Impact of the North Atlantic and the Indian Ocean on the summer hydroclimate over East Central Asia: A case study in the central Tianshan Mountains

Observations have shown that the hydroclimatic variations in East Central Asia (ECA) are a result of the interaction of high-, middle- and low-latitude atmospheric circulation systems. This study aims to explore the potential mechanisms of hydroclimate variability in ECA over a longer timescale than in previous studies. Based on tree-ring stable oxygen isotope (δ¹⁸O), we reconstructed the May–July relative humidity variations in the central Tianshan Mountains from 1661 to 2016, which can represent the dry/wet variations over a large region of ECA. The reconstruction revealed several dry periods that occurred in 1674–1691, 1705–1740, 1750–1770, 1806–1826, the 1940s, and 1960–1980. The reconstructed relative humidity presented an obvious upward trajectory from the 18th to the early 20th century and humidification characteristics in the mid- to late 1980s. The summer North Atlantic Oscillation (SNAO) had a nonlinear impact on the hydroclimate of ECA by modulating the position of the subtropical westerly jet stream (SWJS). Positive sea surface temperature anomalies in the tropical Indian Ocean (TIO), which may enhance the cross-equatorial jet, transport additional water vapour into ECA, and the South Asian summer monsoon (SASM) provides favourable circulation conditions for water vapour transport. Summer hydroclimatic variations in ECA are jointly affected by the North Atlantic and the TIO. The impact of the TIO on the hydroclimate in ECA cannot be ignored.