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Tropical Highlights - March 2000

Mature cold episode conditions continued throughout the tropical Pacific during March, as sea surface temperatures (SST) remained more than 1.0C below normal across the equatorial Pacific between 165E and 135W (Fig. T18). The region of negative SST anomalies has shifted westward over the past few months (Fig. T9), with near normal SSTs observed in the eastern Pacific during March (Fig. T18, Table T2). This westward shift in the negative anomalies has resulted in increases in the indices in the Nio 1+2, Nio 3, and Nio 3.4 regions (Table T2).

The oceanic thermocline was shallower than normal across the east-central and eastern equatorial Pacific during the month, and deeper than normal in the west-central and western Pacific (Fig. T15). The thermocline is more than 30 m deeper than normal in the west Pacific (Fig. T16), which is the largest positive depth anomaly observed in this region in the last several years. This deepening has been associated with oceanic temperatures more than 4.0C above normal at thermocline depth in this region (Fig. T17). Across the east-central and eastern Pacific, the thermocline also deepened during the month (Fig. T15), with anomalies at thermocline depth decreasing from 6-7C below normal during February to 2-3C below normal during March near 120W (Fig. T17).

The pattern of tropical convection during March [as inferred from anomalous outgoing longwave radiation (OLR)] was again consistent with mature cold episode conditions, with suppressed convection observed over the western and central equatorial Pacific and enhanced convection over Indonesia and the Indian Ocean (Fig. T25). This pattern has persisted since the onset of cold episode conditions in late May 1998 (Fig. T8). Elsewhere, convection was again enhanced during the month over southeastern Africa, with rainfall totals ranking above the 95th percentile for the third consecutive month (Figs. E3, E4).

The pattern of tropical convection over the central and western Pacific was accompanied by an enhanced Walker circulation across the equatorial Pacific (Fig. T29). This enhanced circulation was associated with low-level (850 hPa) easterly wind anomalies across the central and western tropical Pacific, with anomalies greater than 6 m s-1 located between 160E and 170W (Fig. T20). In contrast, westerly wind anomalies were observed over the eastern and east-central equatorial Pacific, which contributed to a local deepening of the thermocline and increased SST anomalies in that region. At upper-levels (200-hPa), troughs were observed over the low-latitudes of the central Pacific in both hemispheres, while amplified subtropical and lower mid-latitude ridges were observed across most of the remainder of both hemispheres (Fig. T22). The upper-level troughs were associated with enhanced equatorial westerlies across the central Pacific (Fig. T21), which are also a feature common to cold episodes.

The sea level pressure (SLP) pattern across the Tropics during March featured positive anomalies across the tropical central and eastern Pacific and negative anomalies extending from the Atlantic Ocean eastward to Indonesia (Fig. T19). The pattern across Indonesia and the Pacific Ocean is consistent with the ongoing cold episode (Fig. T19), and was associated with positive values of the Southern Oscillation Index (1.0)(SOI) (Table T1, Fig. T1), and the equatorial SOI (2.5) (Fig. T2).

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