Modeling ice dynamics of coastal seas

Abstract

A coupled sea ice, barotropic ocean model with a 1-km resolution and a seaward domain of 200 km quantifies three coastal processes' coupling of ice motion to wind-driven coastal currents, ice thickness redistribution under compaction at the coast, and formation of coastal shear zones. The model consists of an ice momentum balance, mass concentration and two-parameter ice thickness distribution, and equations for horizontal water motion and continuity using vertical structure functions. An appropriate constitutive law appears to be a hardening plastic based on qualitative observations from Alaskan continental shelves. For first-year sea ice, strength is taken to be proportional to the square of ice thickness. A north wind example of 10 m/s with the coastline to the west shows the depth dependence of rotational shear in the sea ice/ocean boundary layer and sea surface tilt which contributes an alongshore slope current. There is slight convergence of sea ice over the shelf, a coastal shear zone of 4 km, and an alongshore ice spee• seaward of the shear zone of 6% of the wind speed caused by the combination of an under-ice shear layer and a•n alohgshore slope current. For an onshore wind, ice is near free drift at 3% of the wind seaward of a ridging front, which propagates seaward. A square dependence of ice strength on thickness is required for the rubble field to approach a limiting thickness, consistent with observations. The hardening plastic interpretation of the rubble field has the stress state at the yield limit in contrast with a rigi d plastic of high constant strength that yields only at the coast. We conclude that (1) ice thickness/motion feedback is important on scales less than 10 km, (2) the observational base to discriminate between mesoscale constitutive laws is not yet available, and (3) the relation of ice velocity to wind stress is variable because the ocean slope current responds only to the alongshore component of the wind. R = (•tH)•/2/f, where g is gravity, H is water depth, and f is the Coriolis parameter. R is of the order of 180 km for typical high-latitude continental shelves of 60 m depth. Within this distance the sea level tilt term is important to the dynamics in both the sea ice and ocean momentum equations. Finite depth is a consideration for wind-driven ice velocities and the vertical exchange of momentum in water depths less than 50 m during wind events greater than 10 m/s [Overland et al., 1984]. Onshore winds produce large variations in ice thickness approaching downwind coasts [Kot, acs and Sodhi, 1980; Kovacs, 1981]. In coastal seas, ice strength can vary spatially by orders of magnitude due to compactness and thickness gradients; in comparison, models of multiyear pack ice often This paper is not subject to U.S. copyright. Published in 1988 by the American Geophysical Union. Paper number 88JC03197. assume nearly constant ice strength. Modeling the plastic response of sea ice under convergence is influenced by strengththickness-compactness coupling [Hibler et al., 1983]. The offs...

Sea Ice Motion in Response to Surface Wind and Ocean Current in the Southern Ocean

“…The similarity in distribution of the low-value areas is thought to be brought about by physical causes related to internal ice stress. In general, the internal ice stress gradient depends on the ice thickness and concentration (Hibler 1979;Overland and Pease 1988). Thick ice and high ice concentration near the coast cause a large ice stress gradient and enhance nonlinear effects in sea ice, which lead to a small r 2 and act to suppress ice motion, and hence, lower the value of F (Steele et al 1997).…”

The opening of wind-driven polynyas

“…Typical values for n used in the literature are n = 1 (Hibler 1979), n = 3/2 (Hopkins 1998;Wilchinsky, Feltham & Miller 2006) and n = 2 (Rothrock 1975;Overland & Pease 1988). Note that some of the parameterisations just cited use a P that is not constant but depends on other ice field variables.…”

SPH Modelling of Sea-ice Pack Dynamics

“…Non-linearly viscous fluid models are usually derived from a general form of the Reiner-Rivlin constitutive equation (Chadwick 1999). In the context of sea ice applications, the first viscous fluid model was formulated by Smith (1983) and subsequently extended by Overland and Pease (1988) and a few other authors, including Gray and Morland (1994), Morland and Staroszczyk (1998) and Schulkes et al (1998). In general, the Reiner-Rivlin constitutive law expresses the Cauchy stress as a quadratic function of the strain-rate tensor and its three independent invariants.…”

“…Note that (26) is, in general, a non-linear law, since the viscosities ζ and µ are the functions of the strain-rate invariants η and γ. An example of the non-linearly viscous rheology is the constitutive model proposed by Overland and Pease (1988), in which the function φ 1 is assumed to depend on the state variables h and A, and the function φ 2 (describing the deviatoric stress) depends on the shear-rate invariant γ. With the specific forms of φ 1 and φ 2 considered by Schulkes et al (1998)…”