The Asian summer monsoon greatly impacts water resources, agriculture, and ecosystems over the densely populated regions. While precipitation changes over Asia have been extensively studied, the roles of external forcing and internal variability in forming the large-scale Asian precipitation pattern over the past millennium remain underexplored. Here we find that the leading decadal precipitation variability mode over Asia features a mega-tripole precipitation change across South Asia, southeastern Asia, and northern East Asia. This leading mode results from remote forcing from the interdecadal Pacific Oscillation (IPO) but is modulated and synchronized by volcanic forcing. The IPO induces the mega-tripole precipitation mode over Asia by decreasing the tropospheric thermal contrast and driving Walker circulation variations. Volcanic aerosol forcing stimulates IPO-like SST anomalies, which excite the mega-tripole mode through mechanisms like those from the internal IPO. Nonetheless, volcanic forcing effect is distinguishable from the internal IPO due to subtle differences in the resulting large-scale SST and atmospheric circulation anomalies. These anomalies, induced by interhemispherically asymmetric external forcing, differ to the IPO’s more symmetric patterns and provide a pathway to differentiate internal and externally forced climate variations. These findings have important implications on how Asian precipitation may respond to climate interventions such as stratospheric aerosol injection.
 

Mega-tripole precipitation change; last millennium; external forcing; internal variability