Geophysics of Deep Convection and Deep Water Formation in Oceans

Abstract

Abstract An important task for geophysicists studying deep convection and deep water formation in the ocean is to investigate various physical mechanisms generating vertical circulation. Two major types of such mechanisms are available so far, namely, thermodynamical instability, which generates convection and mixing due largely to surface buoyancy flux brought by net brine release or surface cooling, and dynamical instabilities, which also produce various vertical cells exchanging water masses at different depths. After reviewing various mechanisms, in this short note, symmetric instability is found to be very important for both near-boundary and open ocean deep convections. The symmetric instability is a combined inertial (or centrifugal) and baroclinic instability. Vertical cells driven by the symmetric instability are in planes perpendicular to the shear flow. However, vertical cells generated by the baroclinic instability are in the same plane as the shear flow. For the near-boundary deep convection, especially in off-shore wind prevailing regions (e.g., the Weddell Sea, the Ross Sea), where onshore Ekman flux is not available, the symmetric instability plays an important role in driving dense water off the shelf. For the open-ocean deep convection, the symmetric instability of a background cyclonic circulation generates vertical cells, exchanging water masses and reducing the stratification of the water column within the gyre. Therefore, the symmetric instability is a strong candidate for being a dominant mechanism for ‘preconditioning’ for open ocean deep convection.