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WMO Antarctic Ozone Bulletin no. 4, 2015
The area of the region where total ozone is less than 220 DU, the so-called “ozone hole area”, averaged over the 30 worst consecutive days has reached 26.9 million square kilometres according to data from NASA. This places 2015 as the third largest ozone hole on record according to this criterion. One has to go back to 2006 and to 2000 to find a larger ozone hole area for this time period. A stable and large vortex, concentric around the south pole and characterised by low temperatures explains why 2015 experiences the largest ozone hole since 2006. Temperature forecasts from the European Centre for Medium Range Weather Forecasts (ECMWF) show that stratospheric temperature will rise around 5 K over the next week but still remain relatively low for the season. Forecasts of potential vorticity show that the vortex will remain stable and centred around the South Pole but it will stretch out towards the Atlantic sector and the vortex strength will decline. In late September and early October, a time of the year when temperatures normally increase after the polar night, the minimum temperature inside the vortex suddenly dropped and on 3 October in reached 181.9 K, which is about 8 K below the 1979-2014 average. After that the minimum temperature has increased but is still close to the long term (1979-2014)minimum at 50 hPa. The 60-90°S mean temperature at 50 hPa remained well below the long-term mean in September and was close to the long-term minimum on some days. On almost every day during October the mean temperatures at 50 and 100 hPa have been below the long term minimum. On 5th August the NAT area reached a maximum for the season with 28.2 million km2, which is higher than the maximum reached in recent years. One has to go back to 2009 to find a higher PSC area maximum (28.4 million km2). Also in September and so far in October, the NAT area has been well above the long-term mean. Since mid October, the NAT area has oscillated around the long term maximum for this time of the year. It is forecast to drop to zero in early November. During August the NAT volume was similar to that of recent years (2011, 2013 and 2014) and superior to that of 2012. In September, the NAT volume has been close to the long-term mean. Since mid October the NAT volume has oscillated around the long term maximum. In early September the heat flux remained low and on some days early in the month the heat flux was lower than the long-term minimum. During September the heat flux increased somewhat but remained comparatively low. During October the heat flux has been smaller than in 90% of earlier years back to 1979. This is a sign of a stable vortex. However, a wave event is expected in early November and this will lead to a weakening of the vortex and heating of the stratosphere. The whole vortex is now filled with hydrochloric acid (HCl) and essentially all the active chlorine has disappeared. This means that ozone destruction has come to a halt. Satellite observations show that the area where total ozone is less than 220 DU (“ozone hole area”) has been significantly above zero since 18 August. This is a relatively late onset of ozone depletion. During September ozone depletion picked up, and the ozone hole area reached 26.9 million km2 on 2 October according to the analysis from KNMI, which is based on data from the GOME-2 satellite instrument. Analysis by NASA, based on data from the OMI satellite instrument gives a maximum ozone hole area for 2015 of 28.2 million km2, also on 2 October. This are the largest maxima reached since 2006. Averaging the ozone hole area over various time periods also shows that the 2015 ozone hole is one of the largest on record. In 2015, the vortex has been relatively stable and concentric around the South Pole. This can explain the late onset of ozone depletion. The vortex area has been comparatively large, and this can explain the relatively large area of the ozone hole. Measurements with ground based instruments and with balloon sondes show clear signs of ozone depletion at all Antarctic sites and several stations have in late October measured ozone values near historical minima. In this issue data are reported from the following stations: Arrival Heights, Belgrano, Davis, Dôme Concordia, Dumont d’Urville, Halley, Kerguelen, Macquarie Island, Marambio, Mirny, Neumayer, Novolazarevskaya, Río Gallegos, Rothera, San Martin, South Pole, Syowa, Ushuaia, Vernadsky, Vostok and Zhongshan. Ozonesonde data from several stations confirm that ozone depletion has stopped, but the data also show that ozone remains depleted in the 12-20 km altitude range. WMO and the scientific community will use ozone observations from the ground, from balloons and from satellites together with meteorological data to keep a close eye on the development during the coming weeks.