Jacob Wenegrat (Stanford University)
From the submesoscale to the gyre scale: How small-scale fronts modify 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, 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 PV flux framework 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 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. The significance of these dynamics is illustrated using a high-resolution realistic model of the North Atlantic Subtropical Mode Water (18 degree water), where it is demonstrated that injection of PV at the submesoscale reduces the rate of mode-water PV removal by a factor of ~ 2, and shortens the annual period of mode water creation by ~ 3 weeks, with implications for understanding the variability and biogeochemical properties of ocean mode waters globally. These dynamics thus provide a direct link across spatial scales, whereby small-scale boundary layer turbulence, in the presence of submesoscale fronts, can alter the potential vorticity, and hence circulation, of the ocean gyres.