2014 Articles

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

Grise, Kevin M.; Polvani, Lorenzo M.

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.