News – Institute for Quantum Physics of the Atmosphere

At the beginning of December 2020, the ozone hole over the South Pole was bigger than ever at this time of the year. It extended to an area of ​​about 18 million square kilometers and thus clearly towered over the land area of ​​the entire Antarctic (about 14 million square kilometers). It is the largest expansion for this time of year in the last 41 years that the data has collected. The ozone hole over the southern hemisphere should have almost disappeared by December. In the beginning of summer in the Antarctic and the sunshine (polar day) that follows, the pressure and wind conditions normally change, which should have made the ozone hole collapse at the beginning of November latest. This year, however, as researchers discovered, the opposite has been the case.

The extent of the ozone hole over the Antarctic is determined by a polar vortex that extends from the ground 50 kilometers into the stratosphere. This vortex is like a large depression in the stratosphere. In the cold of the polar night, a very strong wind from the West at the edge of the polar vortex largely prevents the exchange of air mass between the equator and the pole. When the sun slowly rises at the South Pole in spring and supplies energy, ozone is broken down in the polar vortex by a chemical reaction. With increasing warmth, the westerly wind weakens. Ultimately, the wind conditions are reversed and the ozone hole becomes smaller. This reversal of the wind conditions took place very late this time of the year. The cause was possibly an unusually strong formation of the polar vortex in the southern hemisphere.

This in return was triggered by the fact that the so-called planetary waves were too weak. In the stratosphere, these planetary waves ensure the exchange of air between the polar regions and the mid-latitudes. They make the polar vortex sway and influence the wind. Because of the low activity of the waves, the polar eddy remained circular over the South Pole. It was not until December 5 that the activity of the waves increased and ushered a change to the south polar summer conditions.

Effects of the situation in the Pacific

A special situation in the Pacific may have an impact on the planetary waves: the El Nino Southern Oscillation (ENSO) takes place near the equator and affects the dynamics around the world every three to seven years. This is based on a coupling between the ocean and the atmosphere. The surface of the Pacific particularly cold off the West Coast of Latin America. Therefore, the ocean provides little energy for the planetary waves. It could be that the polar vortex in the stratosphere over the southern hemisphere was so stable due to the influence of ENSO and thus particularly favored the formation of the ozone hole.

In spring 2020 there was also an exceptionally strong and stable polar vortex in the northern hemisphere. However, it is still unclear whether there is a connection between the current values ​​over the Antarctic.