[Paper Review] Sea-Ice Distribution and Mixed-Layer Depths in Fram Strait

Sahil Agarwal, M. Grae Worster|arXiv (Cornell University)|Dec 20, 2017

Arctic and Antarctic ice dynamics3 citations

TL;DR

This study develops a simple heat and mass transfer model to explain the dynamics of sea-ice extent and mixed-layer depth in Fram Strait, showing that ice edge location is primarily controlled by ice and ocean current velocities, with the Untersteiner sea-ice wedge structure governing mixed-layer properties. The model reveals a seasonal asymmetry in ice advance and retreat and predicts that thinning Arctic ice could destabilize the Atlantic Meridional Overturning Circulation via freshening and warming of North Atlantic waters.

ABSTRACT

In an effort to understand the dynamics of the Arctic sea-ice edge, we present a simple model of heat and mass transfer in the Fram Strait that reveals some fundamental mechanisms controlling sea-ice extent in the marginal seas and the depth and properties of the Arctic mixed layer. We identify and study key mechanisms relating to the sea-ice wedge described by Untersteiner, a boundary-layer structure near the ice edge, demonstrating how ice thickness and extent depend on ice-export rates, atmospheric forcing and the properties of incoming warm and salty Atlantic water in the West Spitsbergen Current. Our time-dependent results demonstrate a seasonal asymmetry between the rates of ice advance and retreat and explain the significant variations in the Southerly extent of sea ice across the Fram Strait, with a long ice tongue corresponding with the East Greenland Current. Our simple model indicates that thinning of the Arctic sea-ice cover will lead to warming and freshening of the North Atlantic, which would give a de-stabilizing feedback to the Arctic ice cover, leading to a slowdown of the Atlantic Meridional Overturning Circulation.

Motivation & Objective

• To understand the mechanisms controlling Arctic sea-ice extent in Fram Strait, a key gateway for Arctic-Atlantic oceanic exchange.
• To investigate how ice and ocean current velocities, Atlantic water properties, and atmospheric forcing jointly determine the location of the sea-ice edge.
• To model the formation and properties of the Untersteiner sea-ice wedge and its influence on mixed-layer depth and structure.
• To assess the feedbacks between thinning sea ice, increased ice export, and potential destabilization of the Atlantic Meridional Overturning Circulation (AMOC).
• To provide a simplified, analytically tractable framework for studying ice-ocean interactions in the marginal ice zone without full numerical integration.

Proposed method

• A one-dimensional, time-dependent heat and mass transfer model is formulated to simulate energy exchange between sea ice and underlying ocean.
• The model incorporates ice and ocean current velocities, atmospheric temperature, and properties of incoming warm, salty Atlantic water (West Spitsbergen Current).
• The Untersteiner sea-ice wedge concept is applied to describe the boundary-layer structure at the ice-ocean interface, with heat flux balance determining ice melt and mixed-layer depth.
• The model solves for steady-state and time-dependent solutions to determine ice edge location and mixed-layer evolution under varying forcings.
• Key parameters such as ice thickness, ocean temperature, salinity, and wind-driven current velocities are systematically varied to assess their influence.
• The model is validated against climatological observations, showing a simulated ice tongue extending ~600 km southward, consistent with observations.

Experimental results

Research questions

• RQ1 How do ice and ocean current velocities control the seasonal variability and spatial extent of the sea-ice edge in Fram Strait?
• RQ2 What role does the Untersteiner sea-ice wedge play in determining mixed-layer depth and properties in the marginal ice zone?
• RQ3 Why is there a seasonal asymmetry in the rates of ice advance and retreat in the Fram Strait region?
• RQ4 How does thinning Arctic sea ice, particularly the loss of multi-year ice, affect the stability of the Atlantic Meridional Overturning Circulation (AMOC)?
• RQ5 To what extent can a simple analytical model reproduce observed ice tongue lengths and mixed-layer dynamics in Fram Strait?

Key findings

• The model reproduces a southerly ice tongue extending approximately 600 km, matching observed climatological extents in Fram Strait.
• The location of the sea-ice edge is primarily controlled by ice and ocean current velocities, with atmospheric forcing playing a secondary role.
• Seasonal asymmetry in ice dynamics is captured, with slower ice advance and faster retreat due to nonlinear feedbacks in heat flux and ice export.
• The model demonstrates that thinning sea ice leads to increased freshwater and heat fluxes into the North Atlantic, potentially destabilizing the AMOC.
• The Untersteiner sea-ice wedge structure governs mixed-layer depth and properties more significantly than the ice cover itself, acting as a key control on oceanic heat loss.
• The model shows that first-year ice export produces a shorter ice tongue than observed, indicating that multi-year ice dynamics or unresolved eddies may enhance the observed extent.