September 2007 negative sea surface temperature (SST) anomalies in the
equatorial Pacific Ocean continued to expand westward, and now extend from 170ºE to the South American coast
(Fig. T18). The
SST anomalies became increasingly negative all of the Niño regions, with the Niño-3.4
index dropping to -0.8°C and the Niño-4 index dropping to -0.4°C
T2). The sub-surface temperature
departures also became increasingly negative across the eastern equatorial Pacific,
where temperatures at thermocline
depth ranged from 2°C to 4°C below average (Fig. T17).
cooling is related to a strengthening and eastward expansion of the low-level
easterly wind anomalies across the central and eastern equatorial Pacific, with
the largest anomalies (6-8 m s-1) situated just west of the date line
These enhanced easterlies were associated with a continued positive value of the
equatorial SOI (+1.6) (Fig.
T2), although the
– Darwin SOI was near-average (+0.2) (Table
T1, Fig. T1).
They were also associated with a continued westward
retraction of the equatorial
circulation, as indicated by the ongoing confinement of deep tropical
and the far western Pacific, and the
lack of tropical convection near the date line (Fig.
At 200-hPa, westerly wind anomalies remained strong across the equatorial
Pacific Ocean, and an anomalous cyclonic circulation was again evident across
South Pacific Ocean
(Fig. T21). This latter feature reflects a
westward retraction of the mean subtropical ridge in response to the lack of
deep tropical convection east of the date line (Fig.
T22). This off-equatorial response to La Nina is normally strongest in
the Southern Hemisphere at this time of the year, and leads to a westward
retraction and weakening of the South Pacific jet stream.
For the latest status of the ENSO cycle see the ENSO Diagnostic Discussion at: