IASNOVA
Why in news?
Recently, US mid-west experienced sub-zero temperatures due to a breakdown in the polar vortex.
What is a polar vortex?
– It is a large area of low pressure and cold air surrounding the Earth’s North and South Pole. The term refers to the counterclockwise flow (clockwise over south pole) of air that helps keep the colder air close to the poles.
– There are not one but two polar vortexes in each hemisphere.
– One exists in the lowest layer of the atmosphere, the troposphere. The tropospheric polar vortex is the one that affects our weather.
– The other exists in the second-lowest, called the stratosphere. It is much more compact than its tropospheric counterpart.
– If the two polar vortexes line up just right, very deep freeze conditions may occur.
– The boundary of the polar vortex is really the boundary between the cold polar air to the north, and the warmer sub-tropical air (considering Northern Hemisphere). And that boundary is actually defined by the polar front jet stream- a narrow band of very, very fast-moving air, moving from west to east.
– But that boundary shifts all the time. It shrinks in summer, pole-ward, while in winter, the polar vortex sometimes becomes less stable and expands, sending cold air southward with the jet stream. This is called a polar vortex event (“breaking off” of a part of the vortex).
–The break in polar vortex appears to be linked to the long and chilly winter in the north India this year.
Why cold air plunges south (in Northern Hemisphere)?
– Greenhouse gas emissions has amplified Arctic warming resulting into dramatic melting of ice and snow in recent decades, which exposes darker ocean and land surfaces that absorb a lot more of the sun’s heat.
– Because of rapid Arctic warming, the north-south temperature difference has diminished. This reduces pressure differences between the Arctic and mid-latitudes, weakening jet stream winds which tend to meander.
– Large north-south undulations in the jet stream generate wave energy in the atmosphere. If they are wavy and persistent enough, the energy can travel upward and disrupt the stratospheric polar vortex.
– Sometimes this upper vortex becomes so distorted that it splits into two or more swirling eddies.
– These “daughter” vortices tend to wander southward, bringing their very cold air with them and leaving behind a warmer-than-normal Arctic.
