3/29/2012 12:48:03 PM. The 2011–12 La Niña event has ended; neutral conditions expected until at least early winterPacific Ocean key indicators return to neutral (neither El Niño nor La Niña), with values similar to those last seen in August 2011; atmospheric indicators such as cloudiness, trade winds and the Southern Oscillation Index (SOI) have also returned to near-normal values for this time of year. Despite the delayed start of the latest event in autumn 2011, its development has been fairly in line with past events, with a peak in January 2012 and a return to neutral conditions during spring.
El Niño and La Niña events tend to develop during the period April-June and they:
- Tend to reach their maximum strength during December-February;
- Typically persist for 9-12 months, though occasionally persisting for up to 2 years;
- Typically recur every 2 to 7 years.
All major models indicate the transition to neutral ENSO conditions to be concluded during the March-May period. For April-June, only 8percent of them continue to indicate La Niña conditions while 92percent foreseen neutral conditions. During the following period, July-September, 73percent indicate neutral conditions and 27percent predict the beginning of El Niño conditions.
Sea surface temperature (SST) anomalies in the central and eastern equatorial Pacific Ocean continued to warm in February. During the last 2 weeks, both warm anomalies (in the far western Pacific) and cool anomalies (in the central Pacific) are broadly becoming less pronounced with the return of neutral conditions. The SST anomaly map for the week ending 25 March shows mostly near-normal sea-surface temperatures in the equatorial Pacific, with some residual cool anomalies across the central equatorial Pacific.The four-month sequence of sub-surface Pacific Ocean equatorial temperature anomalies to 26 March shows that some cool anomalies remain in the sub-surface of the eastern Pacific, although they have continued to decrease during March. The map for the 5 days ending 25 March shows an increase in the volume of water more than 3 °C cooler than usual for this time of year, while there has been a general warming of waters closer to the surface. Shallow warm anomalies in the far eastern tropical Pacific and warm sub-surface anomalies in the western Pacific remain relatively unchanged compared with two weeks ago.The SOI rose early in the last fortnight before holding around +7 for the past week, remaining within values indicative of neutral ENSO conditions; the SOI has remained within neutral values since mid to late February. Sustained positive values of the SOI above +8 may indicate a La Niña event, while sustained negative values below -8 may indicate an El Niño event. Values of between about +8 and -8 generally indicate neutral conditions.Trade winds have weakened over the past two weeks. Across most of the equatorial Pacific wind anomalies are near normal. During La Niña events, there is a sustained strengthening of the trade winds across much of the tropical Pacific, while during El Niño events there is a sustained weakening of the trade winds.Cloudiness near the dateline has remained suppressed over the past two weeks, but is returning to values typical for this time of year. Cloudiness along the equator near the dateline is an important indicator of ENSO conditions, as it typically increases during an El Niño event and decreases during a La Niña event.
Sources: IRI, NOAA, BOM

compiled by WFP Emergency Preparedness and Response

Source: IRI Columbia.

Source: Climate Prediction Center - NOAA.

Source: Weather Underground.