Although great strides have been taken in the past eight years, we have still a long hike ahead of us before we can say that we understand turbulence and mixing in the ocean.
In field experiments, we have made technical progress to the point where large quantities of data are generated. For example, in upper ocean experiments our profilers run ten casts an hour. Together with continuous ADCP, meteorology, acoustic and thermistor-chain measurements, these profilers make it possible to observe changes in the mixed layer and seasonal thermocline in unprecedented detail. On longer casts, fine-scale current sensors added to microstructure profilers have allowed us to establish relationships between fine-scale shear and microscale dissipation.
Advances in computer capabilities have made simulations possible, and they are giving us some feeling for the phenomena to be expected. Laboratory experiments have given indications of the processes at work, and useful guidance.
But this is still not enough! We still lack a theory for turbulence that can truly guide our investigations of the ocean. Numerical simulations cannot presently encompass a realistic range of scales of motion. Laboratory experiments can be misleading because boundary and initial conditions may fail to simulate the ocean in critical respects. In the many cases in which we don't understand conditions on energy-containing scales in the ocean, we can't set them up realistically in the laboratory. To understand and parameterize geophysical turbulent flows we will have to study them in nature, not just in the laboratory or on the computer.
An example is surface-layer mixing. Even when convection is absent, we don't understand what causes the turbulence to be sometimes more energetic than predicted by constant-stress scaling. Apparently waves are important, but exactly how? We don't know enough about the processes at work to determine which aspects of the oceanic situation are the critical ones to simulate on the computer or in the laboratory. Another example: What characteristics of the flow make mixing efficiency different from one place to another? Here we may get some guidance from the laboratory and computer, but the real battle will clearly have to be fought at sea.
Where does the vertical transport occur that mixes the deep ocean? It appears that it must occur at hot spots or boundaries. It won't be found in the laboratory or computer; a search at sea will have to be mounted. The focus for future studies of the effects of internal waves on mixing will be non-GM environments; these must be investigated on site.
We believe that we are going to achieve a better understanding of the physics of turbulence and mixing only by developing better methods of imaging the small scales so that we can actually see the full evolution of the flow field from initiation of the instability to the end result of a mixed fluid. Best would be a remote-sensing method. If the beating of a fetal heart and the motion of blood in a vein can be imaged so remarkably in hospitals, can we hope to see similar images of turbulence in the ocean?
Acknowledgments. During the period in which this paper was written the authors were supported in part by NSF grants OCE-9314396 and OCE-9110552. Further support for J. N. Moum was supplied by the Office of Naval Research (grant N00014-89-J3211). D. R. Caldwell thanks M. C. Gregg and the Applied Physics Laboratory of the University of Washington for their hospitality during his sabbatical in 1993--1994.