C below normal in the eastern Pacific and 1-2°C above
normal in the western Pacific (Fig. T17). This thermocline
structure in consistent with the ongoing La Niña conditions.
Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was
suppressed over Indonesia, as well as over both the western and central equatorial Pacific
during September (Fig. T25). Convection has been
suppressed across the central Pacific since mid-1998 (Fig. T8).
In contrast, September marked the first time since April 1998 that monthly mean convection
and rainfall was below normal over Indonesia (Fig. E4).
This decrease in convection in the Indonesian sector was associated with strong
intraseasonal activity (Fig. T11). Elsewhere, convection
was again enhanced across the African Sahel, with well above-normal rainfall recorded in
the region for the third consecutive month (Fig. E4).
Enhanced low level (850 hPa) easterly winds also persisted across the western Pacific
during September (Fig. T20). Anomalous easterlies have
prevailed in this region since May 1998 (Fig. T7), in
association with ongoing La Niña conditions. Elsewhere, low-level westerly wind anomalies
again covered the subtropical North Atlantic during the month, consistent with
above-normal rainfall across the Africa Sahel. The upper-level atmospheric circulation
(200 hPa) in the Tropics and subtropics remained consistent with continuing cold episode
conditions, with upper level troughs observed over the low-latitudes of the western
Pacific in both hemispheres (stronger in the Southern Hemisphere) and amplified
low-latitude ridges observed across most of the remainder of both hemispheres (Figs. T21, T22).
The Southern Oscillation Index (SOI) was -0.1 (Table T1, Fig. T1), and the equatorial SOI was 1.3 (Fig.
T2) during September. During the past five months, the SOI has averaged 0.1 (Fig.
T1). However, during the same time period, the equatorial SOI has averaged 1.2, and has
better depicted the ongoing cold episode.
Editors Note: Computer problems at NCEP have resulted in the temporary
loss of certain forecast and monitoring products. The CCA SST forecasts (Figs.
F1-F2) were not run, and the teleconnection indices (Table E1, Figs. E6,
E7) were not computed. In addition, some of the weekly/daily monitoring files
that appear on the CPC web site have also been temporarily lost. These interruptions are
expected to last until mid-November.