Current ENSO Information

Technical ENSO Update

16 May 2003

>	Current conditions
>	Expected conditions
>	Dynamics of La Nina onset

Current Conditions

SST anomalies have been rapidly moving through the neutral category and are nearly cool enough in the NINO3.4 region to constitute El Niño conditions. Residual positive SST anomalies in the central equatorial Pacific (near the dateline) continued to weaken during the last month, and are now close to average. Subsurface indicators confirm that the El Niño episode has ended rapidly and conditions may be reversing to a La Niña condition, as the equatorial thermocline is shallower than normal across the entire Pacific basin and this colder than average water has been upwelling to the surface in the eastern equatorial Pacific and advecting westward. Thus, not only have SSTs off the west coast of South America cooled sharply over the last two months due to the themocline shoaling and increased trade winds, but a rapid cooling of SSTs to below-average levels has extended westward to approximately 150W. Trade winds have been somewhat stronger than average across most of the basin, and the equatorial Southern Oscillation Index has become slightly positive.

Expected Conditions

For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.4 degrees C in Mar-Apr-May and as high as 0.75 degrees C in Oct-Nov-Dec. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced. There is some variation among ENSO model forecasts for the coming several seasons. The number of models that are forecasting El Niño conditions to be occurring in the May-Jun-Jul period, 2003, is 2 out of a total of 16, or 12%. The number of models that predict La Niña conditions is also 2 (12%). 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, 2 out of 12 (17%) call for SSTs of at least a minimum El Niño level for the May-Jun-Jul period, while 2 (17%) predict SSTs cold enough to be considered a La Niña. (Note 1). Overall, most of the models indicate that the now neutral to cool SST conditions in the NINO3.4 region will remain in the lower half of the neutral range, or cool further over the coming several months. The two models that call for warm SST conditions indicate a lesser magnitude of warmth than the two that call for cold SST conditions. For the longer lead forecast for Sep-Oct-Nov 2003, 1 of the 15 models (7%) forecasts El Niño conditions, 11 models (73%) forecast neutral conditions, and 3 models (20%) forecast La Niña conditions. If only the 11 models that use subsurface ocean temperature data are included, these figures become 1 (9%), 8 (73%), and 2(18%). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities for the coming several months. 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. The IRI's probabilistic ENSO forecast takes into account factors in addition to this set of models, and indicates a higher probability for La Niña onset than that seen from the tally of model forecasts discussed here. Some of the reasoning behind this is mentioned in the following subsection.

Dynamics of La Niña Onset

The cold SST anomalies in the east are a result of the cold subsurface temperature anomalies (or equivalently shallow thermocline anomalies) advecting to the surface via the equatorial upwelling currents under the action of the Trade Winds. This process linking the subsurface to the surface is most efficient in the eastern equatorial Pacific. The entire equatorial thermocline is shallower than normal, and shallow anomalies in the thermocline are also seen in the off-equatorial Western Pacific (which, by equatorial wave dynamics, will travel westward and be channeled at the western boundary on to the equator where they will then move eastward). This structure in the ocean will most likely act either to hold the tropical Pacific in neutral conditions if there are forces working to warm it (e.g. weather "noise" related to westerly wind bursts associated with the the Madden Julian Oscillation), or move it to La Niña conditions. The latter possibility will occur if ocean-atmosphere coupling is initiated by the currently observed cold SST anomalies in the eastern Pacific. Some coupling is already being observed currently. Therefore, the above normal temperatures currently remaining in the mixed layer of the central Pacific (near and just west of the dateline) should dissipate further, and possibly cool to below-normal temperatures. At the present time it is difficult to predict the strength of the La Niña if one occurs. Some idea of the strength should be possible once the possible event begins evolving, such as in the next two months.