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Abstract:
The introduction of high-resolution ocean model simulations has significantly improved the fidelity of the pathways constituting the Atlantic Meridional Overturning Circulation (AMOC) at subpolar latitudes. However, the progression from eddy-parameterised to eddy-rich ocean models has also introduced new biases in the subpolar North Atlantic, including larger-than-observed dense water formation over the Subpolar Gyre (SPG). Here, we use Lagrangian trajectories initialised across the Overturning in the Subpolar North Atlantic Program (OSNAP) array to explore the influence of these biases on the structure of the subpolar AMOC in three ocean hindcast simulations at resolutions ranging from 1º to 1/12º.
We show that the strength of the AMOC simulated across OSNAP increases substantially with increasing model resolution. The stronger overturning found at high resolution is explained by an intensification of the SPG, which yields greater dense water formation along the boundary currents. The simulated strength of the SPG circulation is determined by the initial model spin-up, where we find a marked salinification of the upper Labrador Sea at eddy-permitting and eddy-rich resolutions. As a result, the area of wintertime surface density outcrops ventilating the SPG interior increases dramatically during the initial decade, yielding a rapid accumulation of dense water at depth. Isopycnals, therefore, slope more steeply across the subpolar basin at higher resolution, producing a stronger, contracted SPG circulation, which enables a greater inflow of saline subtropical waters into the eastern SPG. The downstream propagation of these salinity anomalies represents an important positive feedback mechanism, reinforcing existing model biases in the upper Labrador Sea.
