The Characteristics Of The Low-frequency Oscillation Of The Tibetan Plateau Monsoon

Based on the daily plateau monsoon index data, this paper explored the inter-annual variability of the low-frequency Oscillation features of the Tibetan plateau monsoon, and the characteristics of the atmospheric low-frequency oscillation of Tibetan plateau monsoon during extreme precipitation year and during extreme temperature year also be revealed. Furthermore, the connection between the low-frequency circulation system in and around plateau and the precipitation and temperature of our country has been studied. The result showed that:(1)The plateau monsoon exhibit a obvious periodic oscillation of 30 to 60 days throughout the year. Compared with summer and autumn, the oscillation characteristic in winter and spring is stronger.(2)IN the year of the strong(weak) low-frequency oscillation of the Tibetan Plateau monsoon, most region of southwestern China, northwestern China and Hubei,Henan are mainly show positive(negative) precipitation anomalies, but in Yunnan,central China and Northeast China show negative(positive) precipitation anomalies, at the same, the high(low) temperature in our country is given priority to.(3)As for low-frequency feature of 30 to 60 days, in the range of plateau latitude(between 25°N to 35°N), there is a low-frequency cyclone(anticyclone) system form plateau to the western pacific at the phases of 1-3. when the low-frequency oscillation of the Tibetan plateau monsoon is strong, The range of the average daily rainfall more than 8 mm is great and the big value center of rainfall concentrated in Fujian, Zhejiang;when the low-frequency oscillation of Tibetan plateau monsoon is weak, The range of the average daily rainfall more than 8 mm decreased significantly.(4)The plateau monsoon in the winter of 1997-1998 and in the summer of 1998 exhibit a strong periodic oscillation of 30 to 60 days and it also has a quasi-biweekly low-frequency oscillation feature; the relevant upper troposphere circulation system in areas of 200 hPa mainly shows a periodic variation of 30 to 60 days; In the summer of1998, there are two low-frequency oscillations of different frequencies in plateau surface pressure and its strength has significant longitudinal change, which means thatthe periodic oscillation of 30 to 60 days gradually reduces from south to north, while the feature of quasi-biweekly oscillation becomes stronger and stronger; As for low-frequency feature of 30 to 60 days, when plateau summer monsoon is strong(weak), plateau surface is characterized by low-frequency low-pressure(high-pressure)circulation system, and the zone in the east of china and along the western pacific of the same latitude has a strong low-frequency north-wind(south-wind) and low-pressure(high-pressure) circulation system; between 80°E to 90°E, from the Bay of Bengal to the central region of Northwest China there is a transversal low-frequency wave chain that is low-frequency anticyclone—low-frequency cyclone—low-frequency anticyclone;influenced by low-frequency circulation system, the precipitation becomes more(less)over eastern plateau and the middle and lower Yangtze river region, while less(more)over the west Sichuan plateau and southwestern Yunnan.(5)The plateau monsoon and the southward/northward shift of western pacific subtropic high exhibit obvious low-frequency oscillation of 30 to 60 days in the summer of 2013, and there is a significantly positive correlation between the low-frequency oscillation sequence of them. As for low-frequency feature of 30 to 60 days, when the low-frequency oscillation of plateau monsoon at the strong(weak) phase, form Tibetan plateau to the western Pacific exhibit a obvious low-frequency cyclonic(anticyclonic)circulation system. When the low-frequency of plateau monsoon is strong, the western pacific subtropic high is located to more northern latitude, this made the large range of high temperature appeared in the Northeast China and eastern Inner Mongolia. When the low-frequency of plateau monsoon is weak, the western pacific subtropic high is located to more southern latitude, under the control of subtropic high, there are large areas of high temperature weather appeared in the middle and lower Yangtze river region.