The evolution of a shallow front in the Arctic marginal ice zone

Samuel Brenner,Jim Thomson,Craig Lee,Luc Rainville,Mary-Louise Timmermans,Jody W Deming

doi:10.1525/elementa.413

Samuel Brenner, Jim Thomson + Show 4 more

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https://doi.org/10.1525/elementa.413

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Abstract

The high degree of heterogeneity in the ice-ocean-atmosphere system in marginal ice zones leads to a complex set of dynamics which control fluxes of heat and buoyancy in the upper ocean. Strong fronts may occur near the ice edge between the warmer waters of the ice-free regions and the cold, fresh waters near and under the ice. This study presents observations of a well-defined density front located along the ice edge in the Beaufort Sea. The evolution of the front over a ~3-day survey period is captured by multiple cross-front sections measured using an underway conductivity-temperature-depth system, with simultaneous measurements of atmospheric forcing. Synthetic aperture radar images bookending this period show that the ice edge itself underwent concurrent evolution. Prior to the survey, the ice edge was compact and well defined while after the survey it was diffuse and filamented with coherent vortical structures. This transformation might be indicative of the development an active ocean eddy field in the upper ocean mixed layer. Over the course of hours, increasing wind stress is correlated with changes to the lateral buoyancy gradient and frontogenesis. Frontal dynamics appear to vary from typical open-ocean fronts (e.g., here the frontogenesis is linked to an “up-front” wind stress). Convective and shear-driven mixing appear to be unable to describe deepening at the heel of the front. While there was no measurable spatial variation in wind speed, we hypothesize that spatial heterogeneity in the total surface stress input, resulting from varying ice conditions across the marginal ice zone, may be a driver of the observed behaviour.

Highlights

IntroductionArctic sea ice is rapidly changing, with summer sea ice extent and thickness declining more quickly than winter (Perovich et al, 2018)
Arctic sea ice is rapidly changing, with summer sea ice extent and thickness declining more quickly than winter (Perovich et al, 2018). These changes are pronounced in the Chukchi and Beaufort seas of the Western Arctic which have increased open-water in the summer while maintaining winter ice cover, and have much higher seasonal ice variability than in the past (Thomson et al, 2016; S troeve and Notz, 2018; Peng et al, 2018). Accompanying these changes is an increase in width of the summer marginal ice zone (MIZ; Strong and Rigor, 2013): the band of low concentration, highly dynamic ice that separates the open ocean from the ice pack
The frontal adjustment appears to be linked to the atmospheric forcing, but dynamics differ from traditional descriptions associated with submesoscale frontal evolution

Summary

Introduction

Arctic sea ice is rapidly changing, with summer sea ice extent and thickness declining more quickly than winter (Perovich et al, 2018) These changes are pronounced in the Chukchi and Beaufort seas of the Western Arctic which have increased open-water in the summer while maintaining winter ice cover, and have much higher seasonal ice variability than in the past (Thomson et al, 2016; S troeve and Notz, 2018; Peng et al, 2018). Accompanying these changes is an increase in width of the summer marginal ice zone (MIZ; Strong and Rigor, 2013): the band of low concentration, highly dynamic ice that separates the open ocean from the ice pack. Submesoscale processes with Rossby and Richardson numbers of order one may dominate the momentum balance (Thomas et al, 2008)

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