Reposted from The Cliff Mass Weather Blog

Earlier this month, stratospheric temperatures warmed by roughly 100F over a period of a few days, in what is known as a Sudden Stratospheric Warming (SSW).  Should you be concerned?  The stratosphere, by the way, is the layer of the atmosphere from roughly 10 to 50 km above sea level.

As we will discuss, such stratospheric warmings are sometimes associated with distortions and alterations of the winds and temperatures in the lower atmosphere, resulting in anomalous weather from heat waves to snow storms.  And major changes in the infamous polar vortex.

There are actually two polar vortices:  one high in the stratosphere and another in the troposphere

Based on this incipient warming, some media has been warning about severe weather for several weeks, including major snowstorms over the eastern U.S. (see below).  

The Warming

    Early in January, the temperatures high in the polar stratosphere started to warm suddenly and profoundly.    Take are look at this NASA plot of temperatures at a pressure at 10 hPa (roughly 85,000 ft above the surface) at the North Pole for this year (red/pink) and last year (blue) by date.  The solid black line shows the average temperatures and the gray shading illustrates typical variability of the temperature at that location.

There was quite a warming in early January to about 250 K, roughly 50 K (or Celsius) above normal.  That is 90F above normal!  Wow.  

This is a sudden stratosphere warming, something we typically see once a year in winter.  You can view another such warming last winter in March of last year (blue colors).  

Importantly, this month’s warming did not last long and is predicted to be gone completely during the next week.   Why do such warmings occur?  A lot of research has shown they are associated with high amplitude waves in the troposphere that propagate vertically into the stratosphere and disturb the flow there.  Waves caused by mountain ranges and land-water contrasts, among other reasons.   I have personally done research on such issues, but I won’t get into the details here.
To understand the potential impacts of the stratosphere warming, let’s go back to the figure at the top of the blog, showing the two polar vortices (shown again below).  During a normal winter period, there is an area of very cold air in the upper stratosphere near the North Pole, which is not surprising consider since there is virtually no solar heating!   A jet westerly (from the west) jet stream (the polar night jet) surrounds the cold air and in fact creates a protective barrier around it.

In the lower atmosphere (troposphere), there is also cold air near the pole and a westerly jet at its southern boundary:  commonly called the jet stream and strongest from roughly 25,000 to 35,000 ft above the surface.   
The undulations of the jet stream and the associated cold air to the north have a HUGE impact on weather near the surface.   For example, a southward undulation can bring a cold wave and snow;  a northward undulation, warm temperatures.How stratospheric warming affect surface weather
When a polar stratosphere warming occurs, it is associated with strong sinking (which causes warming by compression); the weakening of the temperature difference between a cold pole and warmer air to the south causes the polar night jet to weaken and buckle, and the stratospheric polar vortex circulation weakens and can get distorted or even move off the pole.
The effects of the stratosphere polar warming and undulations/distortions of the cold air tends to propagate downward into the troposphere, where the tropospheric polar vortex can weaken and the tropospheric jet stream become wavier.
For the event of this month, it was clear that the upper jet  stream weakened during the warming. To illustrate, here is the eastward-directed (zonal) winds at 60N for a level near the boundary between the stratosphere and troposphere (150 hPa pressure, about 45,000 ft).  The winds (purple) dropped below normal after the warming began early in this month.

But has this stratospheric warming had a major impact on the undulations of the jet stream well down in the troposphere where we are?
To examine this, I plotted the difference from normal of the heights of the 500 hPa pressure surface (roughly 18,000 ft).  You can think of this as pressure. And the plots show averages over one week.
For the week prior to the warming (Dec. 26- January 1), there are a lot of strong anomalies (difference from normal)…including some anomalous ridging (high pressure over the pole)

The week following the warming (January 10-16th), the polar warming has weakened but perhaps has spread around a bit.  Nothing to write home about.  One big changing was the ridging (high pressure) over the West Coast and troughing over the eastern US., but hard to point at the warming as the cause.  And, in fact, the eastern half of the U.S. has been relatively mild this winter…no severe cold outbreaks.  And persistent blocking (locking up of the atmospheric flow) has been relatively absent this winter.

Looking ahead this week, the models are going for cold weather later in the week…but not in the eastern U.S, BUT OVER the Northwest.

The latest European Center forecast for 500 hPa height anomalies (think of difference from normal of pressure at 18,000 ft), shows higher than normal heights over near the pole and a major trough (low pressure) over the western US.  The kind of pattern that occurs more frequently in La Nina years (as this year is).

Let me make it clear, this pattern will not produce colder than normal temperatures over the eastern U.S. as some of the media were calling for.

And yes…this IS a snow threat for the Northwest lowlands. But I will wait until Tuesday to talk about that in depth.

via Watts Up With That?

January 18, 2021 at 08:56AM