THE MEAN STRUCTURE AND TEMPORAL VARIABILITY OF THE SEMIANNUAL OSCILLATION IN THE SOUTHERN EXTRATROPICS

The semiannual oscillation of the pressure and mid-tropospheric barocl inicity in the southern extratropics has been studied using 21 years ( 1973-1993) of numerical Southern Hemisphere analyses and long series o f pressure data from a number of mid-and high latitude stations. Using recent data and techniques not previously documented, this study has verified that the semiannual oscillation is an important but highly va riable feature of the annual cycle of pressure and meridional temperat ure gradient. In the pressure, the half-yearly wave attains its greate st amplitudes in the mid-latitude ocean basins and on the Antarctic pe riphery, with a minimum near 55 degrees S. The semiannual oscillation of the temperature gradient is strongest near 60 degrees S, where it e xplains typically 50% of the mean annual variance of monthly data, and the harmonic maxima (strongest gradients) occur during the transition seasons. Analysis of the temporal behaviour of the half-yearly wave h as revealed considerable variability on interannual to decadal time sc ales. The comparison and correlation of the temporal variability of th e mid-and high latitude components of the semiannual oscillation of th e pressure has revealed that these are statistically related and the s trength of the relationship is greatest for variations occurring on lo nger time scales. In both the mid-and high latitudes the temporal vari ability of the semiannual oscillation of pressure has been found to be statistically related to the variability of the high latitude tempera ture gradient. These observations suggest that the differing annual cy cles of temperature in the mid-and high southern latitudes not only gi ve rise to the semiannual oscillation of the pressure in the mean, but , in addition, the variability of this dynamic phenomenon is linked to the variability of the thermal gradients. These findings suggest that a key to understanding the temporal variability of pressure in the so uthern extratropics on annual to decadal time scales lies in the under standing of the large scale variability of the temperature gradients. (C) 1998 Royal Meteorological Society.