Slope-Enhanced Fission of Salty Hetons under Sea Ice

Abstract

Ocean responses to a single brine source under ice and over a sloping bottom are investigated in numerical experiments. Brine sources considered herein are often much stronger than that anticipated from a single seawater freezing event in a time span of about 10 days. The authors have no evidence that such strong sources exist in the ocean, but the consequent heton-like eddies manifest interesting features over a bottom slope. The numerical model contains a stratified ocean capped by an ice layer. The convection initially generates a top cyclone and a submerged anticyclone vertically stacked together. Under sea ice, the top cyclone dissipates in time and often breaks up into several distinct cyclonic vortices. Through heton-type couplings, the breakaway shallow cyclones are often able to tear the underlying anticyclone apart to form distinct anticyclones. Top cyclones are eventually annihilated by ice-exerted friction, leaving submerged anticyclones in stable existence. Fission from a pair of vertically stacked baroclinic vortices is a fundamental process associated with a strong brine source under sea ice. A bottom slope generally enhances fission, often increasing the number of subsurface anticyclones or causing the resulting anticyclones to break farther away from the source. The slope enhancement is consistent with the potential vorticity conservation requirement and a changing Rossby radius with water depths. The foregoing conclusions remain the same in cases with a stationary brine source moving rigidly with a uniform current. Under the less likely scenario of a stationary source embedded in a mean flow, brine waters spread downstream and become less effective in producing distinct vortices. Granting the occurrence of strong baroclinic vortices under sea ice, the preferable increase of anticyclones at depths may help explain the overwhelming predominance of submerged anticyclones in the ice-covered Arctic Ocean.