The Role of Atmospheric Rivers in Linking Weather Extremes: East Asian Cold Surge and High Impact Precipitation over the West Coast U. S

Atmospheric Rivers (ARs) over the North Pacific are narrow ribbons of concentrated moisture in the atmosphere and transport water vapor from the tropics to extratropics and  are responsible for most hazardous weather over Pacific Northwest. In this study, an AR detection algorithm is developed to investigate the downstream modulation of North Pacific ARs by another weather extreme, known as East Asian Cold Air Outbreak (or East Asian Cold Surge, EACS), in both reanalysis data (MERRA) and global model (CCSM4) output. Specifically, after the approaching of an EACS event, the succeeding atmospheric disturbances induced by this event propagate eastward. A cyclonic circulation anomaly and associated moisture transport is built up, which significantly increases the AR probability, and leads to an increase of precipitation over west coast between 30o N and 50o N. The temporal frequency decomposition and geopotential tendency equation suggest that a large portion of the anomalous circulation is generated by intermediate frequency (IF) disturbances. This downstream impact is examined in two simulations of CCSM4 with different horizontal resolutions (T85 and T341) for the same period (1979 - 2005). The feature of downstream propagation, AR probability, and coastal impact is successfully captured by the high resolution version (T341) with certain delay, whereas the wave propagation is not captured in the coarse resolution simulation (T85). The comparison of AR probability and high impact precipitation between observation and simulation suggests that the model 1) correctly represents IF disturbances, including their propagation and generation, and identifies them as a key factor for large scale phenomena over North Pacific and, 2) with the high resolution GCM, accurately produces AR amplitude and propagation, which is critical for simulating and projecting the future climate change, particularly for regional weather and hydrology extremes over west coast of U. S.