Dynamics and Impacts of Northeast Pacific Wintertime Atmospheric Blocking

Persistent, large-scale high-pressure systems called atmospheric blocks lead to anomalous weather patterns capable of persisting for several weeks. Using two different blocking indices, the impacts of northeast Pacific dipole and omega blocks are investigated for the extended boreal winter, including A 2-dimensional blocking index identifies dipole blocks characterized by a reversal of the climatological 500mb geopotential height gradient. Omega blocking events are identified where regions in which 500mb geopotential height anomalies above 100m overlap by at least 50\% for 5 consecutive days. To different the impacts of omega and dipole blocking events, additional criteria are applied to the identification of omega blocks, such as the requirement that less than 50\% of blocked area must show a 500mb geopotential height reversal. Composite analysis is performed to determine the impact of blocking events on temperature, precipitation, and column-integrated water vapor transport (IVT). Results show that precipitation is suppressed along the western Canadian coast, with increased precipitation over Alaska and the Bering Sea. During dipole blocking events, a strong increase in precipitation persists over California that is not apparent during omega blocking events. Both blocking types are associated with warmer temperatures in Alaska and northern Canada, with cold temperatures persisting throughout southern Canada and the continental United States. The physical mechanisms leading to these blocking impacts are investigated, including the role of the storm track and atmospheric rivers.