Well, it's not of annual occurrence, so I guess it's a bit of a 'mystery'. Souther Ocean polynya, maybe abnormally large, but Weddel Gyre is one of the places melting early in the Southern Ocean spring. Pretty close to the mainland Antarctica too, so maybe the use of the adjective 'mysterious' is warranted. (Saw on an article about this, and misread it) Upwelling area known for millennia by the southern whales and krill.