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Tropical Highlights - May 1999

Cold episode (La Nia) conditions weakened throughout the tropical Pacific during the past few months, as sea surface temperatures (SSTs) have increased throughout the central Tropical Pacific and SST anomalies have decreased in the east-central and central Pacific (Table T2, Fig. T5). During May 1999, SSTs more than 1.0C below normal were only observed along the equator between 165E and 170W (Fig. T18 ). However, the oceanic thermocline remained shallower than normal throughout the central and eastern Pacific east of 165W and deeper than normal west of 165W during the month (Figs. T15, T16). Consistent with this subsurface structure, equatorial ocean temperatures at thermocline depth remained more than 3C below normal in the central and eastern Pacific, and more than 3C above-normal in the western Pacific (Fig. T17 ).

Tropical convection during May [as inferred from anomalous outgoing longwave radiation (OLR)] remained suppressed across the western and central equatorial Pacific and enhanced across Indonesia and the Philippines (Fig. T25 ). This pattern of convection has persisted since June 1998 and is consistent with a continuation of mature cold episode conditions (Fig. T11). However, near normal convection was observed over the central equatorial Pacific during the latter part of the month (Fig. T11) for the first time since August 1998.

Low-level (850-hPa) equatorial easterly wind anomalies were again observed over the western equatorial Pacific, where maximum anomalies were near 6 m s-1. In contrast, the easterlies remained slightly weaker than normal (westerly anomalies) over the eastern Pacific (Table T1, Fig. T20). Westerly wind anomalies were also observed north of the equator over the Indian Ocean, which contributed to enhanced low-level convergence and convection over Southeast Asia (Fig. T25).

At upper levels of both hemispheres, the circulation reflected amplified troughs over the low-latitudes of the mid-Pacific and a confinement of the low-latitude ridges to the Indonesia and Indian Ocean sectors (Fig. T22 ). This circulation pattern was associated with westerly anomalies at low latitudes of the central Pacific and easterly anomalies over the tropical Indian Ocean (Figs. T21). These features have persisted for several months and are consistent with ongoing La Nia conditions.

The Southern Oscillation Index (SOI) was 0.1 during May (Table T1, Fig. T1), which is a substantial decrease from the May value. However, this decrease is not indicative of a large-scale change in the pattern of sea level pressure anomalies, since the equatorial SOI was 1.5 for the month (Fig. T2). The equatorial SOI continues to better depict the continued strength of the cold episode than the conventional SOI. These index values reflect above-normal sea level pressure (SLP) over the central Pacific and below-normal SLP over Indonesia and the eastern Indian Ocean (Fig. T19).

Editor's Note: Fig. E3 in the March and April 1999 Climate Diagnostics Bulletin and Fig. T26 in the April 1999 Bulletin were incorrect. Corrected versions in gif format are available on the CPC anonymous ftp site at in directory pub/cpc/bulletin under the names E3_mar99.gif, E3_apr99.gif, and T26_apr99.gif

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