The Influence of Antecedent Atmospheric River Conditions on Extratropical Cyclogenesis

Abstract Some extratropical cyclones (ETC) begin their development in close proximity to a preexisting atmospheric river (AR). This study investigates the differences in the cyclogenesis stage between these cyclogenesis events and those that begin without an AR nearby. Well-established ETC and AR detection methods are applied to reanalysis over the North Pacific during the 1979–2009 cool seasons (November–March). Of the 3137 cyclogenesis cases detected, 35% are associated with a nearby AR at the time of initial cyclogenesis. Of all 448 cyclones that deepened explosively in the 24 h after their initiation, 60% began with a preexisting AR nearby. The roles of both dry and diabatic processes that contribute to cyclogenesis are examined, specifically, low-level baroclinicity, upper-level forcing, water vapor inflow, and latent heating. ETCs that develop associated with a preexisting AR receive nearly 80% more water vapor inflow on average, enhancing latent heating and intensifying cyclone deepening in the genesis stage. In contrast, neither low-level baroclinicity nor upper-level potential vorticity exhibit statistically significant differences between cyclogenesis events with and without an AR. Cyclogenesis events associated with an exceptionally strong AR at the ETC initial time deepen even more rapidly in the genesis stage, indicating that the intensity of an antecedent AR can modulate cyclogenesis. About half of the cyclogenesis cases off the U.S. West Coast are associated with ARs at their initial time. These results imply that errors in initial conditions related to ARs can contribute to errors in both AR and ETC predictions, as well as their concomitant impacts.