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 episode conditions.

The upper-level circulation during December featured an anticyclonic circulation anomaly in the subtropical Northern Hemisphere over the central Pacific (Figs. T21, T22). This 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.