An investigation of internal mixing in a seasonally stratified shelf sea

Abstract

The shelf sea seasonal thermocline is a critical interface within the marine environment, separating the euphotic zone from nutrient-rich deep water. Fluxes across the thermocline therefore represent a key biogeochemical pathway. In this paper we quantify the rate of mixing across the seasonal thermocline for a location in the Celtic Sea and investigate the processes responsible for driving thermocline fluxes. Profiles of the rate of dissipation of turbulent kinetic energy (ε) show enhanced dissipation within the thermocline region (∼6 x 10-5 W m-3). The diffusivity implied by these measurements is ∼0.5 cm2 s-1, similar to previous shelf sea studies, and is sufficient to explain the observed warming of the deep water, suggesting that vertical mixing is the dominant control on water column structure. Two potential sources of mixing energy are identified, the internal tide and near-inertial waves. The mechanism of energy transfer from the candidate mixing mechanisms to turbulence is not clear. Thermocline dissipation rates were found to have no Richardson number dependence, but scaled positively with N-2 and S2, in agreement with a previous turbulence parameterization. Application of this model to our data does a good job of capturing the mean characteristics of the observed heating flux across the thermocline, although none of the short-term fluctuations in mixing were reproduced. Copyright 2008 by the American Geophysical Union.