FLUID-FLOW IN LOOPS DRIVEN BY FRESH-WATER AND HEAT FLUXES

Thermohaline convection in a salt water loop is discussed. Fluid tempe rature is affected by relaxation on the loop surface and fluid salinit y by a freshwater flux through the loop surface. In addition, other bo undary conditions on salinity, such as the equivalent virtual salt flu x or salinity relaxation condition, are examined and the dynamic role of diffusion in thermohaline convection is analysed. Both analytical a nd numerical analyses indicate that the system behaviour depends sensi tively on the nature of the salinity boundary condition. For the salin e-only loop model, analysis indicates that perturbations are advected by the mean flow, and flow stability is independent of the strength of the boundary forcing. In the full thermohaline loop problem, the virt ual salt flux formulation accurately mirrors the freshwater flux resul ts when the system is in the thermal mode. However, these formulations can differ substantially when the system is in the haline mode, espec ially in the strongly forced, weakly diffusive limit. For both types o f loop configuration, salinity profiles governed by freshwater flux ha ve scales determined by the internal parameters, while virtual salt fl ux profiles necessarily reflect the lengthscales of applied boundary c onditions. Negative salinities can also appear under virtual salt flux owing to the inaccuracies inherent in the approximation, while freshw ater flux ensures positive-definite salinity values. Our analysis supp orts the use of the physically more accurate freshwater flux boundary conditions when simulating thermohaline circulation.