Study proposes an new indicator of sea ice variability for the Antarctic marginal ice zone
15 May 2023

The Southern Ocean holds the largest circumpolar marginal ice zone (MIZ) in the global ocean, while the Arctic MIZ regions are mostly confined to the Bering Sea and the Greenland and Norwegian seas. 

In a new paper from the CRiceS project, Marcello Vichi from the University of Cape Town shows why we need a deeper focus on the Antarctic marginal ice zone to better understand the future climate changes and to develop more precise adaptation policies. 

We need more research on the Antarctic marginal ice zone. Up until now most models handles the Antarctic the same way as the Arctic, while the ice behaves in a different way with greater seasonal changes without that being taken into consideration in the models, says Vichi. 

In most general terms and independently of the hemisphere, the MIZ can be depicted as a band of young or fractured ice with floes from one to a few hundred metres, which is continuously affected by air–sea interactions in the form of heat exchanges, wind and current drag, and wave action.

Remote-sensing records over the last 40 years have revealed large year-to-year global and regional variability in Antarctic sea ice extent. Sea ice area and extent are useful climatic indicators of large-scale variability, but they do not allow the quantification of regions of distinct variability in sea ice concentration (SIC). This is particularly relevant in the marginal ice zone (MIZ), which is a transitional region between the open ocean and pack ice, where the exchanges between ocean, sea ice and atmosphere are more intense, says Vichi.

The MIZ is circumpolar and broader in the Antarctic than in the Arctic. Its extent is inferred from satellite-derived SIC using the 15 %–80 % range, assumed to be indicative of open drift or partly closed sea ice conditions typical of the ice edge. 

This proxy has been proven effective in the Arctic, but it is deemed less reliable in the Southern Ocean, where sea ice type is unrelated to the concentration value, since wave penetration and free-drift conditions have been reported with 100 % cover. The aim of this paper is to propose an alternative indicator for detecting MIZ conditions in Antarctic sea ice, which can be used to quantify variability at the climatological scale on the ice-covered Southern Ocean over the seasons, as well as to derive maps of probability of encountering a certain degree of variability in the expected monthly SIC value. 

The proposed indicator is based on statistical properties of the SIC; it has been tested on the available climate data records to derive maps of the MIZ distribution over the year and compared with the threshold-based MIZ definition. 

The results present a revised view of the circumpolar MIZ variability and seasonal cycle, with a rapid increase in the extent in austral autumn and a plateau in winter, as opposed to the steady increase from summer to spring reported in the literature. It also reconciles the discordant MIZ extent estimates using the SIC threshold from different algorithms. 

This indicator complements the use of the MIZ extent and fraction, allowing the derivation of the climatological probability of exceeding a certain threshold of SIC variability, which can be used for planning observational networks and navigation routes, as well as for detecting changes in the variability when using climatological baselines for different periods.