Polar vortex

Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Beyond NCERT

Meaning

The polar vortex is a large area of low pressure and cold air that circles each pole, high up in the stratosphere and upper troposphere. Its edge is a ring of strong westerly winds. It matters because normally this ring keeps the coldest air locked over the pole. When the ring weakens or splits, freezing Arctic air can pour south into the mid-latitudes and cause record cold waves.

Explanation

What it is and when it exists

• Polar vortex: a large "circumpolar westerly circulation" (a ring of west-to-east winds going all the way around the pole). It exists between autumn and spring [2].
• Height: it sits in the stratosphere (the layer above the tropopause) and the upper troposphere (the top of the lowest, weather-making layer).
• Parts:
• Core: cold, low-pressure air over the pole.

• Edge: swirling westerly winds. Normally these winds are strong enough to keep the coldest air inside the Arctic in winter [3].

• Why it forms: in winter the pole gets very little sunlight, so the air above it becomes very cold. The air pressure falls high above the pole. Air moving towards this low is turned by the Coriolis force (the turning effect of the Earth's spin), so it circles the pole as westerly winds instead of flowing straight in.

How it breaks down: sudden stratospheric warming

• Sudden stratospheric warming (SSW): a rapid rise in temperature in the polar stratosphere [2].
• Cause → process → result:
• Cause: Rossby waves (giant loops in the upper westerly winds) rise from the troposphere into the stratosphere.
• Process: the waves break there and slow down the vortex winds, much as a breaking sea wave loses its energy [1].
• Result: the vortex weakens, shifts or splits. Cold Arctic air spills south into the mid-latitudes, and warmer air moves into the Arctic [3].

Conditions that help cold air escape

• Arctic Oscillation (AO): a see-saw of air pressure between the Arctic and the mid-latitudes.
• In its negative phase, cold air escapes south more easily.

• Before the Texas cold wave, the AO had been strongly negative since early December 2020 [3].

• Polar front jet stream dips: when this jet bends towards the equator, cold polar air flows south behind it. This brings cold outbreaks to North America, Europe and Central Asia.

• Arctic warming debate:
• The Arctic is warming faster than the rest of the world. This is called Arctic amplification.
• So the temperature gap between the pole and the mid-latitudes may get smaller.
• A smaller gap may make the jet weaker and wavier, with more cold outbreaks.
• This is still debated. Observations and models do not yet agree.

Where it occurs

• Both poles: there is one vortex over the Arctic (North Pole) and one over the Antarctic (South Pole).
• February 2021, Texas (USA):
• An SSW about 30 km above the North Pole weakened the vortex [3].
• Dallas fell to −16 °C on 15 February 2021. This was its coldest since 1989. The normal temperature for that time of year is about 15.6 °C [3].
• About 4 million people in Texas lost power [3].

• By midnight on 16 February 2021, 73% of the continental USA was under snow. This was the largest snow cover since the record began in 2003 [3].

• January 2024: another SSW disturbed the polar vortex [2].

• Regions hit by cold outbreaks: North America, Europe and Central Asia in the mid-latitudes, when Arctic air escapes south.
• India: the study note gives no Indian polar vortex event. North India's winter rain and snow come from western disturbances, which are steered by the southern branch of the subtropical westerly jet, not by the polar vortex.

Don't confuse with

• Polar front jet stream: a jet in the troposphere (9–12 km) at about 40–60° latitude, above the polar front. It guides temperate cyclones. The polar vortex is a much larger ring of low pressure around the pole, reaching up into the stratosphere.
• Sudden stratospheric warming (SSW): this is the trigger (a rapid warming of the polar stratosphere). It is not the vortex itself. The SSW weakens or splits the vortex.
• Polar high: the surface high-pressure belt at the poles, made by cold, heavy, sinking air (a textbook pressure belt). The polar vortex is low pressure high up in the air above the pole.
• Arctic Oscillation: a pressure see-saw pattern between the Arctic and the mid-latitudes. It is not a wind system. Its negative phase makes it easier for cold air to escape south.

Prelims Hooks

• The polar vortex is a low-pressure, cold system in the stratosphere and upper troposphere. It is not a surface high-pressure system.
• It is a "circumpolar westerly circulation" that exists between autumn and spring [2]. Its winds blow west to east.
• Process pair: Rossby waves rise and break in the stratosphere → sudden stratospheric warming → the vortex weakens or splits → cold air escapes south [1].
• Event–cause pair: the February 2021 Texas cold wave followed an SSW about 30 km above the North Pole [3]. Do not confuse it with the 2010 Russian heat wave, which was caused by Rossby wave blocking.
• Arctic Oscillation in its negative phase makes cold outbreaks easier. It had been strongly negative since early December 2020, before the Texas freeze [3].
• Trap: "Arctic warming has been proven to make the jet wavier." This is false. The link is still debated.

Mains Points

• Polar vortex disruption as a disaster driver (GS-III):
• Texas in February 2021 had record cold, about 4 million people without power, and record snow cover over the USA [3].
• Power grids and water systems built for a "normal" climate fail in rare extremes.

• Cold-proofing infrastructure, better seasonal forecasting and impact-based early warnings are part of climate adaptation.

• Links within the global circulation (GS-I):

• The vortex is connected to the troposphere below. Rossby waves rising from below can break it down through an SSW.

• Because of this link, one stratospheric event can bring cold spells to North America, Europe and Asia at the same time.

• Climate change link (GS-I/III):

• Arctic amplification may weaken the pole-to-equator temperature contrast. This could make the jet wavier and the vortex easier to disturb.
• An answer should present this as a hypothesis, not a settled fact, and stress the need for long-term observation.

Related concepts

• Jet stream
• Polar front jet stream
• Subtropical westerly jet stream
• Tropical easterly jet stream
• Somali jet
• Rossby waves

Read more

• Jet streams, Rossby waves and the polar vortex