Dynamical processes of transfer at the sea surface

Abstract

This review describes the dynamical processes of transport from, and immediately below, the sea surface, particularly those which involve convergence and the separation of flow, and which result in the renewal of surface water at horizontal scales ranging from millimeters to hundreds of meters. Turbulence at or near the sea surface derives from several processes - breaking waves and the bubbles they may produce, precipitation and spray, Langmuir circulation and thermal convection, and turbulence which is internally generated by shear. Interest in the subject derives from the requirements to predict air-sea fluxes of heat, momentum and gases, to develop climate models, to interpret satellite images of the sea surface, including those of ship wakes, and to predict upper ocean structure, including mixing layer depth, in models of phytoplankton blooms and acoustic propagation. The general effect of subsurface turbulence on the sea surface, and the effects of surfactants, is described, and each process is discussed in turn. Laboratory experiments and theoretical studies have contributed particularly to the understanding of the interaction of vortices and turbulence with the surface and to the consequences of breaking waves. They point to the development of instability in the flow ahead of steep waves carrying parasitic capillary waves, which may contribute to the onset of flow separation on the leading face of the waves and the development of a rotor, or ‘roller’, below the wave crest, shown most clearly in the pattern of streamlines in a frame of reference moving forward with the wave. The conditions near the flow separation line on the wave surface ahead of the rotor may be similar to those produced by vortices approaching a free surface. Detailed observation of breaking waves at sea is lacking, but some progress has been made using acoustics to detect the clouds of subsurface bubbles formed by the larger breakers and the depth to which they penetrate. The possible importance of three-dimensional effects in breaking waves is discussed, and a conjectural sketch of the surface flow field and the surface separation and reattachment pattern, following a plunging breaker, is described. Recent observations show elevated levels of turbulence close to the sea surface. The depth of the layer in which this occurs is shown to be consistent with information derived from sonar studies of the depth of bubble clouds, the observed frequency of breaking waves and laboratory observations of breaking waves. Rain is known to ‘knock down the sea’, reducing the frequency and intensity of breaking waves. The precise mechanics of the phenomena are uncertain but an enhancement of near-surface turbulence and the damping of short surface gravity waves has been observed in laboratory experiments. Langmuir circulation, temperature ramps and convection, all associated with coherent structures, are processes which mix the upper ocean, although their relative importance is unknown. Connections between studies with quite disparate motivations are found to help provide descriptions of the upper ocean processes and estimates of the time scale of surface renewal associated with each of the processes identified. Many quantities being transferred across the sea surface appear to be transported by dynamical processes of scale much smaller than that of the dominant surface waves, although their subsequent diffusion in the mixing layer may be strongly influenced by the larger scale processes. Further careful measurements in the laboratory and at sea are required to quantify the fluxes associated with the processes and to relate them to external parameters such as wind speed.