Table of Contents
- In the previous posts, we have studied about tropical cyclones and extra tropical cyclones (Temperate Cyclone). Here we will study Polar vortex (circumpolar vortex) which is a polar cyclone.
- A polar vortex is a large pocket of very cold air, typically the coldest air in the Northern Hemisphere, which sits over the polar region during the winter season.
- Polar Vortex is a
- Upper tropospheric: sometime extending till the lower levels of stratosphere (At poles, the troposphere extends up to 8-9 km);
- Low pressure;
- Large cyclonic parcel of air [1000 km] (counter-clockwise in the Northern Hemisphere)
- Polar vortex is closely associated with jet streams [Rossby waves].
- It is formed mainly in winter and gets weaker in summer.
- It surrounds polar highs and lie within the polar front (boundary separating the temperate and polar air masses).
Polar Vortex Cold Wave
How Polar Vortex slips towards Midlatitudes,
Breakdown of the polar vortex,
Sudden stratospheric warming,
Polar vortex event.
All the above phrases mean the same – Polar Vortex Cold Wave.
- The polar vortex will remain in its place when the Westerlies along with the polar jet are strong (Strong polar vortex means there is huge temperature contrast between the temperate and polar regions).
- When the polar vortex is weak, it intrudes into the midlatitude regions by buckling the general wind flow pattern. This leads to significant cold outbreaks in the midlatitude regions.
- The vortex is capable of delivering subzero temperatures to the United States and Canada where is occurs the most.
How it slips
- The Polar jet traverses somewhere over 650 N and S latitudes. When the temperature contrast between polar and temperate regions is maximum, the jet is very strong and the meandering is negligible.
- But when the temperature contrast is low (doesn’t mean it’s summer), the jet starts to meander (Rossby waves).
- Meandering jet creates alternating low and high pressure cells. High pressure cells are created below the ridges and the low pressure cells below the troughs (This is because of the upper air circulations created by the jet).
- With severe meandering, the high pressure cells push over to north and displace the polar cyclone from its normal position i.e. the cyclone moves away from the pole and slips into the temperate regions where there is an intense low pressure.
- With the strengthening of the jet, the high pressure cells become weak and retreat to their normal latitudinal positions. With the retreat of the high pressure cells, the polar cyclone moves back to its normal position – poles.
Ozone Hole [Ozone Depletion at South Pole]
- Polar vortex and ozone depletion are two distinct but related phenomena.
- There is a steady decline of about 4% in the total volume of ozone in Earth’s stratosphere.
- Much larger decrease in stratospheric ozone is observed around Earth’s polar regions.
- Depletion of ozone is due to increase in halocarbons in the atmosphere.
Halocarbon == a compound in which the hydrogen of a hydrocarbon is replaced by halogens like chlorine, bromine, iodine etc.
Halogen == group of reactive non-metallic elements like fluorine, chlorine, bromine, iodine, and astatine.
Halogen atoms like chlorine destroy ozone
- Photodissociation (under the influence of sunlight) of ozone-depleting substances (ODS) like halocarbon refrigerants, solvents, propellants, and foam-blowing agents (CFCs, HCFCs, carbon tetrachloride and trichloroethane, freons, halons) creates free chlorine atoms that destroy ozone.
But how does a chlorine atom reach to such high levels of atmosphere?
Polar Stratospheric Clouds (PSCs)
- Extend from 12 km – 22 km above the surface.
- They are nacreous clouds.
- Polar Stratospheric Clouds or nacreous clouds contain water, nitric acid and/or sulfuric acid.
- They are formed mainly during the event of polar vertex in winter; more intense at south pole.
- The Cl-catalyzed ozone depletion is dramatically enhanced in the presence of polar stratospheric clouds (PSCs) [Finally this how polar vortex leads to ozone depletion]
- Polar Stratospheric Clouds convert “reservoir” compounds into reactive free radicals (Cl and ClO).
- These free radicals deplete ozone as shown in the animation below.
- So Polar Stratospheric Clouds accelerate ozone depletion.
Prelims question: The formation of ozone hole in the Antarctic region has been a cause of concern. What could be the reason for ozone depletion at poles?
- Presence of prominent tropospheric turbulence; and inflow of chlorofluorocarbons
- Presence of prominent polar front and stratospheric Clouds and inflow of chlorofluorocarbons
- Absence of polar front and stratospheric clouds; and inflow of methane and chlorofluorocarbons
- Increased temperature at polar region due to global warming
Primary References: Encyclopedia Britannica