atmosphere dynamical feedbacks are mutually enhancing, as follows:  The convection associated with regions of positive SST anomalies east of the dateline leads to a pervasive weakening of the easterly trade winds. The convection adds heat to the atmosphere, resulting in a weaker east-west equatorial temperature gradient.  The weaker trade winds then allow additional warm water from the western Pacific to move eastward, further weakening the temperature gradient, leading to further reduction in the trade winds, etc.  Uncertainty still lies in whether the above-described sequence of coupling events will take place between the tropical Pacific ocean and atmosphere to a sufficient extent to allow the warm SST to continue (or strengthen further) rather than return to more climatological values.  The variation among the forecasts of the different models is a reflection of this uncertainty.  The next one to two months are the time of year in which the climate system is most amenable to the growth of El Niño events through the above-described positive feedback of ocean and atmosphere dynamics.  If such feedback occurs, there would come a 'point of no return', normally by June, in which an El Niño would definitely be in progress and would most likely last for the the next several seasons thereafter. In the next month or two, the certainty will increase as to whether the events leading to a positive ocean-atmosphere feedback are taking place.  This will enable us to state more definitively whether the current warming in the central and eastern half of the tropical Pacific basin, and an as-yet nonexistent widespread pattern of westerly wind anomalies and eastward-expanded tropical Pacific convection (the hallmarks of El Niño) will be with us for the rest of 2002 and into early 2003.

The Probability of an El Niño in 2002

IRI's assessment is that there is a 70% probability of an El Niño by mid-2002, lasting into early 2003. This assessment is based on the collective forecasts of many computer models of various types, as well as on the experience of the several oceanographers and atmospheric scientists familiar with the ENSO phenomenon. This is far above the climatological probability of about 20-25% (i.e. one El Niño per 4 to 5 years), and is also a slight increase relative to the IRI's March statement in which the probability was estimated at 65%. Nonetheless, it still leaves some uncertainty, since substantial participation of the atmosphere in this potential El Niño has not commenced. If an El Niño does occur, its likely strength is uncertain at this time, but on the basis of the current forecasts of dynamical and statistical ocean/atmosphere models as well as on empirical indicators it would appear weak to moderate rather than very strong. It would likely form during the late April through June period, and last until March 2003.