(Note: The Geopotential Height (GPH), generated by computer simulations, implies temperature of the air mass below the indicated altitude. An altitude of 500 mb, i.e, 5.5 km is typically used for surface weather forecasts. The word anomaly means deviation from normal values).
The Polar Vortex
What is the underlying cause of this switch in weather pattern? It is the good old “Polar Vortex” which became a household name few years ago. The polar vortex is a low pressure region high above the Earth’s poles (in the stratosphere), that rotates West to East. Normally, in winter in the northern hemisphere, the polar vortex forms a tight (1000 km) circle around the Arctic region, keeping cold air below it confined around the North pole. The jet stream, which lies below the polar vortex, in the troposphere, rotates in a wider circle and meanders North and South a bit in a wavy pattern. The jet stream generally brings cold air from the upper latitudes into the mid latitudes. When the polar vortex moves away from the North Pole or disintegrates or splits into two or more pieces, it allows cold arctic air to ooze out of the region and get transported by the jet stream into North America and Europe.
Here is a normal Polar Vortex from Dec 6, 2015. In this map of wind speed and direction at an altitude of 10 hPa (hecto-Pascals) or 31 km, the polar vortex is tightly located around the North pole, confining arctic air inside the inner green area.
You can see current and historical animations of wind speeds (and other metrics) at different altitudes from different vantage points at the link below. It is mesmerizing to watch; do take a look.
The Polar Vortex Today
Here is the Polar vortex today — we can see two weak circulating vortices, one over North Eastern Canada and one over Siberia. The North pole itself is “bare” and the air there is free to be carried by the jet stream. Also, the air currents now flow East-to-West in certain northern regions of the globe. The splitting of the polar vortex and its effect on weather in North America and Europe this winter was predicted by climate researchers in December.
Here is a map of today’s jet-stream, i.e, winds at an altitude of 10 km. You can see the jet-stream streaming from the arctic region, traveling in a U-shape path across Canada into North-Eastern US, traveling just outside the Polar Vortex located in north-eastern Canada. It is this cold jet stream which will dip deeper in the midwest and Eastern US in the next two weeks and linger around until the polar vortex moves out back towards the pole, sometime in late February.
Will this bring lots of snow? Yes, but not directly. Snow will be formed as a result of moist air from the Pacific and the Gulf traveling North and East and meeting the cold jet-stream. The El Nino effect this year will increase total snow amounts.
How We Got Here
What caused the polar vortex to split? Sometime in late December, temperatures in the stratosphere above Siberia rapidly spiked, in what is known as a sudden stratospheric warming (SSW) event. This is not unusual and occurs every few years with differing intensities. This caused the polar vortex to weaken in intensity and break up into two (some say three) separate vortices. The graphic below shows the average wind direction in the high latitudes (red is West-to-East, blue is East-to-West) and it is clear that the (average) wind direction changed in late December.
The graph below clearly shows the sharp spike in temperature in the stratosphere, that triggered the splitting of the polar vortex.
This is an interesting simulation based 3D visualization of the polar vortex, splitting into three sections, around the end of December.
Polar Vortex and Climate Change
The polar vortex is not new. Its variations and meandering is not new. What is new is the higher frequency at which it has been changing in the past few decades, although it has been stable for many years in between.
Yes, there is a link between global warming, arctic ice decline, warm surface arctic temperatures and sudden stratospheric warming (SSW) leading to the weakening of the polar vortex. However, scientists are hesitant to draw strong conclusions until they have more data and a better understanding of the underlying physics.
From en.wikipedia.org/… -
The general assumption is that reduced snow cover and sea ice reflect less sunlight and therefore evaporation and transpiration increases, which in turn alters the pressure and temperature gradient of the polar vortex, causing it to weaken or collapse. This becomes apparent when the jet stream amplitude increases (meanders) over the northern hemisphere, causing Rossby waves to propagate farther to the south or north, which in turn transports warmer air to the north pole and polar air into lower latitudes. The jet stream amplitude increases with a weaker polar vortex, hence increases the chance for weather systems to become blocked. A recent blocking event emerged when a high-pressure over Greenland steered Hurricane Sandy into the northern Mid-Atlantic states.
Fun fact — Other astronomical bodies are also known to have polar vortices, including Venus (double vortex—that is, two polar vortices at a pole), Mars, Jupiter, Saturn, and Saturn's moon Titan.
Epilogue
It will be a cold and snowy ride into February, thanks to the split polar vortex playing havoc with the weather system.
Yes, global warming can cause extreme cold weather; it may be difficult to explain this to the average conservative climate denier relative, but we should try nevertheless. This troglodyte administration obviously couldn’t care less about this topic. NASA and NOAA web and twitter sites are not posting any new data due to the government shutdown.
Meanwhile, the Arctic and the Antarctic are melting, sea ice is declining and average temperatures there have been rising steadily over the decades. Oscillations between warm and colds winters for different parts of the world are yet another consequence of climate change. Let’s keep fighting, for humanity.
References
- www.carbonbrief.org/…
- en.wikipedia.org/...
- The polar vortex has fractured, and the eastern U.S. faces a punishing stretch of winter weather — www.washingtonpost.com/…
- Polar Vortex: How the Jet Stream and Climate Change Bring on Cold Snaps — insideclimatenews.org/…
- Altitude to standard pressure conversion tables — meteorologytraining.tpub.com/...
- Thirty Mile High Wave Encircling Earth to Break over North Pole on Christmas Day — by FishOutofWater — www.dailykos.com/...
P.S. I am hardly an expert in climate or atmospheric sciences; please point out any errors in the diary or additional information that would be useful to our readers.
As predicted, the cold wave is upon us for the next few days.
The polar vortex segment hanging over NE Canada is expected to dissipate soon, allowing above normal temperatures to return to central and eastern US by the weekend.