Dynamical Downstream Modulation of the eastern North Pacific Atmospheric River Activity by East Asian Cold Surge

East Asian cold surge (EACS) is an efficient mechanism transporting cold and dry air from Siberia to the east coast of the Eurasian continent. Extratropical cyclone genesis, thus the amplitude of the upper level, high-frequency baroclinic disturbances over the east coast are immediately enhanced as the cold air approaches. Utilizing a newly developed algorithm for atmospheric river detection, our study reveals that the occurrence probability of atmospheric rivers over the eastern North Pacific near the west coast of North America is significantly modulated by EACS. In particular, this dynamical downstream modulation goes through two stages where high-frequency, baroclinic disturbances propagating northeastward contributes to the formation of atmospheric rivers in the Gulf of Alaska, and low-frequency, barotropic disturbances developed following the passage of high-frequency disturbances play a more important role in organizing extreme moisture transport and atmospheric rivers along the west coast of U.S. The modulation mechanism discussed above is successfully reproduced in a high-resolution GCM initialized at the time of the peak of EACS, suggesting EACS and its dynamical triggers as potential source of predictability for the winter extreme precipitation events in the western U.S.