Study On The Relationship Between10-30Day Low-frequency Oscillation And The Persistent Rain Events Over Changjiang-Huaihe River Valley During Meiyu Period And Its Previous Signals

Based on the756stations daily precipitation data in China, the NCEP/NCAR reanalysis data, Outing Longwave Radiation (OLR) data and sea surface temperature (SST) data, using the wavelet analysis method, Lanczos filter and other statistical methods, the relationship between10-30days low-frequency oscillation and the rainfall over Changjiang-Huaihe river valley during Meiyu period is analyzed. The results show that:(1) A10-30days period is obvious in the flood years during Meiyu period. The intensity of rainfall is influenced by the low-frequency circulation system. The active and break of rainfall over Changjiang-Huaihe river valley during Meiyu period is also closely associated with the low-frequency oscillations northward and westward propagation. In the flood years active period, the10-30days low-frequency oscillation exhibited as a low-frequency anticyclone located over the South China Sea in the lower troposphere, which modulate the western Pacific subtropical high (WPSH) extending westward. The confluence of cold and warm air in Changjiang-Huaihe river valley is influenced by low-frequency anticyclone located over the Sea of Japan. In the upper troposphere, the low-frequency anticyclone in middle latitude area over Asian mainland influences the position of South Asian High (SAH), leading to excessive rainfall over Changjiang-Huaihe river valley. Opposite circulation anomaly is for break period.(2) The10-30days low frequency oscillation is significantly correlated with persistent heavy rain events over Changjiang-Huaihe river valley. In before and after persistent heavy rain events, there is significant difference in the low frequency circulation. During the persistent heavy rain events, the10-30days low-frequency oscillation exhibited as a low-frequency anticyclone located over the southeast China in the upper troposphere, leading to SAH extending eastward. At the same time, there is divergence circulation over Changjiang-Huaihe river valley, forming upward movement, In the middle troposphere, there is anomaly geopotential height center in middle and high latitude area over Eurasia mainland, leading to cold and warm air convergence in Changjiang-Huaihe river valley. In the lower troposphere, there is a significant P-J wave train, which modulates WPSH extending westward. At this time, convergence exists in low layers, divergence exists in upper layers, and there are significant Upward vertical movement and abundant water vapor from the South China Sea over Changjiang-Huaihe river valley. Opposite circulation anomaly is for before and after persistent heavy rain events. During the persistent heavy rain events, the positive vorticitiy northward and westward propagation in the lower troposphere, coupled with the negative one southward and eastward propagation in the upper troposphere. They are baroclinic structure over Changjiang-Huaihe river valley.(3) The SST anomaly of Kuroshio and its extension and WP teleconnection pattern are the previous key area and strong influence signal. They through affecting the atmospheric circulation field, leading to the intensity of low-frequency precipitation intensity over Changjiang-Huaihe river valley, especially in April. In the positive anomaly years, the Walker circulation and Hadley circulation are increase. In the upper troposphere, the SAH and westerly jet both strengthen. In the middle troposphere, there is a "two ridge and one trough" circulation n middle and high latitude and the position of WPSH is more western. In the lower troposphere, the intensity of Somali cross-equatorial flow and WPSH strengthens, affecting the intensity of the water vapor to Changjiang-Huaihe river valley. So there circulation field lead to abnormal drought in the Changjiang-Huaihe river valley. Opposite circulation anomaly is for negative e anomaly years.