Weakening of the AMOC and Strengthening of Labrador Sea Deep Convection in Response to External Freshwater Forcing

The Labrador Sea open-ocean deep convection was often thought to play an important role in AMOC changes. For example, in some previous water hosing experiments, the prescribed external freshwater flux is released broadly and covers the entire subpolar North Atlantic, which causes the weakening and shutdown of the Labrador Sea open-ocean deep convection and associated subsurface warming. The simulated subsurface warming over the subpolar North Atlantic due to the shutdown of the Labrador Sea open-ocean deep convection further drives the AMOC weakening. However, the importance of the Labrador Sea open-ocean deep convection in the AMOC has been challenged by theoretical, modeling, and observational analyses. In this study, an ensemble of water hosing experiments is conducted to examine mechanisms of AMOC weakening and its subsequent impact on the Labrador Sea open-ocean deep convection. The results show that the subpolar AMOC decline in response to the external freshwater flux released over the southern Nordic Sea is dominated by that across the eastern subpolar North Atlantic, and the largest subpolar AMOC decline is at the relatively dense level. The AMOC decline is associated with subsurface cooling in the subpolar North Atlantic and the decline in the deep ocean west–east density contrast across the subpolar basin. Contrary to previous studies showing that the AMOC decline is caused by subsurface warming through the shutdown of the Labrador Sea open-ocean deep convection, our results reveal a novel response, i.e., a strengthening of the Labrador Sea open-ocean deep convection, which is not a cause of the AMOC decline. We illustrate the key mechanisms causing the strengthening in the Labrador Sea open-ocean deep convection and the relationship with the AMOC weakening. This convection strengthening is mainly due to the relatively stronger freshening in the deep Labrador Sea associated with the freshening/weakening of the Iceland-Scotland Overflow, and thus reduced vertical stratification in the central Labrador Sea.

Wei, X., & Zhang, R. (2024). Weakening of the AMOC and strengthening of Labrador Sea deep convection in response to external freshwater forcing. Nature Communications, 15(1), 10357.