Seasonal trends of climate change in the Yangtze Delta and its adjacent regions and their formation mechanisms

Summary Based on the monthly mean values of 10 meteorological elements from 53 surface weather stations in the Yangtze Delta and its adjacent areas during 1961–1997, the part played by linear climate trends has been analyzed to explore the characteristics of the regional climate change. The results show that the annual air temperature, and maximum and minimum air temperatures, have all increased rapidly in the Yangtze Delta, but there has been a reverse trend in its adjacent areas, i.e., the air temperatures have gradually decreased. Thus the Yangtze Delta emerges as a regional heat island in relation to its adjacent areas. The regional heat island consists of several urban heat sub-islands in southern Jiangsu Province and northern Zhejiang Province, including Shanghai, Wuxi, Changshu, Nanjing, Hangzhou and Ningbo. It appears most obvious in winter and least obvious in summer. An intensity of the regional heat island is defined, and it is shown that the variation of the heat island is positively correlated to the economic development of the region, and is assumed to be related with the increased consumption of energy due to economic development. The results of a climatological analysis of the other annual meteorological elements have indicated the tendency that the soil temperature of the Yangtze Delta has a weak cooling process, its precipitation has evidently increased, sunshine duration and visibility have apparently decreased, and the trend centers appear mainly in the cities. All these phenomena imply that air pollution and aerosols increase with economic development, leading to a cooling mechanism, which in turn suppresses the enhancement of the heat island caused by increasing consumption of energy. On the other hand, an intensified heat island also makes convection and precipitation increase. For the adjacent areas whose economic development is less intense than the Yangtze Delta, a weak cooling appeared there because their heat island caused by increasing consumption of energy resources could not offset the cooling due to aerosols. It can be seen from satellite images that the increase of aerosols in the Yangtze Delta is more obvious than its adjacent areas. By use of a three-region nested mesoscale model, a series of simulations are made with no-forcing and forcing of the optical depth of aerosol and the heating due to energy consumption by human activities. The results of the analysis and the simulation coincide very well. The simulations support the above-mentioned mechanisms of the regional heat island in the Yangtze Delta of China.