Tropical Highlights - December 2002
Pacific warm episode (El Niño)
conditions continued during December 2002 as sea surface temperatures
(SST) anomalies across the equatorial Pacific remained more than 1°C
above normal between 170°E
and the west coast of South America (Fig. T18, Table T2).
Positive SST anomalies exceeded +2°C
in the equatorial Pacific between the 175°W
and 140°W and also between
120°W and 105°W
(Fig. T18). Consistent with this warmth the Niño
3.4 SST index was 1.6 for the month, the third consecutive month that
this index exceeded 1.5 (Table T2).
Consistent with this evolution the oceanic thermocline remained
deeper than normal across the central and eastern equatorial Pacific
and was shallower than normal in the western Pacific (Figs. T15,
T16). At thermocline depth sub-surface ocean temperatures were 5-6°C
above normal in the eastern equatorial Pacific between 120°W
and 100°W, and were 2-3°C
below normal in the western equatorial Pacific (Fig. T17). This
pattern of sub-surface ocean temperatures is consistent with
developing mature warm episode conditions.
Convection remained enhanced across the central equatorial Pacific
and suppressed over Indonesia and the western Pacific during December
(Fig. T25). This pattern has persisted since August (Fig.
T11) and is also consistent with ongoing warm episode conditions.
Elsewhere, convection was enhanced over most of the tropical Indian
Ocean, extending northward into extreme southern India and Sri Lanka.
This region is typically wetter-than-normal in association with warm
The upper-level circulation during December featured an
anticyclonic circulation anomaly in the subtropical Northern
Hemisphere over the central Pacific (Figs. T21,
anomaly flanked the region of enhanced convection over the central
equatorial Pacific, and is consistent with the atmospheric response to
mature El Niño conditions.
The northern flank of the anomaly was also associated with an enhance
subtropical jet stream that extended from the eastern Pacific across
the extreme southern tier of the United States (Fig. T21).
Tropical intraseasonal oscillations associated with the
Madden-Julian Oscillation (MJO) strengthened during November and
December, after remaining weak during the period from August - October
(Figs. T11, T12). During December, they contributed to a
strengthening of the low-level (850-hPa) winds across the western
equatorial Pacific during second half of the month (Fig. T13).
The tropical sea-level pressure (SLP) pattern featured positive SLP
anomalies over most of the global Tropics during December (Fig. T19).
This pattern was associated with negative values of both the
Tahiti-Darwin Southern Oscillation Index (SOI) (-1.4) and the
equatorial SOI (-0.3) (Figs. T1, T2, respectively). The SOI has
fluctuated between -0.5 and -1.5 since March 2002 (Table T1),
with the most recent five-month running mean of -1.0.