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Tropical Highlights - May 2002

The evolution toward a Pacific warm episode (El Nio) 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 120W during the month, with the largest anomalies exceeding 1C near the date line (Fig. T18). By the end of the month, SSTs were more than 1.0C above normal across most of the equatorial Pacific between 160E and 105W (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 120W (Fig. T15) with the largest depth anomalies located between 170W and 140W (Figs. T15, T16). Ocean temperatures at thermocline depth remained above normal west of 120W, and were slightly below normal between 120W 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 Nio 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).


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