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Tropical Highlights - December 1999

Mature cold episode conditions prevailed throughout the tropical Pacific during December 1999, as SST anomalies were more than 1.0C below normal across most of the equatorial Pacific east of the date line, and more than 2.0C below normal near 120W (Fig. T18). The strengthening of the cold episode during the past few months is indicated by decreases in the Nio region indices to levels comparable to those observed last December (Table T2, Fig. E4).

The equatorial oceanic thermocline remained shallower than normal across the east-central and eastern Pacific during the month, and deepened in the west-central and western Pacific (Fig. T15). Consistent with this pattern, temperatures at thermocline depth remained more than 5C below normal in the east-central Pacific, and increased to 3-4C above normal in the western Pacific (Fig. T17). This thermocline structure has been extremely persistent for the past 18 months, and remains consistent with the ongoing La Nia conditions.

Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was suppressed over the western and central equatorial Pacific during December, and enhanced over Indonesia (Fig. T25). Convection has been suppressed across the central equatorial Pacific since the onset of cold episode conditions in late May 1998 (Fig. T8). During December, the suppressed convection yielded an OLR index of 2.6 (Table T1), which is the second largest value in the historical record dating back to 1974, surpassed only by the December 1975 value.

The pattern of tropical convection over the central and western Pacific was again accompanied by low-level (850 hPa) easterly wind anomalies across the central and western tropical Pacific during December (Fig. T20). Anomalous easterlies have prevailed in this region since May 1998 (Fig. T7). The upper-level atmospheric circulation (200 hPa) in the Tropics and subtropics also remained consistent with mature cold episode conditions during December, with well-developed upper-level troughs observed over the low-latitudes of the central Pacific in both hemispheres and enhanced equatorial westerlies observed across the central and eastern Pacific (Fig. T21).

The pattern of sea level pressure (SLP) anomalies during December was also consistent with cold episode conditions, with positive anomalies observed across the tropical Pacific and negative anomalies observed over Indonesia and the eastern Indian Ocean (Fig. T19). This pattern is indicated by strong positive values of the Southern Oscillation Index (SOI) (Table T1, Fig. T1) and the equatorial SOI (Fig. T2) during December, which were 1.5 and 3.2, respectively. Both of these indices have increased over the past few months, consistent with strengthening cold episode conditions. The December value of the equatorial SOI is the largest in the historical record back to 1958, eclipsing the previous record value of 3.1 set in January 1999.

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