BUBBLE CLOUDS AND THE DYNAMICS OF THE UPPER OCEAN

This is a review of turbulence in the upper ocean close to the sea surface, particularly of the information that has been obtained from sonar observations of bubble clouds produced by breaking wind waves. These clouds provide tracers of the turbulent motions and are important, especially at high wind speeds, in the process of air-sea gas transfer. The observations of bubble clouds are here related to other measurements of turbulence, particularly to direct measurements of currents and temperatures in lakes or at sea, and to laboratory studies. Some novel observations of bubble clouds and breaking waves, their frequency and relation to Langmuir circulation, are presented. There is now emerging a pattern of clues that point to the dominance of breaking surface gravity waves as a source of turbulence to a depth below the surface of 0.04 to 0.2 times the wavelength of the dominant breaking waves, although the effect of swell has yet to be clarified. The relative depth appears to increase with increasing values of W10/c, where W10 is the wind speed and c the phase speed of the dominant waves. Below this region the generation of turbulence may be dominated by shear-stress or convection. Here, turbulence is generally similar to that in the atmospheric boundary layer. There are, however, coherent motions on the scale of the mixing layer that persist for periods of a few minutes to an hour or so, to which the transport of a large part of the momentum and heat fluxes can be attributed, and which strongly affect the dispersion of buoyant particles or bubbles. These motions deserve special study to establish their contribution to heat and momentum transport, and hence to determine if, or when, they should be specifically represented in models of the upper ocean devised, for example, to describe the dispersion of passive and non-passive tracers or the air-sea transfer of gases.