It’s been one of the wettest springs and early summers I can remember and the variability in weather from hour to hour has been very unusual as well. Day after day for nigh on two months we’ve seen rain showers, thunderstorms, and clouds punctuated infrequently by short periods of beautifully cool and sunny weather. Summer seems to have been put on hold in favor of a perpetual April. All in all it’s been more like England and Ireland around here than New York of late. But why is this happening? Here’s what I found:

It was actually harder to get a straight answer to this question than I thought it would be. Considering how annoyed everyone has been with the weather you would think it would be front page news on a lot of weather sites.

Believe it or not my first answers came from a post by Rich Duprey on Motley Fool (an investing site I read once in a while) where the discussion was really about futures, and why the poster is thinking we’re likely to have a shorter growing season this year. I get the feeling I wouldn’t agree with Rich’s politics, but at least he offered some possible reasons to investigate. He listed a recent eruption of a volcano in Alaska, El Nino, the NAO (North Atlantic Oscillation), and Solar activity as the main reasons for our current weather pattern. This at least gave me something to go on even if I decide not to blame it entirely on Sarah Palin as he suggests we can.

You can think of the NAO as an atmospheric verison of the El Nino you may already know something about only it’s in the North Atlantic and not off the coast of Peru. You can read a little about the NAO on NOAA’s site but it’s technically defined as “a hemispheric meridional oscillation in atmospheric mass with centers of action near Iceland and over the subtropical Atlantic” in a very informative paper found on the National Academy of Sciences site. The NAO is also described on Wikipedia, and on a page at Columbia. A positive NAO index is associated with wetter weather in the eastern US and the claim is that over the last 30 years the NAO has been shifting to a more positive phase such that on average the NAO index is more positive with each successive year. An up to date NOAA plot of NAO index appears to show this trend reversed over the last 10 years, in fact it looks like we’ve been a more negative phase (on average) for a few years. 2009 *does* appear to be a bit more on the positive side. I’m going to keep my eyes on this chart over the next few years to see if I can really discern any correlation.

Mount Redoubt in Alaska has indeed been erupting of late. There is an entire page on Wikipedia to record the Stratovolcano’s recent activity. There is a long discussion thread on this blog post on whatsupwiththat.com with links to more information about the volcano and it’s possible effects on weather – but it seems that the eruptions at Mt. Redoubt haven’t been sufficient to have very much effect anyway. This could change easily as it does seem to be in an active phase.

The El Nino seems to be on the way back. El Nino (Southern Oscillation) is the increase in sea temperatures in the Pacific Ocean of greater than 0.5 degrees C averaged over three months. The El Nino and the El Nina are expressions of the oscillation of this warmer water and atmosphere between South Asia and South America over the course of years. El Nino coming back could mean colder, wetter weather for the Southeast and less chance of hurricanes hitting that region. NY is affected only peripherally, but on average we might indeed begin to see wetter, colder weather on average as well as a result of a strengthening El Nino.

We’ve been in an unusual period of solar inactivity until very recently prompting some scientists to raise the specter of another “little ice age” recalling the Maunder Minimum which occurred in the late 1600s and ended in 1715 when Greenland was largely cut off by ice and Holland’s canals routinely froze solid. Luckily it appears that the lull has ended so with any luck we won’t have to consider solar activity in our calculations about why the weather has been so crappy of late.