Interannual to centennial variability of the South Asian summer monsoon over the past millennium

Proxy-based reconstructions have indicated that the South Asian summer monsoon (SASM) has shown interannual- to centennial-scale oscillations over the past millennium; however, the variability and mechanisms that operate over different timescales remain to be explicitly identified. This is firstly because of the inadequate spatial representation within previous SASM reconstructions, which is caused by the scarcity of tree-ring records from the core monsoon region. This study used eight additional Indian tree-ring width chronologies from the core region of the SASM to update the reconstructed SASM index that covers the past 1105 years. We found that the most significant interannual variability of SASM is mainly related to the El Niño–Southern Oscillation (ENSO) over the past few hundred years. The decadal/multidecadal oscillations show a high negative/positive correlation with the Pacific Decadal Oscillation (PDO)/Atlantic Multidecadal Oscillation (AMO) after the late 19th century. The centennial component of the SASM, which accounts for 19.4% of the total variance, begins to weaken from the mid-13th century and reaches a minimum in the mid-15th century. The component gradually strengthens again to reach its peak in the early 17th century, followed by a decline trend toward recent. The centennial variations agree well with historical changes in solar activity before the nineteenth century that caused changes in land–sea thermal contrast. However, the close linkage between the SASM and solar activity has weakened since the Industrial era, probably because of the enhanced influence of anthropogenic aerosol emissions.