Dynamics of extreme negative stratospheric planetary wave heat flux events in an idealized model

Recent work has shown significant coupling between the stratosphere and troposphere during extreme total (climatology plus anomaly) stratospheric planetary wave-1 heat flux events. In particular, extreme negative heat flux events are associated with a poleward shift of the tropospheric jet in the Atlantic basin. Here a dry dynamical core is used to investigate the mechanisms underlying negative heat flux events. Ensemble spectral nudging experiments are performed to isolate the roles of i) the tropospheric wave source, ii) the stratospheric reflective layer and iii) higher order wavenumbers. The events are partially reproduced when simultaneously nudging the stratospheric reflective layer via the zonal-mean flow and the tropospheric wave source via wavenumber 1 whereas weak responses are found when nudging either in isolation. The events are almost entirely reproduced when nudging both the tropospheric wave source and stratospheric wavenumbers 2-4. Idealized experiments with imposed stratospheric torques equal to the wavenumbers 2-4 EP-flux divergence show higher wavenumbers help to shape the stratospheric reflective layer during the event. Wavenumbers 2-4 also play a role during negative heat flux events in reanalysis data. Taken together, the nudging and torque experiments support the role of wave reflection during extreme negative stratospheric heat flux events.