The Southern Hemisphere Evolution of ENSO during 1981–99

Improvements in observing systems over the last two decades now permit more detailed examinations of variability in the mid- and high-latitude Southern Hemisphere circulation. In this study the leading modes of variation in the extratropical sea surface temperature are identified from the Reynolds optimal interpolation (OI) dataset between November 1981 and December 1999. Although attempts were made to exclude the strong contribution due to El Niño–Southern Oscillation (ENSO) by confining the analysis between 60° and 15°S, the three leading modes were all ENSO-related. The first EOF with 13.9% of the monthly variance lags the Southern Oscillation index (SOI) by 2 months and depicts sea temperature variations characteristic of mature ENSO events. The second EOF, accounting for 8.8% of the variance, shows a “precursor” pattern that is strongly correlated with the SOI and with EOF1, which it leads by 9–10 months. This pattern is characterized by anomalies west and east of Australia (extending north of New Zealand), a dipole in the Atlantic, and a further center to the southwest of South America. The third EOF expresses variations in the amplitude of the southern arm of the “horseshoe” pattern of the opposite sign surrounding the ENSO anomaly in the central Pacific. In view of these strong links to ENSO and identification of the precursor pattern, further analysis was made of ENSO development patterns over the 1980s and 1990s in combination with datasets expressing lower- and upper-tropospheric flow patterns, outgoing longwave radiation, sea ice extent, and cyclone frequency. These show the expected pattern of behavior in the tropical belt. Farther south, the sea temperature anomaly off the east Australian coast is seen to strengthen and propagate eastward over the Pacific near 30°S. During warm events Rossby waves, initiated by convection in the mid-Pacific, apparently force an anomalous anticyclonic circulation near 60°S in the eastern Pacific. This in turn leads to above-normal temperatures on its western side and a southward displacement of the sea ice boundary. Cyclone frequencies are only weakly related to the SOI and tend to be negatively correlated with the sea level pressure anomalies. Overall, these results show that, on interannual timescales, large-scale extratropical sea temperature variations in the Southern Hemisphere are primarily driven by ENSO, and are unlikely to make a significant independent contribution to seasonal climate prediction.