Good Sunday evening.
As February draws to a close, I’ve decided to take a look back at the Winter – or essentially non-Winter – of 2011-1012, with an emphasis on why we saw such an extensive snow drought.
Here’s a map of the current snow cover across the lower 48 as of today:
And for exactly a year ago:
It’s easy to see that we’re far below the level of snow cover when comparing this year to the prior. This year, the deepest snows are confined to northern New England, the upper Great Lakes, and the higher elevations of the central and northern Rockies and Sierra Nevada and Cascade mountain ranges out west, while last year, essentially the entire northern half of the country was covered by a decent snowpack.
So how is this year different from last? The are a lot of factors, but I’m going to try to simply it to just a few, which I believe are the most important.
The first factor is the El Nino Southern Oscillation, or ENSO, which fluctuates from:
(1) El Nino, where the equatorial waters of the Pacific west of South America are warm, as depicted by the bright white shading below,
And (2) La Nina, where the waters are cold, as shown by the blue and purple shading:
This year was predicted to be a weak to moderate La Nina, which in fact was pretty close to what verified:
On the graph above, the line through the center is a neutral (or La Nada) ENSO state, whereas the red peaks are El Nino’s, and the blue troughs are La Ninas. One can see how this year was in fact a weak to moderate La Nina (last trough to the right on the graph), and the year before was an even stronger La Nina (second to last blue trough). La Nina’s generally favor warmer conditions along the East Coast, colder conditions across western Canada, including Alaska, as well the Pacific northwest:
It’s hard to argue those facts this year. Alaska had record cold and snows, and well, the East Coast was very far from anything like that. But what gives? 2010-2011 was a La Nina as well, and stronger at that, and the results were much different. There have to be other factors involved, and this is why you can’t define the winter based on the ENSO state – it’s just not that simple.
Another important factor which has garnered significant attention in the weather world over the past several years is the North Atlantic Oscillation (NAO). Those that follow me will recall the NAO is an index which measures the degree of Greenland blocking (in the form of a ridge as seen below) which slows storms down and prevents them from quickly exiting the east coast.
Such a negative NAO, as seen above, promotes a storm track along and paralleling the coast, as well as an increased threat of Arctic air intrusion into the eastern US.
A positive NAO means there is a lack of blocking, the ridge is displaced to the east over Europe, and storms race due east and out to sea, in progressive manner, with the majority of our cold locked up to the north.
Unfortunately for snow lovers, this year has featured an extended positive NAO, exactly the opposite of the previous 2 years:
So, in a weak to moderate La Nina year, the lack of blocking or positive NAO we have had for most of the winter has had a pronounced effect in our overall lack of winter weather.
But there have been other factors as well. The Madden Julian Oscillation, or MJO, has been another major player this year. The MJO is not a standing pattern such as La Nina or El Nino; but rather, it’s a traveling pattern characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed throughout the tropics, but mainly over the Indian and Pacific Ocean. It is divided into 8 phases, each corresponding with the geographical location of the active phase (or greatest convection/rainfall) of the MJO:
Phase1 includes signals both from the initiation of an MJO event in the western Indian Ocean basin and the breakdown of MJO events in the mid-Pacific Ocean. During phases 2 through 8 the MJO travels east at 4-10 days/phase.
To put it more simply, think of phase one of the MJO as a large string of thunderstorms dying in the mid Pacific ocean, and re-firing in the western Indian Ocean. Then, every 4-10 days, this active center of thunderstorms propagates east across the tropical Indian and Pacific Ocean, as the MJO advances from phases 2-3-4-5-6-7-8-1 above on the map, and the cycle can then repeat itself. These thunderstorms can vary in intensity, and can die off at any point in the cycle of the above 8 phases. For example, the MJO may start in phase 1, and every 4-10 days advance through phases 2-4, and then weaken or die off completely. They may re-emerge in the same location for phase 4, which is in the region of Indonesia, or may go back to phase 1, or any other phase for that matter.
As you can imagine, this is extremely difficult to forecast, given the high variability. An example of a plot for the MJO can be seen here:
It’s a little busy, I realize. But the number in each of the 8 octanes represent each of the phases of the MJO. The plotted line shows where the MJO wave has been for each day of the past 40-45 days or so. This line cycles in a counterclockwise fashion as the MJO propagates through each of the phases discussed above. Currently we’re in phase 2, and by simple logic, one would expect the MJO to progress into phase 3 and 4 over the next 8-20 days, as is seen by the GFS model forecast plot:
Notice the GFS has the line going towards the center circle of the chart after going through phase 4 – this reflects what can happen when the thunderstorms weaken or even die off completely. It may stay rather dormant, can re-emerge in phase 4, or in any other of the 8 phases.
So, why the hell is this so important? I mean, it’s half way around the world, it’s very confusing, and we don’t forecast it very well. The reason is simply this – because each of those 8 phases has a rather strong influence in the weather here more locally in North America during the winter months of January to March, as illustrated by the graphic below:
The maps above represent the temperature composites across North America from January to March for each of the phases of the MJO. As you can see, phases 8, 1, 2, and 3 correlate with colder temperatures for the east coast, while phases 4, 5, 6, and 7 are significantly warmer for those of us in the east. With that information, take a wild guess as to what phases we’ve spent most of the winter in – and here’s a hint, it wasn’t the first group. And for bonus points, guess what phases we were in the majority of the winter of 2010-2011 (that’s right, it was the first group).
So keeping track so far, in this winter with a weak to moderate La Nina, we have had a mostly a positive NAO, and lack of blocking, and an unfavorable MJO.
And finally, we come to the Pacific. The pattern over the eastern Pacific has been rather dreadful all year. We’ve been largely stuck in a progressive, fast paced flow in the upper levels of the atmosphere:
This firehouse of warmer Pacific air has kept our temperatures rather warm for nearly the entire winter. True Arctic air has been bottled up in Canada and Alaska, minus a few days here and there.
This of course is a somewhat simplified (and I know, some of you are thinking this post was probably too complicated) look at what went lead to the lack of winter weather for 2011-2012. The largely positive NAO, unfavorable MJO, and poor Pacific pattern have all had a hand in it.
La Nina is starting to fade and we will likely be in a weak El Nino come next winter. What does that mean? Well, we’ll leave that for a future post. For now, I leave you with what I always think of whenever I hear the words “El Nino:”