Cold episode conditions continued across the
tropical Pacific during October, as sea surface temperatures (SSTs) averaged more than
1.0°C below normal across the central and eastern tropical Pacific (Fig.
T18). The equatorial oceanic thermocline remained shallower than normal across the
east-central and eastern Pacific during the month, and deeper than normal in the western
Pacific (Fig. T15). Consistent with this pattern,
temperatures at thermocline depth remained more than 4°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 Niņa conditions.
Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was
suppressed over the western and central equatorial Pacific, and enhanced over Indonesia
during October (Fig. T25). Convection has been suppressed
across the central equatorial Pacific since the onset of cold episode conditions in late
May 1998 (Fig. T8). Elsewhere, convection was enhanced over
much of the northern Indian Ocean and over the Caribbean Sea, both in association with
periods of enhanced tropical cyclone activity (Fig. E4).
Enhanced low level (850 hPa) easterly winds also persisted across the central and
western tropical Pacific during October (Fig. T20).
Anomalous easterlies have prevailed in this region since May 1998 (Fig.
T7), in association with ongoing La Niņa conditions. Elsewhere, low-level
westerly wind anomalies again covered the subtropical North Atlantic during the month,
consistent with above-normal rainfall across the African Sahel. The upper-level
atmospheric circulation (200 hPa) in the Tropics and subtropics also remained consistent
with continuing cold episode conditions, with well-developed upper-level troughs observed
over the low-latitudes of the western and central Pacific in both hemispheres and
amplified subtropical and lower mid-latitude ridges observed across most of the remainder
of both hemispheres (Figs. T21, T22).
The Southern Oscillation Index (SOI) was 0.9 during October (Table
T1, Fig. T1), and the equatorial SOI was 1.0 (Fig. T2), indicating continued cold episode conditions. The
pattern of sea level pressure (SLP) anomalies during the month was also consistent with
cold episode conditions, with positive anomalies observed across the tropical Pacific and
negative anomalies observed over Indonesia and the Indian Ocean (Fig.