Hydroclimatic variations during the summer monsoon season (June–September) across the Himalaya are examined over the past several hundred years using tree-ring oxygen isotope records. Owing to their strong associations with hydroclimatic variables including precipitation, relative humidity, and the Palmer Drought Severity Index, tree-ring δ18O chronologies from the Himalaya can be used to reconstruct summer monsoon intensity precisely. A regional chronology derived from five local chronologies across the Himalaya shows a significant correlation with Indian summer rainfall data. One of the most noteworthy features of the regional chronology is a drying trend over the past 180 years, indicating that summer monsoon intensity in the Himalayan region has weakened. A declining land–ocean thermal gradient over South Asia seems to be responsible for the weakened summer monsoon. By analyzing spatio-temporal correlations between zonally distributed tree-ring data over the Himalaya, we also explore possible changes in the relative contributions of source water originating in the Arabian Sea and the Bay of Bengal.
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