Tropical Highlights - August 2004

Sea surface temperature (SST) anomalies continued to increase across the central and east-central equatorial Pacific during August 2004, and remained negative in the eastern equatorial Pacific (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 August featured positive anomalies greater than +1.0�C between 175�E and 130�W and negative anomalies less than -1.0�C east of 100�W (Fig. T18).  The SST anomaly in the Ni�o 3.4 region increased to 0.8, the highest value since January 2003, and the Ni�o 4 anomaly increased to 0.9, the highest value since November 2003 (Table T2).  In contrast, the anomaly in the Ni�o 1+2 region remained negative (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 temperature anomalies at thermocline depth increased to 3-4�C above average in the western and central equatorial Pacific and remained 2-3�C below 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 activity (Table T1).  During August, near-normal low-level winds were observed over the central and eastern equatorial Pacific (Fig. T20), in contrast to the large low-level westerly wind anomalies over the central equatorial Pacific observed during July.

 The global Tropics featured near-normal convection for the second month in a row (Fig. T25).  Across the tropical Pacific, negative OLR anomalies near 15�N indicate a northward shift of the ITCZ.  MJO activity has contributed to increased variability over the equator between 60�E and the date line since November 2003 (Fig. T11).

The Tahiti - Darwin SOI was -0.8 during August (Table T1, Fig. T1), and the equatorial SOI decreased to -0.2 (Fig. T2).  The Tahiti - Darwin SOI has exhibited large month-to-month variability since November 2003, in response to MJO activity (Fig. T10), although it has been negative during the past 3 months, in response to higher than normal pressure over Darwin.