Modeling the North Atlantic and Labrador Sea dynamics with the global high-resolution ocean model FESOM

The subpolar regions of the North Atlantic ocean are crucial for the global climate in terms of deep water formation, which is a major driver for the Atlantic Meridional Overturning Circulation (AMOC) that transports heat into northern latitudes and returns cold deep water masses southward. The influence of a high horizontal resolution (5-15 km) on the general circulation and hydrography in the North Atlantic is investigated using the finite element sea ice-ocean model FESOM. A stronger shift of the upper ocean circulation and water mass properties during the model spinup is found in the high-resolution model version compared to the low-resolution (ca. 1 deg) control run. In quasi-equilibrium, the high-resolution model is able to reduce typical low-resolution model biases. Especially, it exhibits a weaker salinification of the North Atlantic subpolar gyre and a reduced mixed layer depth in the Labrador Sea. However, during the spinup adjustment, initially improved high-resolution features partially reduce over time: the strength of the Atlantic overturning and the path of the North Atlantic Current are not maintained, and hence hydrographic biases known from low-resolution ocean models return in the high-resolution quasi-equilibrium state. Long baroclinic Rossby waves are identified as a potential cause for the strong upper ocean adjustment of the high-resolution model. In addition, the high-resolution model is able to represent turbulent processes on the meso- and submesoscale within the Labrador Sea interior. Mesoscale eddies transport buoyant seawater into regions of strong convection, thereby contributing significantly to restratification. In particular, ageostrophic velocities associated with baroclinic instability were found to play a crucial role on length scales on the order of O(10) km. Until now, the dynamics on such scales were rarely modeled with a realistic global high-resolution ocean model in quasi-equilibrium.