Extratropical Cyclogenesis Changes in Connection with Tropospheric ENSO Teleconnections to the North Atlantic: Role of Stationary and Transient Waves

This study investigates mechanisms for changes in wintertime extratropical cyclogenesis over North America and the North Atlantic during different phases of El Nino-Southern Oscillation (ENSO). Insights into the relationship between the ENSO-North Atlantic teleconnection and the cyclogenesis changes are provided by diagnosing the relative roles of stationary wave propagation and transient eddies in setting cyclogenesis-conducive large-scale circulation anomalies. During La Nina winters, Rocky Mountain and Greenland cyclogenesis are enhanced, while Gulf Stream cyclogenesis is reduced. Diagnostics suggest that stationary waves of tropical origin work in tandem with transient eddies to amplify the ridge over the northeastern Pacific, establishing background flow anomalies that favor Rocky Mountain cyclogenesis; downstream, more transient eddies with an anticyclonic tilt push the North Atlantic jet poleward, favoring cyclogenesis near Greenland, while contributions from stationary waves are small. During central Pacific El Nino winters, the cyclogenesis situation is essentially the opposite: Rocky Mountain and Greenland cyclogenesis are reduced, while Gulf Stream cyclogenesis is enhanced. The analyses are consistent with stationary waves and transient eddies acting to weaken the climatological ridge over the northeastern Pacific, creating a more zonal Pacific jet; downstream, transient eddies with a cyclonic tilt push the North Atlantic jet equatorward, favoring Gulf Stream cyclogenesis. Anomalies in cyclogenesis frequencies, and the relative roles of transient and stationary waves, during eastern Pacific El Nino winters are associated with larger uncertainties.