Comparing the impacts of vertical mixing enhancement on AMOC at the eastern and western boundaries of Atlantic

Diapycnal mixing is recognized as a critical driving mechanism of the Atlantic Meridional Overturning Circulation (AMOC). This study employs the MIT General Circulation Model (MITgcm) to investigate the differential impacts of vertical mixing changes in the eastern and western boundaries of Atlantic on the AMOC. The results reveal that increased vertical mixing in the eastern boundary significantly strengthens AMOC, whereas analogous changes in the western boundary have a markedly smaller effect. Numerical model outputs suggest that this disparity is linked to the sea surface height gradient in the northeastern Atlantic. Enhanced mixing in the eastern boundary intensifies this gradient, facilitating increased water transport from the subtropical to the subpolar gyre, thereby strengthening subpolar gyre and enhancing deep water formation, which collectively contribute to a stronger AMOC. In contrast, enhanced mixing in the western boundary has minimal impact on the sea surface height gradient, resulting in negligible changes to AMOC. Moreover, we find that the differences between the impacts of eastern and western boundary mixing on AMOC persist with a more realistic background mixing distribution. Further analysis shows that AMOC responds similarly to mixing changes in the South Atlantic, regardless of which boundary region, highlighting the necessity of considering background large-scale circulation when assessing the effects of regional vertical mixing on AMOC.