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Tropical Highlights - February 2002

The evolution toward a Pacific warm episode continued during February 2002, with an area of SST anomalies exceeding +1.0°C persisting near the date line (Fig. T18) and the corresponding Niño-4 region SST index increasing to +0.8 (Fig. T5, Table T2). SST anomalies across the east-central and eastern equatorial Pacific returned to normal during February, as indicated by increases in the SST index values to -0.2 in the Niño-3 region and to 0.0 in the Niño 1+2 region (Fig. T18, Table T2). By late in the month, SST anomalies exceeded +1.0°C in the extreme eastern equatorial Pacific (not shown).

The warming of the eastern equatorial Pacific resulted from the arrival of a strong oceanic Kelvin wave that had been triggered during December in association with an MJO-related westerly wind burst over the west-central equatorial Pacific (Figs. T13, T20). This Kelvin wave has resulted in an anomalous deepening of the oceanic thermocline throughout the equatorial Pacific from 170°E to 90°W (Figs. T15, T16), and by an increase in subsurface temperatures to 4-5°C above normal in that region (Fig. T17, bottom). This increasingly large build-up of anomalously warm waters at depth is consistent with the continued evolution toward a Pacific warm episode.

Accompanying this evolution deep tropical convection extended across the central equatorial Pacific to between 160°E- 180° during February (Fig. T8). This location represents the farthest eastward extent of the deep tropical convection recorded since the end of the last warm episode in mid-1998 (Fig. T8, top). However, equatorial convection has remained absent east of the date line throughout this 3 ½ year period (Fig. T25, top), which has contributed to the persistence of cyclonic streamfunction anomalies at 200-hPa in the subtropics of both hemispheres (Fig. T22, bottom) flanking the region of suppressed convection [indicated by negative anomalies of outgoing longwave radiation]. During February, a strong anomalous cyclonic couplet over the central Pacific was associated with a large westerly anomalies (Fig. T21) and a large value of the 200-hPa wind index (Table T1, Fig. T3).

Tropical convection and the associated atmospheric circulation anomalies have also exhibited strong intraseasonal variability during the past several months (Figs. T11, T12, T13). MJO-related tropical convection during February was enhanced over the central Pacific (Fig. T25).

The sea level pressure (SLP) pattern during February featured positive anomalies throughout most of the global Tropics (Fig. T19). This pattern was associated with a positive value of the Tahiti - Darwin Southern Oscillation Index (SOI) (0.9) (Table T1) and a near-zero value of the equatorial SOI (–0.1).

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