Tracer Measurements in Growing Sea Ice Support Convective Gravity Drainage Parameterizations

M Thomas,J Kaiser,J L France,M Vancoppenolle,D C E Bakker,R Von Glasow,W T Sturges

doi:10.1029/2019jc015791

Abstract

AbstractGravity drainage is the dominant process redistributing solutes in growing sea ice. Modeling gravity drainage is therefore necessary to predict physical and biogeochemical variables in sea ice. We evaluate seven gravity drainage parameterizations, spanning the range of approaches in the literature, using tracer measurements in a sea ice growth experiment. Artificial sea ice is grown to around 17 cm thickness in a new experimental facility, the Roland von Glasow air‐sea‐ice chamber. We use NaCl (present in the water initially) and rhodamine (injected into the water after 10 cm of sea ice growth) as independent tracers of brine dynamics. We measure vertical profiles of bulk salinity in situ, as well as bulk salinity and rhodamine in discrete samples taken at the end of the experiment. Convective parameterizations that diagnose gravity drainage using Rayleigh numbers outperform a simpler convective parameterization and diffusive parameterizations when compared to observations. This study is the first to numerically model solutes decoupled from salinity using convective gravity drainage parameterizations. Our results show that (1) convective, Rayleigh number‐based parameterizations are our most accurate and precise tool for predicting sea ice bulk salinity; and (2) these parameterizations can be generalized to brine dynamics parameterizations, and hence can predict the dynamics of any solute in growing sea ice.

Highlights

Vast areas of the world's oceans freeze each year
Gravity drainage exerts a control on the habitable volume (Vancoppenolle et al, 2013), nutrient budget (Fripiat et al, 2017; Fritsen et al, 1994), and inorganic carbon budget (König et al, 2018; Moreau et al, 2015) in growing sea ice, as well as determining the buoyancy forcing provided by growing sea ice to the ocean (Pellichero et al, 2018; Worster & Rees Jones, 2015)
The second dissolved tracer equation is formally similar to the salt equation, except that bulk salinity is replaced by bulk rhodamine concentration, C, and brine salinity is replaced by brine rhodamine concentration, Cbr

Summary

Introduction

Vast areas of the world's oceans freeze each year. Some of the salt originally contained in the freezing seawater is retained in the resulting sea ice, concentrated into liquid brine inclusions, and some is lost to the ocean below. The dominant process redistributing brine within growing sea ice and exchanging brine with the ocean is gravity drainage (Notz & Worster, 2009). Gravity drainage is driven by the greater density of brine relative to the ocean below. In growing sea ice the coldest (and most saline and densest) brine lies above less dense brine and ocean (Cox & Weeks, 1975). Laboratory (Eide & Martin, 1975; Middleton et al, 2016; Niedrauer & Martin, 1979) and field

Methods

Results

Conclusion