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Tropical Highlights - August 2001

ENSO-neutral conditions again prevailed across the tropical Pacific during August 2001, with sea surface temperatures (SSTs) generally within 0.5°C of normal across the basin (Fig. T18). All four Niņo region anomaly indices were the same as those recorded during July (Table T2). However, the actual SSTs decreased across the entire equatorial Pacific during August, consistent with the approach of the climatological minimum in the annual cycle of SSTs.

The equatorial oceanic thermocline (indicated by the 20°C isotherm) remained deeper than normal in the western and central Pacific during the month and near normal in the extreme eastern Pacific (Fig. T15). Consistent with these conditions, temperatures at thermocline depth were above normal throughout the western and central Pacific and slightly below normal in the extreme eastern equatorial Pacific (Fig. T17). Since January 2001, the positive subsurface equatorial ocean temperature anomalies have slowly shifted eastward and upward (Fig. T17). This evolution is consistent with the decay of the subsurface thermal structure that characterizes the mature phase of cold episodes and the development of conditions usually found just prior to warm episodes (see the Forecast Forum for a discussion of the most recent SST forecasts).

Tropical convection during August was near normal across the Pacific, and again slightly enhanced over parts of the African Sahel and the northern Indian subcontinent (Fig. T25). Convection across the tropical Pacific has been significantly influenced by intraseasonal (Madden-Julian Oscillation-MJO) activity during the past few months.

During The first half of August, the MJO contributed to enhanced convection over the central equatorial Pacific, while during the second half of the month it contributed to suppressed convection in this region (Fig. T11). The MJO has also impacted the 850-hPa zonal wind anomalies (Fig. T13), which also showed a reversal in sign over the central and eastern tropical Pacific during August.

The sea level pressure (SLP) pattern during August featured negative anomalies across most of the tropical Pacific and positive anomalies over the tropical Indian and Atlantic Oceans (Fig. T19). This pattern was associated with a negative value of the Tahiti - Darwin Southern Oscillation Index (SOI) (-1.0) (Table T1), which has been negative for the past five months. In contrast, the equatorial SOI has remained positive during this period, although the August value of 0.6 is the smallest value since September 2000, and the 5-month running mean of the equatorial SOI is the lowest since September 1998 (Fig. T2).

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