Wednesday, March 28, 2012

Are Things Finally Changing?

One of the hallmarks of the winter & spring of 2011-12 has been relentless warmth and lack of snow across a large chunk of North America. But things may finally be changing (just in time for true spring) to allow this pattern to break down a bit and perhaps offer up some more variability. Let's look at some of the things at play.

First, one of the main drivers of the pattern the last couple of weeks appears to be the MJO, or convective patterns in the Indian and Pacific Oceans. To the left is what we call a "Wheeler Diagram," which plots the behavior of the MJO. We've seen the MJO sitting firmly in phases 5-7 of late (you can see that on the diagram by following the singular line). These notoriously correspond to very warm US patterns in March. Check out this site (scroll down) for month by month composites of what each MJO phase climatologically results in for the US. Prior to the epic torch of March, the MJO had been moving around phases 1-4, generally weak in nature. As the MJO was weak, it was not a tremendously dominant influence on the overall pattern (though it still had some influence at least).

But trumping the MJO, was the fact that we've also had a raging +EPO pattern in place (East Pacific Oscillation), which has helped tremendously to keep us warm. To the right is a composite map of the +EPO correlation to US surface temperatures (bottom map). Essentially, you can see a positive EPO typically leads to above normal temperatures in the Eastern 2/3 of the US. We've been dealing with this off and on for much of winter. Hence, it has been warm.

There are other factors as well, but these two appear to be very substantial. So where is it going? Well at least in the medium range, from the Wheeler Diagram above, which shows yesterday's European ensemble, you can see the yellow lines (which represent various ensemble members) take the MJO into phase 8. Logical progression. From the link I highlighted in the second paragraph, you can see that in March and April, phase 8 of the MJO corresponds to a slightly cooler than usual Eastern US, especially the Southeast. Additionally, we're starting to see good model agreement that the EPO is going to flip back to a negative sign. Take the map above and flip the temperature anomalies if that's the case (much cooler Eastern US...though not quite as cold as it was warm in March). So we have two things in the corner of cooler temps becoming more prevalent in the coming week or two and perhaps a shift of the warm pattern into the interior West.

Additionally, all winter (and for that matter going back to summer 2010), we've been dealing with a Pacific Ocean dominated by a La Niña pattern. There have been many rumors of La Niña's demise of late. And there has indeed been a significant shift in many aspects of the Pacific. You can see water temperature anomalies are back to neutral at best in much of the main Niña/Niño region. You can also look at a more graphic display of the erosion of subsurface cold water temperatures in much of the Equatorial Pacific as well. Additionally, here's a look at the European model's forecast for the Niño 3.4 region anomaly (from March 1), which shows a clustering into weak El Niño conditions by June. A slightly different calculation is done by the Columbia University IRI model, but it also shows the same general idea...weak Niño by summer.

The bottom line from all this is that it appears we're turning the corner on what has been an interesting and anomalously warm last several months. Now, let's be clear...this doesn't mean we won't revert back into another major warm spell next month or in May, June, etc. But, there are enough changes taking place on a lot of different levels that would argue that, at the very least, persistence is probably not a great forecast going forward...and more variability should be anticipated.