Deep convection variability across strong and weak AMOC states in an eddy-resolving ocean simulation

Water mass transformation in the Subpolar North Atlantic strongly influences the strength of the Atlantic Meridional Overturning Circulation (AMOC), oceanic heat and carbon uptake, and regional climate variability. Despite its importance, the variability of the Subpolar Gyre (SPG), its potential regime transitions, and its coupling to the AMOC remain poorly constrained, particularly regarding the role of mesoscale eddies. While advective-convective feedbacks have been proposed to lead to bistability in the SPG, it is unclear whether such behavior persists in strongly eddying ocean models.

Here, we examine SPG variability and deep water formation for distinct AMOC regimes in the stand-alone global ocean and high-resolution (0.1°) version of the Parallel Ocean Program. The POP was integrated for 600 years under a slowly increasing freshwater flux forcing over the North Atlantic, featuring a strong (20 Sv) AMOC state and, after the AMOC collapse, a weak (5 Sv) AMOC state. Monthly-averaged model output is used to construct composites that contrast SPG circulation and convective activity with particular emphasis on the role of resolved mesoscale variability.

There are  pronounced contrasts in regional convection and SPG behavior between the strong and weak AMOC states. In the strong overturning regime, deep convection across the Labrador Sea and Irminger Basin exhibits relatively low variability, while mixed layer depth variability is more pronounced in the Nordic Seas. In the weak overturning regime, deep convection in the Labrador Sea and Nordic Seas is strongly reduced to shallow mixed layer depths (< 150 m). In contrast, the Irminger Basin exhibits enhanced decadal variability and increased mixed layer depths. Notably, an accompanying low-resolution (1°) simulation does not reproduce this feature and lacks a sustained weak AMOC state after its collapse, highlighting the potential importance of eddy processes that are parameterized in coarse-resolution models. 

These results underscore the sensitivity of SPG dynamics and AMOC stability to model resolution and motivate further investigation into the representation of mesoscale processes in climate models and their role in shaping North Atlantic variability across distinct AMOC states.