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Tropical Highlights - May 2000

Negative sea surface temperature (SST) anomalies have weakened across the tropical Pacific during the past few months, with equatorial SSTs of more than 1.0C below normal now confined to the region between 170E and 160W (Fig. T18). As a result, the Nio 3.4 and Nio 4 region indices were only -0.5C and -0.8C, respectively, during May (Table T2, Fig. T5). These were the smallest negative values for these indices since May 1998 and November 1999, respectively (Fig. T5). Across the eastern Pacific SST anomalies returned toward normal during May, after becoming positive in April (Table T2).

The oceanic thermocline remained much deeper than normal in the equatorial west-central and western Pacific during the month (Fig. T15), with temperatures again averaging more than 4.0C above normal at thermocline depth (Fig. T17). Over the eastern Pacific, the thermocline continued to deepen (Fig. T15), resulting in a lessening of negative temperature anomalies at thermocline depth (Fig. T17). As a result subsurface temperatures more than 1.0C below normal were confined to the region between 100W and 140W. The recent evolution of the oceanic thermocline and subsurface temperature anomalies is similar to that observed during March-May 1999, with the major differences being an increase in the magnitude of the positive subsurface temperature anomalies in the western Pacific and a decrease in the magnitude of the negative anomalies in the eastern Pacific. This basic east-west dipole of subsurface temperature anomalies, which is typical of the mature phase of La Nia episodes, has been highly persistent since late 1998.

The pattern of tropical convection during May [as inferred from anomalous outgoing longwave radiation (OLR)] remained consistent with cold episode conditions, with suppressed convection over the western and central equatorial Pacific and enhanced convection over Indonesia (Fig. T25). Elsewhere, convection was enhanced over southeast Asia and India during the month (Fig. T25), implying a strong start to the Indian/Southeast Asian summer monsoon system. Convection was also enhanced over southern Mexico/Central America, in association with an amplified monsoon circulation in that region. Strong monsoonal circulations in each of these regions are consistent with ongoing La Nia conditions.

The pattern of tropical convection over the central and western Pacific was again accompanied by an enhanced Walker circulation across the central equatorial Pacific (Fig. T29). However, consistent with the decrease in magnitude of the negative SST anomalies, the low-level (850 hPa) easterly wind anomalies across the central and western tropical Pacific decreased to 3-6 m s-1 between 150E and 170W (Fig. T20). At upper-levels (200-hPa), well-developed mid-Pacific troughs were again observed over the low-latitudes of both hemispheres, with anticyclonic circulation anomalies dominating the subtropics and lower mid-latitudes of both hemispheres (Figs. T21, T22). This anomaly pattern has also persisted since mid-1998, in association with ongoing La Nia conditions.

The sea level pressure (SLP) pattern across the Tropics during May featured positive anomalies across the tropical central and eastern Pacific and weak negative anomalies over Indonesia (Fig. T19). This pattern is consistent with ongoing cold episodes, and was associated with a positive value of the equatorial Southern Oscillation Index (SOI) (1.4) (Fig. T2). The May value of the Tahiti-Darwin SOI was only 0.2 (Table T1, Fig. T2), as SLP at Darwin was above normal in association with a strong mid-latitude ridge that extended into the subtropics over Australia (Fig. T19).

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