I suspect the whole Arctic Sea is hot, but it looks like there's a Barents hot spot because that's where there happens to be open ocean. Most of the rest of the Arctic is white, meaning the sea surface is obscured by ice. As John notes, this is likely just polar amplification (enhanced high-latitude warming). The causes of polar amplification appear to be complex. I went to an AGU session on this a few years ago, and was surprised that there were still a number of competing explanations, with little apparent cross-dialogue between proponents of different hypotheses. Some of those hypotheses include surface albedo feedback (from melting sea ice, land ice, or snow), changing cloud feedback, and alterations in poleward heat transport of the atmosphere-ocean circulation.