Although I also think the thinness of sea ice is the reason here, I am sure other factors can also play into this such as snow darkened by some substances like ash and smoke particles that are fallen onto snow and ice surfaces - even during winter. These could alter the radiative balance (flow) of both visible light and long wave spectrum. There is also possibility of properties of silt from the rivers and residual heat from turbulent and mixing break water (altering densities temperature-salinity) that can push more or less heat through ice cover. Indeed, in summer season I have seen rebounce radiation from sea water managing to make its way through snow covered ice to surface making slight variations in the colour of sea ice/snow cover as colour of water shines through ice and snow. (The water beneath ice is illuminated in colours of blue, green, brown or gray depending on riparian discharge and algae growth - noticing these through ice and snow requires a good eye to notice the colours against the immense glare of snow in satellite images.) How significant these are in long-wave region of spectrum, I do not know. In visible light the colours of water beneath ice and snow are hard to see on many occasions.
In addition, the barren ice, thickness of snow cover, and its compactness all change heat signature from sea (besides sea ice thickness). These complications could be more pronounced in interpreting future images as there might be heavier snowfalls on ice ("lake-snow effect on sea ice off the open areas" was, after all, once proposed as the cause of immense snow falls of the Ice Ages, proposed by Maurice Ewing and William Donn. [Ewing-Donn Lake-Snow Effect of the Arctic as cause of open Arctic Ocean flipping into the Ice Age in 1950's. Their case failed in sediment cores leading to reversion back to earlier Milutin Milankovic's orbital forcing effects as driver of glaciations-deglaciations]. Methane laden sea water also has different radiative properties, but whether it is significant in water is another matter as thermal inertia of water is massive in comparison to air. If it were, then water from above methane clathrates would appear warmer.
I've been looking at ASCAT radar images and saw some interesting features. Here's an animation for the past week, and those features are in the first frame (white circle and rectangle):
Usually, dark means thin in radar images, and these features have been there since November. The one in the circle looks especially interesting, but I don't know if it means these regions are thinner. And I haven't compared to other years yet either, but I thought I'd throw it out here to see what you guys think.
tracking the area in the circle back to september I can confirm what SIS spotted in the russian ice chart: this was open water on Sept 17th (changing shape somewhat) the outline seems to be formed by ice movement at the time when it froze over a few days after that.
Clouds obscure the area in the visible images but AMSR brightness temp helps to track it and relate worldview to the ASCAT images.