Spaceborne infrared imagery for early detection and cause of Weddell Polynya openings

• Main Authors: Heuzé, Céline, Lemos, Adriano
• Format: Text
• Language: English
• Published: 2020
• Subjects: Southern Ocean; Weddell; Sea ice
• Online Access: https://doi.org/10.5194/tc-2020-123; https://tc.copernicus.org/preprints/tc-2020-123/

When will sea ice open is a crucial information for navigation and scientific deployments. This became painfully obvious when the Weddell Polynya, a large hole in the winter Southern Ocean sea ice, unexpectedly re-opened in 2016 for the first time in forty years. With no early warning, observations were limited to chance autonomous sensors, so the much-debated cause of the opening still cannot be determined accurately. We aim here to create such an early warning system. From the full historical sea ice concentration record, we find in fact 30 polynyas since 1980. Then, using the full time series of the spaceborne infrared Advanced Very High Resolution Radiometer, we determine that these events can be detected in the two weeks before the polynya opens. Area-average median brightness temperature larger than 253 K in all three bands and area-maximum larger than 269 K along with a footprint at least larger than 4000 km2 successfully forecasts the polynyas and does not return any false positive. Or rather, it returned false positives that were in fact events that the sea ice concentration threshold had missed. Moreover, we find temporal oscillations in brightness temperature that could indicate upwelling of warm water, but also changes of sign in T45 (band 4 – band 5) which could indicate a lead. We hence combine the spaceborne infrared data with atmospheric reanalysis, hydrographic mooring data and Sentinel-1 radar imagery and find that all events, including the 2017 Weddell Polynya, are caused by both atmospheric divergence and oceanic upwelling. That is, the debate is closed: both parties are correct; the Weddell Polynya is a hybrid.