Martin Vancoppenolle

Full-depth desalination of warm sea ice

[1]xa0The large-scale Arctic sea-ice retreat induces a gradual replacement of thick, multi-year sea ice by thinner first-year ice. The latter has distinctive physical properties and is in particular substantially saltier. It is generally thought that while salt rejection occurs primarily during ice formation in winter, most of the remaining brine is flushed out of the ice by the percolating surface melt water in summer. Here, it is argued that a substantial part of this residual desalination of first-year sea ice can occur well before summer melt, due to brine convection over the full thickness of the ice, once the ice temperature is higher than a threshold that depends on bulk salinity and thickness. This critical temperature is substantially higher than the permeability threshold. The argument stems from a theoretical analysis of the porous Rayleigh number depicting the propensity for convection in the mushy-layer theory. It is supported by simulations performed with a state-of-the-art 1-D sea-ice model. The study was initially motivated by observations collected in March 2007 in Storfjorden, Svalbard. Those are indirect, however, and are thus presented here as a possible example. Two sporadic anomalies of seawater salinity were recorded close to the base of 40u2009cm thick ice in temperature conditions that are incompatible with ice formation. Analyses and simulations forced with observed atmospheric conditions suggest that the second peak is caused by flushing of meltwater, while the first and most intense peak is likely associated with an episode of brine convection over the full depth of the ice, yielding significant desalination.