Tropical Highlights - September 2002

Pacific warm episode (El Niño) conditions continued during September 2002 as sea surface temperature (SST) anomalies greater than +1°C persisted across the central equatorial Pacific between the date line and 130°W, and anomalies greater than 2°C developed between 170°W-160°W (Fig. T18). The corresponding SST index for the Niño 3.4 region increased to 1.2 during the month, which is the largest value of that index since March 1998, the end of the 1997-98 El Niño episode (Fig. T5, Table T2)..

Consistent with this ongoing warmth the oceanic thermocline remained deeper than normal across the central and eastern equatorial Pacific (Figs. T15, T16). Sub-surface ocean temperatures in the equatorial central and east-central Pacific also remained 4-5°C above normal at thermocline depth (Fig. T17).

Convection was enhanced across the central equatorial Pacific during September, resulting in the largest negative value of the OLR index since October 1997 (Figs. T1, T11). During the past two months convection has been enhanced in this region, and suppressed over Indonesia and the western Pacific (Fig. T25). These conditions are consistent with a strengthening of the El Niño conditions during the period. Elsewhere, monsoon rainfall was below normal during September over the Indian peninsula and the African Sahel (Figs. E3, E4). Both regions experienced below normal rainfall during their June - September wet season.

The low-level (850-hPa) easterly winds were weaker than normal across the entire equatorial Pacific during September 2002 (Figs. T13, T20). For both the western and central regions these conditions produced the largest negative 850-hPa zonal wind index values recorded since the 1997/98 warm episode. The low-level easterlies have been weaker than normal across the equatorial Pacific since July 2002 (Table T1, Figs. T4, T7)The tropical sea-level pressure (SLP) pattern again reflected an anomalous wave-1 pattern during September, characterized by below-average pressure over most of the tropical Pacific and above-average pressure elsewhere (Fig. T19). This pattern resulted in negative values of both the Tahiti-Darwin Southern Oscillation Index (SOI) (-0.7) and the equatorial SOI (-0.8) (Figs. T1, T2, respectively). The SOI has been negative since March 2002 (Table T1), and the five-month running mean of the SOI reached -1.0 during the May - September 2002 period.