The semiannual oscillation and Antarctic climate, part 5: impact on the annual temperature cycle as derived from NCEP/NCAR re-analysis

We use NCEP/NCAR reanalysis data to study the impact of the semiannual osci llation (SAO) on the annual cycle of Antarctic near-surface temperature. Wh en the SAO is weak, the contracted phases (March/ April and September/Octob er) are warm and the expanded phases (December/January and June/July) cold. This pattern is explained in terms of the changing meridional fetch of the circumpolar pressure trough. Because of the wave number three character of the SAG, large regional deviations are found. For instance, enhanced north -westerly flow in the second expansion phase (June/July) of weak SAO years limits the growth of the sea ice in the Amundsen and Bellingshausen seas, l eading to anomalously high temperatures in the Antarctic Peninsula region. The short (<50 year) temperature records at Antarctic stations still carry the fingerprint of decadal SAO variability. By matching the observed monthl y temperature trends to the patterns derived from the gridded re-analysis, we propose a background Antarctic warming trend for the second expansion ph ase (June/July) of 4.62 +/- 1.02 degrees C per century, four times the annu al value.