Observationally analyzing and numerically simulating the impacts of the South China Sea summer monsoon on summer monsoon rainfall in the middle-lower reaches of the Yangtze River

The ERA-40 and NCEP/NCAR reanalysis datasets, the NOAA’s Climate Prediction Center’s merged analysis of precipitation (CMAP), and the fifth-generation PSU/NCAR Meso scale Model version 3 (MM5v3) were used to define a monsoon intensity index over the East Asian tropical region and to analyze the impacts of the summer (June-July) South China Sea (SCS) monsoon anomaly on the monsoon precipitation over the middle lower reaches of the Yangtze River (YR) from observations and numerical simulations. The results from the reanalysis data show that the interannual variations of the tropical monsoon and precipitation over SCS are negatively correlated with the southwestern winds and precipitation over the YR middle-lower reaches during June and July. Corresponding to stronger (weaker) tropical monsoon and precipitation, the southwesterly is weaker (stronger) over the YR middle lower reaches, with the local less (more) precipitation. The simulation results further exhibit that when the SCS monsoon (SCSM) intensity changes, there are significant variations in the monsoon and precipitation over the YR middle-lower reaches. The simulated anomalies are generally consistent with the observational results, which support the argument concerning the impact of SCSM on the monsoon precipitation over the YR middle-lower reaches. This impact can be elucidated by physical processes. Moreover, when the summer SCSM is stronger, tropical anomalous westerly winds and positive precipitation anomalies generally maintain in the tropics and do not move northwards into the YR middle-lower reaches, not strengthening the transportation of water vapor toward southern China and not causing the strengthening of the southwest flow and precipitation over southern China. On the contrary, the stronger SCSM may result in the weakening and southward shift of the western Pacific subtropical ridge through adjusting the atmospheric intrinsic processes, leading to the weakening of the monsoon and precipitation over the YR middle-lower reaches.