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Tropical Highlights - November 2001

ENSO-neutral conditions prevailed across the tropical Pacific during November 2001, as indicated by near-normal equatorial atmospheric (Table T1) and sea surface temperature (SST) indices (Table T2). ENSO-neutral conditions have persisted since May 2001, although a non-significant weak dipole pattern of positive SST anomalies (0.5 to 1.0C) across the western and central Pacific and negative SST anomalies (1.0C) across the eastern Pacific has persisted since September 2001 (Fig. T18). During November, the SST anomalies reflected an expansion of the 30C isotherm in the region just west of the date line, and the persistence of anomalously warm SSTs in the Nio 4 region (Table T2).

The oceanic thermocline (indicated by the 20C isotherm) remained deeper than normal across nearly the entire equatorial Pacific, and slightly shallower than normal in the extreme eastern Pacific (Fig. T15). Consistent with these conditions, temperatures at thermocline depth were 2-3C above normal throughout the western and central equatorial Pacific and 2-3C below normal in the extreme east (Fig. T17). This overall anomaly pattern has persisted since August, although the positive anomalies have slowly expanded eastward during the period.

Tropical convection during November was enhanced over the eastern Indian Ocean and Indonesia and near-normal across the rest of Pacific (Fig. T25). However, this pattern of anomalous tropical convection was again modulated by intraseasonal (Madden-Julian Oscillation-MJO) activity during the month, especially over the Indonesian sector (Fig. T11).

The low-level (850-hPa) westerly winds were slightly weaker than normal across the western and central equatorial Pacific during November, with 3 ms-1 easterly anomalies recorded between 175E and 160W (Fig. T20). The 850-hPa indices in the western (0.8) and central (0.9) Pacific are the largest values recorded in each region since April 2001 (Table T1). At upper levels (200-hPa) westerly anomalies were observed across most of the equatorial Pacific in association with cyclonic circulation anomalies straddling the equator in the central Pacific (Fig. T21, T22).

The sea level pressure (SLP) pattern during November featured negative anomalies extending from the eastern Indian Ocean across Indonesia to the western Pacific and positive anomalies across the central and eastern Pacific (Fig. T19). This pattern was associated with a positive value of the Tahiti - Darwin Southern Oscillation Index (SOI) (0.7) (Table T1). The equatorial SOI also remained positive during November (1.2), and has been positive since the beginning of the 1998-2001 cold episode (Fig. T2).

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