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

Abstract

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 (3D) 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, which is dependent on w* and w*,L. In this work, the dynamics of frazil ice is investigated numerically with the 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 a separation distance of a few hundred 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 in which frazil modifies the net density, effective viscosity, and 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.

Spatial characteristics of frazil streaks in the Terra Nova Bay Polynya from high-resolution visible satellite imagery

“…In a wider perspective, in situ observations and very-high-resolution, coupled wave, current and ice models of polynyas, nested in regional models, are essential for extending the present, surface-based view to a full three-dimensional (3D) picture. Idealised numerical models (e.g., Herman et al, 2020) clearly show complex 3D patterns of ice concentration related 470 to the OML dynamics. At the same time, recent high-resolution modelling in the TNB basin provides new insights into its 3D circulation, with OML in front of the Nansen Ice Sheet influenced by circulation under the ice sheet (Na et al, 2022).…”

“…Thus, a feedback mechanism is formed, with sea ice production and distribution both affected by and affecting the dynamics and heat transfer in the ABL-sea ice-OML system. On one 40 branch of that feedback loop, formation and growth of ice crystals and their agglomerates, as well as their transport within the OML, are governed by OML's temperature structure and dynamics: Ekman currents, Stokes drift, buoyancy-induced mixing and Langmuir turbulence (e.g., Belcher et al, 2012;Herman et al, 2020; in the context of OML material transport in general, see Chamecki et al, 2019). As a result, complex, three-dimensional patterns emerge, with ice crystals reaching depths of meters or even tens of meters (Drucker et al, 2003, Ito et al, 2019, Ohshima et al, 2022.…”

“…In a wider context, bands of positively buoyant particles (so called windrows) accumulating within surface convergence zones associated with Langmuir turbulence are common at all latitudes. However, the accounts listed above (for a more complete overview, see Herman et al, 2020) are limited to qualitative reports of the presence of frazil streaks or their most basic characteristics, like e.g. the average/typical distance 90 between neighbouring bands, or probability distribution of distances (which, according to observational and theoretical studies, often is log-normal, see, e.g.…”

Typical and anomalous pathways of surface-floating material in the Northern North Atlantic and Arctic Ocean

“…It has been shown for several types of particles, e.g. oil spills 32 and frazil ice 33 , that the spatio-temporal three-dimensional patterns of particle concentration, as well as the overall transport speed and direction, are very sensitive to particle size. Another issue worth mentioning is related to the beaching and de-beaching processes.…”

Wave-in-ice: theoretical bases and field observations

“…Floating sheets manufactured with specific viscoelastic properties have been used to test some theories (as mentioned in [11]). In addition, wave-ice interactions have also been studied using remote sensing data [10] and numerical simulations [60][61][62]. The discussion below is limited to field work.…”