Abstract

The Antarctic marginal ice zone (MIZ) is a highly dynamic region where sea ice interacts with ocean surface waves generated in ice-free areas of the Southern Ocean. Improved large-scale (satellite-based) estimates of MIZ width and variability are crucial for understanding atmosphere-ice-ocean interactions and biological processes, and detection of change therein. Legacy methods for defining the MIZ width are typically based on sea ice concentration thresholds, and do not directly relate to the fundamental physical processes driving MIZ variability. To address this, new techniques have been developed to determine MIZ width based on the detection of waves and calculation of significant wave height attenuation from variations in ICESat-2 surface heights. The poleward MIZ limit (boundary) is defined as the location where significant wave height attenuation equals the estimated satellite height error. Extensive automated and manual acceptance/rejection criteria are employed to ensure confidence in MIZ width estimates, due to significant cloud contamination of ICESat-2 data or where wave attenuation was not observed. Analysis of 304 MIZ width estimates retrieved from four months of 2019 (February, May, September and December) revealed that sea ice concentration-derived MIZ width estimates were far narrower (by a factor of ~7) than those from the new techniques presented here. These results suggest that indirect methods of MIZ estimation based on sea ice concentration are insufficient for representing physical processes that define the MIZ. Improved measurements of MIZ width based on wave attenuation will play an important role in increasing our understanding of this complex sea ice zone.

Highlights

  • Understanding the nature and drivers of the Earth’s sea ice system is a high priority in climate science (Meredith et al, 2019)

  • Sea ice forms a key habitat for a diverse range of marine biota, from micro-organisms to whales (Massom and Stammerjohn, 2010)

  • An important element of this complex air-sea ice-ocean interaction system is the outer part of the sea ice zone, termed the marginal ice zone (MIZ)

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Summary

Introduction

Understanding the nature and drivers of the Earth’s sea ice system (and change and variability therein) is a high priority in climate science (Meredith et al, 2019). Long-period surface gravity waves have been observed to penetrate hundreds of kilometers into sea ice before their energy is fully attenuated (Liu 30 and Mollo-Christensen, 1988; Asplin et al, 2012; Stopa et al, 2018a), where they substantially impact the sea ice cover and the size distribution of ice floes. This is especially the case in the circum-Antarctic sea-ice zone, where long period and high amplitude waves from the surrounding high-energy Southern Ocean (Young et al, 2020) attenuate within the MIZ (Weeks, 2010; Horvat et al, 2020; Alberello et al, 2021)

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