Modeling of deep-water renewal through cold convective plumes

In only a few regions in the world ocean, surface water is transformed into deep water by open ocean convection. In this paper, small areas of vertical flow, or plumes, are studied using a steady-state model, integrated in streamline coordinates. We use an entrainment hypothesis based on knowledge about in-mixing of water across the shear zone from an area of low turbulence into an area of high turbulence. This results in a model using conservation equations for momentum, heat and salt, and a nonlinear equation of state. Effects of earth rotation are included. The depth reached by surface water when moving through stationary plumes is predicted, and found strongly dependent on the properties in the surface layer. This layer has to be colder but only slightly denser than the water just below it to be a source for steady-state plumes. The thermobaric effect, the dependency of compressibility on the temperature, plays a major role for the plume penetration depth. In weakly stratified cold water, like in the Greenland Sea, deep convection is found to be possible and efficient through steady-state plumes. Initial excess density of less than 5×10−3 kg m−3, caused by low temperature, can give convection deeper than 1500 m through plumes with horizontal scales of a few hundred meters and vertical velocities of order a few tens of centimeters per second.