Impacts of AMO on the interdecadal variability of South China Sea summer monsoon withdrawal and associated mechanisms

The present study investigates the interdecadal variability characteristics of South China Sea summer monsoon withdrawal (SCSSMW) and associated impacts of the Atlantic Multidecadal Oscillation (AMO) based on the NOAA-CIRES 20th reanalysis data reconstructed by the Physical Sciences Laboratory (PSL) of the National Oceanic and Atmospheric Administration (NOAA) and the Cooperative Institute for Research in Environmental Sciences (CIRES), the Extended Reconstructed Sea Surface Temperature (ERSST) dataset from the International Comprehensive Ocean Atmosphere Data Set (ICOADS). Numerical experiments are also implemented. The results show that the timing of SCSSMW has obvious interdecadal variability. During the late (early) SCSSMW years, there are significant cyclonic (anticyclonic) circulation anomalies and more (less) convective precipitation over the South China Sea and its nearby areas. Further studies suggest a significant positive correlation between the AMO and the interdecadal variability of SCSSMW: When the AMO is in the positive phase, SCSSMW is later and vice versa. The SST warming over the North Atlantic (i.e., AMO in the positive phase) releases more heat flux from the ocean to the atmosphere, leading to a significant increase of convective activities in the troposphere over the North Atlantic, thus triggering abnormal Rossby wave activities over the North Atlantic through sea-air interaction and enhanced convective activities. Such Rossby wave activities can affect the formation and propagation of the mid-latitude Eurasian teleconnection wave train that is closely related to the variations of atmospheric circulations over key areas in the Northeast Asia, causing positive geopotential height anomalies and significant descending motion anomalies throughout the troposphere and producing divergent motion anomalies in the lower troposphere. The energy is thus transmitted to the South China Sea and its adjacent areas with the anomalous divergent wind flows, producing convergent and ascending motion anomalies. The cyclonic circulation anomaly over the South China Sea further enhances, leading to later SCSSMW. Roughly opposite mechanism works during the negative phase of AMO, leading to earlier SCSSMW.