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Tropical Highlights - September 2001

ENSO-neutral conditions prevailed across the tropical Pacific during September 2001, with sea surface temperatures (SSTs) 0.5 to 1.0°C above normal across the western and central Pacific and more than 1.0°C below normal across the east-central and eastern Pacific (Fig. T18). This SST anomaly dipole was also reflected in the four Niņo region indices, with an above-normal value in the Niņo 4 region, and below normal values in the Niņo 1+2, 3, and 3.4 regions (Table T2).

The equatorial oceanic thermocline (indicated by the 20°C isotherm) remained deeper than normal in the western and central Pacific during the month and was slightly shallower than normal in the extreme eastern Pacific (Fig. T15). Consistent with these conditions, temperatures at thermocline depth were 2-3°C above normal throughout the western and central Pacific and 2-3°C below normal in the eastern equatorial Pacific (Fig. T17). This pattern is very similar to the pattern of ocean anomalies observed last month.

Tropical convection during September was near normal across the Pacific, slightly enhanced over parts of the African Sahel, and slightly below normal over northern India (Fig. T25). Convection across the tropical Pacific continued to be influenced by intraseasonal (Madden-Julian Oscillation-MJO) activity during the month.

Both the low-level (850-hPa) and the upper-level (200-hPa) winds were near normal across the equatorial Pacific during September (Figs. T20, T21). The low-level winds have been normal across the equatorial Pacific since April 2001 (Table T1, Fig T4), while the upper-level winds have been near-normal in every month (except for August) since February 2001 (Table T1, Fig. T3).

The sea level pressure (SLP) pattern during September featured small, positive anomalies across the east-central and eastern tropical Pacific and small, negative anomalies throughout most of the rest of the Tropics (Fig. T19). This pattern was associated with a slightly positive value of the Tahiti - Darwin Southern Oscillation Index (SOI) (0.2) (Table T1). The equatorial SOI remained positive during September (1.0). This index has been positive for every month since the beginning of the cold episode in the middle of 1998 (Fig. T2).

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