Will spring 2005 be a bad one for Arctic ozone? Le printemps 2005 comptera-t-il parmi les mauvais pour l’ozone arctique ?

The other key factor is that even if chemical conversion into reactive forms occurs during the cold, dark polar winter, the reactive chlorine must stick around until sunlight returns to the polar region for ozone destruction to take place. This is why ozone depletion over the poles is a springtime phenomenon. Even following a very cold winter, if temperatures warm quickly during spring very little ozone loss may take place. Alternatively, a milder winter, provided it was still cold enough to lead to chemical processing of halogens, could be followed by greater springtime ozone losses if temperatures stayed cold longer. Thus temperatures during the period when a lot of sunlight first returns to the polar areas following winter, March in the Arctic and September in the Antarctic, are crucial.

This year has seen an exceptionally strong polar vortex over the Arctic (see the red line in the figure). Undoubtedly, chemical processing of halogens into reactive forms has taken place and the Arctic is primed for ozone depletion. Now that we’re in March, sufficient sunlight is available to cause sizeable ozone losses. Should cold temperature persist for another couple weeks, ozone depletion could reach record levels for the Arctic. While the vortex was weakened and pushed to the side of the Arctic during the last week of February, temperatures below the critical freezing point are still present as of March 9 (see figure). The displacement off the pole also pushes the colder air into latitudes with more sunlight, enhancing ozone depletion in the short term. Ozone measurements from the first week of March already show a region over the North Atlantic with very low ozone levels (<250 Dobson units, versus minimum values of ~300 in the early 1980s).

There is much debate over whether this has anything to do with climate change. Some climate models suggest that increasing greenhouse gases may be leading to a gradual strengthening of the Arctic vortex and hence increasing ozone losses, while others do not. Observations show that the vortex was typically more stable in the 1990s than during the 1980s, but the present decade has been mixed thus far. Temperature during the winter as a whole have generally decreased over the past two decades, likely as a result of climate change, but the sensitivity of ozone loss to the exact timing of March warming events makes ozone depletion a much more variable quantity. With only one winter vortex per year, it will take many more years to determine if the exceptionally cold 2004-2005 winter is part of a trend or simply a single cold event. Prepare to see a lot of press coverage if this ends up being a big year though….

Update: Indeed it was.

Min Arctic Strat Temp 50N-90N 2004/2005(conditions actuels)

Tout d’abord, passons en revue certains fondamentaux. La recette pour une perte massive d’ozone au printemps dans les régions polaires comme le trou annuel d’ozone au dessus de l’Antarctique au cours des deux dernières décennies est très simple. Les deux ingrédients principaux sont des gaz halogènes réactifs comme le chlore ou le brome, et de la lumière solaire. Pour la préparation, gardez les halogènes à des températures extrêmement basses, typiquement au-dessous de –78°C (195 °K). Utilisez un vortex polaire puissant pour mélanger les halogènes afin d’atteindre plus facilement la température requise. Quand le mélange a été correctement refroidi, ajoutez la lumière solaire, et vous obtiendrez rapidement la destruction de l’ozone.

Dans le monde réel, le chlore et le brome sont tous les deux facilement disponible partout dans la stratosphère en particulier le chlore à partir des chlorofluorocarbures qui sont bien répartis dans la basse atmosphère (où ils sont stables), avant d’entrer dans la stratosphère où ils sont décomposés photochimiquement.

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