Abnormal weather patterns in the upper atmosphere over Antarctica dramatically limited ozone depletion in September and October 2019, resulting in the smallest ozone hole since 1982, NASA and NOAA scientists reported.
The annual ozone hole reached its peak extent of 16.4 million square kilometers (6.3 million square miles) on September 8, 2019, and then shrank to less than 10 million square kilometers (3.9 million square miles) for the remainder of September and October. During years with normal weather conditions, the ozone hole typically grows to a maximum area of about 20 million square kilometers.
“It’s great news for ozone in the Southern Hemisphere,” said Paul Newman, chief scientist for Earth Sciences at NASA’s Goddard Space Flight Center. “But it’s important to recognize that what we’re seeing this year is due to warmer stratospheric temperatures. It’s not a sign that atmospheric ozone is suddenly on a fast track to recovery.”
NASA and NOAA monitor the ozone hole via complementary instrumental methods. NASA’s Aura satellite, the NASA-NOAA Suomi NPP satellite, and NOAA’s JPSS NOAA-20 satellite all measure ozone from space. Aura satellite’s Microwave Limb Sounder also estimates levels of ozone-destroying chlorine.
At the South Pole, NOAA staff launch weather balloons carrying ozone-measuring sondes, which directly sample ozone levels vertically through the atmosphere. Most years, at least some levels of the stratosphere are found to be completely devoid of ozone.
“This year, ozonesonde measurements at the South Pole did not show any portions of the atmosphere where ozone was completely depleted,” said atmospheric scientist Bryan Johnson at NOAA’s Earth System Research Laboratory.
This is the third time in the past forty years that weather systems have caused warm temperatures that limit ozone depletion, said NASA Goddard atmospheric scientist Susan Strahan. Similar weather patterns in the Antarctic stratosphere produced small ozone holes in September 1988 and 2002.
“It’s a rare event that we’re still trying to understand,” said Strahan. “If the warming hadn’t happened, we’d likely be looking at a much more typical ozone hole.” There is no identified connection between the occurrence of these unique patterns and changes in climate.
The weather systems that disrupted the 2019 ozone hole are typically modest in September, but this year they were unusually strong. At an altitude of about 20 kilometers (12 miles), temperatures during September 2019 were 16 degrees Celsius (29 degrees Fahrenheit) warmer than average—the warmest in the 40-year historical record for September by a wide margin. In addition, these weather systems weakened the Antarctic polar vortex, knocking it off its normal center over the South Pole and reducing the strong September jet stream around Antarctica from a mean speed of 260 kilometers (160 miles) per hour to a speed of 110 kilometers (70 miles) per hour. This slowing vortex rotation allowed air to sink in the lower stratosphere, where it had two impacts.
The sinking air warmed the Antarctic lower stratosphere, minimizing the formation and persistence of polar stratospheric clouds, a main ingredient for the ozone-destroying process. Second, the strong weather systems brought ozone-rich air from elsewhere in the Southern Hemisphere to the area above the Antarctic ozone hole.
As of October 16, the ozone hole remained small but stable. It is expected to gradually dissipate in the coming weeks.
NASA Earth Observatory image by Joshua Stevens, using data courtesy of NASA Ozone Watch. Story by Ellen Gray, NASA Earth Science News Team.