Atmospheric and oceanic conditions in the tropical Pacific indicated a further strengthening of warm episode conditions during may. Positive sea surface temperature (SST) anomalies have increased dramatically in the central and eastern equatorial Pacific during the past few months, particularly near the South American coast. In May, SSTs greater than 29C were observed from Indonesia to 160W. Equatorial SST anomalies exceeded +1.0C from 175E eastward to the South American coast, with values greater than +4.0C observed in the extreme eastern Pacific. Consistent with this warmth, the average anomaly in the Niño 1+2 region was +2.9C during May, which is the largest value observed since August 1983.

All of the atmospheric indices typically used to monitor the Southern Oscillation are consistent with strengthening warm episode conditions. During May, enhanced tropical convection was observed near the date line, with suppressed convection over the western equatorial Pacific and Indonesia. Consistent with this pattern, large low-level westerly wind anomalies were observed from Indonesia to just west of the date line.

The sea level pressure (SLP) pattern during May featured negative anomalies throughout the central and eastern tropical Pacific and positive anomalies over the western Pacific. Consistent with this pattern, the Southern Oscillation Index (SOI) was -1.8, with negative SLP anomalies observed at Tahiti (-1.8) and positive anomalies observed at Darwin (+1.0). This is the third consecutive month with a large negative value of the SOI.

In keeping with recent trends, the latest NCEP statistical (CCA) and coupled model predictions indicate that SST anomalies in the tropical Pacific should continue to increase, with warm episode conditions intensifying during the remainder of the year. Given the magnitude of the SST and OLR anomalies, it is likely that atmospheric circulation anomalies typical of those generally experienced during warm episodes should develop during the next several months. In particular, it is expected to remain drier than normal over Indonesia and India, and wetter than normal over the central tropical Pacific. Also, a stronger than normal South Pacific jet stream should develop and extend farther east than normal. Thus, central Chile and central Argentina, Uruguay and southern Brazil are likely to experience a wetter than normal winter.

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