A statistical definition of the Antarctic marginal ice zone

Abstract

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.

Marginal ice zone dynamics: history, definitions and research perspectives

“…A similar analysis carried out recently for the period 1978–2018 found no clear trend in MIZ extent [23] and authors called for caution when interpreting such results. Methods based on SIC criteria have also been used to study the Antarctic MIZ and polynyas [24,25]. Although these definitions may be appropriate in some contexts (primary production, marine ecosystem dynamics, ship navigation, model skill assessments, etc.…”

Altimetric observation of wave attenuation through the Antarctic marginal ice zone using ICESat-2

“…Indeed, SIC is influenced by a wide range of concomitant processes including winds and ocean currents, air temperature, upper-ocean heat storage, turbulent and radiative heat exchange , and snow cover (Sturm and Massom, 2017). SIC-based MIZ retrieval is thought to inaccurately represent MIZ extent, particularly in the Antarctic (Vichi, 2021), as significant wave penetration can occur in areas of 100 % ice coverage (Liu and Mollo-Christensen, 1988;Vichi et al, 2019) and, conversely, waves may not be present in all low-concentration sea ice.…”

Atmospheric drivers of a winter-to-spring Lagrangian sea-ice drift in the Eastern Antarctic marginal ice zone

“…This outer region is conventionally between 5 and 100 km wide (Feltham, 2005; Heorton and others, 2014), and is usually defined as the area of the ocean covered by 15–80% sea-ice concentration (SIC; Strong and others, 2017; Rolph and others, 2020). The use of the concentration-based definition is a conventional operational choice derived from Arctic consideration, which may be less suitable in the Antarctic (Vichi, 2021, and references therein). Its application to the Antarctic MIZ is subject to the limited validation of remote-sensing products by in situ observations.…”

“…Very little field data of metocean (meteorological and oceanographic) conditions are available in the Southern Ocean and even less in the MIZ (Derkani and others, 2020). Therefore, although the operational definition is applicable in the Arctic, where there is a good agreement between the ice type and ice cover, it is less reliable in the Southern Ocean, where the ice type is less related to the concentration value (Vichi, 2021). This study is therefore not based on a fixed definition of the MIZ, as constrained by concentration thresholds, but it rather considers the Antarctic MIZ extent based on the region of young and/or fractured sea ice that is continuously affected by atmosphere–ocean interactions, in the form of heat and momentum exchanges, wind and ocean current drag, and wave–ice interactions (Kohout and others, 2014; Zhang and others, 2015; Vichi, 2021).…”