The wave-zonal mean flow interaction in the Southern Hemisphere

This study examines the wave-zonal mean flow interaction in the Southern Hemisphere (SH) and in an SH-like model atmosphere. During the SH winter, when there exist both subtropical and polar front jets, growing baroclinic waves are found between the two jets as well as along the polar front jet. These baroclinic waves between the two jets pump westerly momentum into the interjet region flanked by the subtropical and polar front jets. As a result, these baroclinic waves blend the two jets, preventing the longtime occurrence or establishment of two well-defined jets. It is also shown that during the SH winter, the deceleration of the westerlies on the equatorward side of the subtropical jet is mostly associated with the above baroclinic waves in the interjet region, rather than with eddies that originate along the polar front jet. A set of idealized numerical model experiments shows that as the value of the surface friction is increased, the direction of the baroclinic wave momentum flux in the interjet region changes, resulting in a drastically different climatological flow.