This study examined how the jet stream - a fast-moving river of air in the upper atmosphere - behaves at the boundary between tropical and mid-latitude regions in the Northern Hemisphere. We used the 5,800-meter height line as this boundary marker.

The jet stream typically flows near this boundary but can be pulled away when ocean temperature patterns are particularly strong. During spring, when the jet stream shifts northward, it increases rainfall in western Japan and South Korea. This happens more often when ocean surface temperatures are warm in the Kuroshio Extension region (a warm ocean current east of Japan).

We found that warm ocean surfaces release more heat and moisture into the atmosphere, which influences the jet stream’s path. When ocean temperatures are warm, the jet stream is twice as likely to move northward, bringing more precipitation to these regions. The jet stream’s strength also depends on high-altitude wind patterns in the stratosphere working together with surface ocean conditions.