Tropical
Highlights - September 2001
ENSO-neutral conditions prevailed across the
tropical Pacific during September 2001, with sea surface temperatures (SSTs) 0.5 to 1.0°C
above normal across the western and central Pacific and more than 1.0°C below normal
across the east-central and eastern Pacific (Fig. T18).
This SST anomaly dipole was also reflected in the four Niņo region indices, with an
above-normal value in the Niņo 4 region, and below normal values in the Niņo 1+2, 3, and
3.4 regions (Table T2).
The equatorial oceanic thermocline (indicated by the 20°C isotherm) remained deeper
than normal in the western and central Pacific during the month and was slightly shallower
than normal in the extreme eastern Pacific (Fig. T15).
Consistent with these conditions, temperatures at thermocline depth were 2-3°C above
normal throughout the western and central Pacific and 2-3°C below normal in the eastern
equatorial Pacific (Fig. T17). This pattern is very
similar to the pattern of ocean anomalies observed last month.
Tropical convection during September was near normal across the Pacific, slightly
enhanced over parts of the African Sahel, and slightly below normal over northern India (Fig. T25). Convection across the tropical Pacific continued
to be influenced by intraseasonal (Madden-Julian Oscillation-MJO) activity during the
month.
Both the low-level (850-hPa) and the upper-level (200-hPa) winds were near normal
across the equatorial Pacific during September (Figs. T20, T21). The low-level winds have been normal across the equatorial
Pacific since April 2001 (Table T1, Fig T4),
while the upper-level winds have been near-normal in every month (except for August) since
February 2001 (Table T1, Fig. T3).
The sea level pressure (SLP) pattern during September featured small, positive
anomalies across the east-central and eastern tropical Pacific and small, negative
anomalies throughout most of the rest of the Tropics (Fig. T19).
This pattern was associated with a slightly positive value of the Tahiti - Darwin Southern
Oscillation Index (SOI) (0.2) (Table T1). The equatorial
SOI remained positive during September (1.0). This index has been positive for every month
since the beginning of the cold episode in the middle of 1998 (Fig.
T2). |
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