Intraseasonal Interactions between the Tropics and Extratropics in the Southern Hemisphere

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  • 1 Department of Meteorology, University of Utah, Salt Lake City, Utah
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Abstract

The mechanics of the interaction between tropical beating [estimated from outgoing longwave radiation (OLR)] and Southern Hemisphere (SH) subtropical and extratropical circulations on intraseasonal time scales are discussed. Base points are selected from teleconnectivity and teleconnection maps between OLR and zonal wind, heights, and meridional component of the divergent wind. Then, composites are formed for pentads with OLR anomalies at the base point greater in magnitude than one standard deviation.

Enhanced convection over Indonesia is found to be associated with increases of both the southward component of the meridional divergent wind and of the westerly, zonal wind to the south of the heating region during the SH summer. The increased westerly wind gradients, resulting to a certain extent from strengthened northerly flow, together with increased values of the southward component of the divergent wind, lead to an enhancement of the Rossby wave source in the vorticity equation in the vicinity of Australia. Streamfunction anomalies indicate that a wave train evolves from this region, following the typical ray path expected from linear theory.

Tropical-extratropical connections are less pronounced during SH winter than during summer, though an increase of westerly winds in the SH is found associated with enhanced convective activity in the Northern Hemisphere. The increase of the zonal wind during winter is again explained by meridional overturnings that emanate from the heating regions. Isentropic trajectories are used to show that the zonal accelerations caused by the poleward motion at upper levels are in agreement with observed values. The enhancement of convective activity is also related to a southward increase of the meridional component of the divergent wind that maximizes near the equator. However, since the latitudes of maximum southward component of the meridional divergent wind differ from those with maximum changes in the gradient of absolute vorticity, no increase of the Rossby wave source or excitation of Rossby waves due to tropical heating is found during this season.

Abstract

The mechanics of the interaction between tropical beating [estimated from outgoing longwave radiation (OLR)] and Southern Hemisphere (SH) subtropical and extratropical circulations on intraseasonal time scales are discussed. Base points are selected from teleconnectivity and teleconnection maps between OLR and zonal wind, heights, and meridional component of the divergent wind. Then, composites are formed for pentads with OLR anomalies at the base point greater in magnitude than one standard deviation.

Enhanced convection over Indonesia is found to be associated with increases of both the southward component of the meridional divergent wind and of the westerly, zonal wind to the south of the heating region during the SH summer. The increased westerly wind gradients, resulting to a certain extent from strengthened northerly flow, together with increased values of the southward component of the divergent wind, lead to an enhancement of the Rossby wave source in the vorticity equation in the vicinity of Australia. Streamfunction anomalies indicate that a wave train evolves from this region, following the typical ray path expected from linear theory.

Tropical-extratropical connections are less pronounced during SH winter than during summer, though an increase of westerly winds in the SH is found associated with enhanced convective activity in the Northern Hemisphere. The increase of the zonal wind during winter is again explained by meridional overturnings that emanate from the heating regions. Isentropic trajectories are used to show that the zonal accelerations caused by the poleward motion at upper levels are in agreement with observed values. The enhancement of convective activity is also related to a southward increase of the meridional component of the divergent wind that maximizes near the equator. However, since the latitudes of maximum southward component of the meridional divergent wind differ from those with maximum changes in the gradient of absolute vorticity, no increase of the Rossby wave source or excitation of Rossby waves due to tropical heating is found during this season.

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