Tropical Highlights - September 2004

Positive sea surface temperature (SST) anomalies persisted across the central and east-central equatorial Pacific and expanded eastward into the eastern equatorial Pacific during September 2004 (Fig. T18, Table T2).  Meanwhile, the atmospheric indices continued to show month-to-month variability associated with intraseasonal (Madden-Julian Oscillation - MJO) activity (Figs. T11, T12, T13, Table T1).  The pattern of Tropical SSTs during September featured positive anomalies greater than +1.0°C between 170°E and 130°W (Fig. T18).  The SST anomaly in the Niño 3.4 region remained 0.8, the highest value since January 2003, and the Niño 4 anomaly increased to 1.1, the highest value since November 2003 (Table T2).  The negative SST anomalies in the eastern equatorial Pacific decreased in magnitude from those observed during the last few months, with a September value of  -0.4 in the Niño 1+2 region (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 deepened in the eastern Pacific (Figs. T15, T16). This resulted in a decreased slope of the thermocline relative that observed in August. Consistent with these conditions, oceanic temperature anomalies at thermocline depth increased to 1-2°C above average in the 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 (Madden-Julian Oscillation) activity (Table T1).  During September, near-average low-level winds were observed over the equatorial Pacific (Fig. T20).

 The global Tropics featured near-average convection during September, with the exception of enhanced convection over the western Pacific and Indian Ocean (Fig. T25). MJO activity has contributed to increased variability over the equator between 60°E and the date line since November 2003.  Across the tropical Atlantic, negative OLR anomalies extended north to 30°N, in a region that experienced numerous tropical cyclones during the month (Fig. T11).

The Tahiti - Darwin SOI was -0.4 during September (Table T1, Fig. T1), and the equatorial SOI increased to -0.1 (Fig. T2).  The Tahiti - Darwin SOI has exhibited large month-to-month variability since November 2003 in response to MJO activity (Table T1). However, the SOI has been negative during the past 4 months, with less month-to-month variability, in response to higher-than-average pressure over Darwin.