The formation of a haline shelf front in wintertime in an ice-covered arctic sea

Abstract

During some winters, the water overlying Mackenzie shelf in the southeastern Beaufort Sea becomes quite saline (33–35) and at freezing temperature throughout. Although this water is found at the surface, its density is that of waters within the halocline of the arctic basin, and ventilation of the halocline occurs. The formation of sea ice is a necessary, but not a sufficient condition for the production of this water. Observations from the winter of 1980–1981 are presented which illustrate that a two-stage preconditioning of shelf waters is also required: first, surface waters of low salinity must be driven from the shelf by strong westerly winds; then, more saline water must upwell onto the shelf in response to prolonged easterly winds. A haline front forms on the outer shelf separating cold, saline shelf waters from slightly warmer, less saline slope waters. The characteristics of this front are controlled by the input of negative buoyancy from ice growth over the shelf, and by turbulent entrainment and mixing driven by under-ice convection. A simple model to predict the position, dimension and buoyancy contrast of this front is presented. The cross-isobath circulation is examined and found to be relatively ineffective at flushing dense waters from the shelf over a winter. Thus shelf waters accumulate almost all the salt expelled from growing sea ice over the same time. The magnitude of the contribution by this shelf to renewal of waters in the arctic halocline is estimated to average 0.04 × 10 6m 3s −1 over a period of years. Although small relative to the overall rate of renewal (1 × 10 6m 3s −1), this contribution is in proportion to the fraction of the arctic shelf area which this region represents.