Catastrophic events in a surface mixed layer

Abstract

THE formation and evolution of mixed layers in the surface waters of the oceans are important in many fields of study and much effort has been expended towards the development of predictive models of the wind-mixed surface layer^1,2. These models parameterise small-scale high-frequency turbulent mixing processes, which are difficult to measure, in terms of more easily observed variables such as the wind stress. Because this parameterisation depends on hypotheses about the character of the turbulent mixing which dominates the transport of heat and momentum, it is vital to have a realistic concept of the turbulent processes in active mixing layers. It is often assumed that a major source of turbulence at the base of the mixed layer is the shear between the mean currents in the mixed layer and those in the thermocline. Simple shear turbulence of this sort often may be described by a flux-gradient relationship³. We have now found that although a flux-gradient description is adequate most of the time at the base of an actively mixing layer, during a storm isolated and relatively rare large-scale catastrophic mixing events, which cannot be described by a flux-gradient relation, can dominate transport. Although these catastrophic events occur infrequently, the vertical heat transport associated with them can dominate the continuous transport associated with flux-gradient processes.