Recent Northern Winter Climate Trends due to Ozone Changes and Increased Greenhouse Gas Forcing

Besides the increase in greenhouse gas concentrations and aerosols, considerable trends in ozone concentration were recently shown, with positive values mainly in the upper troposphere, and decreasing concentrations in the lower stratosphere of the Northern Hemisphere middle and high latitudes (WMO, 1992). The data basis consists approximately of 15 to 20 years of observations at various locations. The ozone changes are discussed against the background of observed variations of temperature and circulation during the last decades and are compared with the results of General Circulation Model (GCM) simulations of the effects of the increased greenhouse gas concentration, additional aerosols and the modified ozone profile itself. The observed increased lower stratospheric high latitude westerlies are part of a natural coupled mode of lower stratospheric and tropospheric circulation. This coupled mode has its strongest tropospheric effects over the North Atlantic with increased westerlies, and positive air temperature anomalies over Scandinavia and Siberia. The part of this natural coupled mode which is associated with enhanced polar stratospheric westerlies has increased in intensity during the last decades. A primary contribution to this intensification by the low latitude greenhouse-gas effects is suggested. The observed ozone trends, when integrated into a GCM, do not produce substantial changes in circulation supporting the initial greenhouse gas effects. It is concluded that a comprehensive climate model is needed to study future climate scenarios. This should include atmospheric chemistry in combination with a good representation of the whole stratosphere. It appears most probable that greenhouse-gas effects and ozone concentration are not independent of one another.