Thank you for this interesting research. I live in Europe and I noticed that the winter 2013/14 was very strange, because of the persistence of wet, mild conditions in many parts of the continent, probably a consequence of the 2 wave pattern around the northern hemisphere in association with the extreme +ve NPM. I have a suggestion in response to citizenschallenge and a question for Dennis. Suggestion: as Kevin Trenberth suggests, I think that instead of questioning if the probability or intensity of a pattern might or not be influenced by global warming, if is it due to global warming or to natural variability, could be better to assume that it could be both. I think that changes in atmospheric circulation is a product of internal, free variability (as spontaneously occurring changes in the amplitude and polarity of preferred atmospheric circulation patterns) but this product can carry an externally forced dynamically response (think for instance the changes in the wintertime circulation over high northern latitudes induced by large volcanic eruptions or the widening of the tropical Hadley cells in response to human-induced global warming or in response to changes in solar irradiance). The challenge is to find if/how much this is completely internal or/and forced. Question: I read (for instance Seager et al. 2007) that the Medieval Climate Anomaly - which was connoted by a series of pluri-decadal severe droughts that afflicted western North America - was part of a pattern characterized. among other things, by a La Nina-like tropical Pacific and by the warm phase of the Atlantic Multidecadal Oscillation. If I rememberer well, reconstructions tell us that even the IPO was in her -ve phase. Do you think that this particular period might well have been influenced by the NPM, too?