Distinct impacts of the boreal summer <scp>Asian–Australian</scp> monsoon and tropical sea surface temperature anomalies on the interannual variability of Southern Hemisphere circulation

Abstract The boreal summer Asian–Australian monsoon (BSAAM) is a powerful yet intricate climate system comprising several interacting subsystems. Moving beyond traditional subsystem‐based approaches, this study treats the BSAAM as an integrated entity and elucidates its coupled relationships with tropical sea surface temperature anomalies (SSTAs) and Southern Hemisphere (SH) atmospheric circulation. The first leading mode of the BSAAM (BSAAM1) corresponds to anomalies of the Western Pacific Subtropical High, mainly modulated by Niño3.4. The second leading mode of the BSAAM (BSAAM2) represents anomalies of the Australian High, primarily driven by the Indian Ocean Dipole (IOD). Interestingly, both BSAAM1 and BSAAM2 exhibit pronounced impacts on SH circulation, but with distinct spatial patterns. While monsoon variability and tropical SSTAs both act as key tropical forcings, their influences on SH circulation differ markedly. BSAAM1 independently generates the Rossby wave source (RWS) west of Australia, exciting southeastward‐propagating Rossby wave trains, whereas Niño3.4 induces a RWS east of Australia, leading to a meridional circulation response over the southern Pacific. In contrast, BSAAM2 and IOD are highly coupled, jointly producing a RWS over the subtropical Indian Ocean and triggering southeastward Rossby wave propagation. Eliminating either signal markedly weakens these effects, though BSAAM2 exerts a stronger independent role. These distinct SH circulation responses arise from different precipitation forcings and ultimately lead to contrasting Antarctic climate impacts. These observational results are further supported by the linear baroclinic model experiments. These findings provide new insights into the distinct pathways through which the BSAAM and tropical SSTAs modulate the interannual variability of SH circulation.