Sudden Stratospheric Warming , the Polar Vortex , and what it all means

Lately I've been talking a lot about Sudden Stratospheric Warming (SSW), the Polar Vortex (PV), and many teleconnections.  Karen Drosehn asked me to go into how SSW and PV work and how they affect our weather.  She isn't the only one asking questions about all of this. So here is a post that will hopefully clear up some of the mystery surrounding all of that. This is all very complex, but I will tone it down as much as I can.

Global wind patterns and Jet Streams:

Layers of the atmosphere

  

Before we get to the Polar Vortex, I want to quickly mention Jet Streams.  Air flows from high pressure to low pressure. This creates a force called a pressure gradient, which we just call wind.. This is because warm air is less dense than cold air, since nature likes balance, wind flows from higher pressure (warm air) to lower pressure (cold air) to try and restore this balance. When warm air rises colder air will rush in to replace it.  Since the tropics around the equator are warmer than the Arctic and Antarctic near the poles. winds move from north to south. They want to go in a straight line but since the Earth is rotating they can't go straight. The spinning Earth's momentum causes something called the Coriolis Force . This force is created, because the Earth's surface rotates faster at the Equator than it does at the Poles (Because the Earth is wider at the Equator). Because of this force the trade winds in the Northern Hemisphere and Southern Hemisphere curve to the left (east to west), this is why they are called the Easterlies. The trade winds sit at 30⁰ north and 30⁰ south  . In the Northern Hemisphere the Westerlies move  west to east (from southwest) and sit between 30⁰ north and 60⁰ north . In the southern Hemisphere they blow from the northwest. It should be noted that there are three global circulations in the Northern Hemisphere, and three global circulations in the Southern Hemisphere. The polar winds sit at 60⁰ to the Pole, they blow from the northeast in the Arctic, and from the southeast in the Antarctic, These winds are called the Polar Easterlies.  The farther north we go the cooler the air gets. The Jet Streams are created from this difference in temperatures, and form along the boundaries of the warmer and colder air masses. The larger the temperature difference the stronger the jet.  

The Jet Streams Are like narrow rivers of strong wind in the upper atmosphere.  In the northern hemisphere and Southern Hemisphere the major jet streams blow from west to east.   The main Jet Streams drive high and low pressure systems.  The location of high and low pressure centers, warm and cold air, and seasonal changes force the Jet Streams to wonder north and south, and form troughs (dips) and ridges (rising). These spinning eddies can disappear in one spot and reappear somewhere else.

Here are some illustration that show how all of this works.

That covers a lot of ground in a very short time...but it covers the basics of how things work.

What is the Polar Vortex?

The Atmosphere is composed of many layers; the closest to the surface where we live is called the Troposphere. The PV's are really just a set of two jet streams, two over the northern Hemisphere and two over the Southern Hemisphere.   The jet stream most of us  refer to as the Polar Vortex resides in the Stratosphere ( a layer above the one where we live), and is technically called the Stratospheric Polar Vortex (SPV) . The Northern Hemisphere SPV is an area of low pressure that consists of a strong circular  westerly wind field over the Arctic. The SPV keeps the bitter Arctic air bottled up over the North Pole. As long as the SPV stays strong enough, the frigid cold stays locked up in the Arctic.  But sometimes the SPV weakens enough for some of this cold to escape and head south into the mid latitudes; this is where the SSW comes into play.

The second polar vortex is called the Tropospheric Polar Vortex (TPV). Whereas the SPV only appears during the dark arctic winter, the TPV is a yearlong atmospheric element that sits over the Mid Latitudes. It is driven by the differences in temperature between the Arctic and mid latitudes.  This vortex is the one that is commonly called the polar jet stream or northern jet stream.  The TPV is farther south during the winter, and farther north during the summer.  

The subtropical Jet Streams are south of the Polar Jet Stream.

Here is an illustration that shows both of these polar vortexes.   

The distinction between the two is important. But for the purpose of this post I don't really need to go into all of that. To keep things easier to understand, from here out, I will refer to the Stratospheric Polar Vortex as just the Polar Vortex (PV)

Sometimes the normally western flow of the PV is disrupted by weather patterns or strong disturbances in the lower atmosphere.  These things can cause the PV to wobble (become wavy). If you could see it, the process would look like waves breaking on the beach.  Sometimes these waves can become strong enough to weaken the PV , reverse the easterly flow around the Pole, or even cause the one main vortex to split into two or more separate vortices.  This is exactly what happened in 2014, The PV dramatically weakened. This allowed a large area of high pressure to develop over Greenland, which blocked the cold air from escaping .  So a piece of the PV broke off and plunged the East Coast of the CONUS into extreme cold.     

What is a Sudden Stratospheric Warming?

A SSW is a very rapid warming in the Stratosphere over both poles. It is one of the most extreme changes of weather on the entire planet.  In the space of a week  arctic temperatures at the pole between 10 km and 50 km can increase by more than 90 ⁰F.   

Stratospheric warming is caused by atmospheric waves in the troposphere. The waves are forced by things like mountain ranges and the contrast between land and oceans. These planetary atmospheric waves can only move around the Earth's atmosphere in westerly winds. Weather systems and other lower atmospheric fluctuations force the waves to rise into the stratosphere, where they can go no farther and the waves break and reinforce the easterly winds making these easterlies to lower. This causes the PV to wobble. As this process continues the easterly winds are forced even lower into the troposphere as the air in the stratosphere collapses and compresses.  The result of this, pushes the PV away from the Poles, forcing warmer air in the mid latitudes to move north to replace the colder that has been displaced,  and as the air moves downward the polar region warms.

None of this happens overnight. It can take a few days to a few weeks for a SSW to take place.  It also takes a week to three weeks for the arrival of arctic cold into  North America, Europe, and Asia.  There is no guarantee that a SSW will result in arctic cold in the CONUS, it can to into Europe, or Asia instead. 

What is happening right now and analysis on where I think we're heading?

I know many of the snow lovers are preaching doom and gloom. But IMO this is all totally uncalled for.  I was saying well before Christmas, that we would see changes around Christmas and soon after. I said then the models would start to see the atmospheric pattern change  around Christmas/New Years, and that is exactly what is starting to happen. We're about to see arctic air enter the pattern. I think that is a good thing.

This current SSW is very impressive. Here are some images from the Japanese Meteorological Association showing the warming and the sharp spike in the temperatures.   

Currently the stratospheric PV is very elongated. The axis is from around Norway down to Southeast Canada. We also have ridging sitting over eastern Siberia and Alaska.  I think we will see the PV split , over the next few days.  Then we will see the cold air spread out and blocking should set up.  I would estimate that to occur between January 10th and 20th. As I've been saying for weeks, the timing looks to be around mid month.

Right now, we have a lot of low pressure sitting over Canada. This allows the air over the Pacific to enter the North American pattern.  So we end up with the mild conditions we've been experiencing the last two / three weeks.

As this current Stratospheric Warming collapses and works its way down, the stratosphere expands and the troposphere compresses. So we see the temperatures  climb dramatically in the stratosphere, like we see in the image. This will cause the pressures to rise. This results in high pressure over Canada into the CONUS, the exact opposite of where we're at right now.

The data suggest that after the PV splits, the heights will lower and coldest temperatures will sit over three regions in the stratosphere....western Asia/eastern Europe, the North Pacific, and Southeast Canada and the Northeast.  It looks to be a slam-dunk for western Asia/eastern Europe, the North Pacific. But for Southeast Canada and the Northeast the odds are a little less sure.  But before we can count our chickens, the troposphere over the arctic needs to warm.  A warmer arctic means a much higher likelihood for harsher winter weather here on the East Coast.  A split will also favor the cold sticking around for awhile, something else I've been talking about. The models have been having a hard time figuring out the pattern. But they are starting to see what is in the near future. Once the arctic warms a bit, the models will be able to handle everything much better.    

I've been talking a lot about the MJO and all the other teleconnections. Most of these things are going in the direction they should be, if we want cold and snowy. The truth is the SSW is just icing on the cake. Even without the SSW, these other factors would make for a colder and stormier winter pattern for us in the Northeast and Mid Atlantic.