The Relationship between Extratropical Cyclone and Atmospheric River over the Northeastern Pacific and U.S. West Coast

The atmospheric rivers (ARs) play a key role in the total precipitation amount and the extreme precipitation events along the U.S. West Coast during cool season. Meanwhile, these ARs are often dynamically related to the extratropical cyclones (ECs). It is important to understand the physical processes associated with ECs and ARs, as well as their impacts on the precipitation. This study investigates the relationship between the ECs and ARs over the U.S. West Coast using CFSR reanalysis for 31 (1979-2009) cool seasons (November-March). A cyclone relative approach is employed to quantify the contribution of the ARs associated with ECs to the precipitation. These ARs contribute 60%-80% of the total precipitation amount and over 80% of the extreme precipitation over the U.S. West Coast. Most of the EC centers associated with the extreme ARs (integrated water vapor transport, IVT > 1000 kg m^-1 s^-1) are at the relatively lower latitudes (< 50°N) over ocean, while the maximum of EC center density over ocean is located at the relatively higher latitudes (> 50°N). The ECs are categorized based on the maximum IVT around their centers. Then the composite fields (IVT, IWV, SLP, wind, et al.) around EC center are created for the different EC categories using the cyclone relative approach to examine the related physical processes. We find that the maximum IVT is highly correlated with the intensity of ECs, although the distance from the maximum IVT to the EC center varies from about 200 km to over 1000 km. The upper/lower level jet and some other related fields are also explored to understand the relationship between ECs and ARs.