ENSO 2009-10: Chances of El Niño increased

Summary: Recent development of oceanic and atmospheric processes in the equatorial Pacific corresponds to a transitional phase to an El Niño event. SSTs in the equatorial Pacific Ocean continue to increase, with positive SST anomalies across much of the region. The upper ocean heat content also continues its upward trend. The Hawaiian index has dropped sharply in May, and the westerly winds intensified. All these increased the confidence factor that El Niño conditions are going to be present during the winter of 2010 (CF = 70). Most likely, however, it will be a weak El Niño event, similar to that of 1963-64.

Discussion

Fig. 1. SST anomalies in the 3.4 and other Nino regions.

SSTs in the equatorial Pacific Ocean continue to increase. Since the beginning of May 2009, positive SST anomalies have been observed in all Niño regions (Fig. 1). The upper ocean heat content also continues its upward trend. However, the warm water volume (WWV) in May 2009 was at about the same level as this month last year. As one may remember, during the summer of 2008  the transition to El Niño conditions halted, and a weak La Niña event developed instead. On the other hand, the value of the WWV index in May 1982 was also about the same as this year, and a strong El Niño event developed that year. Therefore, despite the high correlation coefficient between the May WWV index and Niño-3.4 (N3.4) index in October-February (r = 0.69 for the period 1980-2008), this relationship is not very reliable for the middle range of variations of the WWV index.

There continues to be considerable spread in the model forecasts for the Niño-3.4 region. Most dynamical models, including the NCEP Climate Forecast System, predict the onset of El Niño during June - August 2009. However, all statistical models predict that ENSO-neutral conditions will continue for the remainder of 2009. Based on recent trends in the observations and model forecasts, the CPC forecast calls for ENSO-neutral conditions to continue into the Northern Hemisphere Summer.

Fig. 2. Correlation between the N3.4 index in May (first differences) and October-February.

Some of our statistical rules suggest that El Niño conditions are likely during the winter of 2009/10. For example, the correlation coefficient between the N3.4 index in October-February and the same index in the preceding May is 0.56 for the period 1950-2008. However, the correlation coefficient increases to 0.80 if the first difference of May N3.4 index is used instead (rule 570). Based on this rule, the October-February value of the index is projected to be close to +1 (Fig. 2).

In our previous forecast issued on April 21st, the Hawaiian index was showing a possibility of La Niña conditions to develop again by the winter of 2010. In May 2009, the index sharply decreased, thus shifting the odds in favor of El Niño.

The westerly wind index also increased recently; however, it still remains below the long-term average. This suggest that, although El Niño conditions are expected during the winter of 2010, it will likely be a weak El Niño event, similar to that of 1963-64. That was the only El Niño event in the past 60 years that occurred with the westerly wind index that low during the month of May.