Effect Of The Interaction Between Mid-High Latitude And Tropic On The Low-Frequency Precipitation In Jiangnan From April To June

Based on daily precipitation of gauge observations in China, daily outgoing longwave radiation data from NOAA, and reanalysis data from NCEP/NCAR, a non-filtering method was applied to extract the low-frequency signals between 10-30 days from the daily rainfall and atmospheric circulations. The daily low-frequency rainfall in Jiangnan during April to June (AMJ) for the period of 1979-2013 is viewed both in terms of its characteristics and the large-scale circulations associated. Specifically, focus on the influence of different configurations of mid-high latitude and tropic on Jiangnan rainfall. Results show that:(1) The AMJ precipitation in Jiangnan mainly displays the spatial structure of consistent variation over the whole region, the larger variance contribution rates of 10-30d low-frequency rainfall appear in the southeast coastal area, which reaches 20%.(2) The low-frequency precipitation anomalies are significantly related to the tropical convection, as well as the atmospheric circulation anomalies over mid-high latitude. When the convective activities propagate eastward and locate over the eastern Indian Ocean and Indonesia, and the geopotential height anomalies over the high latitude of Europe and Siberia areas appear to be active, whereas the anomalies over North America, Baikal and Jiangnan appear to be negative, Jiangnan rainfall turns abundant.(3) Combined effect of mid-high latitude and tropic acts on the low-frequency rainfall in Jiangnan:When the active convection locates over the eastern Indian Ocean and Indonesia, diabatic heating over the tropic causes significant upward movement, while meridional Rossby wave is excited by the convective activities to propagate northward. At the same time, the negative geopotential height anomaly over Europe and Siberia propagates southeastward. These anomalies propagating from northward and southward superpose over the upstream of Jiangnan, characterized by the low pressure. The southwester flows at the front and bottom of the cyclone anomaly reinforces the moisture supply from the Bay of Bengal, vertically integrated moisture flux over Jiangnan becomes convergence, in favors of the abundant low-frequency precipitation in Jiangnan.(4) In the mixed effect of tropic and mid-high latitude:the characteristics different from those in combined effect may associated with the westward-propagated tropical convection. When active convection propagates westward, the 200hPa geopotential height inspired propagates to the upper reaches of Jiangnan. Meanwhile, negative geopotential height anomaly near the eastern Siberia spreads southwestward and superposes with that propagates form the tropic over the upper reaches of Jiangnan, influence the low-frequency precipitation in Jiangnan. Due to the different starting times of the propagating tropical convections between the combined and mixed types, it takes different times for the negative geopotential height anomalies inspired by the convections to spread to the upper reaches of Jiangnan, and then cause the differences in AMJ low-frequency rainfall.