Impact of the East Asian subtropical westerly jet on the interannual variability of the South China Sea summer monsoon onset and associated precursory thermal forcing role of the Tibetan Plateau

Abstract Based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and the Global Precipitation Climatology Project (GPCP) data, this study investigates the meteorological anomalies tied to the interannual variability of the South China Sea summer monsoon onset (SCSSMO), and its relationship with the change of the subtropical westerly jet position (SWJP), and the potential influence of the precursory thermal forcing over the eastern Tibetan Plateau (TP). The results show that during the earlier (later) SCSSMO years, there exist significant cyclonic (anticyclonic) circulation anomalies over the South China Sea (SCS) and its adjoining areas, featuring enhanced (suppressed) precipitation. The earlier SCSSMO years correspond to the southward-shifted upper-level subtropical westerly jet position to the north of the SCS. This favors the occurrence of non-geostrophic southward winds in the upper troposphere, the occurrence of upper-level divergence (convergence) and low-level convergence ascending (divergence) over the SCS and its nearby areas (Yangtze River basin) on the south (north) side of the jet axis, and the strengthening of the meridional circulation anomaly with anomalous ascending over the low-latitude and descending over the middle-latitude East Asia. Further analysis suggests that the anomalous heating over the eastern TP is significantly related to the SCSSMO and the change of SWJP. When the previous first-two-pentad heating anomalies over the eastern TP are positive, an anomalous anticyclonic circulation is formed in the upper troposphere, stimulating an eastward-propagating wave train. This, in turn, generates an anomalous cyclonic circulation downstream of the TP in the upper troposphere. As a result, the subtropical upper-level westerly jet downstream of TP shifts southward, which further affected the variation of atmospheric circulation over East Asia, ultimately leading to the earlier SCSSMO.