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ENSO status and forecast for summer 2009 – winter 2010
The latest ENSO status is in a transition from a weak-to-moderate La Niña event to a neutral state. Negative SST anomaly in the Nino3.4 region weakened from -0.95 in January to -0.5 in March (Fig. 1). The SOI dropped sharply from 1.8 in February to -0.1 in March (Fig. 2). According to the CPC ENSO Advisery a transition to ENSO-neutral conditions will continue.
There is much uncertainty, however, about where the ENSO cycle is heading. The signals are mixed, with some predictors indicating a possibility of an El Niño event later this year, while others suggesting that the La Niña event may linger even longer.
Thus, the animation of subsurface temperature anomalies along the equator shows that a positive anomaly, which was sitting for a long time in the area west of 170°W at the depth of about 100m, recently shifted eastward and closer to the surface, as prior to El Niño events. The evolution of the SOI so far is similar to a typical transition pattern to an El Niño event (Fig. 2). The fastest rate of change from a positive to negative SOI values usually occurs during spring months prior to the onset of a El Niño event. Warm water volume (WWV) between 120°E and 80°W increased in the past several moths and its March anomaly value became positive. The WWV and Nino 3.4 indices are highly correlated (r ~ 0.7), with WWV leading by 8-12 months. Also, the global temperature and precipitation anomaly pattern in recent months are more closely resembles the ones prior to the onset of an El Nino event.
On the other hand, some other predictors suggest either ENSO-neutral conditions or a repetition of the previous year scenario, when the La Niña weakened by mid-summer 2008, but then, quite unexpectedly, strengthened again. One of the main ENSO predictors is the Hawaiian index that is significantly correlated with Nino 3.4 SST anomalies at a lead time of about one year. Historically, when the Hawaiian index was greater than 0.4 (there were a total of 20 such years since 1949), no El Niño events were observed one year later. The index value in November-March 2009 was 0.89, i.e., the chances of having an El Niño event during the winter of 2010 are quite slim.
A majority of model forecasts issued by the CPC for the Niño-3.4 region show that once ENSO-neutral conditions are reached, it will continue through the remainder of 2009. The CPC consolidated forecast suggests ENSO-neutral conditions may continue through Apr-May-jun 2010. Outputs of 22 models provided by the IRI show a wide spread of possible scenarios. The CFS (which is probably the best model currently available) predicts La Niña will end during April 2009, followed by positive SST anomalies in the Niño 3.4 region during the last half of 2009.
In the next 2-3 months it will become more clear which route will be taken by the ENSO cycle. Stay tuned.