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HOME > Expert Assessments > Climate Diagnostics Bulletin > Tropical Highlights

Tropical Highlights - July 2004

Equatorial sea surface temperature (SST) anomalies increased across the central and east-central Pacific during July 2004, and remained below average in the eastern Pacific (Fig. T18, Table T2). Despite the increase in SST anomalies, the atmospheric indices continue to be dominated by month-to-month variability associated with intraseasonal (Madden-Julian Oscillation - MJO) activity (Figs. T11, T12, T13, Table T1). The pattern of Tropical SSTs during July featured positive anomalies greater than +1.0C across the central Pacific between 175E and 140W and below average SSTs east of 125W (Fig. T18). The SST anomaly in the Nio 3.4 region increased to 0.6, the highest value since October 2003, and the Nio 4 anomaly increased to 0.8, the highest value since December 2003 (Table T2). In contrast, the anomaly in the Nio 1+2 region remained less than -1.0 for the third consecutive month (Table T2).

The oceanic thermocline, measured by the depth of the 20 C isotherm deepened in the central Pacific and remained shallower than average in the eastern Pacific (Figs. T15, T16), resulting in an increased slope of the oceanic thermocline relative to June. Consistent with these conditions, oceanic temperature anomalies at thermocline depth increased to 2-3 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 July, low-level westerly wind anomalies (more than 3.0 m s-1) were observed over the central equatorial Pacific (Fig. T20), which resulted in the lowest value of the 850-hPa zonal wind index in this region since June 2003 (Fig. T4).

The global Tropics featured near-normal convection during July (Fig. T25). Across the eastern Pacific, negative OLR anomalies near 15N and positive anomalies near 10N indicate a northward shift of the ITCZ. MJO activity has contributed to increased variability over the equator between 60E and the date line since November 2003 (Fig. T11).

The Tahiti - Darwin SOI was -0.7 during July (Table T1, Fig. T1), and the equatorial SOI increased to 0.4 (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 2 months, in response to higher than normal pressure over Darwin.

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