Double-Diffusive Convection

Abstract

Even in systems with a negative vertical density gradient, instability is possible if the density is controlled by two components (e.g. temperature T and salinity S) that diffuse at different rates [24]. In the oceans, temperature diffuses approximately 100 times faster than salt, and so many regions of the ocean are potential candidates for so-called “double-diffusive instability”. Doubly-diffusive effects were observed by several authors before Stern [24] explained the physical mechanism responsible. Jevons [4] and Ekman [1] had previously observed instability at the interface between temperature stratified water and an overlying layer of denser fluid, but neither recognised the significance of double-diffusion. Later, Stommel et al. [28] showed that a “perpetual salt fountain” can arise when a tube is inserted vertically through the interface between a layer of warm, salty water overlying cold, fresh water. The fountain persists until the system becomes well mixed. This experiment has recently been realised on an industrial scale [29]. There are two forms of double-diffusive instability, referred to as “salt fingers” and “diffusive convection”.