Preprints
https://doi.org/10.5194/tc-2021-307
https://doi.org/10.5194/tc-2021-307

  27 Oct 2021

27 Oct 2021

Review status: this preprint is currently under review for the journal TC.

A statistical definition of the Antarctic marginal ice zone

Marcello Vichi1,2 Marcello Vichi
  • 1Department of Oceanography, University of Cape Town, 7701, Rondebosch, South Africa
  • 2Marine and Antarctic Research centre for Innovation and Sustainability (MARIS), University of Cape Town, 7701, Rondebosch, South Africa

Abstract. The marginal ice zone (MIZ) is a transitional region between the open ocean and pack ice. This region is circumpolar in the Antarctic, with different sea ice types depending on the season and the sector of the Southern Ocean. The MIZ extent have traditionally been inferred from satellite-derived sea-ice concentration (SIC, one of the essential climate variables), using the 15–80 % range as indicative of sea ice with MIZ characteristics. This proxy has been proven effective in the Arctic, where there is a good correspondence between sea-ice type and sea-ice cover. It is less reliable in the Southern Ocean, where sea-ice type is less linked to the concentration value, since wave penetration and free drift conditions have been reported with 100 % cover. I propose an alternative definition of the MIZ that is based on statistical properties of the SIC and its spatial and temporal variability. The indicator is derived from the standard deviation of daily SIC anomalies, which is often employed in the climate sciences. The use of a monthly climatological mean as the baseline allows to capture changes due to both the seasonal advancement/retreat and the local weather-driven variability typical of less consolidated sea-ice conditions. This method has been tested on the available climate data records to derive maps of the MIZ distribution over the year. It reconciles the discordant seasonal extent estimates using the SIC threshold, which is now independent of the used algorithm. This indicator also allows to derive the climatological probability of exceeding a certain threshold of SIC variability, which can be used for ship navigation, design of observational networks and for testing the skills of sea-ice models in forecasting or climate mode.

Marcello Vichi

Status: open (until 31 Dec 2021)

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Marcello Vichi

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Short summary
The marginal ice zone in the Antarctic is the largest in the world ocean. Antarctic sea ice has large year-to-year changes, and the marginal ice zone represents its most variable component. The use of sea ice concentration (SIC) from space is inadequate, since areas with 100 % cover can show large changes. A new method based on SIC variability around a long-term mean has shown to address previous limitations and to allow constructing probability maps to assist navigation and model assessment.