An empirical one-dimensional parameterization for the temporal evolution of sea-ice salinity

1. An empirical one-dimensional parameterization for the temporal evolution of sea-ice salinity Data • Model • Variations in salinity • Entrapment of salt during ice formation • Mainly Gravity drainage (winter) • Flushing (summer) • Vertical profile • constant if no flushing has occurred • linear with 0 at the surface otherwise • Thermodynamic model of sea ice (Bitz and Lipscomb, 1999) • 1 layer of snow and 10 layers of ice • Effect of salinity (brine-pockets) on temperature and freezing/melt rates through salinity and temperature-dependent thermal properties • Experimental setup • 50 years of simulation • Climatologic forcing of Fletcher (1965) • First year of simulation -> first-year ice (FY) Last year of simulation -> multi-year ice (MY) winter Fig 1. Salinity versus ice thickness regressed from Arctic and Antarctic winter FY cores. Kovacs, 1996 Arctic Antarctic summer • mb freezing rate at the bottom • mtot total freezing rate • S ice bulk salinity • Sw mixed-layer salinity • h ice thickness • Tsu surface temperature • S0 equilibrium summer ice salinity [=2 ‰ ] • tf flushing time scale [=14d] • a winter drainage coefficient [ = 2.0 ‰ m] Fig 2. Salinity profiles from different ice cores in the Fram Strait. Martin Vancoppenolle(1) and Thierry Fichefet(1) (1) Institut d’Astronomie et de Géophysique Georges Lemaître, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium vancop@astr.ucl.ac.be Eicken, 1998 Fig 3 :Salinity profiles from different ice cores in the Weddell Sea. Tucker et al., 2007 • Conclusions • The new parameterization give roughly realistic results • Winter drainage takes 2-3 weeks to lower FY sea-ice salinity • MY equilibrium cycle arises from the balance of winter entrapment and summer flushing. This cycle could explain why no significant S(h) relation has never been found. • The shape of the salinity profile is important for the ice thickness • The absence of flushing can result in large differences in ice thickness (~1 m) Results Fig. 2. MY sea ice thickness seasonal cycle. Three cases are shown : “PROF(t)” designates the model with the full salinity parameterization, “BULK(t)” corresponds to the model with a vertically constant profile with a salinity given by the salinity parameterization. “SCH” refers to the case of the stationnary MY ice salinity profile (Schwarzacher, 1959). • References • Bitz, C.M., and W.H. Lipscomb, An energy-conserving thermodynamic model of sea ice, Journal of Geophysical Research, 104, 15,669-15,677, 1999. • Eicken, H., Deriving modes and rates of ice growth in the Weddell sea from microstructural, salinity and stable-isotope data, in Antarctic sea ice: Physical processes, interactions and variability, Antarctic Research Series, 74, 89-122, 1998. • Fletcher, J. O., The heat budget of the Arctic Basin and its relation to world climate, Thec. Rep. R-444-PR, 179pp., The Rand Corp., Santa Monica, Calif., 1965. • Kovacs, A., Bulk salinity versus ice floe thickness, CRREL Rep., 96-7, 1996. • Tucker, W. B. III, A. J. Gow, and W. F. Weeks, Physical properties of summer sea ice in the Fram strait, J. Geophys. Res., 92, 6787-6803, 1987. Fig. 1. (a) FY and MY ice salinity seasonal cycle. (b) and (c) Seasonal cycle of the sources and sinks of salinity as described in 1) for the cases of FY (b) and MY ice (c). Fig. 3. Comparison between modelled S(h) with a = 2.0 ‰ m, a = 1.75 ‰m, and observations of Kovacs (1996). Lower branches correspond to ice being flushed.