Analysis on Variation Trend of Season Start Dates over the Tibetan Plateau during 1961-2007

The variations in the trends for the start dates of the seasons in the Tibetan Plateau over 47 years (1961-2007) were analyzed using the daily temperature data of 1961-2007 provided by the National Meteorological Information Center, China Meteorological Administration. The results show that spring and summer are starting earlier while autumn and winter are starting later; the changes in autumn and summer were not as significant as those of spring and winter. This trend is most obvious in the 1990s and the beginning of the 21st century as the effect of global warming increases. This trend has different seasonal spatial distributions in the Tibetan Plateau; spring and winter have the same spatial distribution—larger changes in the central region than in the surrounding area summer and autumn are similar but different than spring and winter. Before the 1990s, the start date of spring showed little change, but the trend of earlier start date became obvious from the 1990s onward; the trend of earlier start date of summer is more stable than that of spring but the interannual variability is greater. The trend of the earlier start date of autumn is also more stable than that of spring with greater interannual variability; from the 1990s, the start date of autumn shows a more obvious delay trend than that of winter, while in the early phase of 21st century the delay trend is strong in winter.