Baroclinicity, meridional temperature gradients, and the southern semiannual oscillation

It has long been known that a strong half-yearly oscillation exists in surf ace pressure at high southern latitudes. There are two minima during the ye ar, the one occurring in October being more intense than that in March. The semiannual oscillation (SAO) has been related to the different surface typ es and inertia at 50 degrees and 65 degrees S. These conspire to produce a semiannual wave in the meridional temperature gradient between those two la titudes; the timing of the maxims for these is as above, but the stronger i s in March. However, if the arguments of van Loon are applied literally, th en the seasonal evolution of temperature gradient should follow a comparabl e seasonal evolution of baroclinicity and surface pressure. A full calculat ion of baroclinicity shows that the seasonal evolution of static stability, combined with the twice-annual maxima of meridional temperature gradient, produces a larger peak of baroclinicity in October consistent with the SAO in surface pressure.