Tropical
Highlights - May 2002
The evolution toward a Pacific warm episode (El
Niño) continued during May 2002 as sea surface temperature (SST) anomalies increased
throughout the central equatorial Pacific (Table T2, Fig. T9). Sea surface temperatures averaged above normal
across the equatorial Pacific west of 120°W during the month, with the largest anomalies
exceeding 1°C near the date line (Fig. T18). By the end
of the month, SSTs were more than 1.0°C above normal across most of the equatorial
Pacific between 160°E and 105°W (not shown).
Although the oceanic thermocline shoaled across much of the equatorial Pacific during
mid-April to mid-May, it remained deeper than normal west of 120°W (Fig.
T15) with the largest depth anomalies located between 170°W and 140°W (Figs. T15, T16). Ocean temperatures at
thermocline depth remained above normal west of 120°W, and were slightly below normal
between 120°W and the South American coast (Fig. T17).
Tropical convection during May continued to be strongly influenced by the Madden-Julian
Oscillation (MJO). Convection was enhanced over the western equatorial Pacific during the
month and suppressed over the remainder of the Tropics, especially in the region extending
from central equatorial Africa eastward to Indonesia (Fig. T25).
The atmospheric circulation across the tropical Pacific has been strongly influenced by
MJO activity for the past six months. The strong MJO activity during late April and May (Fig. T12) contributed to a weakening of the low-level
(850-hPa) easterly winds across the equatorial Pacific during the second half of May (Fig. T13), although the monthly mean easterlies in this
region were only slightly weaker-than-normal (Table T1, Fig. T20). The upper level (200-hPa) winds were also near normal
across the equatorial Pacific during May (Fig. T21). For
much of the past year the upper-level winds over the equatorial Pacific have fluctuated
between near-normal and stronger-than-normal westerlies, with an El Niño signal not yet
evident.
The tropical sea level pressures (SLP) during May reflected negative anomalies over the
Pacific Ocean and positive anomalies over the remaining two-thirds of the global Tropics (Fig. T19). This anomaly pattern was associated with negative
values of both the equatorial Southern Oscillation Index (SOI) (-0.5), and the Tahiti -
Darwin SOI (–1.2) (Table T1). The SOI has been
negative for three consecutive months (Fig. T1). |
