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Climate Prediction Center

Climate Diagnostics Bulletin
Climate Diagnostics Bulletin - Home Climate Diagnostics Bulletin - Forecast Climate Diagnostics Bulletin - Extratropics


  Tropical Highlights

  Table of Atmospheric Indices  (Table 1)

  Table of Oceanic Indices  (Table 2)

  Time Series

  Time-Longitude Sections

  Mean & Anomaly Fields

  Appendix 1: Outside Contributions

Tropical Highlights


Forecast Forum

During 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 (Table 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).

This 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 (Fig. T20). These enhanced easterlies were associated with a continued positive value of the equatorial SOI (+1.6) (Fig. T2), although the Tahiti – Darwin SOI was near-average (+0.2) (Table T1, Fig. T1). They were also associated with a continued westward retraction of the equatorial Walker circulation, as indicated by the ongoing confinement of deep tropical convection to Indonesia and the far western Pacific, and the ongoing lack of tropical convection near the date line (Fig. T25).

            At 200-hPa, westerly wind anomalies remained strong across the equatorial Pacific Ocean, and an anomalous cyclonic circulation was again evident across the subtropical 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:

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