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Tropical Highlights - November 2002

The evolution toward mature Pacific warm episode (El Nio) conditions continued during November 2002 as sea surface temperatures (SST) and SST anomalies increased across the equatorial Pacific east of the date line (Table T2). Positive SST anomalies exceeded +2C across much of the equatorial Pacific between the date line and 110W, and exceeded +1C between 170E and the west coast of South America (Fig. T18). Consistent with this warmth the Nio 3.4 SST index increased to 1.8, its largest value since February 1998 near the end of the 1997-98 El Nio episode (Fig. T5, Table T2).

Consistent with this evolution the oceanic thermocline has steadily deepened across the eastern half of the equatorial Pacific during the past few months, and was more than 30 m deeper than average between 160W and 100W during November, and more than 50 m deeper than average between 135W and 120W (Figs. T15, T16). In the western Pacific the thermocline has shoaled in recent months, and was 10 m shallower than normal during November. At thermocline depth sub-surface ocean temperatures have increased to 6-7C above normal between 155W and 130W, and decreased to 1-2C 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 November (Fig. T25). This pattern has persisted since August (Fig. T11) and is also consistent with ongoing warm episode conditions. Because of the enhanced convection near the date line the central Pacific OLR index has averaged -1.5 since August (Table T1), its lowest value since late 1997 (Fig T1).

The upper-level circulation featured anticyclonic circulation anomalies in the subtropics of both hemispheres over the central Pacific (Fig. T22). These anomalies flanked the region of enhanced convection over the central equatorial Pacific, and are consistent with the atmospheric response to mature El Nio conditions.

The tropical sea-level pressure (SLP) pattern reflected an anomalous wave-1 pattern during November, with below-average pressure over most of the tropical Pacific and above-average pressure elsewhere (Fig. T19). This pattern again resulted in small, negative values of both the Tahiti-Darwin Southern Oscillation Index (SOI) (-0.6) and the equatorial SOI (-0.4) (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 -0.9.


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