Oceanic and atmospheric conditions in the tropical Pacific
during May were consistent with a developing cold episode. Negative
sea-surface temperature anomalies strengthened across the central and
eastern equatorial Pacific during May, as significant decreases in SST
anomalies occurred in all of the Ni�o
regions (Fig. 1). By early June equatorial SSTs were near or below normal
between 165�W
and the South American coast, with only a small area of residual positive
SST anomalies west of the date line between 155�E
and 175�E (Fig. 2).
Consistent with the trend in SST anomalies, the equatorial easterlies
have been near or slightly stronger than average over the central and
west-central equatorial Pacific, and the equatorial SOI has been slightly
positive during the last two months. In addition, in recent months the depth
of the oceanic thermocline has steadily decreased across the central and
eastern equatorial Pacific, and negative subsurface temperature departures
have developed and intensified in the upper ocean of this region (Fig.
3).
By mid-May subsurface temperatures at thermocline depth were below average
throughout the eastern Pacific, with negative anomalies ranging between -1�C
and -4�C (Fig. 3, bottom panel). These observed trends in oceanic and atmospheric variables
indicate that a transition to La Ni�a
is underway and that La Nina conditions are likely to develop over the next
few months.
Most statistical and coupled model forecasts indicate that either
near-neutral or La Ni�a
conditions will occur during the last half of 2003. However, current
conditions and recent trends favor the development of cold episode (La Ni�a)
conditions in the tropical Pacific during the next few months.