Wednesday, December 21, 2011

Not Your Typical La Nina

As you may or may not be aware, the tropical Pacific Ocean is presently in a colder state...a La Nina, and it has been in such a state since the summer of 2010. Last year, it was substantially stronger than it is this year, but nonetheless, a Nina it is. So, has this been a typical La Nina winter thus far? Do Ninas typically lead to what we can probably safely refer to as a "blowtorch" pattern in the Eastern US? The short answer: Not exactly.

Check out the image at the left, created from NOAA's ESRL site. That is a "typical" moderate La Nina temperature pattern for November. What's typical? I used what we call the MEI, or the Multivariate ENSO (El Nino-Southern Oscillation) Index. This index takes two months of sea-surface temperatures (SSTs) and other meteorological data in the ENSO regions of the tropical Pacific and develops a number that basically gives you a much better representation of how the tropical Pacific is behaving than you would get if you just looked at SSTs.

At any rate, I looked at the MEI value for the Oct-Nov period, the most recent available (usually updated for the prior two month period around the 10th of the month). I then took the 4 closest matching MEI years on either side of this year's value. And what you have is the map above and to the left...which suggests that in a typical Nina of this strength, you end up with a cold Southeast, a cool Northeast, and a rather warm West.

So how did this past November do? At the right you see what November 2011 looked like. It's basically exact opposite of what you would typically see in a Nina, right? A warm Northeast, neutral to warm Southeast, and a chilly West. Clearly, something more is at play here.

So, for kicks, I took the seven strongest El Ninos (I left out the 1982 and 1997 Ninos, as they were ridiculous, one of a kind events) from the MEI value dataset, and I checked to see how Novembers looked in those years. I knew the answer, but it's nice to see verification of your thoughts sometimes. That image is at the left just below. November 2011 matches up really well with moderate to strong El Nino years...and we're in a weak to moderate La Nina year.

In my recent previous entries, I discussed the behavior of the Arctic Oscillation, North Atlantic Oscillation, and the PDO, and how those indices may be playing a very substantial role in our very mild winter thus far. One of the keys to successfully figuring out a seasonal forecast is to try and figure out what the main driving force behind it is going to be. Clearly, the last couple winters, blocking has been the name of the game. And with those strong blocks in place, we had extended periods of cold. It's not at all a surprise that since this winter is featuring what's being coined in some parts of the weather-world as "antiblocking," that we're seeing almost complete opposites show up. Why this is happening is still something we honestly don't know. We figured we were in a long-term negative NAO period, but it's well known that within those cycles, you can still get shorter duration, positive NAO periods. What causes situations like last winter vs. this winter thus far is somewhat of a meteorological puzzle that's still being hashed out...and probably will be for awhile. And so we sit and wait.

Looking ahead, the maps for Ninas and Ninos in December don't change much. What you see for a November average, you normally see for December average. And thus far, December has not changed much at all either, with a strongly positive AO, NAO, and a completely muted La Nina signal. And looking ahead, the news doesn't get any better. At right is a map of the forecasted 8-10 day 500 mb anomalies from this morning's GFS and Euro models from the Penn State E-Wall. That is not at all a cold pattern developing, with Alaska getting hammered again and the US being flooded with mild to warm air. So the beat goes on, and winter stays mostly on hiatus.

Just as a quick reminder....while it has been and will be a mild week or two ahead, as we saw this past Sunday/Monday and may see again (especially in New England) on Saturday, it can still get cold. And yes, it can still snow. But it cannot sustain itself yet. We have a long way to go before we can lock in any sustained cold and/or snow.