Decadal Variation of Summer Dryness/Wetness over the Middle and Lower Reaches of the Yangtze River and the Evolution of Atmospheric Circulation in the Last 52 Years

Monthly precipitation records at 353 stations in the middle and lower reaches of the Yangtze River during 1961-2012, provided by the National Climate Center of the China Meteorological Administration, were used to calculate the Standardized Precipitation Index in summer. The Mann-Kendall (MK) statistical test was applied to detect the trend in the number of stations at which summer SPI reached a value of -1.0 or less. According to the MK test results, the whole time series could be divided into three stages, separated by abrupt change points. Furthermore, based on National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data and National Oceanic and Atmospheric Administration extended reconstructed sea surface temperature (SST) data, the evolution of the large-scale atmospheric circulation was analyzed from pre-winter to summer, and a conceptual model established for each stage. The results showed that: (1) the first stage (1961-1973) was in an obvious state of drought, the second stage (1974-1986) was a dry-to-wet transition phase, and the third stage (1987-2012) was basically wet; (2) the atmospheric circulation changed notably in the second stage, leading to an opposite phase of the circulation anomaly between the first and third stages; and (3) global SST remained abnormally cold from pre-winter to summer, and Indian Ocean SST was especially colder in summer, which led to a dramatically weaker South Asian high and western Pacific subtropical high. The high pressure ridge over the northern Tibetan Plateau was weaker, resulting in weaker high pressure over Mongolia in pre-winter. Low pressure over India was stronger, the southern branch of the trough deepened, and southerly moisture transported by the summer monsoon prevailed over eastern China. Meanwhile, there was straight westerly wind over the mid-high latitudes of Asia, which was unfavorable for the cold air to extend into southern China. Under the above evolution and configuration of the atmospheric circulation, the middle and lower reaches of the Yangtze River were prone to widespread drought in summer during the first stage. However, the circulation from pre-winter to summer during the third stage was opposite to that during the first stage. Global SST was warmer from pre-winter to summer, especially in the Indian Ocean, resulting in a stronger western Pacific subtropical high. The stronger high pressure ridge over the northern Tibetan Plateau reinforced the high pressure in pre-winter over Mongolia. Moreover, the low pressure over India and the southern branch of the trough weakened, and water vapor stagnated over the Yangtze River basin because of the weaker summer monsoon; plus, the high ridge over Lake Baikal was favorable for the cold air to extend into southern China. Therefore, the circulation from pre-winter to summer during the third stage led to more precipitation in summer over the middle and lower reaches of the Yangtze River.