Unlocking 640 years of summer monsoon precipitation variability in the Bhutan Himalayas using tree-ring records

Tree-ring records from the Himalayas provide useful insights into historical climatic variability in a region strongly influenced by the Asian summer monsoon. In this study, we reconstructed 640 years of summer monsoon precipitation (May-August) for Bhutan using tree ring records of Himalayan larch ( Larix griffithii ). The tree ring width index (RWI) revealed a significant positive relationship with summer monsoon (May – August) precipitation, indicating that radial growth is controlled by summer monsoon moisture availability. Despite ecological complexity and limited local climatic records, the reconstruction model demonstrated statistically acceptable levels of fidelity. The reconstruction revealed substantial interannual-to-decadal variability in monsoon precipitation, with increased fluctuations after 1900 CE, reflecting significant changes in precipitation patterns in the twentieth century. Spectral and comparative analyses indicated that the reconstructed precipitation is closely aligned with global-scale climate modes, particularly the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), which independently and synergistically modulate Himalayan summer monsoon precipitation. Several well-documented Asian megadroughts were clearly seen in the reconstruction, including the Ming Dynasty drought (1638–1641 CE), the Strange Parallels drought (1756–1768 CE), the East India drought (1790–1796 CE) and the late Victorian Great Drought (1876–1878 CE). Comparison with other Himalayan hydroclimatic reconstructions revealed coherent regional precipitation signals, underscoring the record's broader climatic significance. The study contributes valuable baseline data to a growing network of Himalayan dendroclimatic records and provides insights into long-term monsoon variability, enhancing regional climate risk preparedness.

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