Highlights - January 2001
Mature cold episode conditions continued throughout
the tropical Pacific during January 2001, as sea surface temperatures (SSTs) remained more
than 1.0°C below normal across the central equatorial Pacific between the date line and
160°W (Fig. T18). Significantly, SSTs in this region
remained well below 28°C, 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 5°C above normal in the western Pacific, and remained
more than 3°C below normal in the eastern Pacific (Fig. T17).
Over the past few months, the return to La Niña 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 Niña 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.