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Tropical Highlights - August 2002

Warm episode (El Niņo) conditions continued during August 2002, as sea surface temperature (SST) anomalies greater than +1°C persisted across the central equatorial Pacific between 175°E and 130°W (Fig. T18). SST anomalies increased in the central equatorial Pacific during August, and the Niņo 3.4 region index reached 1.1. This is the largest value of the Niņo 3.4 index since March 1998, the end of the 1997-98 El Niņo episode (Fig. T5, Table T2).

Accompanying the ongoing El Niņo the oceanic thermocline deepened across the central and eastern equatorial Pacific during August (Figs. T15, T16), as a Madden-Julian Oscillation (MJO)-induced oceanic Kelvin wave crossed the Pacific. This deepening was associated with an increase in sub-surface ocean temperature anomalies to 4-5°C at thermocline depth in the equatorial central and east-central Pacific (Fig. T17).

The tropical sea-level pressure (SLP) pattern during August featured a wave-1 pattern with negative anomalies over the tropical Pacific and positive anomalies elsewhere (Fig. T19). This pattern was associated with the largest negative value of both the Tahiti-Darwin Southern Oscillation Index (SOI) (-1.6) and the equatorial SOI (-1.1) since end of the 1997/98 warm episode (Figs. T1, T2, respectively). The SOI has remained negative since March 2002 (Table T1), and the five-month running mean of the SOI reached -0.9 during the April - August period.

Tropical intraseasonal oscillations associated with the Madden-Julian Oscillation (MJO) continued to strongly influence the atmospheric variability in the Tropics and subtropics during August, again contributing to a weakening of the low-level (850-hPa) easterlies across the equatorial Pacific (Figs. T13, T20). These low-level winds have been weaker than normal for the past two months (Table T1, Figs. T4, T7).

Tropical convection during August was enhanced over the central tropical Pacific and suppressed over Indonesia and the western Pacific (Fig. T25). The distribution of tropical convection has also been strongly modulated by MJO activity since May 2002 (Fig. T11). Elsewhere, monsoon rainfall over the Indian peninsula returned to normal during the month, but remained below normal over the African Sahel (Figs. E3, E4).