Changes in the Upper-Air Wind Speed over China Linked to the Intensification of Tropical Pacific Zonal SST Gradient in Boreal Winter

Abstract Upper-air wind speed (WS) is critical for aviation safety and surface climate, yet its variability and associated physical mechanisms in China remain poorly understood. This study uses radiosonde data and ERA5 reanalysis to examine changes and variability in boreal wintertime WS at different pressure levels over China. A significant decrease in WS near the surface, a phenomenon known as “terrestrial stilling,” was observed from 1979 to 2023. In contrast, WS in the mid- and upper troposphere exhibits varying trends across different regions. Over central and northern China (north of 30°N), upper-air WS shows increasing trends, with the strongest trends around 38°N at 200 hPa. Conversely, decreasing trends are observed over southern China (south of 30°N at 200 hPa). Further analysis reveals that these recent changes in winter-tropospheric WS are closely linked to the northward shift of the East Asian subtropical westerly jet stream. Observational data and model experiments indicate that an enhanced zonal gradient of sea surface temperature in the tropical Pacific strengthens convection and generates divergent wind anomalies in the upper troposphere over the western Pacific. These anomalies trigger Rossby waves that propagate northeastward across East Asia and the North Pacific, producing anticyclonic circulation anomalies over southern China. This leads to the poleward shift of the jet stream and corresponding changes in upper-air WS over China during winter. Significance Statement Changes in upper-air wind speed significantly impact aviation safety and surface climate. With the rapid expansion of low-altitude activities (0–3 km), including the increasing use of drones and unmanned aerial vehicles in China’s airspace, understanding upper-air wind speed variations and their underlying causes is essential for forecasting changes and mitigating associated risks. This study identifies significant trends in upper-air wind speed during boreal winter from 1979 to 2023, highlighting increasing trends over central and northern China and decreasing trends over southern China. Moreover, the analysis reveals that the northward shift of the East Asian subtropical westerly jet stream, driven by an enhanced zonal sea surface temperature gradient in the tropical Pacific, plays a key role in shaping these trends. These findings are not only scientifically important but also have far-reaching socioeconomic implications.