Overflows and Cascades

Abstract

The flow of relatively dense water down continental slopes into the deep ocean forms an important component of the global thermohaline circulation. The dense waters can be formed by evaporation in marginal seas (such as the Mediterranean Sea and the Red Sea) or by cooling on higher-latitude continental shelves. In polar regions brine rejection during sea ice formation can also contribute to the increased density. The dense waters flow down the continental slopes into the deep ocean as intermittent events (cascades) or more continuous flows (overflows). As they descend the slope the currents of dense water (known as gravity currents or density currents) are affected by the Earth’s rotation, turning to the right in the Northern Hemisphere (left in the Southern Hemisphere) to flow predominately along, rather than down, the slope. However, the along-slope flow is continuously drained by a bottom boundary layer that flows more directly downslope. Overflows and cascades entrain ambient seawater from above, mixing until the current has the same density as the surrounding seawater. In certain circumstances, overflows may generate eddies with strong cyclonic vorticity in the overlying water, thus influencing the flow and mixing well above the dense current itself.