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Abstract

Climate Models consistently project an acceleration of the shallow branch of the BDC under increasing greenhouse gases. Changes in the deep branch have been less explored and remain uncertain. Since the BDC is wave driven, both changes in wave generation and wave dissipation can lead to changes in the residual circulation. Here, we investigate the timescales of the BDC response to an abrupt 4x CO₂ increase in CESM1-WACCM simulations with and without a coupled ocean to distinguish changes in effective radiative forcing, which occur on much shorter timescales, from those in the ocean.Our results show that the response in the shallow branch is driven by warmer tropical sea surface temperatures (SSTs) in response to increasing CO₂. About 75% of the total response occurs in the first 25 years consistent with changes in the shallow ocean. The deep branch response develops more quickly than that in the shallow branch, with almost equal contributions from effective radiative forcing and warmer SSTs, constituting a rapid adjustment to increasing CO₂. While changes in the former favor a larger contribution of resolved wave driving, changes in the stratospheric winds due to warmer SSTs modulate the filtering of frontal gravity waves and increase their contribution to the intensification of the deep branch. Finally, we show that O₃ feedbacks strengthen the response of the deep branch but their impact is negligible in the shallow branch, suggesting that in CESM1-WACCM interactive ozone chemistry is needed for correctly modeling the deep branch response, but not for the shallow branch.