At the beginning of December, the ozone hole over Antarctica was bigger than ever before. It occupied an area of about 18 million square kilometers. It thus towered over the land area of the entire Antarctic (about 14 million square kilometers). It is the most extreme extent for this time of year in the past 41 years. Since then, the atmospheric researchers at the German Aerospace Center (DLR) have been collecting the data. In fact, the ozone hole over the southern hemisphere should have almost disappeared by December. Because summer begins in the Antarctic: with the sunshine on polar day, the pressure and wind conditions normally change, causing the ozone hole to collapse at the beginning of November at the latest.
But not this year, as the scientists at the German Remote Sensing Data Center (DFD)of the DLR. The shape of the ozone hole above the South Pole is determined by a polar vortex that extends from the ground 50 kilometers into the stratosphere. “You can imagine this vortex as a large area of low pressure in the stratosphere,” explains Lisa Küchelbacher from DFD. “A very strong westerly wind at the edge of the polar eddy largely prevents air mass exchange between the equator and the pole in the cold of the polar night.” With increasing warmth, the west wind decreases. Ultimately, the wind conditions reverse and the ozone hole becomes smaller. “The changeover from the west to the east wind system took place very late,” says Lisa Küchelbacher.
Planetary waves too weak
The reason for this is again a weakness of the so-called planetary waves. In the stratosphere, these ensure the exchange of air between the polar regions and the middle latitudes. They make the polar vortex sway and influence the wind. Because of the low activity of the waves, the polar vortex remained circular over the South Pole. The activity of the waves only increased from December 5th, which has now initiated a change to the south polar summer conditions.
What does the Pacific have to do with the ozone hole?
It is possible that a special situation in the Pacific is affecting the planetary waves: the El Niño Southern Oscillation (ENSO) takes place near the equator and affects dynamics around the world every three to seven years. The mechanism is due to a coupling between the ocean and the atmosphere. The surface of the Pacific is especially cold off the west coast of Latin America. That means the ocean provides little energy for the planetary waves. “So it could be that the polar vortex in the southern hemisphere stratosphere was also so stable due to the influence of ENSO. That may have particularly favored the formation of the ozone hole, ”explains Lisa Küchelbacher.
In spring 2020 there was also an exceptionally strong and stable polar vortex in the northern hemisphere: record values were also measured in March . It is still unclear whether there is a connection with the current values over the Antarctic.