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. 

The tropics and it's going to get chilly next week.

The Tropics in the Atlantic Basin  are quiet, and look to stay that way for at least the next five days. The reason is  strong wind shear and dry stable air.  When you have this kind of setup the tropical waves moving off of Africa heading across eastern and  the central Atlantic have no chance of forming tropical cyclones.  

El Nino is still missing in action. But the teleconnections and sea surface temperatures ( SST's) show a good likelihood of El Nino.  The Climate Prediction Center (CPC) still has the chances for El Nino at 70% for later this Summer and up to 80% for the Fall. As I've been saying, I think it will happen later this Fall. Some are calling for a strong El Nino. But I don't think its going to be that strong.... more likely weak to maybe moderate.   As I've said in many blog post and on my accompanying Facebook page...eastern based El Nino's cause wind shear to increase over the tropical Atlantic Ocean.  So with the Setup in the Pacific still strongly pointing toward El Nino, it is really no surprise to see all the wind shear over the Atlantic.  

 

 

I've also mentioned quite a few times that  Saharan Dust inhibits tropical formation as well. It typically brings dry air that is not conducive to storm development, and some studies have suggested the dust itself has an effect on cloud formation, dust that is suspended in the wind absorbs and scatters solar radiation. This means less sunlight reaches the ocean surface,  resulting in cooler temperatures in the tropical Atlantic Ocean, therefore cutting off the fuel supply to any waves moving over the tropical Atlantic, and inhibits development of tropical cyclones.  Just remember inhibits doesn't mean excludes tropical cyclone formation in the Atlantic Basin.  

 

 

Any tropical waves that do make it into the Caribbean and Gulf; they can become tropical Storms and hurricanes. As was the case of Arthur.  Because of the setup over the Atlantic Basin, the East and Gulf Coast are more at risk for land falling tropical systems.  

 The week of July 13th-19th.

This coming week is going to see an old friend return. That being the Polar Vortex.  I'm sure most of us can remember the havoc the polar vortex caused last winter..........

Next week unseasonable very chilly air looks to move into the Midwest and Northeast. The vortex is going to allow a very cool air out of the Gulf of Alaska  to plunge into the northern tier of the U.S. typhoon Neoguri that just hit Japan, is going to play a huge role in the pattern, by amplifying  the area of low pressure over Alaska. The west coast trough will act like a slide giving a clear path for the cooler air to the south.  Right now it looks like the Midwest will take the brunt of the cold. However the Northeast will see temps 10-20 degrees below average, especially in the northern parts of New York and New England. I expect to see record lows set this coming week.  

 

Current surface chart, note the low in the Gulf of Alaska.

 

Here is the position of the vortex on Monday.

For Tuesday

And for Wednesday.

 

We had a very volatile  Winter....The Spring saw the same type of validity... I expect Summer of 2014 to continue the same theme ...long range teleconnections and models are showing a pattern of wild temperature swings and possibly an active severe season this Summer. One only has to look back to this past Tuesday to see just how severe it could get.......  

Will the midmonth arctic air outbreak be record breaking?

With the high pressure building in and a southwest flow.....we will see fairly warm temps going into the upcoming weekend....Tomorrow we will see a strong low pressure center in Canada....it will cause the wind to be  bit breezy and gusty wind gust could be in the strong breeze slot on the beaufort wind scale 25-31 mph. There could be a little precipitation over the Tug Hill, Adirondacks, and  northern Greens. Some of this could be in the form of snow...but accumulations will be light.  Then Thursday night, we will have another system approach the region. The Euro is running a little slower on timing then the GFS, Canadian, and Nam So any precipitation should be late.  During the overnight, the precipitation could fall as freezing rain and sleet. The precipitation on Friday should be mainly rain. Temperatures on Saturday will be in the 40's and perhaps low 50's.

OK let's get to the arctic cold outbreak for midmonth.

As I’ve been saying there are several signals pointing at a dynamic cold air outbreak.

Polar vortex:

If you look polar vortex (PV) you will see something like this; the large scale cyclonic circulation in the middle and upper troposphere located near the polar regions. in other words it's a strong wind flowing around a low-pressure system normally present over the Arctic in winter. I think that reads a little easier.  The PV is analyzed at 500 millibar level of the atmosphere. It can often be located over Canada or Siberia since the coldest surface air is often found over these regions.

Here's a look at the 500 mb chart showing the PV

  
How does the polar vortex influence our weather?



Strong Polar Vortex:

Strong is the more common state of the polar vortex. When the polar vortex is strong, this creates strong low pressure in the Arctic region.  Because of the pressure difference between the Arctic and mid-latitudes, air flows into low pressure and this confines the cold air to high latitudes closer to the Arctic. Therefore it is often mild across the Eastern US, Europe and East Asia during winters when the polar vortex is strong.

During strong polar vortex, the air flow is fast and in a direction from west to east.

Low pressure in the Arctic region is referred to as the positive phase of the Arctic Oscillation AO and the North Atlantic Oscillation NAO.

 Weak Polar Vortex:

 

When the polar vortex is weak or “perturbed”, the flow of air is weaker and meanders north and south (rather than west to east). This allows a redistribution of air masses where cold air from the Arctic spills into the mid-latitudes and warm air from the subtropics is carried into the Arctic. This mixing of air masses also favors more storms and snow in the mid-latitudes.

 

During a weak polar vortex, high pressure occurs in the Arctic region and is referred to as the negative phase of the AO and NAO. Air flows away from the high pressure Arctic. The north to south direction of the polar vortex carries cold Arctic air into the mid-latitudes of Eastern US, Europe and East Asia. Therefore it is cold across the Eastern US, Europe and East Asia during winters when the polar vortex is weak.

 

   

Here’s a look at the NAO and AO are forecast to be.

   
 
    
As you can see they are positive now ...but they will be negative by midmonth
     

Here are the current and the next weeks Euro 30 Hpa geopotential height.


 

In the image for next week you can see the vortex over Canada and the Northern U.S. If we see a negative NAO and AO this could  mean we will see near record cold around mid month into the end of January.

I saw a blog post today where they were talking about the arctic air outbreak of January 1985. So I wanted to compare and show you how the two might stack up. Here is an image from January 1985.

If you look at the Euro forecast for next week and compare it to the one from 1985. you will notice that they look very similar. The arctic outbreak in 1985 also was accumulated by a negative NAO and AO. The arctic outbreak 28 years ago was record breaking. This outbreak was the result of a shifting of the polar vortex farther south than is normally seen. Much the same as the potential outbreak beyond midmonth looks to be. Back in 1985 hundreds of cities in the United States saw record low temperature readings. The cold encompassed the entire Eastern Seaboard, Midwest down into Texas and the Gulf Coast. In fact the West Coast was cold too. Here is a temperature map from January 21, 1985. one other interesting fact is it was unusually warm weather in the eastern U.S. in December 1984, suggesting that there was a build-up of cold air that was suddenly released from the Arctic. Sounds a lot like the Summer of 2012. I'm not saying that this January 2013 outbreak. But you had to admits there are a lots of similarities, between this year and January, 1985.

   

Here’s a temperature map from January 21, 1985.
 
     

 

Sudden stratospheric warming:

 

Something known as a sudden stratospheric warming has occurred in the Northern Hemisphere arctic region over the last few days. This most likely the same thing that happened back in 1985.

what is a sudden stratospheric warming ( SSW)? This is when the winds in the PV slow down or reverse from the normally westerly direction. Frequently when a SSW takes place, it displaces cold air in the mid and upper levels and then build in the lowest layer of the atmosphere; once this happens it will move southward into the continental U.S.

  
  

As you can see from the image above that there was a another burst of cold at 150hpa giving the extra momentum. I see it nearing the end of its rule so will be looking for effect from that warming to reach further down through the stratosphere.

   

The outbreak will most likely move into the Northwest U.S and into the northern Plains. From here it will gradually move south and east. We could have storms impacting the East Coast. Because of this, the Cold could take awhile to build into the Northeast and the rest of the East Coast. Right now, most likely reaching the Northeast around the 23 to the 26th. We will see several waves of cold air move out of Canada, each one colder than the last. Once the arctic air reaches the Northeast it could take awhile for it to leave...it could be longer than ten days.  It's too early to know if we will see storms during the outbreak...but we could very well see storms ride along with the cold air.

That’s it for now. I just wanted to give you the latest information.

Rebecca.