Further evolution toward a Pacific basin-wide warm episode (El Niño) was observed during May 2002, as sea surface temperature (SST) anomalies increased throughout the equatorial Pacific between 170°E and 95°W (Fig. 1 and Fig. 2). By the end of May warmer-than-normal SSTs were observed over a large portion of the equatorial Pacific (Fig. 2, bottom panel), with departures from average greater than +1°C in the region from 165°E to 135°W and also locally between 130°W and the South American coast. This warming represents a significant transition from the localized warmth (central equatorial Pacific and South American coast) earlier this year, observed during recent months, toward a more extensive basin-wide warming typical of El Niño/Southern Oscillation (ENSO) events.

This warming resulted from a significant weakening of the equatorial easterly winds throughout the Pacific related to the Madden-Julian Oscillation (MJO). The MJO is an important source of variability that can contribute to the evolution toward El Niño through related fluctuations in low-level winds and precipitation over the western and central equatorial Pacific. An eastward-propagating oceanic Kelvin wave, initiated by strong MJO activity in late 2001, resulted in the rapid warming that was observed along the coasts of Ecuador and northern Peru in early February. Subsequently, MJO activity was relatively weak during early 2002, with no additional significant Kelvin wave activity during that period. However, in late April 2002 the MJO intensified and during the last half of May the low-level equatorial easterly winds substantially weakened throughout the Pacific, accompanied by an increase in equatorial SST anomalies (Figs. 1, 2, and 3). NOAA TAO buoy data indicate that the oceanic thermocline has deepened east of the date line (180°W) since mid-May, consistent with the basin-wide weakening of the low-level equatorial easterly winds. This deepening may be an indication of renewed Kelvin wave activity.

Given the recent strength of the MJO and its period of about 40 days, it is likely that significant month-to-month fluctuations will continue to occur in many atmospheric indices used to monitor the ENSO cycle. In spite of this variability, the overall trends in SSTs and some atmospheric indices (Southern Oscillation Index, 850-hPa zonal wind) in recent months indicate that further development of El Niño will continue. Consistent with this assessment, most statistical and coupled model forecasts indicate that weak-to-moderate El Niño conditions will continue through the end of 2002.