Imminent re-opening of the Weddell Polynya detectable days ahead by spaceborne infrared

The Weddell Polynya, a large hole in the winter sea ice cover, has intrigued researchers since satellite observations began in the late 70s. There is no consensus regarding the mechanisms leading to its opening, not least because there never was an instrument deployed early enough at the right location and with the right sampling interval. But what if we could predict imminent openings, by detecting early-warning signs from space?

The leading theory among oceanographers is that the polynya opens after the sea ice is melted from below by upwelled warm waters. We argue that such upwelling, or at least the increased heat flux through a thinning ice, should be visible on spaceborne thermal infrared imagery. Using microwave-based sea ice products to determine past polynya openings, we first found that there were in fact 83 Weddell / Maud Rise Polynya occurrences since winter 2000, 19 of which reaching an area larger than 1000 km². We then created a timeseries of (cloud-filtered) daily mean brightness temperature at 3.7, 10.5 and 12 μm from Advanced Very High Resolution Radiometer datasets and found a significant warm temperature anomaly at least 10 days before the polynya opened, peaking at 4K for all bands 5-6 days before the opening. The anomaly is on average 2K stronger for the large polynyas (> 1000 km²). Moreover, the band ratios brutally change magnitude, which suggests lead formation rather than progressive melting – a hypothesis that would agree with meteorologists' theory that the polynya opens because of winds, and that we are now checking with spaceborne radar.

Six days is not much, but it would be enough to re-route expeditions or autonomous sensors so that the opening can be monitored in details. And this is only the first step of our ongoing project... stay tuned to see if we can predict weeks or months ahead!