Influence of Zonal Variation of the Subtropical Westerly Jet on Rainfall Patterns and Frequency of Heavy Precipitation Events over East Asia

Abstract

Abstract Understanding the effects of zonal variation of the East Asian subtropical westerly jet (EAWJ) on spatial features of heavy precipitation events requires characterization of the shape, orientation, position, and scale of both the EAWJ and rain belts. Applying a rotating calipers algorithm, jet-axis tracking, wavelet analysis, and K-means clustering algorithm, spatial structures of both the EAWJ and rain belts were quantified for each heavy rainfall event lasting 3 days (3-day-HRE) in 1983–2020. The results reveal that approximately 90% of the EAWJs related to 3-day-HREs had a statistically significant wave structure of ∼6000–12 000 km over East Asia and the North Pacific. These EAWJs had tilted, wavy, and flat patterns and strongly affected the position, orientation, and spatial scales of the 3-day-HRE rain belts by modifying the vapor transport trajectory and vertical rising motions. All types of EAWJ had an orientation similar to that of the rain belts and an average distance to the rain belts of ∼500–1500 km at 105°–125°E and ∼500 km at 125°E–180°. Correspondingly, the rain belts of 3-day-HREs had the largest frequency over eastern China and southern Japan. Zonally asymmetric Rossby waves arising from the land–sea thermal contrast, atmospheric diabatic heating, and topography dominantly contributed to the formation of a meandering or flat EAWJ. A zonally oscillating trough–ridge system, featuring an equivalent barotropic structure with large geopotential height anomalies reaching the lower troposphere, weakens or blocks vapor transport and is ultimately responsible for the strongly varying spatial scales and orientations of rain belts. Significance Statement A solid theoretical basis that variations in the EAWJ intimately covary with the location and orientation of rain belts means that understanding the relationships between the EAWJ’s zonal variations and the spatial features of monsoonal rain belts is conducive to better predicting the weather and climate over East Asia. We quantitatively explored the effects of EAWJ zonal variations on the position, orientation, and scale of rain belts and found that a tilted, wavy, or relatively flat pattern of the EAWJ strongly affected the rain belt spatial features by modifying the vapor transport trajectory. A zonally oscillating trough–ridge system, featuring an equivalent barotropic structure throughout the troposphere, is responsible for the varying spatial scale of rain belts.