The Conditional Frazil Ice Instability in Seawater
Wednesday, 17 December 2014
It has been suggested that the presence of frazil ice can lead to a conditional instability in seawater. Any frazil forming in the water column reduces the bulk density of a parcel of frazil-seawater mixture, causing it to rise. Due to the pressure-decrease in the freezing point, this causes more frazil to form, causing the parcel to accelerate, and so on. We use a primitive-equation ocean model to study this instability. We find that frazil ice growth caused by rising of supercooled water is indeed able to generate a buoyancy-driven instability. Even in a gravitationally stable water column, the frazil ice mechanism can still generate the convection. The instability does not operate in the presence of strong density stratification, high thermal driving (warm water), a small initial perturbation, high background mixing or the prevalence of large frazil ice crystals. The instability is not necessarily expressed in frazil ice at all times; an initial frazil perturbation may melt into a freshwater perturbation that drives re-growth of ice. Given a large enough initial perturbation this instability can allow significant ice growth. A model shows frazil ice growth in an Ice Shelf Water plume several kilometres from an ice shelf, under similar conditions to observations of frazil ice growth under sea ice. The presence of this instability could be a factor affecting the growth of sea ice near ice shelves, with implications for AABW formation.