Nonlinearity of Atmospheric Circulation response to increased CO2

It is well-documented that the atmospheric circulation changes in response to increased CO2, but there remains many uncertainties. The response involved differs among aspects of the circulation, and the cause of these differences is not well understood. Further, it has recently been shown that the atmospheric response can be not only a non-linear but also a non-monotonic function of CO2 forcing. Here we examine the linearity of the response of a wide range of aspects of the atmospheric circulation to an abrupt increase in CO2, using simulations from multiple models. The nonlinearity is shown to differ between atmospheric metrics as well as between models. Non-monotonic behavior is found, in some models, for some aspects of the circulation (e.g., latitude of the eddy-driven jet, strength of the Hadley cell), but there is a near-linear response for other aspects (e.g. strength of the subtropical jet). Further, the critical level of CO2 for nonlinearity varies among models; in some models nonlinearity occurs below 2xCO2, whereas other models exhibit a near-linear response out to 8xCO2. The processes causing the nonlinearity and differences among models will be explored.