The Mechanism Of Tropospheric Biennial Oscillation And Its Inter-decadal Variation Of The Precipitation In Yangtze-Huaihe Region

The precipitation in Yangtze-Huaihe Region is characterized by tropospheric biennial Oscillation (TBO) as analyzing from 1960-2011 rainfall, and there is a decadal change in this TBO, as the period 1960-1974 and the period 2001-2011 witness weak TBO, the period 1975-2000 experiences strong TBO. The mechanism of this TBO and its inter-decadal variation are researched and the results show that:(1) The TBO has close relations with the Indian Ocean heat content. If the heat content in the southern Indian Ocean in prior winter is positive in the west and negative in the east, there will be a pair of anomaly anticyclone circulation above. It is a response in the atmosphere over Indian Ocean which is known as the "Matsuno-Gill Pattern" produced through the heating effect. Under the effect of the Ocean Rossby wave and Kelvin wave generated from "Matsuno-Gill Pattern", the ocean heat content move west and east. The coupled anti-cyclonic circulation anomalies move northward with the ITCZ. It merges with the Western Pacific anti-cyclone in summer and makes it strong. At the same time a similar type of EAP teleconnection wave train emerges, forms a number of conditions that conducive to the formation of precipitation resulting in a rainy summer in Yangtze-Huaihe Region. The anti-cyclonic circulation anomalies over the Indian Ocean turn into cyclonic circulation anomalies in the fall and strengthen in the winter. The heat content in Indian Ocean turns into positive in the east and negative in the west and the reversing circulation configuration makes the following summer drought in Yangtze-Huaihe Region. At this time the heat content again reverses in east-west under the influence of the Ocean Rossby wave and Kelvin wave formed by the "Matsuno-Gill Pattern", and a distribution of heat content contrast with last year and the same as the year before last year occurs. Completing a full TBO.(2) The positive anomaly zone of precipitation in the first and third stage isn’t in Yangtze-Huaihe Region. It can be one reason that weakening the TBO strength in the first and third stage. And the TBO strength of the positive anomaly zone of precipitation in the first and third stage is also weaker than the second stage. We analyze the change of the background field in three stages, and find out that division of three stages is consistent well with the phase change of the PDO. After further analysis we find out that the TBO strength of the first and the third stage weaker than the second stage may be caused by the phase change of the PDO. If the PDO is in warm phase, the SST distribution in central and east Pacific will decrease the temperature gradient, and then weaken the trade winds and air-sea coupling. Such circulation background makes the precipitation influenced more by the Indian Ocean, as the rain belt is located in Yangtze-Huaihe Region, and ENSO has no decisive impact on rainfall TBO. At this stage, the Indian Ocean heat content has the phenomenon of TBO, making the Yangtze-Huaihe Region summer rainfall a strong TBO. Conversely, if the PDO is in cold phase, the temperature gradient in central and east Pacific increases, strengthen the trade winds and air-sea coupling, the East Asian summer monsoon will be stronger, and the rain belt north lift. Such circulation background makes the precipitation influenced more by the Pacific and the selected positive and negative phases of TBO corresponding well with the ENSO year, as the ENSO cycle is 2-6 years, making the Yangtze-Huaihe Region summer rainfall a weak TBO.