Pre-industrial multiple equilibrium sea-ice flow states due to plastic ice mechanics

Abstract

Abstract Because the Arctic Ocean is largely surrounded by land masses, sea ice mechanics significantly affects the outflow of ice from the Arctic Basin. An important legacy of Dr. Max Coon has been the recognition that Arctic ice pack fails and flows in a plastic manner. Building on this concept we demonstrate here that with reasonable simulation of plastic scaling of sea-ice flow through narrow channels, dynamic–thermodynamic sea ice models have the potential to yield multiple equilibrium flow and hence thickness states for appropriate seasonal daily wind and thermodynamic forcing. These multiple states are due to ‘plastic’ ice mechanics and are not ‘Budyko-Sellers’ albedo feedback multiple states. Each state consists of a mean seasonal ice thickness with annual outflow balancing net growth. Low flow states are thicker. In this paper we first demonstrate that with some caveats, a properly formulated ‘viscous plastic’ sea ice model where ‘rigid’ ice is approximated by very slow flow can successfully simulate plastic ice flow and stoppage of ice flow through narrow channels in agreement with observations and theory. The characteristics of this flow and stoppage are methodically examined by a series of initial value problems of ice flow and advection through various narrowing channels with different ice thicknesses being advected into the channel. Different rheologies are examined and the effect of different creep closure rates on stoppage is investigated using several day mechanistic simulations of flow into the channel employing explicit time stepping. In the case of parallel channels, numerical results are examined in light of analytic solutions. Together with idealized growth rates this simple channel model is used to illustrate the principle of multiple equilibrium states due to ice mechanics. Following this we utilize a 40 km resolution dynamic thermodynamic sea ice model with multiple openings from the Arctic Basin to examine the potential for multiple equilibrium flow states under pre-industrial atmospheric thermodynamic forcing together with daily wind field forcing taken from more current conditions. This potential is assessed by carrying out five year seasonal simulations initialized with different ice thicknesses. Intersection of seasonal averaged net growth and outflow thus obtained are then used to identify potential multiple states and assess their stability. With a coulombic rheology with modest cohesive strength, the results show three states two of which are stable. With an elliptical yield curve having higher cohesive strength only one low flow state is identified. Several ~ 100 year simulations are then carried out to show the existence of the two stable states and their seasonal flow and thickness characteristics. The decay time scales between pre-industrial low flow states and present high flow states are also examined via long term simulations. Finally, a simulation from 1960 to 2040 with estimated climatic warming is carried out and compared with equilibrium thickness states at discrete intervals to show the effect of inertia in the rate of ice decay due to ice mechanics. Forcing data reported in this paper are adequate to test the capability of other dynamic thermodynamic sea ice models to yield multiple flow states.