A predominant reversal in the relationship between the SAM and East Antarctic temperatures during the Twenty-First Century

The scientific literature portrays a temporally invariant spatial relationship between the phase of the southern annular mode (SAM) and the sign of surface air temperature (SAT) anomalies across Antarctica. However, here the authors describe a predominant switch from a negative to positive SAM–temperature relationship (STR) across East Antarctica in austral summer/autumn during the first decade of the twenty-first century, when the SAM was generally weakly positive. Of the nine years that had a positive regional STR from 1957 to 2010, seven occurred during the last decade. This reversal appears to be a response to anomalous high pressure over East Antarctica, resulting from variability in the phase and amplitude of the local component of the zonal wavenumber 3 pressure pattern. In years when a reversed (positive) regional STR exists the anomalous circulation is such that there is greater energy flux into the region, while enhanced katabatic drainage across the continental interior disrupts the surface temperature inversion leading to warmer SATs inland, too. The average summer/autumn SAT increase across East Antarctica for years with reversed versus standard STR is ~1°C. Anthropogenically forced models fail to reproduce the trend toward the anomalous high pressure pattern so it is likely that the STR switch is due to natural internal climate variability. That such broadscale STR reversals can take place on decadal time scales needs to be considered when detecting and attributing recent Antarctic climate change and when utilizing isotope data from the East Antarctic ice core record to provide a proxy SAM index prior to the instrumental record.