2 December 2015
THE ELEPHANT IN THE ROOM
Ask me what the Northern Hemisphere winter will be like, and I can easily tell you. As a matter of fact, It’s a slam dunk!
It will be WAY above normal.
I’ll say the same for the whole planet for December through January. As a matter of fact, rising greenhouse gases in our atmosphere make it quite likely that the U.S. will see an above average winter. The devil is in the details of course. Last winter was very cold in the Eastern U.S., while the West and Alaska had a very mild winter.
Since El Nino years are always globally hot years, it’s very likely that the globe will see a very warm boreal winter, and El Nino winters in much of America tend to be warm, even when you ignore climate change. You can’t do that though, so climate change increases the odds even more. It also means that areas that would be near average in an El Nino year will probably be above average. The climate of this planet is changing very rapidly, and this cannot be ignored. Look at the image above. 2015 is the hottest year on record, and by a record amount. December-February will almost certainly be the warmest winter on record globally!
THAT PESKY COLD SPOT
There’s another factor in the winter forecast you probably haven’t heard of. It’s a pesky cold spot of water near Greenland. While the rest of the oceans are at record warmth, there is this area of colder than average water in the North Atlantic, and there is a real possibility that this is from melting glaciers in Greenland (Good summary of this thinking is here). It may also be a symptom of the slowing thermohaline circulation.
It is a myth that this circulation keeps Europe from becoming a deep freeze in the winter (like Eastern Canada), but remember the rule about the heat in the ocean being the real driver of the atmosphere. It’s possible that this colder ocean (and the lack of sea ice in the Barents Sea) may tend to make a blocking weather pattern more likely. This is measured by what meteorologist refer to as the North Atlantic Oscillation. There is a good reason to pay attention to the NAO, and other pressure patterns like the Eastern Pacific Oscillation (Good summary of the EPO here), because they play a much bigger role in our winter weather than El Nino does.
FORECASTING ATMOSPHERIC TRAIN WRECKS
The NAO comes in two flavors, a positive phase and a negative phase. The positive phase in winter allows cold air to stream east into the Atlantic, but when a large blocking high pressure builds over Greenland, an atmospheric train wreck develops, and Polar air is shoved deep into the Eastern U.S. Where there is Arctic air, there will eventually be snow, and a negative NAO is responsible for some of our biggest cold and snow events.
Forecasting the NAO (or its cousin the Arctic Oscillation) is not easy beyond ten days, but one person who thinks he has found some clues is Meteorologist Judah Cohen. He’s discovered a connection between the NAO and ice in the Barents Sea (mentioned earlier), and more importantly the October snow cover in East Asia. Cohen found that when October snows are more widespread than average, the NAO tends to be more negative in the following winter.
Dr. Cohen’s work has gotten a lot of notice, and it will be interesting to see if it continues to be a decent predictor of the winter NAO in the Atlantic. His ideas are not without detractors and frankly, I’ve seen data that makes the connection with Asia snow rather tenuous. I wrote more about Cohen and the NAO in 2010 in this journal, and if you’re wondering how snow in Eurasia could influence our winter in America, remember that cold temps. over snow cover across such a huge land mass, can cause a storm track shift with downstream impacts as well. I’ll say again: Many of my fellow meteorologists (me included) are a bit skeptical of the relationship between snow in Asia and the NAO pattern, so I advise caution!
That said, remember the elephant, less Arctic ice in the fall means more moisture for snow. Dr. Jennifer Francis at Rutgers has also done important research into how the loss of sea ice may be making atmopheric blocking patterns more frequent. There seems to be growing evidence in support of Dr. Francis’s research, and there is far less ice in the Arctic in the Autumn than 25 years ago. This is almost without doubt due to climate warming.
Since we only have two strong El Nino events to look at, we have little idea about how a strong El Nino, combined with a strongly negative NAO (or EPO), will impact our weather. So with that in mind, the best forecast for the winter is to lean toward what we saw during strong El Nino events in 97 and 82-83, but to remember that we could be surprised. Also, I’m forecasting the average over 3 months from December-February, while most people remember the winter for cold snaps and snow storms, not the averages!
The NOAA Climate Forecast System uses an average of long-range model runs to look at long-range temperatures, and the latest forecast for all of December is below. I can tell you that based on the storm track over the past 3 weeks, this makes sense, and I see no signs of it changing.
My Best Guess at Winter
With all of the above said, here’s what I’m comfortable saying about the coming winter. In the eastern U.S. a colder than normal winter is unlikely. I’ll also predict that for the Mid-Atlantic, Northeast, and Gulf Coast , a drier than normal winter is unlikely.I’ll also predict that for the U.S. as a whole (and the planet) the next 3 months will average out to be much warmer than average. I also expect some areas of the east will have a very wet winter, and most likely this includes the Mid-Atlantic to the Southeast.
If the NAO or EPO goes strongly negative, we could have a historic snow event with a period of really cold weather in the East, while the rest of the winter is more El Nino like. Just remember, this is not your grandfather’s atmosphere. Things are different now, and will get even more-so in the coming years…