Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from denstity overturns

Abstract

Observations were made of the dense overflow from Storfjorden from a survey conducted at closely spaced stations in August 2002. The field data set consists of conventional conductivity-temperature-depth profiles and short-term moored current meters and thermistor strings. Finestructure estimates were made by calculating Thorpe scales over identified overturns using 0.1-dbar vertically averaged density profiles. Dissipation rate of turbulent kinetic energy per unit mass, e, is estimated assuming proportionality between Thorpe and Ozmidov length scales. Vertical eddy diffusivity Kz is estimated using Osborn’s model assuming a constant mixing efficiency. Survey-averaged profiles suggest enhanced mixing near the bottom with values of Kz and e, when averaged within the overflow, equal to 10 x 10-4 m2 s-1 and 3 x 10-8 W kg-1, respectively. Kz is found to decrease with increasing buoyancy frequency as N-1.2 (±0.3), albeit values of N covered only 0.5–8 cph (1 cph = 2p/3600 s-1). Values of heat flux obtained using Kz suggest that the plume gains a considerable amount of heat, 45 ± 25 W m2, when averaged over the thickness of the plume, from overlying waters of Atlantic origin. This value is lower than but, considering the errors in estimates of Kz, comparable with 100 W m2, the rate of change of heat in the overflow derived from sections across the sill and 80 km downstream.