INTRASEASONAL INTERACTIONS BETWEEN THE TROPICS AND EXTRATROPICS IN THE SOUTHERN-HEMISPHERE

The mechanics of the interaction between tropical heating [estimated f rom outgoing longwave radiation (OLR)] and Southern Hemisphere (SH) su btropical and extratropical circulations on intraseasonal time scales are discussed. Base points are selected from teleconnectivity and tele connection maps between OLR and zonal wind, heights, and meridional co mponent of the divergent wind. Then, composites are formed for pentads with OLR anomalies at the base point greater in magnitude than one st andard deviation. Enhanced convection over Indonesia is found to be as sociated with increases of both the southward component of the meridio nal divergent wind and of the westerly zonal wind to the south of the heating region during the SH summer. The increased westerly wind gradi ents, resulting to a certain extent from strengthened northerly flow, together with increased values of the southward component of the diver gent wind, lead to an enhancement of the Rossby wave source in the vor ticity equation in the vicinity of Australia. Streamfunction anomalies indicate that a wave train evolves from this region, following the ty pical ray path expected from linear theory. Tropical-extratropical con nections are less pronounced during SH winter than during summer, thou gh an increase of westerly winds in the SH is found associated with en hanced convective activity in the Northern Hemisphere. The increase of the zonal wind during winter is again explained by meridional overtur nings that emanate from the heating regions. Isentropic trajectories a re used to show that the zonal accelerations caused by the poleward mo tion at upper levels are in agreement with observed values. The enhanc ement of convective activity is also related to a southward increase o f the meridional component of the divergent wind that maximizes near t he equator. However, since the latitudes of maximum southward componen t of the meridional divergent wind differ from those with maximum chan ges in the gradient of absolute vorticity, no increase of the Rossby w ave source or excitation of Rossby waves due to tropical heating is fo und during this season.