You're right that any trends in winter temperature are very complex! Especially when you take changes in minimum and maximum temperature into consideration. Here are a couple of articles from our sister blog Beyond the Data that discuss climate trends and placing recent cold-air outbreaks in context. In general, the article cited in this post (Cohen et al. 2014) refers to a trend in the average winter temperature from 1990-2014.

More research is needed to identify the changes in atmospheric circulation that are likely driving the Arctic/midlatitude changes. The Arctic Oscillation is an intra-annual mode, meaning it changes on timescales shorter than a year. However, the Arctic Oscillation may be affected by with the circulation changes from global warming... another thing we need more time and research to untangle.