Climate mechanism for smallest Antarctic ozone hole

The reason for this year's small hole in the ozone layer is that the stratosphere gets warmer which has complex reasons. According to preliminary NASA's data, there were more sudden stratosphere warming (SSW) events in the southern hemisphere from July to September this year (Fig. 1). Stochastic warming in the stratosphere is a sudden change in the large-scale atmospheric circulation structure in the stratosphere.  It was first discovered by Scherhag, a German scientist. When the SSW event occurs, the stratospheric temperature in the polar region suddenly increases; meanwhile, the stratospheric circulation associated with temperature change also drastically adjusts. In addition, the zonal average wind field reverses. The polar vortex is strongly disturbed and deviates from the polar region, even completely collapsing. The yellow area in Figure 1 represents the occurrence of SSW events from July to September in 2017.

The cause of the SSW events may be the significant heat exchange in the southern hemisphere (Fig. 2) and the large amount of planetary wave flux (Fig. 3). The red line in Figure 2 shows the heat flux data in 2017. The negative value in the figure indicates that the heat flux is transmitted to the south. It can be seen that the peak heat flux to the south in 2017 occurred earlier than previous years (blue and black lines), so did the component of the planetary wave to the south, which provided momentum conditions for heat transfer.

Figure 1. Zonal Temperature Anomaly Time Series [-90, -60] of JAS in 2017 (Source: NOAA).

Figure 2. Heat Flux[-75, -45] of 100hPa (Source: NASA).

Figure 3. Heat Flux (Wave 1-3) [-75, -45] of 100hPa (Source: NASA).

It is noteworthy that the mechanism of this year's stratospheric warming is not significantly related to the global warming which however is one of the factors affecting the ozone depletion. The relationship between global warming and the ozone depletion will be covered in the next blog.

News: Smallest ozone hole in 2017

The ozone hole over Antarctica is very small this year.A recent report by NASA and NOAA shows that this year's ozone hole area has been the smallest since 1988  Fig. 1). According to NASA's data earlier of this month, the Antarctic ozone hole, which is nearly 1 million square miles smaller than that of 2016 (Fig. 2), will continue to shrink. However,    according to Dr Matt Tully, Director of the Bureau of Meteorology for the International Ozone Committee, this does not mean that the hole will soon disappear. Scientists think it is mostly caused by natural reasons rather than the human action. NASA regards the smaller ozone hole this year as a factor in natural change, not "a signal of rapid healing."

Figure 1. Maximum of daily ozone hole area (Source: NASA).

Figure 2. Antarctic Total Ozone of 2016 (left) and 2017 (right) (Source: NASA).

NASA announced that the reason for the smaller ozone hole this year was resulted from the warmer vortex over the Antarctic. The Antarctic vortex is a low-lying, low-pressure region rotating clockwise over the Antarctic. However, since the depletion of ozone occurs in cold environment (for a reference to previous blogs), the ozone hole reaches its annual maximum in September or October at the end of the southern hemisphere winter. This year, however, the system is warmer and more unstable than usual and contributes to reducing clouds in the lower stratosphere which is formed during the Antarctic winter. When the temperature drops below -78 degrees, nitric acid or sulfuric acid particles adsorb water droplets to form larger particles, resulting in large-scale polar stratospheric clouds. These clouds store ozone-depleting substances such as chlorine and bromine (Rowland, 1996). When Antarctic spring comes, the ozone depletion in these clouds will consume a large amount of ozone under UV catalysis. Thus, the warmer stratosphere this year weakened the formation of stratospheric clouds and, as a result, reduced ozone depletion.

Scientists expect that around 2070, the Antarctic ozone hole may recover to 1980 levels .