Thermodynamic growth of sea ice: assessing the role of salinity using a quasi-static modelling framework

TL;DR

This study develops a quasi-static model to analyze how salinity affects sea ice growth, revealing that multiple physical mechanisms lead to low sensitivity of growth rate to salinity variations, especially during winter.

Contribution

It introduces a simplified quasi-static framework that explains the physical mechanisms behind the low salinity sensitivity in sea ice growth, bridging simple and complex models.

Findings

Salinity has a low impact on growth rate due to thermal conductivity and heat capacity trade-offs.

Temperature profile feedback reduces sensitivity compared to linear models.

Thicker and thinner ice exhibit opposite sensitivities to salinity, counteracting each other during growth.

Abstract

Sea ice is a mushy layer, a porous material whose properties depend on the relative proportions of solid and liquid. The growth of sea ice is governed by heat transfer through the ice together with appropriate boundary conditions at the interfaces with the atmosphere and ocean. The salinity of sea ice has a large effect on its thermal properties so might naively be expected to have a large effect on its growth rate. However, previous studies observed a low sensitivity throughout the winter growth season. The goal of this study is to identify the controlling physical mechanisms that explain this observation. We develop a simplified quasi-static framework by applying a similarity transformation to the underlying heat equation and neglecting the explicit time dependence. We find three key processes controlling the sensitivity of growth rate to salinity. First, the trade-off between thermal conductivity and (latent) heat capacity leads to low sensitivity to salinity even at moderately high salinity and brine volume fraction. Second, the feedback on the temperature profile reduces the sensitivity relative to models that assume a linear profile, such as zero-layer Semtner models. Third, thicker ice has the opposite sensitivity of growth rate to salinity compared to thinner ice, sensitivities that counteract each other as the ice grows. Beyond its use in diagnosing these sensitivities, we show that the quasi-static approach offers a valuable sea-ice model of intermediate complexity between zero-layer Semtner models and full partial-differential-equation-based models such as Maykut-Untersteiner/Bitz-Lipscomb and mushy-layer models.