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Tropical Highlights - April 2001

Cold episode conditions weakened during April 2001, as sea surface temperatures (SSTs) across the central tropical Pacific returned toward normal (Fig. T18). Since February, SSTs and SST anomalies across the central Pacific have increased significantly, with the Niņo 4 anomaly index value increasing from -0.8 to -0.2 and the Niņo 3.4 anomaly index value increasing form -0.5 to 0.0 (Table T2). SSTs in the extreme eastern Pacific (Niņo 1+2) remained above normal for the month (Figs. T5, T18, Table T2), but had returned to near normal by the end of the month (not shown).

The equatorial oceanic thermocline remained shallower than normal east of 145°W and deeper than normal to the west during the month (Fig. T15). Consistent with this pattern, oceanic temperatures were above (below) normal west (east) of 145°W at thermocline depth (Fig. T17). During the past few months the thermocline has deepened across the central Pacific (Fig. T15). Associated with this deepening, positive temperature anomalies at thermocline depth have spread from the western Pacific into the central Pacific. This deepening and warming is typically observed during the late stages of a cold episode.

Tropical convection during April was slightly below normal over the west-central and central Pacific [as inferred from positive anomalies of outgoing longwave radiation (OLR)] and well above normal over the western Pacific, Indonesia, and northern Australia (Fig. T25). This pattern of anomalous convection is consistent with La Niņa conditions and has largely persisted since mid-1998 (Fig. T8). Elsewhere, convection was also enhanced over the southern part of the Sahel and over southern India, possibly associated with an early start to the monsoon seasons in these regions.

The anomalous tropical convection during April was again associated with low-level (850 hPa) easterly wind anomalies over the western equatorial Pacific and westerly wind anomalies over the eastern Indian Ocean (Figs. T7, T20, and Table T1). Enhanced equatorial easterlies have persisted across the western and central Pacific since the onset of cold episode conditions in mid-1998 (Fig. T7). At upper levels, weak low latitude troughs and westerly anomalies, centered near the date line, were observed farther west during April than during the past few months (Figs. T21, T22).

The sea level pressure (SLP) pattern during April featured near normal SLP throughout most of the global Tropics (Fig. T19). This pattern was associated with a near normal value of the Southern Oscillation Index (SOI) (-0.1) and the smallest value of the equatorial SOI (0.8) since August 2000 (Table T1, Fig. T2). Interestingly, both the SLP anomalies at Tahiti and Darwin were close to zero (Table T1).

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