Characteristics of the Low-Frequency Oscillations over the Tibetan Plateau and Its Impacts on Extreme High-Temperature Events of the Yangtze River Basin

Using the NCEP/DOE (NCEP II) daily reanalysis data and the daily maximum/average air temperature data from China National Meteorological Information Center (CNMIC), the characteristics of Low Frequency Oscillation (LFO) over Tibetan Plateau and their influences on the extreme high temperature events of the Yangtze River Basin in the summer of 2013 have been studied. The results show that: (1) Since 1970, the July and August of 2013 is the hottest summer in the middle and lower reaches of the Yangtze River Basin. The maximum value of the seasonal variation trend of air temperature in the summer of 2013 is about 2 ºC higher than climate temperature. (2) The LFO variability of air temperature contains both the 40-70 days and biweekly oscillation frequency bands in the middle and lower reaches of the Yangtze River Basin. And the LFO variability of the 40-70 days is mainly due to the lower level circulation system (Plateau monsoon) and the upper troposphere circulation system (South Asia high) over the Tibetan Plateau. (3) The abnormal extreme high temperature weather events in the middle and lower reaches of the Yangtze River Basin are the superposition results of both the abnormal trend of seasonal variation and the LFO variability of air temperature. (4) During the LFO variability cycle of 40-70 days, the plateau monsoon of near ground and lower level turns to be weaken (strengthen) since early May of 2013; then the South Asia high of the upper troposphere starts to strengthen (weaken) in 7-10 days since the change of the plateau monsoon started; and the air temperatures of the middle and lower reaches of the Yangtze River Basin starts to decrease (increase) in another 7-10 days since the change of the South Asia high began.