Understanding the Zonal Circulation Response to Climate Forcing Using Feedback Analysis

The zonal flow of the atmosphere often displays an annular mode-like structure in response to different mechanical or thermal perturbations, but it remains challenging to quantify how different physical processes (e.g., radiative forcing or latent heat release) are projected onto the annular mode and its associated eddy-zonal flow interactions. We introduce a new method to separating the direct response and feedback in the zonal flow response to a climate forcing. The direct response is defined as the zonal wind change under a climate forcing when the advecting zonal mean zonal wind is held as in the control simulation. The feedback is the additional zonal wind change due to the change in the advecting zonal mean zonal wind and the resultant eddy feedbacks. Using a Green’s function approach, one can predict the full zonal wind response to a given climate forcing by the direct response through a feedback matrix that is constructed based on the control climate. The framework is applied to a hierarchy of atmospheric models from a dry atmospheric dynamical core to an idealized moist atmospheric model with realistic radiation. It is found that the annular mode-like response is mainly explained by the feedback and that the direct response is important for the sign and magnitude of the full response. This feedback analysis gives new insights on the relative role of forcing versus feedback in the zonal flow changes in a changing climate.