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,
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 Niņo 3.4 region remained 0.8, the highest value since January 2003, and
the Niņo 4 anomaly increased to 1.1, the highest value since November 2003
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
Niņo 1+2 region (Table T2).
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).
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.
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.