SST anomalies decreased quickly

in the
central and eastern Pacific recently

Jun. 17, 2003

Since May 2002 when the 2002/2003 El Nino came into being, the unusual warming
conditions developed gradually in the central and eastern equatorial Pacific
though cooling conditions once appeared in the offshore of Peru. The El Nino
peaked in November 2002. During December 2002, the tropical atmospheric and
oceanic conditions still exhibited characteristics of peak phase. However, the
El Nino weakened quickly since January 2003 and ended in March 2003(Fig.1). This
El Nino event is characterized by onset in spring, short duration, moderate intensity
and the warming center close to the date line.

Fig
1   time-longitude cross section of equatorial SST anomalies(unit: ℃)

Fig
2  distribution of SST
anomalies in May 2003(unit: ℃)

Figure 1
also shows that, after the 2002/2003 El Nino, SST anomalies decreased quickly
in the central and eastern Pacific, and cold water even appeared steadily in
parts of this region. During May, negative SST anomalies across the eastern and
central equatorial Pacific obviously enhanced and extend westward. Negative SST
anomalies below –0.5℃
controlled the region east to 150°W, with the maximum below –3.0℃. The
SST indices of NINO 4, NINO 3 and NINO 1+2 decreased to 0.2℃, –0.9℃and –1.7℃,with drops of 0.3℃, 0.6℃and
0.6℃ from last month, respectively. In
addition, positive SST anomalies about 0.5℃ maintained in the western equatorial Pacific and most
of the equatorial Indian
Ocean. Meanwhile,
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.

2003

2002

Fig.
3  Southern Oscillation Indices(SOI)
since January 2002

However, during recent months tropical
atmosphere shows obvious oscillation. In April, the Southern Oscillation Indices(SOI) was -0.1 which is
close to normal，while it dropped
to a large negative value of -0.5 in May(Fig.3). At the lower troposphere (850hPa), zonal wind anomalies
display very strong intra-seasonal oscillation over the western equatorial Pacific. After
strong easterly anomalies appeared during later March, ever stronger and more
spread easterly anomalies appeared again from end of April to
early May. However, from middle May to early June they changed into very strong westerly
anomalies(Fig. 4). During the same period, convection activities changed
significantly over the equatorial Pacific, with convections being obviously
suppressed over the eastern equatorial Pacific and convections close to normal
over the western and central equatorial Pacific.

In
conclusion, tropical oceanic condition reflects obviously characteristics of
cold phase, while tropical atmospheric condition displays a slow response to
this. It is difficult to
predict whether this cooling is a temporarily seasonal fluctuation or the start
of a La Nina event. The development in the tropical Pacific will continue to be
carefully monitored.

Fig.4 time-longitude cross
section of 850hPa equatorial zonal

wind anomalies Ua（unit：m/s）

Ua >0 means westerly anomaly, Ua <0 means easterly anomaly