Large-scale Atmospheric Variability and the Risk of Atmospheric River-Related Flooding in the West Coast of the United States

Abstract:

Atmospheric rivers (ARs) produce 25% to 50% of the annual precipitation and the major wintertime flooding events in most rivers along the U.S. West Coast. ARs can be significantly modulated by large-scale atmospheric features and have been related with different Rossby wave breaking types. In this work we link large-scale variability with the risk of AR associated flooding by associating AR features with different Rossby wave breaking types directly impacting two river basins: Chehalis basin (Washington State) and Russian river basin (California State).

While the moisture sources for both types of ARs are of tropical-subtropical origin, ARs of anticyclonic wave breaking (AWB) type have ~10% more of tropical-subtropical (<=35°N) sources but ~10% less of extratropical (>35°N) sources than cyclonic wave breaking (CWB) events. In addition, ARs of AWB type have larger impinging angles to the coast and lower wind speed at the 850mb level. Because their impinging angle is almost perpendicular to the mountains, ARs of AWB type have higher impact on precipitation in the Chehalis basin and Cascade Mountains. Snow cover increases 10~20% over the Cascade Mountains after AWB events while the increase after CWB events are below 10%. The extreme streamflow measured by gauges within the Chehalis basin can be mostly explained by AWB events. However, CWB type ARs with larger amount of water vapor transport result in more precipitation over the Russian river basin on the coastal region and the northern Sierra Nevada. Snow cover increases by 20~30% over the northern Sierra Nevada but decreases slightly (<10%) in regions with lower elevation. Slight increases in snow (<10%) over lower elevation regions are caused by AWB events. This is likely the reason why extreme AR-related streamflow vary from station to station. Finally, CWB AR events are more likely to happen during El Nino years in both basins; more AWB AR events tend to happen during La Nina years in the Russian river basin but no strong correlation is found for the Chehalis basin possibly due to the variations of wave breaking positions.