So my mother asked me the question the other day: why is it that the rest of the US (or at least, the lower 48 states) has been so cold lately, but here in Florida, it’s been so hot? Well, my first thought was to ask what ENSO was currently doing to see if that would offer a clue: no dice, ENSO is doing nothing right about now. So I told her “I don’t know mom, weather is just weird sometimes” and let the matter rest there. But the question “Why?” kept eating at me. Finally I decide to do a bit of a “forensic” analysis. First, I decided to ask the question: when, in the past, has the US as a whole, seen Decembers colder than average, while at the same time Florida, as a whole, has seen Decembers warmer than average? It turns out that since 1895 (relative to the 1895-2012 mean), the answer is in the years 1897, 1902, 1911, 1916, 1919, 1924, 1926, 1932, 1948, 1951, 1961, 1964, 1967, 1972, 1978, 1990, 1992, 2008, and 2009. Now, looking at the years since 1948, I can create composites for those from ESRL’s composite page. The result looks like this:
A couple of things stood out to me: that these years tend to feature a warm Antarctic and a cold Arctic, at least in the Reanalysis. But on the other hand, the reanalysis is least reliable in those areas due to sparse data. Moreover, this December has, so far, been warmer than average in Alaska, where the above map shows it cold. One interesting place where the maps *do* match, however, is the North Pacific, where much of it is above average in temperature in both. This got me to thinking, “what about the PDO? ENSO is neutral, but the PDO is probably negative right now.” And sure enough, yup, it is! Which got me wondering, again, how the PDO correlates with December Weather in the US. This is the plot:
Indeed, Florida is in the Negatively correlated region, so a negative PDO would tend to be associated with a warm December in Florida-but oddly enough, also much of the rest of the US. However, negative PDO values are also often associated with La Nina conditions, which presently are not prevailing. This lessens the impact of the PDO in being associated with a cold US *generally*. So the explanation for our warm weather and the rest of the US’s cold weather? I think, but need to investigate further, that we can attribute it to the conditions in the Pacific: the presence of warm water in the middle of the North Pacific, in the absence of cold water in the Equatorial Pacific: Cold PDO-No La Nina pattern.