North Atlantic subsurface temperature response controlled by effective freshwater input in “Heinrich” events

The northern North Atlantic (NNA) subsurface temperature in response to the slowdown of the Atlantic meridional overturning circulation (AMOC) is crucial for ice sheet calving and recovery of the AMOC in Heinrich events. Paleoclimate proxies and modeling studies suggest that the NNA subsurface exhibits a robust warming during Heinrich 1, but with a less clear response during Younger Dryas (YD). The mechanism for the potentially different subsurface responses has remained not well understood. Previous studies show different NNA subsurface response depending on hosing at different locations. Here, by examining a suite of “water-hosing” experiments with different hosing regions and intensities, we show that, regardless of the hosing location, NNA subsurface temperature response is determined by the effective freshening over the NNA deep convective regions through the competition between the warming associated with the suppressed vertical mixing and the cooling associated with the weakened AMOC heat transport. A weak effective freshening favors advective effect and, in turn, cooling, while a strong effective freshening strengthens the mixing effect and leads to a warming. Our results suggest that a cooling may occur in the NNA subsurface during YD.