The Role of Internal Waves in the Layering of Outflows from Semi-Enclosed Seas

Abstract

Outflows from semi-enclosed marginal seas typically involve complex layered structures and inversions in vertical profiles of temperature and salinity. We have carried out experiments with turbulent plumes and their outflows in an enclosed, stratified environment. Low frequency internal waves excited by the plume outflow itself, as it intrudes into the surrounding stratification at its depth of neutral buoyancy, produce horizontal counter-flowing shear layers. These ‘shearing modes’ lead to branching that breaks up the outflow into a number of layers with a vertical scale determined by the structure of the upward propagating wave modes. In the oceans this scale is predicted to be of order 100m. In experiments with two diffusing components (T and S) doublediffusive convection develops thin interfaces and salt fingers between the counter-flowing layers. However, the convection is parasitic and does not generate the layers or influence their thickness. Oceanic outflows too are prone to doublediffusive convection but in this case the convecting layers, 10m to 30m thick, are potentially distinguishable from the larger ‘shearing mode’ structure.