Exchanges at the Sea Surface

While the boundary between the ocean and the atmosphere has been extensively studied it is still not well understood. Heat, mass and momentum cross this boundary at a rate determined by many features of not only the sea surface motion but also the properties of the atmosphere and the ocean boundary layers on each side of the interface. There are some simplifications that can be made because the sharp variations are predominantly in the vertical, but there is a hierarchy of scales and processes at play which cannot be ignored in many applications. Central to understanding the processes at the boundary is gaining knowledge about the flux of momentum between the water and the air. The flux, which is the rate of transport of momentum across unit area, can be in either direction. In a frame of reference fixed to the earth, flux is mostly from the wind to the ocean currents, but less frequently, the flux is from the wind-waves or currents to the atmosphere.

The winds and currents have gradients of horizontal momentum. If we treat the sea surface as a sharp boundary between two fluids of different properties, we can model the flux of momentum from one of the fluids to the other as a drag force per unit area at the sea surface. This is the surface shear stress.