Saturday, August 06, 2011

Latest ENSO Status and Expectations:

Neutral ENSO conditions persist in the tropical Pacific.

The majority of international climate model forecasts of ENSO show that neutral conditions are likely to continue for the remainder of the southern winter and into spring with the chances of an El Niño developing now considered unlikely.

In fact, more models are now predicting further cooling over the coming season.

Most of the set of dynamical and statistical model predictions issued duringlate June and early July 2011 predict neutral ENSO conditions from the July-September 2011 season currently in progress, through the early part of 2012. However, a minority of models call for a re-emergence of La Nina conditions going forward, while even fewer models suggest the development of El Nino conditions. For the July-September season the probabilities for La Nina, neutral and El Nino conditions are estimated at 10%, 82% and 8%, respectively. Probabilities for La Niña rise to 25% or slightly higher from Oct-Dec into 2012.

What is the outlook for the ENSO status going forward?

Here is a summary Chart of what all the models predict.

The currently near-average subsurface temperatures across the equatorial Pacific imply fairly low probabilities of returning to La Niña conditions in the short term, and also only a slight possibility for El Niño development during the coming few months. The continuation of an ENSO-neutral state appears most likely for the remainder of 2011.
However, some uncertainty nonetheless remains, and one cannot rule out the possibility for re-emergence of weak La Niña or development of weak El Niño in the September to November time frame. Between those two minor possibilities, re-emergence of weak La Niña appears relatively more likely, as a pocket of weakly below normal subsurface sea temperature has developed between 110W and 140W longitude. This pocket of cooler water must be monitored carefully in the coming weeks.

The IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Jul-Aug-Sep and the Aug-Sep-Oct seasons are approximately (-0.50C, 0.45) and (-0.55, 0.50), respectively.


The 30-day SOI to 31 July was +10.7.

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