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Tropical Highlights - March 2003

Pacific warm episode (El Nio) conditions continued to weaken during March 2003 as sea surface temperature (SST) anomalies again decreased across the equatorial Pacific (Table T2). SSTs remained more than 1.0C above normal in the central Pacific between 170E and 140W, but were slightly cooler than normal east of 120W (Fig. T18). The anomalies in all four Nio region indices decreased for the third consecutive month (Table T2, Fig. T5). SST anomalies across the central and eastern equatorial Pacific have steadily decreased since reaching maximum values in late 2002 (Fig. T9).

Consistent with weakening El Nio conditions, the depth of the equatorial oceanic thermocline during March remained near normal across the central Pacific and was slightly below normal in the eastern Pacific (Figs. T15, T16). Ocean temperatures at thermocline depth were 1-2C above normal in the central equatorial Pacific and 2-4C below normal in the eastern Pacific (Fig. T17).

Tropical convection (precipitation and cloudiness) was above average across the western and west-central equatorial Pacific and near average over Indonesia during March (Fig. T25). Although above-average convection has persisted over the central equatorial Pacific since August, there has been a significant decrease in the departures from average since the beginning of year, which is consistent with weakening warm episode conditions (Fig. T9). The Outgoing Longwave Radiation index during March (-0.5) was the smallest negative value since July 2002 (Table T1). Elsewhere, precipitation deficits occurred over much of southern Africa and northern Australia.

The low-level (850-hPa) equatorial easterly winds were stronger than normal over the west-central Pacific and weaker than normal over the eastern Pacific (Fig. T20). The enhanced easterlies were associated with the largest value of the 850-hPa zonal wind index in the western Pacific region since April 2001 (Fig. T4).

The tropical sea-level pressure (SLP) pattern during March featured near-normal SLP throughout the entire global Tropics (Fig. T19). This pattern was associated with a near-zero value of the equatorial SOI (-0.2) (Fig. T2).

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