The phrase "winter sea" refers to the sea or ocean during the winter season. It implies colder temperatures, rougher waves, and a different atmosphere compared to the sea in other seasons. Full definition
The big difference compared to a tipping point scenario is the prolonged presence of winter sea ice. (bitsofscience.org)
In fact, there has been a slight gain in winter sea ice since 1979: about 0.2 million square miles. (realitydrop.org)
The top plot is the seasonal averages through 2009 plus the yearly average, and the bottom plot is average winter sea ice cover through 2010. (realclimate.org)