Tropopause-aware IVT and IWV sharpen atmospheric river detection over North America.

Atmospheric rivers are among the most consequential moisture-transport systems shaping flood risk, hydroclimatic extremes, and water cycles across North America. Standard IVT/IWV threshold methods, however, fail to capture slender ARs or accurately gauge intensity due to neglected upper-tropospheric thermodynamics. Here, using ERA5 reanalysis (1984-2024), we modified IVT and IWV by incorporating a tropopause-aware thermodynamic weighting function calibrated with radiosonde profiles. Validation against radiosondes and SSMIS imagery confirms that the modified indices improve AR intensity and geometry estimation by 8.7% and narrow-filament detection by 12%. Four-decade seasonal trends reveal robust AR increases in spring, summer, and winter over eastern North America, and in winter and summer over the west. Furthermore, the modified IVT and IWV better revealed the dominant role of remote drivers in North American AR activity. Accordingly, stepwise regression findings showed that ENSO, PNA, PDO, and NAO explain more than 45% of regional AR variance. These findings emphasize the urgent need to embed upper-tropospheric thermodynamics in AR detection and their precise spatiotemporal activity for improved flood forecasting and climate projection.