Tropical Highlights - January 2004

Surface and sub-surface oceanic conditions across the Tropical Pacific remained slightly warmer than average during January 2004 (Table T2), while atmospheric indices have featured increased variability associated with strong intraseasonal (Madden-Julian Oscillations - MJO) activity (Figs. T11, T12, T13, Table T1). Although sea surface temperature (SST) anomalies remained above average across the entire equatorial Pacific, the departures across the tropical Pacific decreased slightly from December (Table T2, Fig. T5). The largest SST departures (exceeding 1°C) remained in the western Pacific between 160°E and the date line (Fig. T18).

The strong MJO activity during December and January was instrumental in initiating an eastward-propagating oceanic Kelvin wave, which has resulted in an eastward shift of positive sub-surface temperature anomalies (Fig. T17) (deeper-than-average thermocline depths, Fig. T15). Consistent with these conditions, oceanic temperatures at thermocline depth increased to 2-3°C above average in the west-central equatorial Pacific (Fig. T17).

The low-level (850-hPa) and upper-level (200-hPa) equatorial winds were near-normal across the Pacific during January (Table T1, Figs. T20, T21). Tropical outgoing longwave radiation (OLR) anomalies were negative (enhanced cloudiness and precipitation) over the central Pacific between the equator and 10°S, equatorial Africa and northeastern Brazil and positive over the eastern Indian Ocean and Indonesia (Fig. T25). The pattern of convection over the western and central Pacific represents an eastward shift of the South Pacific Convergence Zone, while the enhanced convection over northeastern Brazil was associated with well-above normal precipitation (Figs. E3, E4). Both tropical convection and the low-level winds continued to be influenced by MJO activity (Figs. T11, T12).

The Tahiti-Darwin SOI was -1.7 during the January, the largest negative value of this index since April 1998 (Table T1, Fig. T1). This index has exhibited large week-to-week variability since late November, in response to strong MJO activity (Fig. T10). The equatorial SOI has exhibited less variability during the past few months, with the index remaining near 0.5 since November 2003 (Fig. T2).