Strengthened Linkage between QBO and Peak Summer Precipitation in East Asia since the 1990s

Abstract This study investigates the interdecadal stability of the relationship between the quasi-biennial oscillation (QBO) and East Asian peak summer (July–August) precipitation using long-term observational and reanalysis datasets from 1959 to 2020. Results show a significant strengthening of the June QBO’s influence on regional precipitation over East Asia after the 1990s. Before the 1990s, the QBO index at 30 hPa showed weak correlations with peak summer precipitation. However, after the 1990s, it exhibits significantly positive correlations with precipitation over South China (SC) and negative correlations over the Yangtze–Huaihe River basin (YHRB), including associated shifts in precipitation extremes. This change is primarily linked to changes in the vertical structure of the QBO. After the 1990s, QBO-related westerly anomalies extended deeper into the troposphere, facilitated by a transition from northerly to southerly wind anomalies in the lower stratosphere. These structural changes promoted enhanced downward propagation of zonal wind anomalies, leading to subtropical cyclonic anomalies and increased vorticity. The Rossby wave responses and wave activity flux divergence/convergence produced a robust zonal wind dipole over East Asia, reinforcing moisture transport and vertical motion. These circulation changes gave rise to a meridional dipole in precipitation centered near 30°N, with increased precipitation over the SC and decreased precipitation over the YHRB. The findings highlight a significant shift in the QBO–precipitation relationship, suggesting the enhanced predictive potential of the QBO for East Asian summer rainfall in recent decades. Significance Statement The stability of the QBO–precipitation relationship has received limited attention. Our study highlighted that the modulation of East Asian peak summer precipitation—including extreme rainfall events—by the QBO in June has become more pronounced since the 1990s. This enhancement is primarily driven by changes in the vertical structure of the QBO. Our results will improve the understanding of the mechanisms through which the QBO modulates East Asian summer precipitation.