Combined effects of mid-to high-latitudes and the tropic on the low-frequency precipitation in Jiangnan from April to June

Based on gauge observations of daily precipitation in China, daily outgoing longwave radiation data from the National Oceanic and Atmospheric Administration (NOAA), and reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), a non-filtering method is applied to extract low-frequency signals of 10-30 days in the daily rainfall and atmospheric circulations. The daily low-frequency rainfall in Jiangnan during the period of April to June (AMJ) for 1979-2013 has been revealed in terms of its characteristics and associated large-scale circulations. Specifically, this study focuses on the combined effects of mid-to high-latitudes and the tropic. Results show that:(1) the first EOF mode of the AMJ rainfall in Jiangnan demonstrates a consistent variation over the entire region. The variance accounted by the low-frequency precipitation in southern area of Jiangnan is about 20%, which is larger than that in other areas; (2) low-frequency precipitation anomalies are significantly correlated with convective activities in the tropics and atmospheric circulation anomalies in the mid-to high-latitudes. When convective activities are located over the eastern Indian Ocean and Indonesia, positive geopotential height anomalies occur over the high-latitudes of Europe and Siberia areas, whereas the negative anomalies occur over Jiangnan with abundant rainfall; (3) the combined effects of mid-to high-latitudes and the tropics affect the low-frequency rainfall in Jiangnan. When active (suppressed) convective activities are located over the eastern Indian Ocean and Indonesia, the diabatic heating over the tropics causes significant ascending (descending) motions, and triggers northward propagation of the Rossby wave. At the same time, the negative (positive) geopotential height anomalies over Europe and Siberia propagate southeastward. These northward and southward propagating geopotential height anomalies superpose on the upstream of Jiangnan, leading to the formation of a low-pressure (high-pressure) center. The southwesterly (northeasterly) flows ahead of the cyclone (anticyclone) anomaly reinforces (weakens) the moisture transport from the Bay of Bengal, leading to convergence (divergence) of the vertically integrated moisture flux over Jiangnan and abundant (scarce) low-frequency precipitation in this region.