North Pacific meltwater weakens the Atlantic Meridional Overturning Circulation and preconditions Heinrich Stadial 1.

Heinrich Stadials provide insights into a potential future weakening of the Atlantic Meridional Overturning Circulation (AMOC), yet the mechanisms driving AMOC weakening during these intervals remain elusive. While Heinrich Stadials are associated with iceberg-discharge events over the North Atlantic (i.e., Heinrich events), AMOC weakening is known to precede Heinrich events. Here we run freshwater hosing experiments to show that contemporaneous Northeast Pacific iceberg-discharge events (i.e., Siku events), which consistently occur at the onsets of Heinrich Stadials, could trigger AMOC weakening by transporting freshwater to the North Atlantic deepwater formation regions. This initial weakening may subsequently be amplified by further meltwater release from the British and Laurentide Ice Sheets, induced by stepwise subsurface warming in the northeastern and northern North Atlantic. Our findings suggest that Siku events may have played a critical role in preconditioning the North Atlantic for Heinrich events, underscoring the interconnected nature of the global ocean-cryosphere system.