Under‐ice salinity transport in low‐salinity waterbodies

In cold and temperate climates, ice typically covers the surface of waterbodies during winter. Many of these systems are also weakly saline where, unlike seawater, the temperature of maximum density, T-md, is higher than its freezing temperature, T-f. This feature of the equation of state results in a stable temperature stratification when surface waters cool below T-md. Conversely, salts excluded from the growing ice can destabilize the underlying water. Previous laboratory and field experiments demonstrated that excluded salts generate localized overturning and downward transport of salt, despite the persistence of a stable temperature gradient. Those experiments were not able to determine the processes responsible for this transport. Here, we use direct numerical simulations to visualize and characterize the plumes generated when ice excludes salt into a stable temperature gradient. We restrict our analysis to times much earlier than the diffusion timescale of temperature over the domain. We define a mass flux parameter R that considers the strength of the reverse-temperature stratification relative to the rate of salt exclusion. We identify two types of plumes whose characteristics depend mainly on R: double-diffusive salt-fingering plumes and convective plumes. The former encourages transport of salt to the bottom without significantly mixing the temperature stratification, while the latter tends to mix the water column. We apply a scaled mass flux parameter to published laboratory and field observations in low-salinity systems. These limited observations compare favorably with our numerical analysis.