Mechanisms controlling the jet stream response to global warming in the presence of zonal asymmetries

A robust poleward shift of the jet stream occurs in response to global warming in zonally-symmetric aquaplanet models. In comprehensive climate models, however a poleward shift does not occur at all longitudes and in all seasons in the Northern Hemisphere largely due to the presence of stationary waves.

Here we show that in the presence of thermally induced subtropical stationary waves the jet stream can shift poleward, equatorward or not at all in response to surface warming. The different jet responses to surface warming depend on 1) the stationary wave response to global warming, which is connected to changes in low-level equivalent potential temperature via quasi-equilibrium thermodynamics and 2) stationary-transient interactions with transient eddies responding to the upper-tropospheric torques induced by stationary eddy momentum fluxes. The influence of stationary wave location (tropical, subtropical, high latitude) and the interaction with clouds and water vapor radiative feedbacks are also considered. The results can be used to interpret the jet stream response to global warming in comprehensive models and the jet stream response to El Nino events.