Vertical Flow in the Southern Ocean Estimated from Individual Moorings

Abstract

his study demonstrates that oceanic vertical velocities can be estimated from individual mooring measurements, even for nonstationary flow. This result is obtained under three assumptions: (i) weak diffusion (Péclet number ? 1), (ii) weak friction (Reynolds number ? 1), and (iii) small inertial terms (Rossby number ? 1). The theoretical framework is applied to a set of four moorings located in the Southern Ocean. For this site, the diagnosed vertical velocities are highly variable in time, their standard deviation being one to two orders of magnitude greater than their mean. The time-averaged vertical velocities are demonstrated to be largely induced by geostrophic flow and can be estimated from the time-averaged density and horizontal velocities. This suggests that local time-mean vertical velocities are primarily forced by the time-mean ocean dynamics, rather than by, for example, transient eddies or internal waves. It is also shown that, in the context of these four moorings, the time-mean vertical flow is consistent with stratified Taylor column dynamics in the presence of a topographic obstacle.