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Tropical Highlights - November 2000

Weak cold episode conditions re-developed across the tropical Pacific during November 2000, as sea surface temperatures (SSTs) dropped to 0.5°-1.0°C below normal across the central and eastern equatorial Pacific (Fig. T18). This evolution is reflected by decreases in all four Niņo region indices, with the Niņo 3.4 index recording its lowest value since March 2000 and the Niņo 3 index recording its lowest value since February 2000 (Fig. T5, Table T2).

Accompanying this anomalous cooling the equatorial oceanic thermocline shoaled across the east-central and eastern Pacific and deepened throughout the western and central Pacific (Fig. T15). This evolution resulted in an increase in both the slope of the oceanic thermocline (Fig. T16) and the magnitude of east-west temperature gradient across the equatorial Pacific, with oceanic temperatures increasing to more than 2°C above normal in the western Pacific and decreasing to more than 3°C below normal in the eastern Pacific (Fig. T17).

Tropical convection during November was below normal over the central Pacific [as inferred from positive anomalies of outgoing longwave radiation (OLR)] and above normal over the western Pacific, Indonesia, and northern Australia (Fig. T25). This pattern of anomalous convection is consistent with La Niņa conditions and has largely persisted since mid-1998 (Fig. T8). Intraseasonal activity (Madden-Julian Oscillation-MJO) with a period near 45 days also continued to impact tropical convection. During November, this activity contributed to increased convection over the Indian Ocean and Indonesia (Fig. T11).

The anomalous tropical convection during November was associated with increased low-level (850 hPa) easterly wind anomalies over the western equatorial Pacific (Figs. T7, T20, and Table T1). Easterly wind anomalies have been observed over the western and central equatorial Pacific since the onset of cold episode conditions in mid-1998 (Fig. T7). At upper levels, westerly wind anomalies persisted over the central equatorial Pacific, consistent with cyclonic circulation anomalies flanking the region of suppressed convection (Fig. T21, T22).

The sea level pressure (SLP) pattern featured positive anomalies over the eastern half of the tropical Pacific during November, and negative anomalies throughout the remainder of the global Tropics (Fig. T19). This pattern was associated with large positive values (2.0) of both the Southern Oscillation Index (SOI) and the equatorial SOI during the month (Table T1, Fig. T2).

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