A modulation of the mechanism of the semiannual oscillation in the Southern Hemisphere

The local pressure changes associated with the twice-annual contraction/intensification and expansion/weakening of the circumpolar trough of low pressure around Antarctica, termed the semiannual oscillation (SAO), was the dominant signal in the annual cycle at mid and high southern latitudes before 1979. The mechanism, as shown by Van Loon (1967), arises from different response to the surface heat budget over the polar continent and the midlatitude ocean. It has subsequently been shown that in most years since 1979 the SAO has weakened considerably. Evidence is presented here from surface temperature data, 500 mb temperatures from a station pair and zonal mean 500 mb temperatures from the NCAR/NCEP reanalyses to show that a warming trend since 1979 has not been evenly distributed through the year at each latitude. Thus an anomalous change in the temperature gradient between 50 degrees S and 65 degrees S, with peaks in roughly May and November, has modulated the mechanism that produces the SAG, with its peaks in March and September. Consequently, the magnitude of the SAO has decreased in the more recent period.