Effects of the Submesoscale on the Potential Vorticity Budget of Ocean Mode Waters

Ocean mode waters are defined by their anomalously low potential vorticity (PV), and represent a key pathway for communicating a history of air-sea interaction into the ocean interior, exporting heat and carbon, and influencing the gyre scale circulation. The formation of ocean mode waters is fairly well explained by air-sea buoyancy fluxes, however the destruction of upper-ocean mode water during the seasonal transition from winter to spring is less thoroughly understood. In particular, recent observational and numerical modeling work suggests that the mode-water formation regions are sites of active submesoscale turbulence, which is not generally resolved in observational or modeling studies of the mode water seasonal cycle. Here we use the framework of PV fluxes to assess the role of submesoscale processes on the seasonal cycle of mode water formation and destruction. Particular focus is given to the competing effects of diabatic PV removal associated with surface buoyancy loss, and the frictional injection of PV due to boundary layer turbulence acting on the geostrophic shear of submesoscale fronts. Comparison of theory and idealized models suggest that the frictional injection of PV in regions of sharp fronts can effectively offset the PV removal due to diabatic processes, leading to a net injection of PV onto outcropped isopycnals, even during times of surface buoyancy loss. A high-resolution numerical model is used to assess the effects of these processes on the North Atlantic subtropical mode water, with results suggesting that frictional injection of PV at the submesoscale alters the seasonal mode water PV budget, and shortens the annual period of mode water creation by several weeks to a month, with implications for understanding mode water variability and properties.