Sunday, September 14, 2014

The Story Behind The Polar Vortex

Ever since last winter, the polar vortex has taken over as the go-to thought as soon as the prospect of cold weather is brought up. Let's take this time today to go over what the polar vortex actually is, and disprove some inaccuracies I've seen swirling around the world.

1. The Polar Vortex Exists Year-Round
The phrase 'polar vortex' is used to describe a strong low pressure system (vortex) that is semipermanently placed over the Arctic region (polar). In the summer, the vortex significantly weakens, and is replaced by high pressure. During the fall months, however, the vortex gains strength yet again and low pressure dominates the Arctic as the polar vortex gears up for winter.

2. The Polar Vortex Extends into the Stratosphere, Not Just the Troposphere

The image above shows the 10-day forecast from the GFS model in the stratosphere, at the 30 millibar level. That's pretty high up the sky when you consider us humans are at the 1000 millibar level. In this forecast, we see relatively weak low pressure establishing itself over the Arctic Circle, with weak high pressure surrounding it. That low pressure body is the "birth" of the polar vortex for this cold season, as it begins to come back to life and strengthen for the winter.

Some people assume the polar vortex is no different than your average storm system that brings snow to your neighborhood. The vortex is actually quite different. In order for it to be the polar vortex, it must exist at both the tropospheric and stratospheric levels. That's how big and controlling it is in the atmosphere.

3. There is only ONE Polar Vortex

This is something I've been hearing ever since January 2014, when this polar vortex business really took off. The belief is that there is more than one polar vortex in the atmosphere. This might stem from the aforementioned idea that some believe the polar vortex is no different than your average storm system, but regardless of its origins, the belief is false.

Take a look at the image above. In this image, we see a view of 500mb height anomalies across the Northern Hemisphere on January 7th, 2014. Warm colors correspond to high pressure, which brings warmth and generally quiet conditions. Blues and purples indicate the presence of low pressure, which permits stormy and chilly weather. Just taking a glance over this map, we see one strong upper level low that seems to be the strongest one in the entire hemisphere. That upper level low, seen scraping the northern US and inundating Canada, is the tropospheric version of the polar vortex. As you can see, there is only one low as strong as the one in North America, hence there is only one polar vortex. Those other low pressure systems are just disturbances compared to the polar vortex.

To give you a better idea of how there's only one polar vortex, let's take a look at the stratosphere on January 7th, 2014. In this image, we see what appears to be two strong areas of negative height anomalies, being pinched by two bodies of ridging/high pressure on either side of the globe. If you thought that this image shows two polar vortexes, you are incorrect!
What's actually happening is the two bodies of high pressure are trying to tear apart the polar vortex, something that happens rather often each year. The difference between other years and this past year is that the ridging forced the main part of the polar vortex into North America; that other area of negative height anomalies in Asia is just a weaker part of the single polar vortex that was torn off of the main body. Some of you more experienced weather enthusiasts may know this type of set-up as Wave-2 stratospheric activity, as two bodies of high pressure push into the Arctic.

To close out this post, here's a stellar graphic made by the folks at the National Weather Service office in New York in early January, when the hype over the polar vortex was in full swing.

To summarize:

• The Polar Vortex exists year-round, varying in strength. It does not (and will not) "come back" for the winter, as it's already present and cannot "come back".

• The Polar Vortex exists across both the troposphere (where we live) and the stratosphere (where Felix Baumgartner jumped from his capsule in October 2012); it does not exist at the surface.

• There is only one polar vortex; our world would be much different if more than one polar vortex existed.

Additional points of summary are included in the NWS graphic above.


SOI Continues To Plummet As El Nino Emerges

The Southern Oscillation Index, a key indicator for the emergence of an El Nino or La Nina, continues to free-fall as our El Nino looks to finally come to the surface.

The image above shows a 30-day average of the SOI values from January 2012 to the present day. The Southern Oscillation Index is calculated by examining the pressure differences between Tahiti and Darwin, Australia. When values drop below -8, conditions are considered optimal for El Nino formation. Values above +8 are considered optimal for La Nina conditions.

 As the chart shows, we had been seeing predominantly-positive anomalies this past spring and early summer, which is likely a reason why the the historic Kelvin Wave never brought about the Super El Nino so many (myself included) had discussed. However, beginning right around the start of July, we saw SOI values tank, and they've remained below zero for the entire period since. Values have gone both above and below the El Nino 'benchmark' of -8 since that time, but have remained below zero the entire time.

Due to potential copyright infringement concerns, I will refrain from posting a forecast of the SOI I came across yesterday. What it shows, however, is continued negative SOI values for the next few weeks, and possibly beyond.

The animation above shows a depth-by-longitude animation of equatorial temperature anomalies, in Celsius, since mid-July to present day. In this animation, we see the Kelvin Wave-induced warm waters spreading east and pushing to the surface as they do so, now just beginning to hit the surface.

The SOI being negative indicates that trade winds along the Equator and areas in the neighborhood of the Central Pacific have shifted to a state that will allow this warmth to stay and ferment on the surface, likely into an El Nino. Back in April, with the historic Kelvin Wave warmth, trade winds wee unfavorable, and prohibited the warmth from sustaining itself on the surface.

In coming days and weeks, we'll likely see this winter's El Nino bring itself to the surface, likely at a weak, potentially moderate strength. I'll have more on the projected El Nino strength in the near future.