More Technical ENSO Comments

Current Conditions

The basic ENSO indices moved decisively toward those associated with El Niño during May 2002. Between the third week of May and mid-June, the equatorial SSTs from 160 east to 115 west longitude rose to more than 1 degree C above normal. This warming, which included all of the NINO3.4 region and much of the NINO3 region, was a result of a large-scale weakening of the trades in late May and early June, mainly in association with a strong pulse of Madden-Julian Oscillation (MJO) activity. Above normal SST also continued in the western tropical Pacific, as well as in much of the Indian Ocean. In parallel with the weakening of the trades, the Southern Oscillation Index (SOI) decreased to more than one standard deviation below normal. The increased participation of the atmosphere in the developing El Niño condition signifies further movement toward a basin-wide El Niño episode. However, anomalous convection has been absent across much of the central and eastern equatorial Pacific despite a much warmed SST for at least 3 weeks, indicating a lack of a complete coupling of the ocean-atmosphere system expected in a more coherent El Niño episode. This may be a result of the fact that the SST in the western Pacific and also more than 10 degrees off the equator at some east-central Pacific longitudes is also significantly above normal, weakening the gradient of anomalous SST that would favor convergence toward the positive SST anomalies along the equatorial east-central Pacific. By mid-June, the weaker than normal trades had made a recovery toward normal strength, although anomalous westerlies continued in a reduced band within part of the NINO3.4 region. It is possible that anomalous convection will appear over portions of the anomalously warm SST region during the later part of June. This would promote the continuation of anomalous westerly wind, which would be expected to move the ocean-atmosphere system farther toward a more fully coupled El Niño state.

As a result of weaker easterly winds along the equator, the equatorial thermocline became flatter. In mid-June the themocline became deeper than normal in the eastern part of the basin. The deepened thermocline translates into sub-surface temperature anomalies as great as 4 degrees C above normal in the lower part of the mixed layer in the eastern equatorial Pacific. These temperature anomalies at depth will tend to reinforce the currently warm SSTs for at least the next mon th even in the absence of further enforcement by anomalous winds.

As the seasonal window of opportunity for El Niño development began to close over the last month, the increase in SSTs to levels beyond those required for minimal El Niño conditions helped strengthen the likelihood of at least a weak episode this year. The onset of partial participation of the atmosphere, and some buildup of positive subsurface temperature anomalies, are also favorable indicators.
 

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 Aug-Sep-Oct 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 9 months, the spread among the forecasts has decreased somewhat from one month ago. This month, near to or slightly greater than 50% of the models forecast 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, 50% to 60% of them are calling for SSTs to maintain at least a minimum El Niño level between August and October 2002.  Of the models not indicating appreciable warming, all indicate near-neutral conditions, and most of these call for slightly above normal SST (e.g. 0.3 to 0.5 degrees C above normal); 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 for the end of northern summer of 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 13 ENSO forecast models examined, 9 of them forecast at least a 0.7 degree C positive anomaly in the NINO3.4 regions for the end of 2002 (Nov-Dec-Jan), while the remaining 4 predict neutral (but slightly above average) SST levels. If we omit the models that do not explicitly use subsurface sea temperature data, the counts become 7 out of 9 versus 2 out of 9.

ENSO forecast models have a relatively modest level of skill for forecasts made during the February to April period. Now that it is June, skills are higher. 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. 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. Compared to one month ago, there has been a slight increase in the number of models that are forecasting at least weak El Niño conditions persisting into the second half of 2002. This is consistent with the observation of rising SSTs during mid-May to mid-June period, and the onset of at least partial participation of the atmosphere. As the seasonal window of opportunity for El Niño development began to close over the last month, the increase in SSTs to levels beyond those required for minimal El Niño conditions helped strengthen the likelihood of at least a weak episode this year. The onset of at least partial participation of the atmosphere is also a favorable indicator, although we still wait for an eastward shift in anomalous convection into the NINO3.4 region. Compared to one month ago, the range of likely scenarios has continued to narrow. In contrast to last month, when there was a non-trivial chance of having a moderate (or greater) strength El Niño, it now appears that the most likely outcome will be either a weak El Niño or a near-neutral condition during the remainder of 2002. Confidence in the forecasts has increased somewhat compared with what they were one month ago. This is consistent with the statistics of historical ENSO data that 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.
 

Requirements for El Niño Formation

A transition into warm ENSO conditions has progressed appreciably in the last month, 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 this 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 easterly trade winds in the central and eastern tropical Pacific have generally weakened, and 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 growth in area and intensity 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 setting up presently, but have not yet occurred in their entirety yet this year. The decreased variation among the forecasts of the different models reflects that ocean-atmosphere coupling conducive to El Niño is most likely. However, there is also more agreement among models that the strength of such an episode this year would likely be modest, with a slight chance of a moderate event. 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.
 

The Probability of an El Niño in 2002

IRI's assessment is that there is a 75% 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 well above the climatological probability of about 25% (i.e. one El Niño per 4 years), and is also an increase relative to the IRI's May statement in which the probability was estimated at 55%. It clearly still leaves uncertainty, although the range of possibilities has been narrowed, in that the probability of a moderate (or greater) strength El Niño is now reduced. If an El Niño does occur, its likely strength will be weak or possibly the lower portion of the moderate strength category. It would likely persist from the present time (subject to some temporary fluctuations, particularly in June or July) and last through March 2003.