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Early spring turbulent mixing in an ice-covered Arctic fjord during transition to melting

Fer, Ilker and Widell, Karolina LU (2007) In Continental Shelf Research 27(15). p.1980-1999
Abstract

Observations are presented of currents, hydrography and turbulence in a jet-type tidally forced fjord in Svalbard. The fjord was ice covered at the time of the experiment in early spring 2004. Turbulence measurements were conducted by both moored instruments within the uppermost 5 m below the ice and a microstructure profiler covering 3-60 m at 75 m depth. Tidal choking at the mouth of the fjord induces a tidal jet advecting relatively warmer water past the measurement site and dominating the variability in hydrography. While there was no strong correlation with the observed hydrography or mixing and the phase of the semidiurnal tidal cycle, the mean structure in dissipation of turbulent kinetic energy, work done under the ice and the... (More)

Observations are presented of currents, hydrography and turbulence in a jet-type tidally forced fjord in Svalbard. The fjord was ice covered at the time of the experiment in early spring 2004. Turbulence measurements were conducted by both moored instruments within the uppermost 5 m below the ice and a microstructure profiler covering 3-60 m at 75 m depth. Tidal choking at the mouth of the fjord induces a tidal jet advecting relatively warmer water past the measurement site and dominating the variability in hydrography. While there was no strong correlation with the observed hydrography or mixing and the phase of the semidiurnal tidal cycle, the mean structure in dissipation of turbulent kinetic energy, work done under the ice and the mixing in the water column correlated with the current when conditionally sampled for tidal jet events. Observed levels of dissipation of turbulent kinetic energy per unit mass, 1.1×10-7 W kg-1, and eddy diffusivity, 7.3×10-4 m2 s-1, were comparable to direct measurements at other coastal sites and shelves with rough topography and strong forcing. During spring tides, an average upward heat flux of 5 W m-2 in the under-ice boundary layer was observed. Instantaneous (1 h averaged) large heat flux events were correlated with periods of large inflow, hence elevated heat fluxes were associated with the tidal jet and its heat content. Vertical heat fluxes are derived from shear-probe measurements by employing a novel model for eddy diffusivity [Shih et al., 2005. Parameterization of turbulent fluxes and scales using homogeneous sheared stably stratified turbulence simulations. Journal of Fluid Mechanics 525, 193-214]. When compared to the direct heat flux measurements using the eddy correlation method at 5 m below the ice, the upper 4-6 m averaged heat flux estimates from the microstructure profiler agreed with the direct measurements to within 10%. During the experiment water column was stably, but weakly, stratified. Destabilizing buoyancy fluxes recorded close to the ice were absent at 5 m below the ice, and overall, turbulence production was dominated by shear. A scaling for dissipation employing production by both stress and buoyancy [Lombardo and Gregg, 1989. Similarity scaling of viscous and thermal dissipation in a convecting boundary layer. Journal of Geophysical Research 94, 6273-6284] was found to be appropriate for the under-ice boundary layer.

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author
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Arctic, Fast ice, Fjords, Mixing, Svalbard, Turbulence, Van Mijenfjorden
in
Continental Shelf Research
volume
27
issue
15
pages
20 pages
publisher
Elsevier
external identifiers
  • scopus:34547805197
ISSN
0278-4343
DOI
10.1016/j.csr.2007.04.003
language
English
LU publication?
no
id
f102b0dd-3998-4b8d-a10d-584483c969db
date added to LUP
2017-06-14 16:42:55
date last changed
2017-06-16 09:42:51
@article{f102b0dd-3998-4b8d-a10d-584483c969db,
  abstract     = {<p>Observations are presented of currents, hydrography and turbulence in a jet-type tidally forced fjord in Svalbard. The fjord was ice covered at the time of the experiment in early spring 2004. Turbulence measurements were conducted by both moored instruments within the uppermost 5 m below the ice and a microstructure profiler covering 3-60 m at 75 m depth. Tidal choking at the mouth of the fjord induces a tidal jet advecting relatively warmer water past the measurement site and dominating the variability in hydrography. While there was no strong correlation with the observed hydrography or mixing and the phase of the semidiurnal tidal cycle, the mean structure in dissipation of turbulent kinetic energy, work done under the ice and the mixing in the water column correlated with the current when conditionally sampled for tidal jet events. Observed levels of dissipation of turbulent kinetic energy per unit mass, 1.1×10<sup>-7</sup> W kg<sup>-1</sup>, and eddy diffusivity, 7.3×10<sup>-4</sup> m<sup>2</sup> s<sup>-1</sup>, were comparable to direct measurements at other coastal sites and shelves with rough topography and strong forcing. During spring tides, an average upward heat flux of 5 W m<sup>-2</sup> in the under-ice boundary layer was observed. Instantaneous (1 h averaged) large heat flux events were correlated with periods of large inflow, hence elevated heat fluxes were associated with the tidal jet and its heat content. Vertical heat fluxes are derived from shear-probe measurements by employing a novel model for eddy diffusivity [Shih et al., 2005. Parameterization of turbulent fluxes and scales using homogeneous sheared stably stratified turbulence simulations. Journal of Fluid Mechanics 525, 193-214]. When compared to the direct heat flux measurements using the eddy correlation method at 5 m below the ice, the upper 4-6 m averaged heat flux estimates from the microstructure profiler agreed with the direct measurements to within 10%. During the experiment water column was stably, but weakly, stratified. Destabilizing buoyancy fluxes recorded close to the ice were absent at 5 m below the ice, and overall, turbulence production was dominated by shear. A scaling for dissipation employing production by both stress and buoyancy [Lombardo and Gregg, 1989. Similarity scaling of viscous and thermal dissipation in a convecting boundary layer. Journal of Geophysical Research 94, 6273-6284] was found to be appropriate for the under-ice boundary layer.</p>},
  author       = {Fer, Ilker and Widell, Karolina},
  issn         = {0278-4343},
  keyword      = {Arctic,Fast ice,Fjords,Mixing,Svalbard,Turbulence,Van Mijenfjorden},
  language     = {eng},
  month        = {09},
  number       = {15},
  pages        = {1980--1999},
  publisher    = {Elsevier},
  series       = {Continental Shelf Research},
  title        = {Early spring turbulent mixing in an ice-covered Arctic fjord during transition to melting},
  url          = {http://dx.doi.org/10.1016/j.csr.2007.04.003},
  volume       = {27},
  year         = {2007},
}