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Tropical Highlights - May 2001

Conditions across the tropical Pacific returned to normal during May 2001. Sea surface temperatures (SSTs) across the tropical Pacific (Fig. T18) were remarkably normal during the month, with SST anomalies significantly different from zero only along the South American coast, and with all 4 Niņo region indices within 0.1°C of 0°C (Table T2).

The equatorial oceanic thermocline remained deeper than normal in the western central Pacific and generally near normal in the eastern Pacific during the month (Fig. T15). Consistent with this pattern, oceanic temperatures were above normal throughout the western and central Pacific at thermocline depth and slightly below normal in the extreme eastern Pacific (Fig. T17). Tropical convection during May was near normal over the west-central and central Pacific [as inferred from positive anomalies of outgoing longwave radiation (OLR)] and above normal over the western Pacific and Indonesia (Fig. T25). Over the past few months, the pattern of tropical convection over the western and central Pacific, which has persisted since mid-1998 has weakened (Fig. T11) as the cold episode has weakened. Elsewhere, convection was again enhanced over the southern part of the Sahel.

The low-level (850 hPa) winds across the Pacific basin featured slightly enhanced easterlies over the western and central Pacific, and anomalous cross equatorial flow over the eastern Pacific (Fig. T20). Enhanced equatorial easterlies have persisted across the western and central Pacific since the onset of cold episode conditions in mid-1998 (Fig. T7), although the enhanced easterlies have weakened during each of the past three Northern Hemisphere summers (Fig. T7). At upper levels, an anomalous ridge-trough couplet was observed across the subtropical western and central South Pacific (Figs. T21, T22). This was associated with anomalous cross equatorial flow over the central Pacific and with slightly enhanced westerlies across this region (Table T1).

The sea level pressure (SLP) pattern during May featured slightly positive SLP anomalies throughout most of the global Tropics (Fig. T19). This pattern was associated with small values of both the Southern Oscillation Index (SOI) (-0.8) and equatorial SOI (0.8) (Table T1, Fig. T2).

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