The Relationship Between the Autumn Drought in the Eastern Part of Northwest China and the Summer Asian-Pacific Oscillation

The eastern of Northwest China is recognized universally as a sensitive area of climate change and ecologically fragile region. Being adjacent to the Tibetan Plateau, effects of the Plateau terrain on atmospheric heating leads to thermal difference between the atmosphere above the Tibet Plateau and this area. The Asian-Pacific oscillation (APO) is defined as a zonal seesaw of the tropospheric temperature in the mid latitudes of the Asian-Pacific region. When the troposphere is cooling in mid-latitudes of the Asian continent, it is warming in mid-latitudes of the central and eastern North Pacific, and vice versa. In essence, the Tibetan Plateau thermal effects arouse likeness APO large scale teleconnection pattern. Used as an index of the thermal contrast between Asia and the North Pacific, it provides a new way to explore the Asian atmospheric circulations and climate change.Based the NCEP/NCAR reanalysis data, the monthly precipitation and temperature from 589 stations of China during 1961-2010, the relationship between the summer APO and the following autumn drought in China is examined statistically. Results show there are a significantly positive correlation between the APO index and the following autumn in the eastern part of Northwest China. A positive phase of summer APO, characterized by two high ridges strengthened located near the Ural Mountains and east of the Okhotsk Sea, respectively, and a trough deepened between the Balkhash and the Baykal. A positive phase of summer APO associated with east Asian subtropical westerly jet stream turns to be weakened and northward. These changes provide favoring conditions for enhanced wet in the eastern part of Northwest China. The situation is reversed in the negative phase of summer APO, leading to drought in this region. The east wind strengthening at the bottom of the northeastern Pacific anticyclone and the moisture transport that roots in the Arabian Sea and the Bay of Bengal strengthening has very important contribution to the variability of the atmospheric water vapor resource in the eastern part of Northwest China. Moreover, the positive phase of summer APO is followed by increased ascending vertical velocity in autumn especially during the 54th-56th pentad.Specifically, the anomalous signal of the summer APO can persist until the following autumn, accompanying with continuous high correlations between the summer APO index and that in the following autumn. Therefore, the summer APO variation provides a potential valuable signal for predicting the autumn wet/drought in the eastern part of Northwest China.