Nonlinearity of the ITCZ response to climate perturbations revealed by q-flux Green's function experiments

Thursday, December 13, 2018 - 10:35
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The sensitivity of the Intertropical Convergence Zone (ITCZ) position to forcing patterns is important for understanding the variability and long-term changes in tropical rainfall. A set of q-flux Green’s function experiments with NCAR CAM5 reveals that this sensitivity is asymmetric depending on which hemisphere (northern or southern) the forcing is applied. Northern hemisphere forcings produce a much larger response than southern hemisphere forcings of same magnitude. The response of the ITCZ position to forcing can be broken into linear and nonlinear components, and it is shown that the asymmetry arises from the nonlinear component. Moreover, the nonlinear component is insensitive to the location of the forcing, epitomizing a mode behavior of the climate system. Further response function analysis on the nonlinear component reveals an inter-hemispheric asymmetric global cooling pattern as the leading neutral mode of the nonlinear response in the q-flux Green’s function experiments. It is the asymmetry of this pattern with a greater cooling in the southern hemisphere that is behind the nonlinear inclination towards a northward shift of the ITCZ in our current climate.

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