Impacts of Spatial Distribution of Parameterized Local and Remote Tidal Mixing on Large-scale Ocean Circulation and Climate.

Energy extracted from the barotropic tide as it flows over topography can ultimately be used for mixing, which in turn influences large-scale circulation and climate. While the physics of the energy conversion process is now well understood, the eventual location of the resultant mixing is still poorly known, and dependent on a variety of wave breaking processes. Here, we examine the impact of different horizontal and vertical distributions of the mixing by breaking internal tides on the large-scale ocean circulation and climate in a fully-coupled climate model, GFDL’s ESM2G. Internal tides are assumed to dissipate either at the generation site, or in deep ocean basins, over continental slopes, or on continental shelves. The dissipation has a vertical profile which is specified to either: (a) decay exponentially with height above the bottom, or (b) scale with the buoyancy frequency, or (c) scale with the buoyancy frequency squared as suggested by observations of dissipation away from rough topography. The resultant large-scale ocean circulation is highly sensitive to the vertical distribution of dissipation, with bottom intensified mixing leading to enhanced deep overturning and sharper thermoclines, while profiles with greater dissipation near the surface have stronger subtropical cells, and more diffuse thermoclines. The climate appears less sensitive to these idealized horizontal distributions of mixing. An exception is mixing near regions of deep water formation, such as the continental slopes of the North Atlantic and the Antarctic, where substantial mixing in the descending branch of the overturning circulation leads to a dilution of dense water, and corresponding weakening of the overturning. Motivated by this demonstrated sensitivity of the large-scale ocean circulation, we are refining models of the spatial distribution of internal wave breaking, in collaboration with partners in the Internal Wave Driven Mixing climate process team.