Tropical Highlights - January
Pacific warm episode (El Niņo)
conditions continued during January 2003 as sea surface temperatures (SST)
anomalies across the equatorial Pacific remained more than 1°C
above normal between 170°E and 125°W
(Fig. T18, Table T2). However, a weakening of El Niņo
conditions was observed during the month, as SST anomalies decreased across
most of the equatorial Pacific. The largest decreases occurred in the
eastern equatorial Pacific, with anomalies declining more than 1.0°C
between 120°W and 100°W
(not shown). Consistent with this weakening, the anomaly in all four Niņo
region indices decreased, with the largest decrease occurring in the Niņo
3 region (Table T2, Fig. T5)
Consistent with mature El NiZo
conditions, the oceanic thermocline remained deeper than normal across the
central and eastern equatorial Pacific and shallower than normal in the
western Pacific (Figs. T15, T16).
Over the past few months, the thermocline has shoaled across the eastern
half of the Pacific, consistent with a decrease in upper-ocean heat content.
At thermocline depth sub-surface ocean temperatures were 3-4°C
above normal in the east-central equatorial Pacific between 150̊W
and 130̊W, and were 2-4°C
below normal in the western equatorial Pacific (Fig.
T17). This pattern of sub-surface ocean temperatures is consistent
with mature warm episode conditions.
Convection (precipitation and cloudiness) remained enhanced across the
central equatorial Pacific and near-normal or slightly below normal over
Indonesia during January (Fig. T25).
Enhanced convection has persisted over the central equatorial Pacific since
August (Fig. T11), consistent with ongoing
warm episode conditions. Elsewhere, convection was suppressed over
southeastern Africa and over northeastern South America. Both of these
features are typically observed during warm episodes.
The upper-level circulation over the subtropical Pacific during January
featured an anomalous anticyclonic couplet just east of the date line (Figs.
T21, T22). This anomalous couplet flanked the region of enhanced
convection over the central equatorial Pacific, and is consistent with the
atmospheric response to mature El NiZo
The tropical sea-level pressure (SLP) pattern during January featured
positive SLP anomalies over most of the global Tropics, except for small
negative anomalies over the central and eastern Pacific (Fig. T19).
This pattern was associated with negative values of both the Tahiti-Darwin
Southern Oscillation Index (SOI) (-0.4) and the equatorial SOI (-0.9) (Figs.
T1, T2, respectively). This was the smallest negative value of the SOI
since April 2002 (Table T1).