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Tropical Highlights - December 2004

Across the central and east-central equatorial Pacific, sea surface temperature (SST) anomalies remained positive during December 2004.  The SST anomalies in the eastern equatorial Pacific have decreased slightly, with a December value of 0.1 in the Nio 1+2 region (Fig. T18, Table T2).  The pattern of tropical SSTs during December featured positive anomalies greater than +1.0C between 160E and 120W (Fig. T18).  The SST anomaly in the Nio 3.4 region increased to 0.9, the highest value since January 2003 (Table T2).  Meanwhile, atmospheric features continued to show month-to-month variability associated with intraseasonal (Madden-Julian Oscillation - MJO) activity (Figs. T11, T12, T13, Table T1).  The Tahiti - Darwin SOI (latest value -1.1, Table T1, Fig. T1) has been negative during the past 7 months, primarily in response to higher-than-average pressure over Darwin.

The oceanic thermocline, measured by the depth of the 20C isotherm, remained deeper than average across most of the equatorial Pacific during December (Figs. T15, T16). Consistent with these conditions, oceanic temperatures at thermocline depth were 2-3C above average across most of the equatorial Pacific (Fig. T17).

The monthly low-level (850-hPa) and upper-level (200-hPa) equatorial zonal wind indices, the Tahiti-Darwin SOI and the outgoing longwave radiation (OLR) index have all exhibited considerable variability since January 2004 in association with intraseasonal (Madden-Julian Oscillation) activity (Table T1).

Near-average low-level winds were observed over the equatorial Pacific during December 2004 (Fig. T20). The global Tropics featured near-average convection, with the exception of enhanced convection over the Indian Ocean (Fig. T25).  Enhanced convection was found across the subtropical North Atlantic (Fig. T25, Fig. E3), associated with anomalous upper-level and low-level troughs in that region (Fig. T20, T21) and enhanced upper-level divergent flow (Figs. T23, T24).

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