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Tropical Highlights - February 2001

Mature cold episode conditions continued across the tropical Pacific during February 2001, as sea surface temperatures (SSTs) remained more than 1.0C below normal across the central equatorial Pacific between 165E and 165W (Fig. T18). Significantly, SSTs in this region were again well below 28C, which is sufficient to suppress convective activity over the central equatorial Pacific (Fig. T18, top).

Consistent with the pattern of SST anomalies, the equatorial oceanic thermocline remained shallower than normal across the eastern Pacific and deeper than normal in the western Pacific (Fig. T15). Also, subsurface oceanic temperatures remained more than 4C above normal in the western Pacific, and more than 3C below normal in the eastern Pacific (Fig. T17). During February, the thermocline shoaled between the date line and 125W (Fig. T15), which resulted in a lessening of the temperature gradient across the central Pacific at thermocline depth.

Tropical convection during February remained 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 extreme western Pacific, Indonesia, and northern Australia (Fig. T25). This pattern of anomalous convection is consistent with La Nia conditions and has largely persisted since mid-1998 (Fig. T8). Intraseasonal activity (Madden-Julian Oscillation-MJO) intensified during February (Fig. T11), as a strong MJO enhanced convection over the Indian Ocean and Indonsesia.

The anomalous tropical convection during February was 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, large westerly wind anomalies (15 - 20 ms-1) over the central equatorial Pacific were associated with strong cyclonic circulation anomalies flanking the region of suppressed convection (Fig. T21, T22). This resulted in a 200-hPa zonal wind index value of 3.0, equaling the largest value observed (February 1989) in the historical record dating back to 1979 (Fig. T3). This pattern of upper-level and lower level wind anomalies is consistent with an increased strength of the equatorial Walker circulation (Fig. T29).

The sea level pressure (SLP) pattern throughout the global Tropics during February featured negative anomalies throughout most of the Tropics (Fig. T19). This pattern was associated with positive values of both the Southern Oscillation Index (SOI) (1.5) and the equatorial SOI (1.4) during the month (Table T1, Fig. T2), with much of the contribution to these values coming from the large negative values at Darwin (-2.9) and Indonesia. The value at Darwin is the largest negative value since November 1973.

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