Tropical Highlights - May 2004

Sea surface temperature (SST) anomalies remained near-average across the western and central equatorial Pacific during May 2004, and decreased in the eastern Pacific (Fig. T18, Table T2), while atmospheric indices continued to exhibit month-to-month variability associated with intraseasonal (Madden-Julian Oscillations - MJO) activity (Figs. T11, T12, T13, Table T1).   The pattern of Tropical SST anomalies during May featured small positive values over the western and central Pacific and negative values over the eastern Pacific (Fig. T18).  The value in the Niño 1+2 region decreased to -1.2, the lowest value since June 2003, while the Niño 4 value increased to 0.5 (Table T2).

The oceanic thermocline, measured by the depth of the 20°C isotherm remained deeper than average in the western and central Pacific and shallower than average in the eastern Pacific (Figs. T15, T16).   Consistent with these conditions, oceanic temperatures at thermocline depth were 3-4°C above average in the western and west-central equatorial Pacific and 3-4°C below average in the east-central and eastern 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 November 2003 in association with intraseasonal activity (Table T1).  During May, stronger-than-average low-level easterly winds were observed over the western and central Pacific (Fig. T20).  Despite the intraseasonal activity, stronger-than-normal easterlies have been dominant over this region since November 2003 (Fig. T13).

The global Tropics were dominated by slightly below-normal convection during May.  Enhanced cloudiness and precipitation (negative OLR anomalies) were observed over the eastern Indian Ocean, parts of Indonesia and northern Australia, and the western Pacific, while suppressed cloudiness and precipitation (positive OLR anomalies) were observed over much of the rest of the Tropics (Fig. T25).  MJO activity has contributed to increased variability over the equator between 60°E and the date line (Fig. T11).

The Tahiti - Darwin SOI was 1.1 during May (Table T1, Fig. T1), while the equatorial SOI increased to 0.7 (Fig. T2).  The Tahiti - Darwin SOI has exhibited large month-to-month variability since November 2003, in response to MJO activity (Fig. T10).  The equatorial SOI has exhibited much less variability during the past year, remaining between 0 and 0.9 since March 2003 (Fig. T2).