The Weather Guys

The ozone hole occurs high in the stratosphere, about 18 miles high, and over the continent of Antarctica. It is not actually a hole, but the appearance of very low values of ozone in the stratosphere. Typically, the Antarctic ozone hole has its largest area in early September and lowest values in late September to early October.

The Antarctic ozone hole varies in size each year, from nearly zero in 1980 to an area larger than North America in 2000. The amount of ozone in the atmosphere now is routinely measured from instruments flying on satellites. The size of this year’s ozone hole reached a maximum in mid-September of almost 400,000 square miles, which is about six times larger than Wisconsin. It was a little larger than last year’s ozone hole and about the same as in 2009 and 2010. The amount of ozone likely is reaching its lowest values of the year this week.

The winter atmosphere above Antarctica is very cold. The cold temperatures result in a temperature gradient between the South Pole and the southern hemisphere middle latitudes. These temperature gradients lead to a belt of strong westerly stratospheric winds that encircle the South Pole region. These strong winds prevent the transport of warm equatorial air to the polar latitudes. These extremely cold temperatures inside the strong winds help to form unique types of clouds called Polar Stratospheric Clouds, or PSC.

PSCs begin to form during June, winter time at the South Pole. In the winter, chemical reactions on the surface of the particles composing PSCs result in chemical reactions that remove the chlorine from the atmospheric compounds. When the sun returns to the Antarctic stratosphere in the spring (our fall), sunlight splits the chlorine molecules into highly reactive chlorine atoms and ozone is rapidly depleted. The depletion is so rapid that it has been termed a “hole in the ozone layer.”

As the one-year anniversary of Superstorm Sandy approaches, we have news to report regarding current understanding of that tremendous storm.

One of the major questions confronting atmospheric scientists in the face of that unusual event was whether Sandy was a direct result of a warmer climate. This is a difficult question because there are so many elements that conspire to produce major storms.

One condition that influenced the impact of Sandy was the sea level. It is true that the sea level has risen as the planet warms, and so it is likely that at least some of the flooding associated with Sandy would have been less severe in the absence of climate change.

On the other hand, recent research, conducted using numerical forecast models, has suggested that the intensification of Sandy from the Caribbean to the New Jersey coast most likely was not influenced by climate change.

However, the development of conditions that set up this period of intensification may be linked to climate change — though there is not yet experimental or theoretical evidence that clearly makes this case.

The atmosphere-ocean system is complicated, so questions such as whether events like Sandy are direct consequences of the changing climate are also complicated. The interested citizen, like the interested scientist, may be well advised to be skeptical of such a connection while remaining open to being convinced as the evidence and analysis continues to be considered.