Understanding the Role of Tropical Moisture in Atmospheric Rivers

There has been a long-standing debate about whether tropical moisture transport or local moisture convergence plays the dominant role in the development and strength of atmospheric rivers (ARs). In this work, we quantify the contribution of tropical moisture to 29 simulated extreme ARs that affected the U.S. Northwest Coast by "tagging" tropical moisture from the eastern tropical Pacific using water vapor tracers in the Weather Research and Forecast model. From a case study with strong tropical moisture, we find three mechanisms by which tropical moisture can contribute to AR strength: direct contribution of moisture, indirect contributions via thermodynamic feedbacks, and indirect contributions via synoptic-scale dynamic feedbacks. Focusing on these processes, we divide the 29 simulated ARs into two groups (>25% of direct tropical moisture contribution from the eastern tropical Pacific to inland precipitation as DTM-ARs and the other ARs with <25% as OT-ARs) and find that DTM-ARs are characterized by stronger pre-cold-frontal low-level jets and stronger warm air advection. Notably, the ARs with the greatest 2-day precipitation are found to be DTM-ARs but with <50% tropical moisture contributions, indicating a more dominant role of indirect enhancement of local moisture convergence due to tropical moisture than the direct moisture contribution supplying precipitation.