Tropical Highlights - April 2004

Sea surface temperature (SST) anomalies remained near-average across the entire equatorial Pacific during April 2004 (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 April featured small positive values over the western and central Pacific and small negative values over the eastern Pacific (Fig. T18). The values in the four Niño regions were all within 0.3°C of zero (Table T2).

The oceanic thermocline, measured by the depth of the 20°C isotherm remained deeper than average in the western and west-central Pacific and shallower than average in the eastern Pacific (Figs. T15, T16). Consistent with these conditions, oceanic temperatures at thermocline depth were 2-3°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 April, stronger-than-average low-level easterly winds were again observed over the 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 featured small OLR anomalies during April. Enhanced cloudiness and precipitation (negative OLR anomalies) were observed over the west-central Pacific, while suppressed cloudiness and precipitation (positive OLR anomalies) were observed over Indonesia, the western Pacific and northern Australia (Fig. T25). Enhanced convection has been more persistent between 160°E and the date line for the past two months (Fig. T11).

The Tahiti - Darwin SOI was -1.3 during April (Table T1, Fig. T1), while the equatorial SOI increased to 0.8 (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).