In week 10 Mike Blackburn will be writing a blog on early analysis on what was an interesting season in the troposphere. In this blog I’ll give a quick update on the northern hemisphere stratospheric winter and its implications for ozone as we enter spring. Unlike recent stratospheric winters, with dramatic mid-winter major stratospheric sudden warmings, this winter was a rather placid affair with a strong, cold polar vortex dominating the flow throughout the season. The vortex has been particularly anomalous throughout February and into March One sign of this were the very cold temperatures at 50hPa throughout winter 2010/11:
A cold polar vortex indicates a lack of dynamical activity in the stratosphere with the usual vacillations of polar temperatures and jet strength, caused by the interaction of the polar vortex with tropospherically generated planetary waves absent. Recent work has also suggested that an anomalously strong polar vortex can influence the tropospheric jet to be further poleward and stronger than normal. These tropsopheric weather patterns are associated with a positive anomaly in the Arctic Oscillation index (AOI), which measures the pressure gradient between the polar cap and the mid-latitudes. A plausible case could be made for the very strong polar vortex in late winter to have played a role in the shift from the very negative AOI values of early winter which were associated with colder weather over North America and Europe toward the more positive values in late winter, giving us a very mild February.
A cold late winter in the Arctic stratosphere is a warning sign for atmospheric chemists, since cold conditions are the foundation of larger than normal ozone depletion in Arctic spring. The reasons for this dependence on temperature starts with the formation of polar stratospheric clouds. The stratosphere is extremely dry, but when conditions are cold enough at the poles it is possible for clouds to form from either pure water ice or a combination of water and nitric or sulphuric acid particles. Some observations of polar stratospheric clouds from the Calipso lidar are shown here.
The stratospheric temperature trace above shows the thresholds for the formation of two different types of polar stratospheric clouds, and the mean polar temperature remaining below one of these thresholds. Polar stratospheric clouds are important for polar ozone depletion because chemical reactions on their surface allow chlorine compounds to be liberated from inactive reservoir compounds which do not react with ozone to so called ClOx compounds which do. The key ingredients for enhanced polar ozone depletion are cold temperatures which allow polar stratospheric clouds to form and sunlight to provide energy input for the chemical reactions which destroy ozone, hence the formation of the largest ozone holes in stratospheric spring.
The Arctic stratospheric spring, unlike the Antarctic has a great deal of year-to-year variability. This means that in some years with colder temperatures there is significant chemical ozone depletion while in other years with warmer temperatures there isn’t. There is a good empirical relationship between the area of polar stratospheric clouds in the Arctic, as inferred from temperature observations and the amount of ozone depletion in the Arctic (as described in this study).
In general this means a much weaker ozone ‘hole’ in the northern hemisphere compared to the southern hemisphere, although in some years Arctic ozone depletion can be significant. It already looks like the spring of 2011 might be shaping up to be one of those years. Assimilated ozone observations from the MLS satellite show significant regions of ClO concentrations and depleted ozone, look particularly at the 490K plots (follow this link to produce similar plots for previous years).