The response of midlatitude jets to increased CO2: Distinguishing the roles of sea surface temperature and direct radiative forcing

In Coupled Model Intercomparison Project Phase 5 (CMIP5) models, the zonal-mean tropospheric circulation shifts robustly poleward in the Southern Hemisphere extratropics in response to increased atmospheric CO2 concentrations. However, in the Northern Hemisphere (NH) extratropics, the circulation response to CO2 is largely absent in the zonal mean and is instead characterized by complex regional anomalies. This study decomposes the atmospheric circulation response to CO2 forcing in CMIP5 models into two components: a direct component due to CO2 radiative forcing and an indirect component associated with sea surface temperature (SST)-mediated changes. The direct radiative forcing of CO2 drives a weak poleward jet shift in both hemispheres, whereas the indirect (SST) component of the CO2 forcing dominates the total response and drives a zonally asymmetric response in the NH. Hence, understanding the SST-mediated component of atmospheric CO2 forcing appears crucial to unlocking the mechanisms that contribute to forced extratropical circulation changes.