The mean annual temperature is taking the mean average of the coldest month of the year and averaging it with the mean average of the hottest month of the year.
Do volcanoes have mean annual temperatures, Jack?
"....warming over all of West Antarctic in the annual mean, driven by significant warming over most of the region in winter and spring. Summer and fall trends, are insignificant except over the Antarctic Peninsula where they are widespread only in fall. These finding are in good agreement with the 2009 study in Nature, though in general the new results show greater warming in West Antarctica and less warming over East Antarctica as a whole....."
The research shows that the warming trends in W. Antarctica and the Peninsula are seasonal. So unless you can show that volcanoes erupt seasonally...then I think it safe to conclude that volcanoes are not the cause for the warming trend in W. Antarctica...but the rising air and sea water temperatures in winter and spring most likely are.