Non-Rotating Gravity Currents

Abstract

Gravity currents (also known as “density currents”) in the ocean and atmosphere are flows of water (or air) that are principally due to differences in density (due to differing temperature and/or salinity) between neighboring fluid bodies. These density differences cause lateral pressure gradients that produce the horizontal motion. They may occur on length scales ranging from centimeters to hundreds of kilometers, and they have a characteristic structure that is described below. In the oceanic environment there are three main types, as follows. Firstly, gravity currents may move along the ocean bottom, with denser (colder or saltier) water moving under a lighter water mass. Secondly, a body of lighter water may move along the ocean surface, above denser water, and thirdly, a homogeneous body of water of intermediate density may penetrate a larger stratified water body of varying density, with lighter fluid above and denser fluid below. The latter process is termed an intrusion. In the atmosphere, sea breezes and the leading noses of cold fronts often have the character of density currents, driven by the difference in air temperature. If these flows last for more than about a day, the Earth's rotation (via the Coriolis force) has an important effect on the flow, and these rotational effects are discussed elsewhere (MS 116). In this article I give some examples of these flows, and then describe their basic dynamical properties for flows that move horizontally in one direction, and that move radially outward in two dimensions. I then proceed to discuss gravity currents that flow down slopes, taking into account the effects of environmental stratification.