Though Earth’s ozone layer has been depleted over the past four decades by chlorofluorocarbons (CFCs) and similar chemical compounds, the changes are expressed differently at the North and South Pole. While a large ozone hole forms consistently every year over Antarctica, the concentration of Arctic ozone is much more variable. The differences occur because the weather patterns are very different.
In the far south, the ice-covered continent of Antarctica is surrounded by an ocean. Winds circle the continent in a potent eddy-like band—a polar vortex—that promotes the formation of very cold air masses and prevents atmospheric mixing with middle latitudes. Ozone depletion is highly dependent on the formation of polar stratospheric clouds, which accumulate chlorine and bromine compounds in the cold polar night and then release these ozone-eaters when the sunlight of spring returns.
The North Pole, however, is an ocean surrounded by land, and that land is irregular in shape and altitude. This leads to more atmospheric waves and uneven wind patterns that mix the air more between middle and high latitudes and between different layers of the atmosphere. This changes the amount of ozone-depleting substances delivered to and from the Arctic, while also making temperatures more variable. And while polar vortices do form in the Arctic, they do not tend to last as long or stay as stationary as their southern counterparts.
The map above shows the concentration of stratospheric ozone over the Arctic—63 to 90 degrees North—on April 1, 2014. Ozone is typically measured in Dobson Units, the number of molecules required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and an air pressure of 1 atmosphere (the pressure at the surface of the Earth). Reaching 470 Dobson Units, April 1 marked the highest average concentration of ozone over the region so far this year. The average amount of ozone in Earthâ€™s atmosphere is 300 Dobson Units, equivalent to a layer 3 millimeters (0.12 inches) thick—the height of 2 pennies stacked together.
The concentration of ozone over the Arctic varies greatly from year-to-year, and ozone holes do not form consistently like they do in Antarctica. In fact, ozone concentrations over the Arctic have been relatively higher the past three winters after an exceptional low in 2011. It is possible that warmer weather over the Arctic this winter, as well as the polar vortices that wandered down to lower latitudes, led to less ozone depletion in the North this winter.
The map was assembled from observations made by the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite. OMI is a spectrometer that measures the amount of sunlight scattered by Earth’s atmosphere and surface, allowing scientists to assess how much ozone is present at various altitudes, particularly the stratosphere.
NASA animation by Robert Simmon, using imagery from the Ozone Hole Watch. Caption by Mike Carlowicz.