The significance of double-diffuse mixing for water-mass transformation and determining the structure of the temperature-salinity relationship in all oceans is becoming increasingly clear over the last few years. One of the most intriguing aspects of double-diffuse convection is related to its ability to transform smooth vertical gradients into the stepped structures consisting of mixed layers separated by thin stratified interfaces. Recent field data from the tropical Atlantic has shown that vertical fluxes due to salt fingers within thermohaline staircases are significant (by order of magnitude) enhanced over non-staircase regions with similar stratifications. (Schmitt et al., 2005). Thus, it is vital to improve our understanding of the thermohaline staircase formation, evolution and the large scale consequences.
In this CAREER award, a researcher at the Naval Postgraduate School will quantify the double-diffusive transport in both smooth gradients and thermohaline staircases, developing a clearer insight into the origin of staircases and specifying the conditions for their information. The work will involve a combination of numerical process modeling and theoretical analysis. Inferences from the theory and model runs will be used to interpret the oceanic fine-and microstructure data and results of the laboratory experiments. The results from this study will resolve the controversy with regard to the dynamics of the thermohaline staircases, the magnitude of the heat and salt mixing rates, and their variation with environmental parameters. The newly developed theory by the principle investigator will invoke the thickness of the mixed layers and the resulting diapycnal fluxes, will be explored. In addition to the intellectual merit of the work, the proposed teaching activities will target the Naval Postgraduate School students and expose them to the physics of the ocean microstructure by developing a special topics course, expanding the distance learning program, and advising several graduate students.
Double-diffusive convection could be defined as a set of processes related to the presence in fluids of two distinct density components which diffuse at different rates. In the context of ocean circulation, double-diffusive convection is maintained by the distribution of heat (faster diffuser) and salt (slower diffuser) in sea-water. Double-diffusive mixing plays an important role in the ocean's ability to transport and sequester heat, nutrients, pollutants and carbon dioxide. Thus, advances in understanding and quantification of double-diffusive transport will improve climate and ecosystem modeling capabilities that could have wide societal benefits. One of the most intriguing aspects of double-diffusive convection is related to its ability to transform smooth vertical gradients into the stepped structures consisting of mixed layers separated by thin stratified interfaces. The appearance of staircases dramatically increases the vertical mixing rates relative to the levels typically observed in smooth-gradient regions. In this NSF CAREER project, we have quantified the double-diffusive transport in both smooth gradients and thermohaline staircases, developed insight into the origin of staircases and specified conditions for their formation. The equilibration of magnitude of the heat and salt mixing rates, and their variation with environmental parameters was explained using a combination of numerical process modeling and theoretical analysis. Numerical simulations indicate that in time salt fingers evolve from relatively organized vertical filaments to very irregular structures through the action of their secondary instabilities. The vertical transport of heat and salt reaches a quasi-equilibrium level determined by the environmental conditions. To predict and explain the heat/salt mixing rates, we developed a novel analytical theory which emphasizes the role of secondary instabilities of salt fingers in saturation of their linear growth (Radko and Smith, 2012) and successfully tested it by a series of direct numerical simulations. The ability of double-diffusive convection to generate structures on much larger scales, such as intrusions, gravity waves and thermohaline staircases, has also been illustrated by numerical simulations and analytical models. Diagnostics of the numerical simulations suggest that the propensity of double-diffusive fluids to produce well-defined staircases in the ocean is linked to the parametric variation in the heat/salt flux ratio (Traxler et al., 2011; Stellmach et al., 2011). Our results thereby resolve the long-standing controversy with regard to the origin and dynamics of thermohaline staircases. During this project, eight graduate students participated in the planned research activities, developing expertise about oceanic mixing and numerical modeling techniques, and successfully defended their MS theses. Our scientific findings have been reported in eight papers published in leading oceanographic and fluid dynamical journals.
