Salt fingers, diffusive convection, intrusions, and related phenomena have been observed and studied in the laboratory for three decades. The signatures of such phenomena have often been observed in oceanic measurements. However, one question has not been satisfactorily answered: what are the fluxes of heat, salt, mass, and momentum, both lateral and diapycnal, caused in the ocean by double-diffusion? Answering this question is key to assessing the overall importance of double-diffusion and devising realistic parameterizations of the fluxes.
The goal of SCOR WG #108 is to review progress made in the laboratory, ocean observations, numerical models, and theories of double-diffusive phenomena, and to suggest promising future directions of research that will lead to successful quantification and parameterization of fluxes. We particularly wish to understand why laboratory parameterizations of double-diffusion do not always appear to hold in the ocean.
The discovery of fine structure of the ocean stimulates detailed studies of small-scale physical processes and the mechanisms of its layering. Forty years ago it was shown that even for stable stratification, opponent distributions of two component species could drive convection if their diffusivities differed. Two main types of structures were observed - vertically directed narrow cells, i.e. thin salt fingers, and horizontally well-mixed layers separated by sharp interfaces. Recent advances in instrumentation show that the ocean is strongly unstable to double-diffusive processes and seems to be profoundly affected by them. Simple laboratory experiments have been carried out in different countries. They have continued to be important in identifying new phenomena and in their detailed studies, sophisticated theoretical and numerical models are used for calculating flow structure and stability, fluxes of heat and matter.
Active research by sea-going oceanographers, theoreticians and laboratory experimentalists has shown the complicated nature of double diffusive convection in the real ocean and has clarified some important features of a sequence of related phenomena. While much of the basic physics is understood clearly enough, theoretical description of the processes and their parameterization need considerable updating. The dependence of structure, geometrical parameters and fluxes upon external characteristics are to be established.
Recent field work has demonstrated that double-diffusive processes play a more important role in ocean dynamics than was previously suspected. The broader issue of disagreement between measured vertical diffusivity and that derived from large-scale modelling, may be considered as the most important evidence within the impact of small-scale phenomena on the global transfer. Double diffusive convection plays a vital role in heat and mass transfer in enclosed seas and in the deep or polar ocean.
The subject is being studied in different countries (in the USA, UK, Australia, New Zealand, Russia, France and others), but the number of research scientists is not very large. Coordination of efforts will improve the working contact between participants, planning of costly field measurements, use of modern facilities in a few advanced laboratories for cooperative investigations and creation of an adequate description of the phenomenon including methods of extrapolation of laboratory data to the conditions of the real ocean and their assimilation by numerical models.