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HOME > Expert Assessments > Climate Diagnostics Bulletin > Tropical Highlights

Tropical Highlights - April 2003

Conditions across the Tropical Pacific returned to normal for the first time since May 2002, as sea surface temperature (SST) anomalies decreased across the entire equatorial Pacific (Table T2). Since peaking in late 2002, SST anomalies have steadily decreased throughout the central and eastern equatorial Pacific (Fig. T9), with below normal SSTs observed in the eastern Pacific east of 140°W, and slightly above normal SSTs recorded across the western and central Pacific during April (Fig. T18). The anomalies in all four Niņo region indices decreased for the fourth consecutive month (Table T2, Fig. T5), with the value in the Niņo 3 region becoming negative for the first time since February 2002 (Fig. T5).

Consistent with the cooling trend in SSTs, the depth of the equatorial oceanic thermocline has steadily decreased since late 2002, with negative subsurface temperature departures developing in the upper ocean of the equatorial Pacific during April (Figs. T15, T16). Ocean temperatures at thermocline depth were below normal across the central and eastern equatorial Pacific, with negative departures of more than 3°C recorded between 130°W and 140°W and also near 100°W (Fig. T17).

Consistent with weakening warm episode conditions, convection over the central equatorial Pacific has steadily decreased since January 2003 (Fig. T8). Tropical convection (precipitation and cloudiness) was below average over Indonesia and across the central and eastern Pacific and near average over the western Pacific during April (Fig. T25). This is the first time since July 2002 that suppressed convection has been observed over the central equatorial Pacific (Fig. T8, Table T1).

The low-level (850-hPa) equatorial easterly winds were slightly stronger than normal over the central and eastern Pacific during April (Fig. T20). Near-normal to slightly enhanced easterlies have been observed across the equatorial Pacific since the beginning of the year, consistent with weakening warm episode conditions (Fig. T4).

The tropical sea-level pressure (SLP) pattern during April featured slightly above normal SLP over the eastern half of the Pacific Ocean and the Atlantic Ocean and slightly below normal SLP over the Indian Ocean and the western Pacific (Fig. T19). This pattern was associated with a small negative value of the SOI (-0.4) (Table T1) and the first positive value of the equatorial SOI (0.7) since April 2002 (Fig. T2).

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