Persistent Precipitation Anomalies And Quasi-biweekly Oscillation During Pre-rainy Season Over South China

Based on the in-situ daily precipitation from Chinese Meteorological Administration(CMA) and the National Centers for Environmental Prediction– Department of Energy(NCEP-DOE) daily reanalysis data, the low-frequency characteristics of precipitation anomalies and the impact of atmospheric circulation during flood season over South China are investigated. Focused on the quasi-biweekly oscillation, the relationship between the persistent precipitation anomalies and the atmospheric low-frequency oscillation at mid-high latitude and tropical zone during the pre-rainy season are discussed.First of all, on the base of tipical case study of persistent heavy rainfall in 2010, the results show that the precipitation anomalies during flood season over South China in 2010 have obvious low-frequency characteristics. Specifically, the pre-rainy season is controlled by 10–20-day oscillation(quasi-biweekly oscillation), while the after-rainy season is controlled by 20–50-day oscillation(intraseasonal oscillation). The low-frequency circulation at the mid-high latitude can influence the low-frequency circulation over South China by the dispersion of Rossby wave along the low-frequency wave train. The analysis of wave activity flux indicates that the wave energy originated from West Siberia disperse toward the eastern China along the low-frequency teleconnection wave train which stretches across the Eurasian continent, leading to the strengthen of the disturbance over eastern China and the precipitation anomalies by changing the lowfrequency circulation and vertical movement over South China. The quasi-biweekly oscillation over the tropical eastern Indian Ocean(IO) is another low-frequency origination influencing the pre-rainy season of South China. When the convection over the eastern equatorial IO is strong(inhibited), there are strong ascending(descending) movements in this region accompanied with convergence(divergence) at lower layer and convergence(divergence) at upper atmosphere. At the same time, descending(ascending) wind prevails over South China, suppressing(promoting) the precipitation. In this process, QBWO over the tropical eastern IO play a leading role. So the superposition of low-frequency signals from mid-high and low latitude, along with the transportation of anomalous moisture, determined the low-frequency variations of circulation anomalies over South China, causing the low-frequency precipitation anomalies and favoring the persistent rain anomalies.Secondly, the low-frequency characteristics of precipitation anomalies during pre-reainy season over South China during 1981-2010 are investigated and the low-frequency of characteristics of the atmospheric circulation are discussed by composite analysis of many cases. The results suggest that the pre-rainy season is obviously controlled by 10–20-day oscillation(quasi-biweekly oscillation), and there is a good corresponding relationship between them. With respect to the low frequency circulation assosiated with the precipitation over South China, the results of composition are consistent with the analysis of the case in 2010. There are lowfrequency wave trains across Eurasia continent around 60°N during pre-rainy season and the wave energy originated from Mediterranean Sea and then spread to South China along this wave train, which can influence the low-frequency circulation over South China and then cause the precipitation anomaly in this area. Also, there is a significant relationship between the quasibiweekly oscillation of the convecton over tropical eastern IO and precipitation during the prerainy season over South China. When the convection over tropical eastern IO is strong(inhibited), there are negative(positive) anomalies of the precipitation in South China. The results of composition also suggest that the low frequency of convection over South China Sea is associated with precipitation over South China. The convection over South China Sea(SCS) is strong(inhibited) during the dry(wet) phase of the pre-rainy season over South China. However, this frature is not evident in 2010, which means there is annual variation in the main factors affecting the precipitation in the pre-rainy season over South China.To sum up, the superposition of low-frequency signals from mid-high and low latitude, along with the transportation of anomalous moisture, determined the low-frequency variations of circulation anomalies over South China, causing the low-frequency precipitation anomalies and favoring the persistent rain anomalies. Based on the studies above, the index of QBWO teleconnection wave train, the index of QBWO convection over SCS, and the index of QBWO vortexes over tropical eastern IO have been defined, which all have good relationship with the low-frequency characteristics of precipitation anamalies in South China. Usually, the evolution of the circulation is stronger than that of precipitation, and it is more predictable. So the indices mentioned can provide the theoretical basis for the sub-seasonal forecast of the persistent precipitation in the pre-rainy season over South China.