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

Abstract

Abstract 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 CO 2 concentrations. However, in the Northern Hemisphere (NH) extratropics, the circulation response to CO 2 is largely absent in the zonal mean and is instead characterized by complex regional anomalies. This study decomposes the atmospheric circulation response to CO 2 forcing in CMIP5 models into two components: a direct component due to CO 2 radiative forcing and an indirect component associated with sea surface temperature (SST)‐mediated changes. The direct radiative forcing of CO 2 drives a weak poleward jet shift in both hemispheres, whereas the indirect (SST) component of the CO 2 forcing dominates the total response and drives a zonally asymmetric response in the NH. Hence, understanding the SST‐mediated component of atmospheric CO 2 forcing appears crucial to unlocking the mechanisms that contribute to forced extratropical circulation changes.