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The sensitivity of the intertropical convergence zone (ITCZ) position to forcing patterns is important for understanding changes in tropical rainfall. A set of Green's function experiments reveals hat 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 similar 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. The linear and nonlinear response components have similar magnitudes, but the pattern of the nonlinear component is insensitive to the location of the forcing, such that it amplifies the response to northern hemisphere forcings and dampens the response to southern hemisphere forcings. This asymmetry hints at an intrinsic mode of the climate system, depending on the current climate state.

Plain Language Summary The tropical rain belt is a key component of the climate system, and understanding its response to heating and cooling patterns is a necessary step toward understanding tropical rainfall changes. When the atmosphere is heated or cooled in one hemisphere, flow of energy across the equator in the atmosphere shifts the tropical rain belt. Warming the northern hemisphere and cooling the southern hemisphere produces a much stronger (northward) shift of the rain belt than warming the southern hemisphere and cooling the northern hemisphere. The inclination toward a northward shift of the rain belt, irrespective of the location of heating (or cooling), likely arises from the intrinsic nonlinearity of the current climate. In other words, the asymmetry arises because the rain belt response depends on the state of the current climate, the interhemispheric asymmetry of which underpins the asymmetric response to external energy perturbations.