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The increase of atmospheric greenhouse gases is expected to affect the hydrological cycle and large-scale precipitation patterns. In parallel, unforced natural variability on decadal-to-multidecadal timescales can also modulate forced changes at the regional scales. Based on multimember ensembles from a coupled General Circulation Model, we investigate the sensitivity of CO2-forced changes in tropical precipitation and atmospheric circulation to fluctuations of the Atlantic Multidecadal Overturning Circulation (AMOC). We show that contrasted AMOC states yield considerable differences in equatorial Pacific precipitation forced changes, by impacting the direct (within a year) CO2-induced weakening of the Walker circulation. We use global atmospheric energetics, as a theoretical backdrop, to explain the relationship between the tropical atmospheric circulation and the AMOC state. A physical mechanism is then proposed, relating the direct CO2-forced weakening of the atmospheric tropical circulation to its climatological strength in unperturbed climate and indirectly to the AMOC state.

Plain Language Summary The precipitation response to increased greenhouse gases atmospheric concentration is one of the most critical factors of the overall impact of climate change. The presence of natural climate variability on decadal-to-multidecadal timescales is expected to modulate the long-term projections at the regional scales.This can be viewed as an irreducible source of uncertainties for the climate response to GHG forcing, which should be quantified and better understood. Based on an ensemble of experiments from a fully coupled General Circulation Model, we quantify and investigate the sensitivity of the CO2-forced climate response in terms of tropical precipitation and circulation, to the internal fluctuation of the Atlantic Multidecadal Overturning Circulation (AMOC). We show that contrasted states of AMOC, used as initial conditions in increased-CO2 experiments, yield considerable differences in the tropical precipitation forced changes over Indonesia, Southeast Asia, and the equatorial Pacific Ocean. We provide evidence that the AMOC-related influence on precipitation changes is essentially due to the fast and direct (within a year) CO2-induced weakening of the Walker circulation. Our findings are further supported by physical understanding based on large-scale atmospheric energetics.