Instrument- and Tree-Ring-Based Estimates of the Antarctic Oscillation

Abstract

An estimate of the strength of the austral summer Antarctic Oscillation using station sea level pressure records for the period 1878?2000 is presented, the first to the authors' knowledge. The reconstruction was obtained by relating the Antarctic Oscillation (AAO) intensity derived from NCEP?NCAR reanalysis data to the leading principal components of station records using multiple regression analysis. Particular effort has been made to fit the model in a way that is robust to the questionable trends in the NCEP?NCAR data in the Southern Hemisphere. The trends in the reconstruction are derived from the station data, not from the NCEP data. Cross-validation with the NCEP data and comparison with other analyses of the AAO over the late instrumental period give confidence that this station-based reconstruction can be regarded as trustworthy. With regard to the whole reconstruction period, some unquantifiable uncertainty stems from potential instability of the statistical relationships. To extend this record further back, a reconstruction using tree-ring chronologies back to 1743 has also been undertaken. Comparison with the station-based reconstruction shows moderate agreement on interannual and decadal timescales, but the comparison also points toward the inherent uncertainties of proxy-based climate reconstructions. In particular, it was found that this tree-based reconstruction may have been influenced by a warming that is not related to changes in the Antarctic Oscillation index during the twentieth century. Comparison of the tree-based reconstruction with a published reconstruction of zonal flow over New Zealand before the twentieth century shows common features. The temperature and precipitation signals of the Antarctic Oscillation have been calculated and show that the response of the chronologies to Antarctic Oscillation variability is physically plausible. In addition, it was shown that a substantial fraction of the observed warming over much of Antarctica between the late 1950s and the 1980s can be linked to changes in the Antarctic Oscillation, whereas the observed warming over New Zealand is related to other influences.