HR: 0800h
AN: C51A-0098    [Abstracts]
TI: Implications of Increasing Vertical Resolution in an Isopycnal Model of an Ice Shelf Cavity
AU: * Little, C M
EM: cmlittle@princeton.edu
AF: Department of Geosciences Princeton University, Guyot Hall Princeton NJ 08544, Princeton, NJ 08544, United States
AB: Boundary layer dynamics and thermodynamic feedback processes govern the efficiency of oceanic heat delivery to ice shelves. Isopycnal models offer an opportunity to resolve thin meltwater-freshened layers, improving the representation of the oceanic boundary layer and water masses modified in the cavity. However, heat, freshwater, and momentum fluxes between ice and ocean pass through a variable density bulk mixed layer (BML). Increased vertical resolution impacts the properties of the BML, and may modify the rate and spatial distribution of basal melting. To investigate these impacts, a series of simulations using the Hallberg Isopycnal Model, modified to represent sub-ice shelf processes, is conducted in an idealized, strongly forced, east-west aligned ice shelf cavity. Meltwater mixtures remain in the BML or fill intermediate density isopycnal layers (from 1-22 layers, depending on resolution) above a uniform (1.4°C) source water mass. Since mixing may mute the benefits of increased resolution, these simulations incorporate differing parameterizations, including a minimum BML thickness, background (tidal) velocities, and Richardson number-dependent entrainment. Water in the BML and, if present, intermediate layers, is advected weakly (O(10^-2) ms^-1) towards a southern boundary current. Meltwater flux near the ice shelf front is dominated by O(10^-1) ms^-1 flow in this boundary layer. The thickness of the meltwater-enriched outflow (20-160 m) increases in step with vertical resolution; gradients in tracers are apparent in all but the lowest resolution cases. Area-averaged melt rates of 16-35 myr^-1 indicate a strong sensitivity to near-boundary mixing; the spatial distribution of melting reveals the influence of vertical resolution under different regimes. Under uniformly high mixing, melting rates are dominated by "upstream" regions and are insensitive to resolution. Weaker imposed mixing induces a shift to a "shear-driven" regime, with melting intensified in the fast-flowing boundary current. Stronger shear-driven entrainment (which may be supported by observations of relatively dilute outflow) reveals greater resolution- dependence, as entrainment into the BML and isopycnal layers are controlled by different parameterizations. These results provide guidance for the appropriate level of vertical resolution, evidence for the importance of consistency in the BML and isopycnal interior, and suggest observational evidence that might constrain these results.
DE: 0728 Ice shelves
DE: 0762 Mass balance (1218, 1223)
DE: 1622 Earth system modeling (1225)
DE: 4540 Ice mechanics and air/sea/ice exchange processes (0700, 0750, 0752, 0754)
DE: 4572 Upper ocean and mixed layer processes
SC: Cryosphere [C]
MN: 2007 Fall Meeting