Highlights - September 2000
Sea surface temperatures (SSTs) remained weakly
negative over portions of the central and eastern equatorial Pacific during September,
with all four Niño region indices indicating SSTs within 0.5°C of normal for the second
consecutive month (Fig. T18, Table
T2). The oceanic thermocline remained deeper than normal in the west-central and
western equatorial Pacific during September (Fig. T16),
although it has shoaled 20-30 meters in that region during the past 6 months (Fig. T15). Consistent with this evolution, temperature
anomalies at thermocline depth have decreased from 4-6°C above normal in April to 2-3°C
above normal in September (Fig. T17). Over most of the
eastern and east-central equatorial Pacific the depth of the thermocline remained slightly
shallower than normal (Fig. T15), which is consistent with
the persistence of below-average temperatures in that region (Fig.
Tropical convection during September [as inferred from anomalous outgoing longwave
radiation (OLR)] was below-normal over the western and central Pacific (Figs. T1, T25). Tropical intraseasonal
activity (Madden-Julian Oscillation-MJO), with a period near 45 days, has increased over
the past six months (Fig. T11). During September, the MJO
activity was associated with the decreased convective activity over the western and
central Pacific. Elsewhere, convection and rainfall was below normal over the African
Sahel, which continues the drier-than-normal Sahel rainy season.
Low-level (850 hPa) easterly wind anomalies persisted over the western equatorial
Pacific during September (Fig. T20, Table
T1), while large cross-equatorial flow and westerly wind anomalies developed over
the eastern half of the tropical Pacific. This pattern of low-level winds was consistent
with the suppressed convection over the central tropical Pacific, and also with a
strengthening of the ITCZ over the eastern Pacific (Figs. T25,
T23 bottom). At upper levels, easterly anomalies were
observed over the east-central Pacific for the first time since September 1999 (Fig. T3).
The sea level pressure (SLP) pattern across the Tropics during September featured weak
negative anomalies across the Atlantic and Indian Oceans and weak positive anomalies over
the Pacific Ocean (Fig. T19). This pattern was associated
with an increase in both the Southern Oscillation Index (SOI) and the equatorial SOI to
1.0 for September (Table T1, Fig. T2).