Decadal changes of the early summer Asian monsoon and the South China Sea tropical cyclones during 2001–2020

An inter-decadal increase (1990–2009) in the western North Pacific (WNP) tropical cyclone (TC) genesis frequency in May has recently been reported. The TC decadal changes was attributed to an advanced monsoon onset over the Asian Summer Monsoon region, particularly over the South China Sea (SCS) in May. In the present study, we used 60 years (1961–2020) of TC data and 40 years (1981–2020) of the global reanalysis data to investigate the unique changes of the SCS TC activity during May and the associated large-scale environment. We find that the increasing trend was weakened during the most recent decade (2011–2020), and the advanced monsoon onset was not discernable. The period of 2011–2020 was identified as a decade of the minimum TC genesis frequency over the SCS in May since 1961. The extremely inactive TC genesis was attributed to the weak low-level cross equatorial monsoonal flow over the Indo-Pacific warm pool and the weak westerly monsoon (easterly anomalies) north of equator from the western Pacific through the Philippine Sea and the SCS to the Bay of Bengal. The weak tropical monsoon was associated with subtropical anticyclonic and suppressed convection north of the equator and the opposite wind and convection south of the equator. The anticyclonic circulation over the SCS was particularly strong that inhibited TC genesis during the first decade of the twenty-first century. Key points The SCS saw an unprecedentedly quiet decade with no TC genesis in May from 2010–2019. The decadal SCS low-level circulation anomaly in May was more anticyclonic after 2011 than before. The decadal scale variability was driven by Indian Ocean SST and East Asia low-level flow changes.