The thermohaline "conveyor belt" circulation transports heat
and salinity throughout the Earth's oceans, controlling the evolution of global climate. Of particular note is the climate of western Europe where the warm North Atlantic current maintains average temperatures considerably higher than other locations at comparable latitude, e.g., Hudson Bay, Canada. A major component of the thermohaline circulation occurs as cold, heavy water exits the Norwegian-Greenland-Iceland sea on its way to the North Atlantic. This heavy water passes over several low points in the ocean topography, accelerates down relatively steep slopes, and mixes vigorously as shear destabilizes the stable stratification in a slow-moving gravity current. I will describe ongoing laboratory experiments to understand the turbulent mixing process associated with this type of gravity current, generically referred to as an oceanic overflow. In particular, we measure the mean and fluctuating velocity and density of a laboratory gravity current using particle image velocimetry and planar laser induced fluorescence. Our characterization of turbulent mixing yields a compact description in terms of Prandtl mixing length theory, an approach that is currently being tested for use in ocean modeling applied to the national global climate prediction effort.