Is There Interdecadal Variation in the South Asian High?

The decadal intensification of the South Asian high (SAH) after the late 1970s, which is determined based on the geopotential height (H), is suspicious due to the lifting effect upon H caused by global warming. The updated reanalysis datasets ERAS and JRA-55 indicate that the anticyclone in the upper troposphere over the Tibetan Plateau is relatively weak during 1980-2018 compared to that during 1950-79. This decadal weakening of the SAH after 1979 can also be observed in the radiosonde observation data. Correspondingly, the SAH defined by eddy geopotential height (H') reflects a consistent decadal weakening variation. The decadal weakening of SAH detected from H' after the late 1970s matches with a decadal southward shift of the East Asian westerly jet, causing ascending motions over the Yangtze River valley and descending motions over North China. Moreover, the decadal weakening and westward shift of the SAH is accompanied with positive relative vorticity anomalies over the northwest Pacific in the upper troposphere, which implies a declining and eastward shift of the western Pacific subtropical high (WPSH) and a weakened East Asian summer monsoon (EASM). Hence, the decadal weakening of the SAH after the late 1970s may contribute to the Yangtze River flooding/North China drought pattern through its connection with other circulation systems of EASM.

Automated summary

The study found that the South Asian high (SAH) has weakened after the late 1970s, a trend also observed in observational data. The weakening of the SAH is associated with a southward shift of the East Asian westerly jet, increased ascending motion over the Yangtze River valley, and increased descending motion over North China.