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Tropical Highlights - January 2001

Mature cold episode conditions continued throughout the tropical Pacific during January 2001, as sea surface temperatures (SSTs) remained more than 1.0C below normal across the central equatorial Pacific between the date line and 160W (Fig. T18). Significantly, SSTs in this region remained 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 increased to more than 5C above normal in the western Pacific, and remained more than 3C below normal in the eastern Pacific (Fig. T17). Over the past few months, the return to La Nia conditions has been accompanied by an increase in both the slope of the oceanic thermocline (Fig. T16) and the magnitude of the east-west temperature gradient across the equatorial Pacific.

Tropical convection during January remained below normal over the 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). The lack of convection over the central equatorial Pacific was associated with an OLR index value of 2.1, which is the largest value observed since March 2000 (Fig. T1). This pattern of anomalous convection is consistent with La Nia conditions and has largely persisted since mid-1998 (Fig. T8). In contrast, intraseasonal activity (Madden-Julian Oscillation-MJO), which had strongly impacted tropical convection during the fall and early winter, weakened during January (Fig. T11). This decrease in intraseasonal activity is also consistent with mature cold episode conditions.

The anomalous tropical convection during January 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, westerly wind anomalies persisted over the central equatorial Pacific, which is consistent with cyclonic circulation anomalies flanking the region of suppressed convection (Fig. T21, T22). These upper-level wind anomalies are also consistent with an increased strength of the equatorial Walker circulation (Fig. T29).

The sea level pressure (SLP) pattern throughout the global Tropics during January featured negative anomalies over the Indian Ocean, Indonesia and the western Pacific and positive anomalies over the remainder of the Tropics (Fig. T19). This pattern was associated with positive values of both the Southern Oscillation Index (SOI) (1.1) and the equatorial SOI (2.8) during the month (Table T1, Fig. T2). The value of the equatorial SOI is the largest observed since February 2000.

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