2010 ozone hole smaller than usual
The destruction of ozone in the ozone layer over Antarctica this year is about 40 to 60% less compared to the previous five years, according to MACC analyses based on observations from the SCIAMACHY instrument on board the European ENVISAT satellite. Less ozone destruction is consistent with the expectation that the ozone layer will recover during the coming decades due to reduction in the stratospheric amount of chlorine. However, such a large change cannot be attributed to the slow decrease of stratospheric chlorine of 0.5-1% per year. This year’s reduced ozone destruction turns out to be caused by unusual meteorological conditions.
Sudden Stratospheric Warming
In July and August a phenomenon known as a Sudden Stratospheric Warming occurred in the stratosphere above Antarctica: a sudden fast warming in the ozone layer. The stratosphere is the atmospheric layer between 15 and 35 km altitude which contains large amounts of ozone that protects us from harmful ultraviolet radiation. The exact mechanisms of these of warmings are still not fully understood, but they happen quite often and have their origin in the troposphere, the lowest layer of the atmosphere where our weather occurs.
Effect of nitrogen-oxides on chlorine release
The ozone hole appears because of the destructive power of chlorine which is present in the ozone layer but under normal conditions chemically bonded with nitrogen-oxides. However, via complex chemical processes the chlorine can be released during the polar winter above Antarctica. At temperatures below -78°C the nitrogen-oxides are removed from the stratosphere. The remaining chlorine can no longer be bonded and when the sun becomes stronger during spring the chlorine can start to do its damaging work.
Although the temperature in the Antarctic stratosphere sinks below -78°C every July and August, it does not get much colder. A small temperature rise of only a few degrees thus can result in considerably more nitrogen-oxides remaining in the stratosphere, and thus less effective ozone destruction. Recent measurements of the American MLS instrument on the AURA satellite indeed show that there are still nitrogen-oxides present in the stratosphere above Antarctica, unlike previous years.
Late onset of ozone desctruction in 2010
This year’s ozone destruction consequently only started to become visible during September, whereas under normal conditions the first signs already become visible about halfway through August. The observations by SCIAMACHY show that the amount of ozone decreased steadily towards October, but the destruction continued to lag behind that of previous five years. At the beginning of October the amount of ozone destruction was still 40-60% less than in a typical year. In the second half of October the ozone loss became more comparable to previous years. Ozone destruction usually ceases in October after which the ozone hole starts to fill up.
Not the first time
Studying historical satellite observations shows that Sudden Stratospheric Warmings occur quite often. In 2002 there was a particularly large warming that caused the ozone hole to split and disappear already in September. In 1988 there was a sudden stratospheric warming in July and August comparable to this year’s, and also then the first signs of ozone destruction became visible only during September. Small temperature changes the Antarctic stratosphere in July and August of only a few degrees can still have a large impact on the formation of the ozone hole. Such events very much complicate efforts to determine if the expected ozone hole recovery has started.
More information can be found at the MACC ozone hole monitoring pages.