U.S anomalous weather

U.S weather is more bi-polar than ever. Eastern U.S shivers from the polar vortex and California struggles in a historic drought.

The "Polar Vortex", one of the buzzwords of 2014, brought news last week by bringing Arctic temperatures to North America and by providing climate skeptics with arguments against Global Warming.
In fact, the scientific explanation behind this blast of cold weather helps to provide a more troubling evidence about the increasing effects of global warming.

What is a Polar Vortex?

These cyclones (vortex) over the two poles of the earth are located in the middle and upper troposphere and stratosphere and move at different speeds. Thanks to them, the cold, dense air over the poles is confined there.

This movement of strong winds is part of the polar front. It is stronger in the winter and decreases or disappears in the summer.

The Antarctic polar vortex is more pronounced and persistent than the Arctic vortex, this is due to the distribution of land masses at high northern latitudes which leads to an increase of Rossby waves in the jet stream (Rossby waves are oscillations that occur in geophysical fluids and are due to the principle of potential vorticity conservation). These waves shape the polar jet stream, while in the southern hemisphere the vortex remains less affected.

Rossby Waves. Source: knmi.nl

When these polar winds decrease significantly, the vortex may be distorted and the result is that the jet stream plunges deep into the southern latitudes.

This video from  Obama's Science and Technology advisor could help:

When this strong cold current moves towards low pressure areas, winter storms with heavy snow, intense cold and freezing winds may develop. A cold air outbreak caused by a polar vortex is much larger in area and length than a single storm and can lead to cold waves at multiple locations, but this doesn't always happen.

What is the link with Global Warming?

Last January, due to a polar vortex, America experienced a deep freeze. Cities like Chicago recorded a wind chill approaching -60 degrees Fahrenheit (-51°C). Even some parts of Canada reported temperatures colder than Mars!

Homes are covered in snow in West Seneca, N.Y.,Wednesday, Nov. 19, 2014. (AP Photo/The Buffalo News, Derek Gee)

However, the international scientific community points out that global warming can play an indirect role in these occasional freezes.

The fact is that, due to global warming, the Arctic is warming faster than the rest of the planet, warming which is accelerated due to the the polar ice melt (change in the albedo). As a result of this, temperatures recorded in the Arctic are around 2º C warmer than they were in the mid-sixties (compared to the overall increase in the average temperature of the atmosphere of Earth which is about 0.7º C, since 1900).

Therefore, the temperature difference between the Arctic and North America is decreasing. This temperature difference causes changes in the air blast around the polar region. The decrease in the temperature gradient that has made the air jet to slow down and develop Rossby Waves, bringing warmer temperatures farther north and more cold Arctic temperatures further south.

Maybe skeptics who take any chance to deny global warming should read about these mechanisms the climate system has to face imbalances.