More Technical ENSO Comments

Current Conditions

El Niño conditions continued during late June and early August. During this time, equatorial SSTs from 175E to 135W longitude remained at or above1 degree C above normal, and anomalies of 1.5 degrees C or more stretched from the dateline to 155W. This warming, which includes much of the NINO4 and NINO3.4 regions and the western part of the NINO3 region, was fueled by repeated episodes of weak to moderate westerly wind anomalies, some in association with Madden-Julian Oscillation (MJO) activity. The Southern Oscillation Index (SOI) continued to be somewhat below normal. After a slow start, significant anomalous convection has now appeared in the central equatorial Pacific in response to the sustained positive SST anomalies there. By early August, westerly wind anomalies began responding to the shifted convection. The SST, along with the participation of the atmosphere, suggests we are in the early stages of a basin-wide El Niño episode. The eastward migration of deep convection indicates a more fully developed El Niño episode than that of one month ago. Due to this shifted convection and the associated zonal circulation changes, above normal SST that had dominated the far western tropical Pacific has begun weakening.

As a result of the westerly wind anomalies along the equator, the equatorial thermocline has become flatter from west to east. In early August the themocline remained deeper than normal in the central and east-central part of the basin. Sub-surface positive temperature anomalies of 2 to 4 degrees C in portions of the mixed layer accompany the deepened thermocline. These temperature anomalies at depth will tend to reinforce the currently warm SSTs for at least the next month. In the far eastern tropical Pacific, from 80W to 115W, somewhat below normal SSTs and a slightly shallower than normal thermocline developed during July. This feature is not considered at odds with the continued development and maturation of the El Niño, and has already begun to warm again in response to Kelvin waves generated by the above-mentioned westerly wind activity.

There is now only a negligible possibility that the above normal SST in the central equatorial Pacific could weaken toward normal over the next few months. The continued presence positive SST anomalies of greater than 1 degree C, the persistence of westerly wind anomalies in the central Pacific, continued below normal SOI, the newly developed eastward-shifted convection, and the presence of positive subsurface temperature anomalies all favor the continuation of the current SST conditions through the end of the year.
 

Expected Conditions - See summary of model predictions

For purposes of this discussion, neutral SST conditions are defined as the sea surface temperatures (SSTs) in the NINO3.4 region averaged over the Oct-Nov-Dec period (for which model forecasts are highlighted here) being more than -0.6 and less than 0.6 degrees C, while SSTs of at least this magnitude would be indicative of La Niña (if negative) or El Niño (if positive). This definition allows about half of all years to be classified as neutral.  Although there is some variation among ENSO model forecasts for the coming 8 months, the spread among the forecasts is not large. Compared to one month ago, there has been a slight increase in the already high number of models that are forecasting El Niño conditions continuing through the second half of 2002 and into early 2003. This month, 80% of the 15 models forecast a continuation of at least weak El Niño conditions. At lead times of more than 4 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, about 85% are calling for SSTs to maintain at least a minimum El Niño level between October and December 2002.

Of the models not indicating appreciable warming, all indicate near-neutral conditions. (Note 1) Overall, the ensemble of the available model forecasts implies a high likelihood for El Niño conditions of weak to moderate strength to continue through the remainder of 2002. If El Niño conditions are still present at 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 but often persist into several months of the following year. Out of the 14 ENSO forecast models examined, 11 of them forecast at least a 0.6 degree C positive anomaly in the NINO3.4 regions for the beginning of 2003 (Jan-Feb-Mar), while the remaining 3 predict neutral SST anomalies. If we omit the models that do not explicitly use subsurface sea temperature data, the counts become 9 out of 10 warm, versus 1 out of 10 neutral. Because of the persistence of the warm SST observed in the NINO3.4 region since late May, our assessment of the probability of an El Niño is even greater than that reflected collectively in the models. The onset of a more complete participation of the atmosphere is indicative of El Niño maturation. It appears very likely that the El Niño SST conditions seen over the last 2 to 3 months will remain in place for the next 6 to 8 months. Confidence in the forecasts is high, as the statistics of historical ENSO data indicate that the observations during northern summer often set the stage for what is to be expected for the remainder of the year and into the beginning of the following year.

ENSO forecast models have a relatively modest level of skill for forecasts made during the February to April period. Skills are considerably higher for forecasts made in August. Caution is still advised, however, in interpreting the distribution of model forecasts as the actual probabilities for the ENSO conditions expected for the later parts of this year. The expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the above probability estimate from the true probability.
 

Requirements for Basin-wide El Niño Formation Now Nearly Fulfilled

The onset of warm ENSO conditions has taken place over the last two months, as the equatorial SSTs from west of the dateline to the middle of the NINO3 region have exceeded levels that rank in the top 25% of the historical climatological distribution for this time of the year. When top-quartile SST levels are reached during northern spring or early summer, El Niño conditions become attainable, as the Pacific ocean-atmosphere system is most conducive to ENSO phase changes at that time of year. One of the two major atmospheric behaviors that are required in parallel with El Niño-associated SST behavior has taken place: The first is that the easterly trade winds in the central tropical Pacific have somewhat weakened with the repeated occurrences of westerly wind anomalies, and correspondingly, the SOI has begun to assume negative values. However, the second behavior is lacking, as the deep convection that is normally limited to the western and central tropical Pacific has not yet migrated eastward over the warmed SST in the east-central Pacific. The area and magnitude of the warm water, and the associated atmospheric changes, occur if the ocean/atmosphere dynamical feedbacks are mutually enhancing, in the following manner: 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. Such feedbacks appear to be gradually developing, but have not yet been completed due to the lack of a significant weakening of easterly trade winds in the eastern Pacific, and a significant eastward shift of convection. The small to moderate variation among the forecasts of the different models reflects that ocean-atmosphere coupling conducive to El Niño is most likely. There is also agreement among models that the strength of such an episode this year would most likely be much weaker than the episodes of 1982-83 and 1997-98. While El Niño episodes can develop after June and even occasionally after July, their probability of development drops sharply if they have not shown strong signs of development by the end of June. Similarly, existing El Niño SST conditions are unlikely to decay after June or July: they tend to remain at those levels through the end of the year rather than to move back toward average. Fluctuations on a shorter time scale (week to week) are to be expected.
 

The Probability of an El Niño in 2002

IRI's assessment is that there is a 97% probability of an El Niño this year, 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 ENSO. This is an increase relative to the IRI's July statement in which the probability was estimated at 90%. There is still some uncertainty regarding the strength of this El Niño as it fully matures over the coming several months. The probability of a strong El Niño of high strength (e.g. 1997-98) is low. The magnitude of the NINO3.4 anomaly at maturity is likely to be between 0.5 and 2 degrees C, placing it into the weak to moderate range. It would likely persist through March or April 2003.

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