Long-term Variation of Quasi-Stationary Front and Precipitation over East Asia During Boreal Summer

Abstract

This study examines long-term changes in quasi-stationary fronts (QSFs) over East Asia during boreal summer (1979–2023) using objectively identified fronts based on ERA5 reanalysis. Variations in QSF frequency, associated precipitation, and synoptic-scale atmospheric structure are analyzed, with emphasis on the contrast between early summer (ES; June-July) and late summer (LS; August-September). QSF frequency decreased during ES over the Korean Peninsula, eastern China, and southern Japan. During LS, QSF frequency increased significantly over the Korean Peninsula, increased non-significantly over southern Japan, and changed little over eastern China. QSFs account for approximately 25–60% of total summer precipitation across East Asia. The QSF-related precipitation ratio decreased significantly during ES over all three regions, whereas it increased during LS, with the clearest increase over the Korean Peninsula. Three-region composites show that ES QSFs are associated with a deeper baroclinic structure, whereas LS QSFs are characterized more clearly by lower-tropospheric cyclonic development and moisture transport. Additional diagnostics show that this seasonal redistribution occurs together with changes in the Eady growth rate and the precipitable water.