Variations In Aerosol Optical Thickness And Its Effects On Climate Over China

The scientific community has attached great importance to aerosol variations and its direct radiative forcing as well as indirect climate forcing, which have been one of the key topics in variation laws of the global climate system. Investigations suggest that the largest forcing uncertainty is that due to the aerosols, which may influence climate in two ways: directly through scattering and absorbing radiation, and indirectly through acting as condensation nuclei for cloud formation or modifying the optical properties and lifetime of clouds. However, the aerosols' characteristics and effects have not been covered thoroughly heretofore. Among the aerosol studies, aerosol optical thickness (AOT), as an important property of the aerosol, is always used to estimating the variations and climate influence of aerosols. Aiming to investigate the aerosol influence on climate change over China, this paper focuses on the AOT characteristics and its climate effects, which are validated by numerical simulations, with major results as follows.1. Based on TOMS monthly AOTs in 1980-2001 a study is made of space/time patterns of AOT 0.50μm thick over China, which are then divided, using combined method of the clustering analysis with statistic tests and the rotating principal component, into 10 regions in China Mainland and 4 regions over the seas for a regional analysis. Results suggest that the long-term mean AOT over China is characterized by typical geography, with higher cores in the Southern Xinjiang Basin, South-China seaboards, and the Sichuan Basin, high cores in the North China to the mid-lower Yangtze (MILY) valleys, and lower cores in the eastern Tibetan Plateau, east Qinghai and Yunnan. Over the seas the values were a lot lower in comparison to those over the land, with the high-valued center in the South-China Sea. AOT has significant seasonality and its seasonal difference is diminished as a function of latitude, with smaller seasonal difference in the oceanic AOT than the land counterpart. Much of the country shows the greatest AOT in spring but the least not in the same season for different areas. Both the regional mean AOTs over land and sea show linear increasing trends, experiencing the similar evolution processes as two stages of greater and one of smaller values. The AOT variation patterns are the distinct increase in the eastern China and the coastal areas where have advanced economy and more effect of human activity, but marginal decrease over western Xinjiang, Yunnan and southeast Qinghai-Tibetan plateau. AOT increases more in the spring and autumn than in the winter and summer, while the spring AOT are most similar to the yearly situation. AOT over land and sea is marked by conspicuous intra-seasonal and -yearly oscillations, with remarkable periods at one-, two-yr and more (as interannual periods).2. 16 climate change indices derived from daily temperature and precipitation data were calculated by observations of 550 stations in China, primarily focusing on the climate change and extreme events. Evidence of indices variations suggests that daily maximum, minimum and mean temperatures are increased on the annual and seasonal basis, with the maximum temperature increasing less than the minimum, resulting in reduced daily temperature duration (DTR). The maximum temperature is elevated more significantly in the north China and not greatly variable in the south, particularly some cooler regions therein. The minimum temperature shows a coherent warming trend all over China. Comparatively, the increases in the maximum and minimum temperatures are more notable in east China. The yearly and seasonal DTR displays a coherent decreasing trend in China, especially in the north or during the winter. The eight temperature indices indicate an interdecadal warming shift during the 1980s, larger for the maximum than minimum temperature. During 1961-2000, the precipitation indices show weak linear increasing trends in the rainfall intensity and frequency as well as the extreme precipitation percentage, and interdecadal shifts in the late 1980s, followed with more extreme rainfall events but no distinct changes in the precipitation intensity/ frequency. For the whole country, the northwest, MILY valleys and its south areas are dominated by more and strengthened extreme precipitations, as opposed to the southern parts of the northwest China, north China and Sichuan basin.3. Using 1980 - 2000 TOMS AOT and climate change indices, study is performed of relations of AOT and indices to analyze the AOT influence on the climate over China. Results show that the influence of aerosol optical thickness on the Climate has remarkable regional and seasonal characteristics. Generally, the rising AOT may induce decreases in the mean, maximum and minimum temperatures, as well as less rainfall and extreme precipitation percentage, but no distinct effects on the precipitation frequency.4. The regional climate model RegCM3 is applied for the AOT influence on the climate response to the total aerosols. Simulations show that the AOT effects may induce lower temperature and less precipitation than no AOT case, but all the change ranges are feeble. The temperature/precipitation may respond to the AOT situation more seriously in the winter/summer half-year. In AOT case, the precipitation could change in more complex distribution, with greater decrease in the Sichuan basin, the MILY valleys and to its south, but slightly increase in southwest and north China.