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Tropical Highlights - November 1999

Cold episode conditions intensified during November 1999, as sea surface temperatures (SST) became more than 1.0°C below normal across the central and eastern equatorial Pacific (Table T2, Fig. T18). The equatorial oceanic thermocline remained shallower than normal across the east-central and eastern Pacific during the month, and deeper than normal in the western Pacific (Fig. T15). Consistent with this pattern, temperatures at thermocline depth remained more than 5EC below normal in the east-central Pacific and 2-3°C 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 Niņa conditions.

Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was suppressed over the western and central equatorial Pacific during November, 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). Elsewhere, convection and rainfall were also enhanced across northern and eastern Australia during November (Figs. T25, E4). In northeastern Australia, area-averaged precipitation exceeded the 95th percentile, and totals were nearly double the November climatological mean (Fig. E4).

Enhanced low-level (850 hPa) easterly winds persisted across the central and western tropical Pacific during November (Fig. T20). Anomalous easterlies have prevailed in this region since May 1998 (Fig. T7), in association with ongoing La Niņa conditions. The upper-level atmospheric circulation (200 hPa) in the Tropics and subtropics also remained consistent with cold episode conditions, with well-developed upper-level troughs observed over the low-latitudes of the central Pacific in both hemispheres and enhanced equatorial westerlies across the central and eastern Pacific (Fig. T21).

The pattern of sea level pressure (SLP) anomalies during November was also consistent with cold episode conditions, with positive anomalies observed across the tropical Pacific and negative anomalies observed over Indonesia and the Indian Ocean (Fig. T19). As a result, the Tahiti-Darwin Southern Oscillation Index (SOI) was 1.1 (Table T1, Fig. T1), and the equatorial SOI increased to 2.2 (Fig. T2).

Editor’s Note: CDAS has been run for October and ran normally for November. However, the CCA SST forecasts (Figs. F1-F2) continue to be omitted, since the software necessary to run these products has not been transferred to the new computing environment.

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