Sea ice on the Ross Sea. Photo: Craig Potton
Case study: Detecting polynya from space
Antarctic Science Platform researchers are using satellite data to determine the spatial extent of polynya with better accuracy, which unlocks the ability to quantify their role in large-scale processes, such as sea ice formation, ocean circulation and their impact on marine ecosystems.
A polynya is an area of open water or reduced concentration of sea ice surrounded by either concentrated sea ice or land ice. They are essentially ice production factories, and sites of intense ocean-atmosphere heat exchange.
The Ross Sea region, the closest Antarctic region to New Zealand, is home to the Ross Sea polynya, the largest Antarctic offshore polynya with a winter area about 20,000 km2, as well as the Terra Nova Bay polynya (1,300 km2), and McMurdo Sound polynya (900 km2).
Polynya occur where persistent winds and/or ocean currents move sea ice away from the area as fast, or faster than, it is forming. Once opened, the polynya transfers large amounts of heat between the ocean and the atmosphere, particularly in winter. Such intense activity at the relatively ‘small’ scale of a polynya can influence climate and Earth system processes at the regional and even global scale. This includes sea-ice formation, atmospheric circulation, Antarctic deep and bottom water formation, ocean convection, and oceanic carbon uptake.
But observational data on polynya is sparse both spatially and temporally, due to the remoteness of their location and harsh conditions. High-resolution satellite imagery, combined with in-situ observing in the Ross Sea polynya and novel sea-ice modelling capability, is proving to be a key for delineating the extent of polynya.
Polynyas are not simple to detect and quantify. In their early evolution phase, polynya area is generally too small, and edge definition too fuzzy to allow accurately quantification using remote satellite imaging. Innovations in satellite technology and data availability are allowing new strategies that remove some of the limitations in observing extremely dynamic phenomena, like polynya.
No single satellite product can capture the high spatial and temporal resolution required to measure polynya. For example, local winds can change a polynya in a few days – but satellites might only be in position to collect data every 12 days. Antarctic Science Platform researchers have analysed polynya properties within the Ross Sea region using a range of satellite products (read this recent publication for more information). Of particular interest are observing instruments that are unaffected by cloud cover and darkness.
Traditionally, sea-ice remote sensing has used passive microwave (PMW) radiometry, a method that is unaffected by cloud cover and occurs daily but tends to have low spatial resolution (multi-km grid size) and struggles to quantify low ice density zones. New analysis using high-resolution synthetic aperture radar (SAR) data, also unaffected by cloud cover but with a 40-metre grid resolution, has allowed polynya detection and quantification with much better accuracy than PMW sensors. The research has produced algorithms that accurately identify thin-ice zones, separating the polynyas from adjoining pack ice.
McMurdo Sound Polynya (left, outlined in cyan) appears as a bright area, or high backscatter, on the SAR imagery. No polynya (right) is observed at times of the year when the polynya will have transitioned into full ice coverage. Source: Burada et al, 2023
By facilitating high resolution studies of polynya dynamics, this work unlocks and enables other research on a breadth of pressing research areas, including:
- The ability to downscale coarse resolution sea-ice concentration data
- The ability to upscale and generalise precise information on interannual variability and sea ice thickness
- The role of polynya in sea-ice production and energy transfers between the atmosphere and ocean
- Connections between polynya behaviour and larger-scale sea ice variability
- Polynya activity in response to small- and large-scale atmospheric drivers
- Implications for ocean and atmospheric circulation, sea ice and primary productivity if polynya change in a warming world.
This Case Study was prepared as part of the Platform’s annual reporting to MBIE for the 2022-2023 year. It illustrates the importance of advancing both detection techniques and high resolution spatio-temporal data collection to locate and quantify polynya – critical sites of ocean-atmosphere heat exchange, and ice production factories.
Contributors: Girija Kalyani, James Renwick, Adrian McDonald, Ben Jolly