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Tropical Highlights - August 1999

Cold episode conditions intensified across the tropical Pacific during August, as indicated by changes in sea surface temperatures (SSTs) and the subsurface ocean structure. Negative SST anomalies in the central Pacific strengthened to more than 1.0 C below normal between 170-120W, which resulted in a decrease in the Nio 3.4 region SST index to below -1.0 for the first time since February 1999 (Table T2). Elsewhere, SSTs across the equatorial Atlantic east of 30W remained more than 1.0C above normal (Fig. T18). These warmer than normal Atlantic SSTs have persisted since May 1999.

The oceanic thermocline shoaled across the east-central and eastern Pacific during the month (Fig. T15), which resulted in a decrease in temperatures at thermocline depth to more than 4 C below normal in this region (Fig. T17). This evolution is indicative of a strengthening of cold episode conditions and is a reversal of the trend that was observed in this region during the past few months. Farther west, oceanic temperatures at thermocline depth remained more than 2C above-normal (Fig. T17).

Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] during August was suppressed across the western and central equatorial Pacific and enhanced across eastern Indonesia and the Philippines (Fig. T25). This pattern has prevailed since mid-1998, and is also consistent with ongoing cold episode conditions (Fig. T8). Elsewhere, convection was again enhanced across the African Sahel, with well-above-normal rainfall recorded in the region for the second consecutive month (Fig. E4).

The pattern of tropical convection over the western and central equatorial Pacific was again accompanied by anomalous easterly winds at low-levels (850 hPa) over the western equatorial Pacific (Fig. T20). These stronger-than-normal easterlies have prevailed since May 1998 (Fig. T7). Elsewhere, low-level westerly wind anomalies covered the sub-tropical North Atlantic during the month, consistent with above-normal rainfall across West Africa.

The upper-level atmospheric circulation (200-hPa) in the Tropics remained consistent with the pattern of tropical rainfall and cold episode conditions, with well-defined troughs observed over the low-latitudes of the mid-Pacific in both hemispheres and amplified low-latitude ridges observed across most of the remainder of the two hemispheres (Figs. T21, T22).

The Southern Oscillation Index (SOI) was 0.1 (Table T1, Fig. T1), and the equatorial SOI was 0.6 (Fig. T2) during August. Both of these indices have decreased in magnitude over the past few months, as positive sea level pressure (SLP) anomalies have overspread most of the tropical Pacific and Indian Ocean basins (Figs. T6, T19).

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