Atmospheric Low-frequency Oscillation over the Tibetan Plateau During 1997-1998 and Its Effects on Precipitation

Based on NCEP/DOE daily reanalysis data and conventional observations of National Meteorological Information Center, characteristics of the atmospheric low-frequency oscillation of the Tibetan Plateau monsoon in the winter of 1997-1998 and in the summer of 1998 are studied. Furthermore, the configuration of the upper and lower atmospheric low-frequency circulation system on and around the Plateau and its connection to the precipitation of China are also studied. Results mainly show that the Plateau monsoon exhibits not only a strong periodic oscillation of 30-60 days, but also a quasi-biweekly low-frequency oscillation feature, and the relevant upper troposphere circulation system in the same areas at 200 hPa mainly shows a periodic variation of 30-60 days. In the summer of 1998, there are two low-frequency oscillations of surface pressure on the Plateau and its strength has significant longitudinal change, which means that the periodic oscillation of 30-60 days gradually reduces from south to north, while the signal of quasi-biweekly oscillation becomes stronger.As for low-frequency signal of 30-60 days, when the Plateau summer monsoon is stronger (weaker), there is an obvious low-frequency cyclonic convergence (anti-cyclonic divergence) circulation system from the Plateau to the western Pacific between 25°N and 35°N, but the low-frequency anti-cyclonic (cyclonic) circulation system exists in the region south to the Plateau, from the northern Indian subcontinent and the Bay of Bengal to the northern part of South China Sea. Within the longitude scope of the Plateau (between 80°E and 90°E), there is a low-frequency wave chain from the Bay of Bengal to the eastern region of Xinjiang, which ranks as low-frequency anticyclone (cyclone)-low-frequency cyclone (anticyclone)-low-frequency anticyclone (cyclone). Within the latitude scope of the Plateau, there is low-frequency anti-cyclonic (cyclonic) circulation system at 200 hPa from the western part of the Plateau to the Sea of Japan.Influenced by low-frequency circulation system, when Plateau summer monsoon is strong, the low-frequency circulation system configuration within the latitude scope of the plateau converge on the low-level and diverge on the high-level of 200 hPa, which cause more precipitation over the eastern part of the Plateau and the middle and lower reaches of the Yangtze, while cause less precipitation over the western Sichuan Plateau and southwestern Yunnan. When the Plateau summer monsoon is weak, the low-frequency circulation system configuration diverges on the low-level and converge on the high-level, which leads to less rainfall in many parts of the contral-eastern part of the Plateau and the eastern part of China. At this time, the easterly flux from the South China Sea, southwesterly flux from the Bay of Bengal and northerly flux from the Plateau converge in the southwest of Yunnan and corresponding low-frequency circulation system of water vapor transportation also provide moisture conditions in this region, as a result, the precipitation in the southwest of Yunnan becomes more.