On the nature of the sea ice albedo feedback in simple models

TL;DR

This paper analyzes how simplified sea ice models handle ice area evolution and demonstrates that neglecting certain fluxes can significantly impact the understanding of ice-albedo feedback, especially during ice decline.

Contribution

It identifies a fundamental flaw in common two-category sea ice models and proposes the need for more accurate thickness distribution schemes.

Findings

Neglecting area-evolution fluxes leads to significant errors in energy balance models.

The flawed scheme affects the modeling of ice-albedo feedback during ice decline.

Revising models to fully resolve ice thickness distribution is recommended.

Abstract

We examine the nature of the ice-albedo feedback in a long standing approach used in the dynamic-thermodynamic modeling of sea ice. The central issue examined is how the evolution of the ice area is treated when modeling a partial ice cover using a two-category-thickness scheme; thin sea ice and open water in one category and "thick" sea ice in the second. The problem with the scheme is that the area-evolution is handled in a manner that violates the basic rules of calculus, which leads to a neglected area-evolution term that is equivalent to neglecting a leading-order latent heat flux. We demonstrate the consequences by constructing energy balance models with a fractional ice cover and studying them under the influence of increased radiative forcing. It is shown that the neglected flux is particularly important in a decaying ice cover approaching the transitions to seasonal or ice-free conditions. Clearly, a mishandling of the evolution of the ice area has leading-order effects on the ice-albedo feedback. Accordingly, it may be of considerable importance to re-examine the relevant climate model schemes and to begin the process of converting them to fully resolve the sea ice thickness distribution in a manner such as remapping, which does not in principle suffer from the pathology we describe.