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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/tc-2020-189
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-189
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Jul 2020

13 Jul 2020

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A revised version of this preprint was accepted for the journal TC and is expected to appear here in due course.

High-resolution simulations of interactions between surface ocean dynamics and frazil ice

Agnieszka Herman1, Maciej Dojczman2, and Kamila Świszcz1 Agnieszka Herman et al.
  • 1Institute of Oceanography, University of Gdansk, Poland
  • 2Faculty of Physics, University of Warsaw, Poland

Abstract. Frazil and grease ice forms in the ocean mixed layer (OML) during highly turbulent conditions (strong wind, large waves) accompanied by intense heat loss to the atmosphere. Three main velocity scales that shape the complex, three-dimensional OML dynamics under those conditions are: the friction velocity u* at the ocean--atmosphere interface, the vertical velocity w* associated with convective motion, and the vertical velocity w*,L associated with Langmuir turbulence. The fate of buoyant particles, e.g. frazil crystals, in that dynamic environment depends primarily on their floatability, i.e., the ratio of their rising velocity wt to the characteristic vertical velocity, dependent on w* and w*,L. In this work, dynamics of frazil ice is investigated numerically with a high-resolution, non-hydrostatic hydrodynamic model CROCO (Coastal and Regional Ocean COmmunity Model), extended to account for frazil transport and its interactions with surrounding water. An idealized model setup is used (a square computational domain with periodic lateral boundaries; spatially uniform atmospheric and wave forcing). The model reproduces the main features of buoyancy- and wave-forced OML circulation, including the preferential concentration of frazil particles in elongated patches at the sea surface. Two spatial patterns are identified in the distribution of frazil volume fraction at the surface, one related to individual surface convergence zones, very narrow and oriented approximately parallel to the wind/wave direction, and one in the form of wide streaks with separation distance of a few hundreds meters, oriented obliquely to the direction of the forcing. Several series of simulations are performed, differing in terms of the level of coupling between the frazil and hydrodynamic processes: from a situation when frazil has no influence on hydrodynamics (as in most models of material transport in the OML) to a situation when frazil modifies the net density, effective viscosity, transfer coefficients at the ocean--atmosphere interface and exerts a net drag force on the surrounding water. The role of each of those effects in shaping the bulk OML characteristics and frazil transport is assessed, and the density of the ice–water mixture is found to have the strongest influence on those characteristics.

Agnieszka Herman et al.

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Agnieszka Herman et al.

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Latest update: 21 Oct 2020
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Short summary
Under typical conditions favorable for sea ice formation in many regions (strong wind and waves, low air temperature), ice forms not at the sea surface, but within the upper, turbulent layer of the ocean. Although interactions between ice and ocean dynamics are very important for the evolution of sea ice cover, many aspects of them are poorly understood. We use a numerical model to analyze three-dimensional water circulation and ice transport and show that ice strongly modifies that circulation.
Under typical conditions favorable for sea ice formation in many regions (strong wind and waves,...
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