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Tropical Highlights - September 2000

Sea surface temperatures (SSTs) remained weakly negative over portions of the central and eastern equatorial Pacific during September, with all four Nio region indices indicating SSTs within 0.5C of normal for the second consecutive month (Fig. T18, Table T2). The oceanic thermocline remained deeper than normal in the west-central and western equatorial Pacific during September (Fig. T16), although it has shoaled 20-30 meters in that region during the past 6 months (Fig. T15). Consistent with this evolution, temperature anomalies at thermocline depth have decreased from 4-6C above normal in April to 2-3C above normal in September (Fig. T17). Over most of the eastern and east-central equatorial Pacific the depth of the thermocline remained slightly shallower than normal (Fig. T15), which is consistent with the persistence of below-average temperatures in that region (Fig. T17).

Tropical convection during September [as inferred from anomalous outgoing longwave radiation (OLR)] was below-normal over the western and central Pacific (Figs. T1, T25). Tropical intraseasonal activity (Madden-Julian Oscillation-MJO), with a period near 45 days, has increased over the past six months (Fig. T11). During September, the MJO activity was associated with the decreased convective activity over the western and central Pacific. Elsewhere, convection and rainfall was below normal over the African Sahel, which continues the drier-than-normal Sahel rainy season.

Low-level (850 hPa) easterly wind anomalies persisted over the western equatorial Pacific during September (Fig. T20, Table T1), while large cross-equatorial flow and westerly wind anomalies developed over the eastern half of the tropical Pacific. This pattern of low-level winds was consistent with the suppressed convection over the central tropical Pacific, and also with a strengthening of the ITCZ over the eastern Pacific (Figs. T25, T23 bottom). At upper levels, easterly anomalies were observed over the east-central Pacific for the first time since September 1999 (Fig. T3).

The sea level pressure (SLP) pattern across the Tropics during September featured weak negative anomalies across the Atlantic and Indian Oceans and weak positive anomalies over the Pacific Ocean (Fig. T19). This pattern was associated with an increase in both the Southern Oscillation Index (SOI) and the equatorial SOI to 1.0 for September (Table T1, Fig. T2).

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