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This study reveals the remarkable interdecadal changes in the influence of boreal winter Arctic sea ice concentration (SIC) anomalies (ASICAs) in the Greenland–Barents Seas on the subsequent El Niño–Southern Oscillation (ENSO) development. Winter ASICA is strongly associated with the subsequent winter ENSO before the late 1980s and after the late 2000s, but their connection is weak during the 1990s and the 2000s. The interdecadal variations in the influence of ASICA on ENSO are associated with changes in the spatial structure of the ASICA-induced North Pacific atmospheric anomalies. During high-correlation periods, winter SIC increases in the Greenland–Barents Seas lead to tropospheric cooling via the suppression of upward surface heat fluxes, which further trigger an atmospheric teleconnection from the Arctic to the North Pacific. The accompanying North Pacific Oscillation–like atmospheric anomalies result in sea surface temperature (SST) warming in the subtropical North Pacific, which extends southward into the tropical Pacific via wind–evaporation–SST feedback and leads to surface westerly anomalies over the tropical western Pacific in the following summer. The tropical western Pacific westerly wind anomalies impact the subsequent ENSO development via triggering positive Bjerknes air–sea interaction. During low-correlation periods, atmospheric anomalies over the North Pacific generated by the winter ASICA are located more northward and cannot induce marked subtropical North Pacific SST anomalies and thus have a weak impact on the following ENSO development. Numerical experiments suggest that the interdecadal variation in the spatial structure of the North Pacific atmospheric anomalies induced by the winter ASICA is partly attributed to change in the atmospheric mean flow. Significance Statement: Arctic sea ice is an important component of the global climate system. Studies have shown that Arctic sea ice has decreased significantly in recent decades, which is considered to be one of the most important responses of Earth's climate system to global climate change. A number of studies have shown that Arctic sea ice anomalies have significant impacts on weather and climate over mid–high latitudes through tropospheric and stratospheric processes. Recent studies have suggested that the effects of Arctic sea ice anomalies could extend to the tropics via air–sea interactions. El Niño–Southern Oscillation (ENSO) is the strongest air–sea coupled system in the tropics and can have a significant impact on climate over the globe. ENSO is also considered to be one of the most important sources of subseasonal–seasonal climate predictability over many parts of the globe. It is therefore important to study the factors for the ENSO occurrence. A recent study showed that Arctic sea ice anomalies during boreal winter in the Greenland–Barents Seas have a significant impact on the following winter ENSO. In this study, we further reveal that the influence of winter Arctic sea ice anomalies in the Greenland–Barents Seas on the subsequent ENSO is unstable and has undergone significant interdecadal variation. We then investigate the mechanisms underlying the interdecadal variation via observational analysis and numerical simulations. The results of this study not only have implications for improving the prediction of ENSO but also could improve our understanding of the physical link between high-latitude climate systems and tropical air–sea coupling systems. [ABSTRACT FROM AUTHOR]

The changing climate and intensifying human activities have made an impact on the hydrological processes in the upper Yangtze River (UYR), but quantifying their effects remains uncertain. This study used the Budyko framework to investigate the response of runoff (Q) to climate change and human activities during 1956–2017 and evaluate the impacts of human activities, including land use/cover change, water use, dam construction, and vegetation change, on watershed characteristic. Results show that climate change is the dominant driver of Q variations in the Wujiang River (WJR), Jialing River (JLR), and Jinsha River (JSR) watersheds, with contributions of 58.6%, 66.9%, and 67.6%, respectively. However, in Mingjiang River (MJR) and UYR watersheds, human activities contribute more to Q variations with 55.2% and 51.2%, respectively. Human activities play important roles in variation of watershed characteristics, and they can explain 22%, 26%, 36%, 25%, and 53% of the watershed character change in UYR, WJR, JLR, MJR, and JSR, respectively. This study conducts a comprehensive analysis of the causes of Q change in UYR, and provides a new perspective to explore the effects of specific human activities on watershed characteristics. Plain Language Summary: With climate change and intensifying human activities, the water cycle has been changed. Runoff is an essential component of the water cycle, and its response to changing environments has drawn broad attention. This study aims to quantify the contributions of climate change and human activities to runoff changes in the upper Yangtze River (UYR) and its four sub‐watersheds over the past half‐century. According to the results, human activities and climate change have been responsible for approximately 51.2% and 48.8% of the observed runoff changes in UYR, respectively. It suggests that both human activities and climate change have played significant roles, and their contributions are remarkably comparable. In addition, the impacts of human activities on watershed environment changes are analyzed. The results suggest that increased settlement area and industrial water use are the dominant influencing factors. Key Points: Human activities and climate change have made comparable contributions to the runoff reduction in the upper Yangtze RiverWater use, land use/cover change, vegetation change, and dam construction together explain 22% of the watershed environment changesHuman settlements and increased industrial water use have been the principal factors contributing to the changes in watershed environment [ABSTRACT FROM AUTHOR]

This study investigates the interannual variability in autumn precipitation over the Greater Mekong Subregion (GMS) during the period 1981–2019 and its possible causes. Statistical analysis indicates that GMS autumn precipitation showed a seesaw pattern between the central–southern and northeastern GMS over interannual timescales. The tropical sea surface temperature anomalies (SSTAs), reflected in the tropical northeastern Pacific SST index (TNEPI), significantly modulate anomalous GMS autumn precipitation. When the TNEPI is higher than normal, the SSTAs pattern prevails during autumn, with warmer SSTAs in the tropical eastern Pacific and western Indian Ocean, and colder SSTAs in the tropical western Pacific and southeastern Indian Ocean. This SSTAs pattern excites an anomalous anticyclone in the lower troposphere over the central–southern GMS and an anomalous cyclone over the northeastern GMS by the Gill–Matsuno mechanism. Subsequently, the anomalous anticyclone with anomalous descending motion results in below‐normal autumn precipitation over the central–southern GMS via a weakening of the convergence of water vapor from the tropical western Pacific and the Bay of Bengal. The anomalous cyclone on the northern flank of anomalous anticyclone, benefiting from water vapor supply from the Bay of Bengal, leads to above‐normal autumn precipitation over the northeastern GMS. The TNEPI, incorporating signals from the eastern‐Pacific (EP) type of ENSO, Indian Ocean dipole (IOD), and SSTAs associated with the Trans‐Niño index (TNI‐like), shows a stronger relationship with GMS autumn precipitation than individual indexes that describing the EP‐type ENSO, IOD or TNI‐like pattern. Therefore, the TNEPI can be considered an efficient predictor of GMS autumn precipitation. Key Points: The autumn precipitation over the Greater Mekong Subregion (GMS) exhibits an interannual seesaw pattern between the central–southern and northeastern GMSThe EP‐type ENSO, IOD, and TNI‐like SSTA patterns significantly modulate the precipitation seesaw pattern via the Gill–Matsuno mechanismThe tropical northeastern Pacific index reflects the synthetical effects of the EP‐type ENSO, IOD, and TNI‐like pattern on the autumn precipitation over the GMS [ABSTRACT FROM AUTHOR]