The Relationship between Extratropical Cyclone Strength and Atmospheric River Intensity and Position

Atmospheric rivers (ARs) and Extratropical cyclones (ECs) are the main drivers of water vapor transport and precipitation at midlatitudes during the cool season. Although the two are frequently mentioned together, there is a lack of comprehensive study to quantify the linkage between ARs and ECs from both statistical and dynamical perspectives. This study investigates the statistical relationship and the dynamical interaction between AR and EC over the North Pacific. We used the reanalysis data CFSR (1979-2009) from NCEP and focused on the cool season (November-March). Over the North Pacific, 70~85% of ARs are associated with an EC, while roughly 20% of ARs occur without a nearby EC but close to a subtropical/tropical moisture source and/or an anticyclone. The ARs associated with an EC are often intensified by stronger wind-driven meridional water vapor transport and tend to be more meridional due to the stronger cyclonic wind, especially for the intense ARs. On the other hand, 35~70% of ECs are associated with an AR, while over 50% of ECs at high latitudes (> 50°N) do not have a paired AR. Greater EC intensification occurs with stronger ARs since those ARs enhance the precipitation and latent heat release, which generates strong low-level potential vorticity and contributes to the EC deepening. The positive feedback between AR strengthening and EC intensification is critical to the extreme AR and EC cases, and the associated extreme precipitation events.