Brine Channels As Conduits for Convection in Sea Ice

Abstract:

The onset of convection in growing sea ice has been characterized by a critical Rayleigh Number(Ra) of 10, where buoyancy forcing due to brine and seawater density differences overcomes viscous dissipation forces by a large amount. A necessary condition is that the sea ice matrix is also permeable, determined by a brine volume >5%. When the ice warms up in spring or an ice cover freezes back in fall, evidence for brine convection has been inferred from gas and nutrient exchanges. However, computations of Ra in these non-growing sea ice conditions are generally found in the subcritical range, <5. A numerical model of sea ice was constructed to re-examine the convective brine dynamics and whether a new parameterization of the onset of convection can explain these full-thickness convection cases. The hypothesis is that convection may occur at lower Ra if it takes place through macroscopic structural features, such as cm-scale brine channels, instead of a permeable matrix. The sensitivities of Ra to the buoyancy forcing and viscous dissipation parameters used for brine in the two cases, a permeable matrix and a brine channel network, are also examined.