Quantifying the Response of Tropospheric Circulations to Stratospheric Forcing Using a Green’s Function Approach

The dynamical coupling between the stratosphere and the troposphere contributes significantly to climate variabilities on a wide range of timescales. In this two-way coupling, it would be useful to think of the response of tropospheric circulations to stratospheric forcing as the direct zonal wind response to stratospheric perturbations with tropospheric eddy feedbacks. In this work, we will use a Green’s function approach to quantify tropospheric eddy feedbacks in an idealized general circulation model (GCM). More specifically, we separate the direct zonal wind response from the eddy-mediated full response by overriding the zonal advection terms in vorticity and temperature equations. By applying numerous local perturbations to the overriding zonal wind, we can obtain a feedback matrix that enables us to predict the full response to any stratospheric forcing given its direct response. The eigenvector analysis of the feedback matrix reveals both the structure and amplitude of the eddy feedback modes, which give insights on the underlying mechanisms and circumvent the limitations of EOF or Principal Oscillation Pattern (POP) analysis. With this new framework, we carry out a suite of idealized GCM experiments where the stratosphere is disturbed both thermally and mechanically. We evaluate how our approach predicts the full tropospheric responses to these stratospheric perturbations. The contributions of tropospheric eddy feedbacks to these responses are also investigated.