Sea-surface temperature (SST) is crucial for atmosphere–ocean interactions. Since the sea surface is the common interface between the atmosphere and ocean, either system can change the SST through exchanges of heat and momentum between the two. In general, the regions on both sides of the sea surface are turbulent. In order to understand those turbulent fluxes of heat and momentum capable of changing the SST, we have to be able to characterize the boundary layers on both sides of the interface. The SST can change directly because of changes in boundary-layer mixing, and, in addition, changes in boundary-layer mixing can change the fluxes themselves. For example, an atmospheric boundary layer growing into a vertically sheared wind (say the wind increases upward) will entrain more momentum into the boundary layer and increase the stress at the surface. This, in turn, can change the evaporation and sensible heating into the atmosphere and, consequently, the heat flux into the ocean.

The nature of the boundary layers in the tropical atmosphere and ocean is very different and we will have to treat each boundary layer in a manner that respects this difference. The tropical marine atmospheric boundary layer is driven from below by buoyancy forcing at the ocean surface and is unstable. The tropical ocean boundary layer is also heated at the surface, so warm water overlies cold, creating stable conditions. Mixing has to be forced by wind stresses at the surface.