Scaling turbulent dissipation in an Arctic fjord

In September 2003, microstructure and fine-scale shear (vertical derivative of horizontal current) and strain (vertical derivative of isopycnal displacements) profiles were collected in Storfjorden, Svalbard Archipelago. Storfjorden is a sill-fjord of significant dense water production and the period of observations corresponds to weak overflow conditions of the dense water. The data set comprises fine-scale measurements combined with simultaneous dissipation and buoyancy frequency profiles and allows for evaluation of various turbulence scalings and internal wave–wave interaction models. Microstructure profiles identified high levels of background vertical diffusivity, Kρ, O(10−4) m2 s−1, which increased by one order within the overflow. Fine-scale shear and strain spectral levels were found to be elevated above those of Garrett–Munk, consistent with the observed enhanced mixing, whereas their average ratio was comparable to that of Garrett–Munk. Among the internal wave–wave interaction models, a shear–strain model agreed with the observations (between the surface and bottom mixed layers) within a factor of 2, on the average. A scaling of the dissipation, ε, in the form of total-energy×N suggests that approximately 0.5% of the energy is dissipated per buoyancy period, 2π/N. The best-fit power-law scaling between diffusivity and N is Kρ∝N−1.4 (±0.2). For this environment, a parameterization of the form of Pacanowski–Philander is suggested with background levels of eddy viscosity and diffusivity elevated by an order of magnitude.