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Conclusions

Five different indicators, that in different ways describe and characterise the Arctic vortex, have been studied at the 475 K isentropic level. Monthly means of these indicators have been calculated for the months January through April for the years 1980 to 2000. The aim of this study has been to determine if any of these monthly means shows a significant trend during this 21-year period. By a significant trend we here mean a trend that has a larger absolute value than twice its standard deviation. It can be seen from Table 1 that none of these five indicators exhibit any significant trend for the months of January and February. For the month of March, however, all five indicators show trends that are above the 2 slevel. For April only four of the parameters could be studied because the temperatures have never been below the NAT limit in April during the time period in question. Of the four remaining indicators there are three that show significant trends (minimum temperature, mean PV and vortex area), whereas the vortex strength indicator has a trend that is well above the 1 slevel.

From this study it can not be concluded that the Arctic vortex has undergone any significant change during January and February. It is clear however that the vortex in March has become significantly colder and stronger. To a large extent the same also applies to the month of April. It also looks as if the interannual variability for several of the indicators has increased during the last years. The very cold winters of 1995-96 and 1999-00 together with the very mild winters of 1997-98 and 1998-99 are the main reason for this. Our results are in line with the work of Zhou et al. (2000), who showed that the vortex in the northern hemisphere has lasted 4 weeks longer in the 1990s than in the early 1980s.

In the Antarctic only one parameter has been studied, namely the vortex area. Six levels from 400 to 550 K have been studied. For the month of September there is no significant trend for this parameter, except for the 450 K level, where there has been a small increase of 0.4% per year. At the 550 K level there has been a decrease of 0.4% per year. For the month of October there is also no large change, but at 450 K and 500 K there is a significant increase of 1.0 and 0.8% per year, respectively. For the months of November and December there has been a significant increase in the vortex area at all 6 levels. For both these months and for all the six levels the trends are significant to better than 3 s, in many cases better than 4 s.

It is a common feature to both the Arctic and Antarctic that there is a significant trend towards an increasing vortex in late winter, at the time when the vortex is about to break up. It is quite likely that this trend is caused by the destruction of ozone, which leads to a colder vortex that can exist further into the spring. Large vortex areas are found in years with substantial ozone depletion, such as 1995, 96 and 97 in the Arctic and 1987, 93 and 98 in the Antarctic. The results from Antarctica must be considered as preliminary as there was no data from 1994 included in this study. It is unlikely, however, that the inclusion of 1994 will change the general result, namely that November and December have experienced a significant increase in the size of the vortex.