Divergent responses of runoff to climate change in the upper basins of the Third Pole dominated by westerlies and monsoon

The diverse climates, distribution of snow and glaciers, and geographic locations directly affect the runoff response to climate change in the upper basins of the Third Pole. At present, a comprehensive analysis of runoff variations and their distinct responses to climate change in the westerlies- and monsoon-dominated upper basins is still lacking. This study comprehensively analyzed annual runoff variations in westerlies-dominated basins (the upper basins of the Aksu (UAKS), Syr Darya (USRD), Yarkant (UYK), Hotan (UHT), Amu Darya (UAMD), and Indus (UI)) and monsoon-dominated basins (the upper basins of the Yangtze (UYA), Yellow (UYE), Lancang (ULC), Nujiang (UNJ), and Yarlung Zangbo (UYZ)) of the Third Pole from 1961 to 2015. Using multi-source meteorological data and large-scale circulation factors, this study investigated the divergent responses of runoff in the upper basins to climate change, and explored the large-scale circulation mechanisms underlying runoff variations in these upper basins. The results showed that: (1) The annual runoff in the majority of upper basins (except for the UYE and UYZ) exhibited an increasing trend, and the annual runoff in the UAKS, UYK, and UI showed a significant increasing trend from 1961 to 2015. The annual runoff in the upper basins of the Third Pole changed abruptly from decreasing to increasing between the 1980s and 2000s, with the exception of the UYE. (2) The runoff in the monsoon-dominated upper basins has been controlled primarily by changes in precipitation over the past 55 years. In contrast, the runoff in the westerlies-dominated upper basins exhibited three distinct long-term responses to climate change: temperature-dominated (UYK and UHT), precipitation-dominated (USRD and UAMD), and the combined influence of precipitation and temperature (UAKS and UI). Since the 1960s, the sensitivity of runoff to warm season temperature changes in the most westerlies-dominated upper basins has decreased, while the response of runoff to precipitation changes has intensified. (3) The study revealed the connection between large-scale circulation, climate, and runoff in the upper basins of the Third Pole. The Atlantic Multidecadal Oscillation, the Westerly Index, and the El Niño-Southern Oscillation predominantly impact the precipitation or temperature in the upper basins of the Third Pole, which in turn affect the runoff variations in the upper basins dominated by either the westerlies or the monsoon. This study will be a valuable scientific reference for water resource management and climate change adaptation for both the westerlies- and monsoon-dominated upper basins in the Third Pole.