Tropical Highlights - September 2003

Current atmospheric and oceanic conditions continue to reflect neutral conditions across the Tropical Pacific (Tables T1, T2). Sea surface temperatures (SSTs) remained slightly warmer than average across the western and west-central Pacific and were slightly cooler than average along the South American coast (Fig. T18). SST anomalies more than 1°C above normal have persisted in the equatorial west Pacific since July (Fig. T9). SST anomalies in the four Niño regions have exhibited little change since July (Table T2, Fig. T4).

Oceanic thermocline depths, as measured by the depth of the 20°C isotherm (Figs. T15, T16) was slightly deeper than average across most of the equatorial Pacific during September. Consistent with these conditions, oceanic temperatures at thermocline depth were 1-2°C above average across most of the equatorial Pacific (Fig. T17).

Both the low-level and upper-level equatorial winds were near average across the Pacific during September (Table T1, Figs. T20, T21), and the Tahiti-Darwin SOI also remained near zero (-0.1) during the month (Table T1, Fig. T1). None of these indicators has exhibited any trend towards warm or cold episodes during the past four months and all are consistent with near neutral conditions across the Tropical Pacific.

Relatively weak outgoing longwave radiation (OLR) anomalies were again observed throughout much of the global Tropics during September, with the largest negative anomalies observed over the western Pacific (Fig. T25). Enhanced convection (negative OLR anomalies and above-average precipitation) was observed over north-central India (Fig. T25) and also over the west African Sahel (Figs. E3, E4). Rainfall was above-normal over the African Sahel during all four months of its rainy season (Fig. E4).