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Tropical Highlights - August 2000

Oceanic and atmospheric conditions were near-normal during August 2000. Sea surface temperatures across the tropical central Pacific were near-normal, with the indices in the four Nio regions all within 0.5C of normal (Fig. T18, Table T2). The oceanic thermocline remained deeper than normal in the equatorial west-central and western Pacific during the month (Fig. T16), although the thermocline has shoaled 20-30 meters over the past 6 months (Fig. T15). Consistent with this shoaling, anomalies at thermocline depth have decreased from 4-6C above normal during April to 2-4C above normal this month (Fig. T17). Over the eastern and east-central Pacific, the depth of the thermocline (Fig. T15) and thermal structure in the upper 150 m of the ocean (Fig. T17) both remained near normal.

Tropical convection during August [as inferred from anomalous outgoing longwave radiation (OLR)] was near normal over the western and central equatorial Pacific (Fig. T25) for the first time since the beginning of the cold episode in May 1998 (Fig. T1). Convection was enhanced over Indonesia during the month (Fig. T25), after being slightly suppressed during July (Fig. E4). Elsewhere, rainfall was below normal over the African Sahel during August, continuing the pattern of drier-than-normal conditions that has been observed this year and contrasts with the above-normal rainfall which occurred last year. Tropical intraseasonal (Madden-Julian Oscillation-MJO) activity (with a period near 45 days) has increased over the past six months (Fig. T11). During August, the MJO was associated with an increase in Atlantic tropical activity during the beginning and middle of the month and a suppression of activity late in the month.

Consistent with the pattern of small, negative SST anomalies across the equatorial central Pacific, weak low-level (850 hPa) easterly wind anomalies were observed over the central and east-central Pacific (Fig. T20, Table T1). At upper levels, westerly anomalies were observed over the central Pacific, in association with an upper-level trough in the central subtropical South Pacific (Fig. T21).

The sea level pressure (SLP) pattern across the Tropics during August featured weak negative anomalies across most of the global Tropics (Fig. T19). The Southern Oscillation Index (SOI) was 0.4 for August and has been near zero since May (Table T1) The equatorial SOI also decreased to 0.4, the lowest value of this index since August 1998 (Fig. T2).

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