Mechanisms of Global Teleconnections Associated with the Asian Summer Monsoon: An Intermediate Model Analysis*
Abstract The Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) are two subsystems of the Asian summer monsoon, and they exhibit different global teleconnection patterns. The enhanced ISM strengthens the South Asian high and Mascarene high, and the WNPSM excites a meridional tripolar wave train in the Northern Hemisphere and affects the Australian high in the Southern Hemisphere. To understand the dynamics behind these global teleconnections, especially the processes responsible for the cross-equatorial teleconnection, an intermediate model, describing a two-level troposphere and a steady planetary boundary layer (PBL), is linearized from the background horizontal wind field. The model results indicate that the ISM heating, located under the strong easterly vertical shear (VS) and close to the westerly jet in the Northern Hemisphere, can excite a barotropic Rossby wave that emanates northwestward and then propagates downstream along the westerly jet. Since the WNPSM heating is far away from the westerly jet over the North Pacific, it only excites a weak Rossby wave train, which cannot explain the meridional tripolar teleconnection associated with the WNPSM. It is found that both the ISM and WNPSM heating excite strong teleconnections in the Southern Hemisphere via an advection mechanism; that is, the background upper-level northerly winds can transport energy across the equator from the Northern Hemisphere summer monsoon to the Southern Hemisphere westerly jet. In addition, the PBL enhances monsoon teleconnections by trapping more energy in the upper troposphere. The elevated maximum monsoon heating also reinforces upper-level perturbations and enhances the teleconnection pattern.