Increased drought events in southwest China revealed by tree ring oxygen isotopes and potential role of Indian Ocean Dipole

Highlights: • Tree ring oxygen isotopes can be used to reconstruct rainy season precipitation in southwest China. • The reconstructed precipitation reveals an apparent drying trend since 1840. • Increased drought events since 1970 likely due to the increase in positive Indian Ocean Dipole events. -- Abstract: The highlands in southwestern China experience pronounced fluctuations in the hydroclimate with profound impacts on agriculture and economics. To investigate the drought history of this region beyond instrumental records, a tree ring cellulose oxygen isotope (δ¹⁸O_c) chronology was developed for the period 1733–2013 using samples collected from six Larix trees in the low-latitude highlands (LLH) of southwestern China. The analysis revealed that δ¹⁸O_c is significantly correlated with the rainy season (May–October) precipitation and relative humidity, as well as drought severity. The δ¹⁸O_c chronology accounts for 46% of the observed variance in the rainy season precipitation and it was subsequently used to reconstruct precipitation. The reconstructed precipitation reveals an apparent drying trend since 1840, accompanied by increasingly frequent drought events since 1970. Interdecadal variability is also present, characterized with two distinct wet periods in 1740–1760 and 1800–1900 and two drier periods in 1760–1800 and 1900–2013. On the interannual timescale, the LLH precipitation was modulated collectively by the El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD). There appears to be an enhanced precipitation-IOD relationship since 1970 in response to the increase in positive-IOD events, implying an increasing likelihood of drought for the southwest China LLH.