Downstream modulation of North Pacific atmospheric river activity by East Asian cold surges

An East Asian cold surge (EACS) is characterized by rapid advancement of a polar airmass toward the east coast of the Eurasian continent in boreal winter. Over the east coast of Asia, extratropical cyclogenesis and the amplitudes of atmospheric disturbances ranging from synoptic to sub-seasonal timescales are immediately enhanced as the cold air approaches. This study investigates for the first time the impact of these EACS-excited disturbances on the activity of atmospheric rivers (ARs) over the North Pacific. Applying a new AR detection algorithm to the NASA MERRA dataset, we show that the daily occurrence probability of ARs over the eastern North Pacific near the west coast of U. S. is effectively modulated by EACS. In particular, this downstream dynamical modulation goes through two distinct stages: during the period Day 0-3, where Day 0 corresponds to the time of the peak intensity of an EACS event, high-frequency (HF, <6-day) baroclinic disturbances developed over the western North Pacific and Gulf of Alaska lead to significant poleward moisture transport over these two regions, and during the period Day 4-6, intermediate-low frequency (IF-LF, >12-day) barotropic disturbances developed from the merging of high-frequency troughs increase the daily occurrence probability of ARs near the west coast of the U.S. by 50%, relative to the climatological value. The results reported here demonstrate the critical role of IF and LF disturbances in establishing the link between the predictability of EACS and that of the AR-related extreme precipitation events in the western U. S. in boreal winter. Citation: Jiang, T., and Y. Deng (2011), Downstream modulation of North Pacific atmospheric river activity by East Asian cold surges, Geophys. Res. Lett., 38, L20807, doi:10.1029/2011GL049462.