Baroclinicity, Meridional Temperature Gradients, and the Southern Semiannual Oscillation

It has long been known that a strong half-yearly oscillation exists in surface pressure at high southern latitudes. There are two minima during the year, the one occurring in October being more intense than that in March. The semiannual oscillation (SAO) has been related to the different surface types and inertia at 50° and 65°S. These conspire to produce a semiannual wave in the meridional temperature gradient between those two latitudes; the timing of the maxima for these is as above, but the stronger is in March. However, if the arguments of van Loon are applied literally, then the seasonal evolution of temperature gradient should follow a comparable seasonal evolution of baroclinicity and surface pressure. A full calculation 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.