Compensation and Alignment of Thermohaline Gradients in the Ocean Mixed Layer

The surface mixed layer of the ocean is often characterized by thermohaline compensation and alignment. That is, temperature and salinity gradients tend to be parallel and to cancel in their contribution to density. In this paper a combination of theoretical arguments and numerical simulations is presented to investigate how compensation and alignment emerge as a result of processes at work within the mixed layer. The dynamics of the mixed layer is investigated through a simple model that couples a nonlinear diffusive parameterization for the horizontal transports of temperature and salinity with stirring by mesoscale eddies. It is found that stirring quickly aligns the temperature and salinity gradients and that nonlinear diffusion creates compensation. Neither process, by itself, is sufficient to reproduce the observations.