Internal Waves and Ocean Mixing in the Western Arctic: Observations from the SHEBA Drift

An effective eddy diffusivity of order 10^-6 m²/sec has been found necessary to support the large scale thermohaline structure of the Beaufort Sea [Killworth and Smith, 1984]. Measurements in the Canadian basin indicate insufficient internal wave shear to maintain this diffusivity. At issue is whether mid-water processes above the continental slopes and shelves effect the required fluxes or whether the mixing occurs primarily in bottom boundary layers. The dispersal of pollutants from the shelves into the deep basins is significantly affected by the vertical distribution of mixing.

From October 1997 to October 1998 the SHEBA ice camp drifted from the central Beaufort across the Northwind Ridge and over the Chukchi Cap. As an aspect of SHEBA, a ten month record of upper ocean velocity was obtained using repeat-sequence, coded-pulse Doppler sonars. The shear field measured by the sonars, resolved to 3 m vertical scale, consists of sub-inertial (vortical) and super inertial (wave) constituents. An apparent separation between these fields is seen in the wavenumber-frequency domain. In contrast to low latitude observations there is a surplus of wave shear variance with an anti-cyclonic sense of rotation, relative to cyclonic, both above and below the inertial frequency. When the wave variance is displayed as a function of intrinsic frequency (determined by the linear polarization ratio), the entire wavenumber-(intrinsic) frequency spectrum collapses to a single spectral ridge, with group velocity roughly equal to the rms orbital velocity of the wavefield. There is significantly more energy in upward propagating waves over the topography of the Chukchi Cap relative to the central Canadian Basin. However, rms shear levels suggest that, even over topography, there is insufficient mixing in the water column to support a basin-wide average diffusivity of 10^-6 m²/s.