Trends, teleconnections and nonstationary frequency modeling of riverine heatwaves in North American Atlantic salmon rivers (1979-2100).

Freshwater ecosystems play a vital role in supporting cold-water species like Atlantic salmon, providing the essential conditions to complete critical life stages. However, climate change is increasingly disrupting these habitats, driving shifts in river hydrology and rising water temperatures that threaten their survival. This study quantifies the past (1979-2020) and assesses potential future changes (2030-2100) in the hydrological and thermal regimes of 35 Atlantic salmon rivers across northeastern North America. According to the selected climate change scenarios, the results reveal a significant potential rise in water temperature (Tw > 20 °C), drought conditions, and an increasing trend in heatwave frequency, duration, and intensity by the end of the century. Under the most pessimistic SSP5-8.5 scenario, the total duration of summer riverine heatwave (RH) averaged from 2061 to the end of the century is projected to rise 12-fold (98.0 days) compared to the average of 8.5 days, calculated from 1979 to 2020. Consequently, nearly half of the studied rivers are projected to enter a permanent state of summer heatwaves. We also propose a nonstationary riverine heatwave frequency (RHF) modeling framework, integrating both climate change through a temporal trend and climate variability through large-scale atmospheric-ocean oscillations (teleconnection indices) as covariates. Models incorporating climate-related covariates outperform stationary models without covariates, with the North Atlantic Oscillation and the Arctic Oscillation indices emerging as the most influential predictors. However, with the small sample sizes used in this study, the uncertainty can increase especially for extreme non-exceedance probabilities and for the most extreme values of the climate indices.