The Identification Of Four Seasons In China And Its Relationship With Summer Rainfall Under Global Warming

Season division, which has significant sense to agricultural production and the medium-and long-term weather and climate prediction, is one of the most important issues in the study of synoptic climatology. Under the background of global warming, the raising of air temperature has changed the inherent properties of climate system, which will subsequently affect the natural seasons. Thus, it is necessary to investigate the variations of four seasons under global wanning in order to offer the guidance to agricultural production as well as weather and climate prediction. Base on the above understandings, this paper performs three works. First, we propose two new methods, which named similarity method and non-linear similarity method, to distinguish the four seasons in the current climate. Compared to the existing methods, these two methods use multi-factors to divide season, which makes them more objective, and the division results of the seasons are close to the changes of actual atmospheric circulations and atmospheric activity centers. Moreover, the division results of the new methods could also reveal the influences of the differences of local climate systems and topography on the seasons. Second, based on the752stations’daily observational data of temperature, pressure, relative humidity and precipitation in China from1961to2008, and the non-linear similarity method, we further study the spatial-temporal characteristics of four seasons in China under global warming. It is found that the length of summer has extended and the length of winter has shortened in the majority of the country. In particular, the length of summer, autumn, spring has increased2.8,4.7,2.6days respectively since the1960s. However, the length of winter has shortened by5.6days since the abrupt climate change in mid-1980s. These changes in the lengths of seasons are linked to the changes in the starting dates of seasons. Averaged across the whole country, the starting date of autumn and winter has been delayed while spring and summer in China has started earlier; this phenomenon is especially obvious since1980s. On the other hand, the changes exhibit apparent seasonal and regional differences under the background of global warming. The most significant changes occurred in winter, with summer, autumn and spring to be less. They are more significant in the north than the south where the trend of local changes in Heilongjiang, Xinjiang provinces and south China are the most significant. Third, the relationship between the onset date of preceding winter and the following summer precipitation in eastern China is also investigated by using NCEP/NCAR reanalysis datasets. Results indicate that there are two positive correlation bands and two negative correlation bands. The four bands are corresponding with the main rainbelts in eastern China in summer. The positive ones are located in South China, Southwest China, the region north of the Yellow River with latitudes between39°N and42°N, and central and southern Northeast China. The negative ones are in Huanghuai, Jianghuai, the middle and lower reaches of the Yangtze River, and most regions north of latitude42°N. The statistical results show that the frequency of rainfall pattern Ⅱ is larger than that of the climate state, those for both rainfall patterns I and III are less than that of the climate state when earlier preceding winters come. While for later winters, the pattern I is larger than that of the climate state, those for both rainfall patterns Ⅱ and Ⅲ are much smaller than that of the climate state. The rainbelt outcomes from the statistics and composition analysis in the typical years are consistent with each other. The statistical results show that the later preceding winters are prime when it is pattern I, earlier winters for pattern Ⅱ, and there is not significant relationship between the winter starting dates and pattern Ⅲ. In a word, the patterns of precipitation are closely related to the starting dates of the seasons. The different starting dates of winters are corresponding to different air-sea interactions which result in different patterns of precipitation.