May–September precipitation in the Bhutan Himalaya since 1743 as reconstructed from tree ring cellulose δ¹⁸O

We developed a 50‐year tree ring δ 18 O chronology for each of three tree species ( Juniperus indica , Larix griffithii , and Picea spinulosa ) using a total of 12 trees (four trees per species) from the Bhutan Himalaya. Despite originating from different species sampled at two different altitudes, the δ 18 O chronologies are highly correlated with one another ( r = 0.76–0.89). Response analyses reveal that tree ring δ 18 O values are controlled mainly by summer precipitation, irrespective of species. Based on these results, a robust 269‐year δ 18 O chronology was established to reconstruct the amount of May–September precipitation based on data from four larch trees. Our tree ring δ 18 O data show significant correlations with those from other regions of the Himalaya and the Tibetan Plateau, indicating that common signals related to monsoon activity are recorded in the data. However, at centennial timescales, our data from Bhutan show normal conditions during the 20th century, whereas records from sites in western Nepal and the southern/eastern Tibetan Plateau show weakening trends in monsoon intensity during the last 100–200 years; the weakening trends may be the result of a reduction in the meridional sea surface temperature gradient in the Indian Ocean during this time. At continental scales, the tree ring records show that areas more from ocean basins are particularly sensitive to reduced monsoon circulation. Correlation analyses suggest that the El Niño–Southern Oscillation (ENSO) plays an important role in modulating summer precipitation. However, the teleconnected relationship disappears during the period 1951–1970, coinciding with a negative phase of the Pacific Decadal Oscillation (PDO), implying interdecadal modulation of the PDO on the influence of the ENSO on precipitation in Bhutan.