A Study Of Climate Change Impact On Total Ozone Column Over The Northern Hemisphere

Global mean total ozone column(TOC) firstly decreases and then increases over the past decades. However, the ozone trends are asymmetric in different latitudes and at different altitudes. Internal climate variability, climate changes and anthropogenic emissions exert an important influence on long-term ozone trends, which undermines the robustness of ozone recovery in the future. Generally speaking, climate variability and climate changes are closely related with the changes of surface forcing(e.g., sea surface temperature(SST), land temperature and sea ice). This study analyzed the influence of surface forcing in the different latitudes on the TOC in the Northern Hemisphere and its underlying mechanism. The main conclusions are summarized as followings:1. ENSO-type SST in the tropics exerts a significant influence on the mid-latitude ozone in the Northern Hemisphere. El Ni?o events can cause higher TOC values than normal over the North Pacific, southern USA, northeastern Africa, and East Asia, but anomalously low TOC values in central Europe and over the North Atlantic. La Ni?a events have almost the opposite effects on TOC anomalies. ENSO can make a 20-30 % contribution to the interannual variability of the northern mid-latitude TOC, and the contribution can be up to 50 % over the North Pacific Ocean. ENSO events mainly affect the northern mid-latitude ozone through modulating planetary waves structures and changing tropopause heights in the middle latitudes. The anomalous convective activities over the eastern Pacific Ocean can cause planetary wave trains propagating into the upper troposphere in the middle latitudes. The planetary waves include long-wave trains along Pacific-North America continents and short-wave trains along the North African–Asian jet. They can modulate the tropopause height and the local circulations in the upper troposphere and lower stratosphere(UTLS), leading to changes in ozone concentrations and TOC. In addition, El Ni?o(La Ni?a) events will enhance(weaken) the westerly jets over the North Pacific and southern USA, leading to increases(decreases) in Rossby wave breaking frequency to the north of the jets and stronger(weaker) ozone transport by eddy-driven meridional circulation. The above mentioned processes also cause a shift of the maximum TOC and clear-sky ultraviolet(UV) radiation over China associated with ENSO events from South China in winter to North China in summer. Strong ENSO events can lead to 6-10 % changes in clear-sky UV radiation over the middle and low reaches of the Yangtze River during winter and over the northwestern Tibetan Plateau during spring.2. The Cold-Ocean Warm Land(COWL) and North Pacific(NP) teleconnection patterns correspond to the weakening of the Aleutian Low, Iceland Low and Azores High over the past 30 years. The changes of these weather systems are barotropic and are noticeable from the lowermost troposphere to the lower stratosphere. Furthermore, the geopotential heights in the lower stratosphere(70-300 h Pa) cause a zonal asymmetry in TOC trends. The TOC over the North Pacific shows negative trends for the period 1979-2010, while the TOC over the North American continent shows positive trends. Analysis of statistical diagnosis and WACCM3 simulation results further reveal that the SSTs over the North Pacific and North Atlantic Ocean can significantly influence the zonally asymmetric ozone trends in the northern middle latitudes through modulating COWL and NP patterns. This study also found that the upward lifting of tropopause height associated with the surface warming over the Tibetan Plateau and more ozone-poor air transport from the tropics to the UTLS over the Tibetan Plateau lead to the deepening of total ozone low during winter and spring for the period 2000-2009. The Tibetan Plateau warming contributes ~ 50 % to the wintertime TOC decline over the past 30 years.3. In the high latitudes and the Arctic regions, the Arctic sea ice, especially over the Barents-Kara sea, is reduced during the 2000 s as compared with that during the 1980 s. Meanwhile, the snow cover over the Eurasian continent increases over the past decades. They both cause a warming in the middle and lower troposphere over the ocean to the north of the Eurasian continent and lead to more upward baroclinic waves propagation as well as a cold cyclone in the stratosphere over the Eurasian continent. As a result, the Arctic stratospheric vortex shifts towards the Eurasian continent in late winter(February) over the recent decade. The vortex shift also brings more air with active bromide and chlorine to the Eurasian continent and cause more stratosphere ozone depletions, leading to a stronger TOC decline in winter over this region than the other regions at the same latitude.4. The paper finally studied the influence of the Arctic Oscillation(AO) on the TOC and stratospheric ozone over the Northern Hemisphere. The results show that there are three negative centres in stratospheric ozone anomalies in the middle and high latitudes over the Northern Hemisphere during positive AO phases as compared with those during negative AO phases. They are located in the Arctic middle stratosphere and UTLS as well as in the mid-latitude UTLS and are the strongest in winter and spring. Further analysis shows that the negative ozone anomalies in the Arctic middle stratosphere are caused by the weakened meridional transport of the Brewer-Dobson circulation(BDC). The negative ozone anomalies in the Arctic UTLS are caused by the upward lifting of tropopause height, weakened vertical transport of the BDC, weaker exchange between the mid-high latitudes and the Arctic as well as enhanced heterogeneously chemical ozone depletions. On the other hand, the negative ozone anomalies in the mid-latitude UTLS are mainly caused by enhanced eddy transport from middle latitudes to tropics, while the poor-ozone air transport from the Arctic to the middle latitudes makes a minor contribution.