Introduction

Disequilibrium between the fast air stream and sluggish water drives the irreversible process of air–sea momentum transfer, much as temperature difference drives heat transfer. The rate of transfer per unit area and unit time, the momentum flux, equals the tangential force per unit area applied to the air–sea interface, “drag” for short. In laminar flow viscous shear would be the instrument of momentum transfer, an air-side boundary layer the principal resistance, as a solution of the Navier–Stokes equations reveals. If we could magically contrive to sustain laminar flow above and below the sea surface, we would witness momentum transfer rates some two orders of magnitude lower than we actually find. Hydrodynamic instability of laminar shear flow makes this impossible and brings two important ingredients to the air–sea momentum transfer process: Reynolds flux of momentum, which becomes the main route of momentum transport to and from the air–sea interface, and wind-waves on the interface, which play a prominent role in the momentum handover process from air to water.

Pathways of Air–Sea Momentum Transfer

Turbulence in the air and in the water sustains Reynolds fluxes. Anything that tends to suppress turbulence increases the resistance to momentum transfer, as the example of drag-reduction chemicals shows: introduction of high-polymer substances into the viscous sublayer of a streamlined object (such as a submarine) dissipates turbulence energy, thickens the viscous sublayer, and reduces drag.