The Half-Yearly Oscillations in Middle and High Southern Latitudes and the Coreless Winter

Abstract

In mid-tropospheric levels of the Southern Hemisphere, the temperature contrast between middle and high latitudes reaches two maxima near the times of the equinoxes, the strongest contrast being at the autumnal equinox. This variation is dominated by the seasonal changes over the Antarctic Ocean where the second (semiannual) harmonic of the mid-tropospheric meridional temperature gradient has a magnitude exceeding that of the first (annual) harmonic. The existence of the marked semiannual component in this region is shown to be a consequence of the difference between the heating and cooling rates in different latitudes where the mid-tropospheric annual temperature ranges are similar. In latitude 50S cooling in autumn is rapid compared with the warming in spring, the reverse being true at latitude 65S. This behavior is related to the heat budget of the oceanic upper layers. The semiannual maximum temperature gradients in the middle troposphere over the Antarctic Ocean, through increased cyclonic activity, cause the mean position of the circumpolar trough to be closer to the pole during the transition seasons. In turn, this produces a semiannual oscillation in the pressures and winds over the area affected by the trough. While the hemispheric mean pressure in middle latitudes falls with the equatorward shift in the mean position of the circumpolar trough from autumn to winter, the pressure rises over Australia, South America, and Africa. The resulting increased longitudinal pressure contrast between the continents and the neighboring sea is expressed as an amplification of the mean wave pattern at the surface, greatest in the half hemisphere centered on Australia. The enhanced poleward transfer of warm air accompanying this amplification apparently accounts for the cessation of rapid temperature falls over Antarctica in early winter, and for actual temperature rises at some stations such as Little America.