Session F30: Geophysical Fluid Dynamics: Cryosphere

Sea ice dispersion mirrors underlying submesoscale ocean currentsamid strong atmospheric forcing

Marginal ice zones (MIZ) are important regions between the ice  pack and the open ocean. Here, heat and fresh water budgets are strongly influenced by the sea ice field and vice versa. However, a full characterization of these dynamics is still lacking. Employing two decades of continuous optical satellite imagery, we present unique sea ice observations acquired during the spring and summer seasons in Arctic MIZ over the 21st century. We find that the intrinsically strong sea ice-ocean interactions drive sea ice to mirror meso/submeso-scale ocean eddies, even amid strong wind conditions. As such, the sea ice drift field serves as an indicator to gauge the evolution of MIZ dynamics in a rapidly changing Arctic. We analyze the topology of the underlying flow field in the region via one- and two-particle dispersion statistics of sea ice. We show that deviations from a mean flow are driven predominantly by energetic turbulent fluctuations. We also find an anti-correlation between sea ice extent and sea ice fluctuating velocities. This relationship provides direct observational evidence that declining sea ice concentrations have lead to enhanced turbulent activity on the ocean surface. Our results can help achieve a realistic parameterization of sea ice-turbulent ocean interactions.   

Not Participating

Antarctic glaciers slide over subglacial till, a wet mixture of grains and clay, and their dynamics depend strongly on the degree of basal sliding. Hence, understanding the dominant physical processes for till deformation are crucial to modelling ice sheet dynamics. In steady-state experiments, till has been shown to behave plastically with a pressure dependent yield stress.  Many ice streams show velocity variations that have been linked to daily fluctuations in subglacial water pressure. To understand the response of till to periodic forcing requires a model that extends beyond the steady state.

We construct a one-dimensional, two-phase model of coupled fluid and solid flows, using Darcy flow for the fluid phase and a wet granular μ(I_ν) rheological model for the solid part. After verifying our model against steady-state observations of till deformation, we force the model with a fluctuating effective pressure at the ice-till interface and infer the resulting relationships between basal traction, solid fraction, rate of deformation, and till flux. Shear dilation introduces internal pressure variations, leading to hysteretic behaviour in low-permeability materials, which may help explain the large-scale transient response of ice sheets to changes in the subglacial hydrology.   

Non-Gaussian statistics of inelastically interacting ice floes

Sea ice comprises plates (or floes) of ice that are transported on the ocean surface by wind and ocean currents. Despite the typically Gaussian noise of the driving wind, observations show a much longer tailed velocity distribution of the ice, which is not well understood. Here, we present a stochastic dynamics framework of interacting ice floes that resolves the connection between wind and ice statistics. We model the wind as the superposition of a mean and a normally distributed single-correlation-time noise. The noisy wind drives a dynamical equation for the ice floes that includes ice inertia, ocean drag, and inelastic collisions between the floes. Under this framework, we find that the ice velocity distribution is indeed broader than that of the driving wind. This broadening is controlled by collisions, which depend on ice inertia, floe size and area fraction, as well as the wind statistics. Our results for the ice velocity distribution and frequency spectrum are in good agreement with observations in the Fram Strait, without the need to invoke details of the wind turbulence. We rationalize our findings in terms of a coarse-grained Fokker-Planck framework, which can ultimately inform climate-scale parametrizations based on floe-scale physical principles.   

Fluid dynamics of subglacial lakes

Subglacial lakes are isolated, low-temperature and high-pressure water environments hidden under ice sheets. The fluid dynamics of subglacial lakes is controlled by the geothermal flux, which forces vertical convection, and the tilt of the ice-water interface (ceiling), which can drive a baroclinic horizontal flow (due to the pressure-dependence of the freezing point). On Earth, the effect of the geothermal flux is likely to dominate in many subglacial lakes. Thus, in this talk, I will discuss the properties of vertical convection flows in subglacial environments. I will highlight the differences between Rayleigh-Bénard convection and turbulent convection in subglacial lakes due to the nonlinearity of the water equation of state and the fixed-flux bottom boundary condition. I will describe the mixed convective and stably-stratified (two-layer) dynamics of subglacial lakes and discuss how to derive scaling laws for the Nusselt and Reynolds numbers. Implications for flow velocities and the habitability of subglacial lakes on Earth and elsewhere in the solar system (icy moons) will be briefly mentioned.   

Rapid lake drainage: elastic blister dynamics with topography

Geological processes such as subglacial flooding and magmatic intrusions are often modelled as blisters of fluid spreading beneath an elastic sheet. The behaviour of these blisters is determined almost entirely by contact line dynamics, with the interior of the blister remaining approximately hydrostatic. Previous work has described how curvature at the contact line causes the elastic sheet to peel upwards and hence leads to spreading. Here we show that the introduction of along-slope gravity can fundamentally change the dynamics by removing the jump in curvature at the upslope edge. This allows for a receding contact line and a new "translating" regime in which the main body of the blister moves downslope at approximately constant speed, leaving behind a thin trail of fluid. This allows the blister to move much further and faster than in the case where the evolution is determined entirely by spreading, and raises questions about the impact of basal topography and existing drainage networks on subglacial flooding beneath ice sheets.