Characteristics of Low-frequency Circulation Propagation Path of the Main Modes of Sub-seasonal Variation of Precipitation in Rainy Season in Southwest China
The paper discusses the main modes of sub-seasonal variation of precipitation in the rainy season(May-September)in Southwest China,and how the sub-seasonal atmospheric oscillations of mid-high latitudes and tropics affect the persistent abnormal low-frequency precipitation in Southwest China for the purpose of providing some references for understanding and forecasting the sub-seasonal variation of precipitation in the flood season in this region.The daily precipitation data from 111 surface weather stations and JRA-55 reanalysis circulation field data in Southwest China during 1961-2015 as well as the EOF empirical orthogonal decomposition method are adopted in analyzing the features of low-frequency circulation and propagation path of the main modes of sub-seasonal variation of rainy-season precipitation in Southwest China.The results show that the main modes of daily precipitation in rainy season in Southwest China are distributed in a North-South zonal pattern.The high-value region of the spatial feature vector is in the south and southeast of Southwest China,and the correlation coefficient between SWDPI and PC1 in the high-value region is 0.991,which has passed the 0.05 confidence test.The significant period of PC1 climate mean is 12.9 d,and the significant period of annual PC1 is mainly concentrated in the 10～20 d period,accounting for 60.0％.In the typical low-frequency years of PC1,the PC1 in rainy season and each month can well represent the temporal evolution characteristics of the corresponding precipitation anomaly distribution pattern,with the positive correlation of the low-frequency components of the two becoming even higher and having passed the 0.05 confidence test.That is,the low-frequency components of PC1 can better grasp the quasi-biweekly oscillation characteristics of the low-frequency components of SWPDI,especially in June,July and August,which might be related to the more stable low-frequency atmospheric circulation signals that affect the anomaly distribution pattern of the main-mode precipitation in the corresponding period.The sub-seasonal variation of PC1 is not only linked to the convective anomaly dipole in Southeast China and the South China Sea,but also associated with the anomaly dipole of geopotential height in the upper troposphere in Southwest China.The formation of the convective anomaly dipole is caused by the northwest propagation of the positive OLR anomaly in the area east of the Philippines and the southeast propagation of the negative OLR anomaly in the southeast of Balkhash Lake.In addition,the suppressed northwesterly propagation east of the Philippines is always accompanied by local anticyclones in low level,while the southwest convection enhancement(suppression)in Southwest China is always accompanied by the negative(positive)geopotentical height anomaly in the north of upper troposphere.Meanwhile,the formation of anomaly dipole in the upper troposphere in Southwest China is correlated to the clockwise turn of the Rossby-like low-frequency wave train in the upper troposphere in the mid-high latitudes in the east side of the Tibetan Plateau.From the perspective of climate background,the low-frequency wave train propagation that affects the sub-seasonal variation of PC 1 is mainly related to two factors.On the one hand,the main position of westerly jet shifts southward and the strength is enhanced in the upper troposphere,causing the Rossby-like low-frequency wave train to spread eastward and southward in the mid-high latitudes and affecting the distribution of the geopotential height anomaly dipole in the upper troposphere in Southwest China.On the other hand,the northward shifting of the Indian monsoon trough strengthens the southwest airflow in the northwest side of the anomalous anticyclones from the South China Sea to the Philippines,promoting the formation of the low-level anomalous cyclones in Southwest China.Meanwhile,the strengthening of the East Asian monsoon trough also facilitates the development and northwestward propagation of low-frequency convection in the tropical western Pacific,thus affecting the distribution of the convective anomaly dipole in Southeast China and the South China Sea.
precipitationsub-seasonlow-frequency circulationpropagation pathSouthwest China
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万方数据
