Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was suppressed over Indonesia, as well as over both the western and central equatorial Pacific during September (Fig. T25). Convection has been suppressed across the central Pacific since mid-1998 (Fig. T8). In contrast, September marked the first time since April 1998 that monthly mean convection and rainfall was below normal over Indonesia (Fig. E4). This decrease in convection in the Indonesian sector was associated with strong intraseasonal activity (Fig. T11). Elsewhere, convection was again enhanced across the African Sahel, with well above-normal rainfall recorded in the region for the third consecutive month (Fig. E4).

Enhanced low level (850 hPa) easterly winds also persisted across the western Pacific during September (Fig. T20). Anomalous easterlies have prevailed in this region since May 1998 (Fig. T7), in association with ongoing La Niña conditions. Elsewhere, low-level westerly wind anomalies again covered the subtropical North Atlantic during the month, consistent with above-normal rainfall across the Africa Sahel. The upper-level atmospheric circulation (200 hPa) in the Tropics and subtropics remained consistent with continuing cold episode conditions, with upper level troughs observed over the low-latitudes of the western Pacific in both hemispheres (stronger in the Southern Hemisphere) and amplified low-latitude ridges observed across most of the remainder of both hemispheres (Figs. T21, T22).

The Southern Oscillation Index (SOI) was -0.1 (Table T1, Fig. T1), and the equatorial SOI was 1.3 (Fig. T2) during September. During the past five months, the SOI has averaged 0.1 (Fig. T1). However, during the same time period, the equatorial SOI has averaged 1.2, and has better depicted the ongoing cold episode.