TEXAS COLD AND POLAR VORTEX

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 TEXAS COLD AND POLAR VORTEX

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• The instant reason: The freeze in Texas in Feb 2021 was unprecedented. It is not a state that has many of these. From an atmospheric point of view, the Texas deep freeze was caused by a giant bend in the jet stream, the fast-moving band of air 10km above the earth’s surface that plays a central role in the weather of North America and Europe.
• What it does: The jet stream separates the cold polar air of the north from the warmer air of the tropics, and when it gets wavy the cold air can end up farther south than usual (or warm air can end up farther north). The jet stream has been behaving in unusual ways this winter in the northern hemisphere. The seasonal forecast for this winter didn’t really predict it.
• Global warming or not: It was not clear that the Texas freeze had been made more likely because of global warming. Freak weather events will happen, and they are not all necessarily linked to climate change.
• The stratospheric polar vortex is a large-scale region of air that is contained by a strong west-to-east jet stream that circles the polar region. This jet stream is usually referred to as the polar night jet. 
• The polar vortex extends from the tropopause (the dividing line between the stratosphere and troposphere) through the stratosphere and into the mesosphere (above 50 km). Low values of ozone and cold temperatures are associated with the air inside the vortex.
• Seeking answers: One topic of great debate among scientists of the atmosphere is whether the jet stream is becoming weaker and wavier as a result of global warming, or whether the opposite might be true, making the jet stream stronger and pushing it further north. It is wrong to view climate change as simply a gradual warming. In some cases weather becomes more variable, with more extremes in both directions.
• Wobbly: Destabilization of the polar vortex began in late January 2021 when Europe was going through a cold snap. This caused the polar vortex to become “wobblier” and pushed a bulge of frigid arctic air down through the Midwest USA. The freezing temps lingered, thanks to high pressure systems moving warm air farther north on either side of the bulge of cold air. This was happening over the Atlantic basin and the Pacific basin, so it squeezed that bulge farther south, and held that pattern (atmopsheric blocking). The squeezing motion was able to produce a higher amplitude bulge, and that’s the devastating weather events in Texas.
• Stratospheric: One reason for the disturbed jet stream could be an earlier disruption in the stratospheric polar vortex, a mass of cold air that sits more than 30km above the earth’s surface in the artic region. The term “polar vortex” is also used to refer to the cold air that sits on the poleward side of the jet stream, in the troposphere, which is the lowest layer of the atmosphere.
• SPV: In the atmospheric layer above it, about 10 to 30 miles up, is the stratospheric polar vortex, where every winter a sunlight-starved Arctic spins up a mass of cold air that, ultimately, dissipates in the spring. It's much smaller than the vortex below it, and typically spins west to east above the North Pole.
• As with the northern hemisphere and the Arctic, there is also a southern hemisphere polar vortex. This polar vortex also extends through the stratosphere and into the mesosphere. In contrast to the northern vortex, the southern vortex is stronger, larger, and longer-lasting. In addition, temperatures are colder and ozone levels are lower than their northern counterpart.

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