Northern Hemisphere Tropopause Folds Exhibit Increasing Frequency and Deepening Intrusion with Long-term Spatiotemporal Characteristics

Tropopause folds (TFs) fundamentally facilitate stratosphere-troposphere exchange (STE), impacting tropospheric thermodynamics, atmospheric composition and extreme weather. Using hourly ERA5 data from 1979 to 2025 and a 3D labeling model, this study systematically analyzes the updated 3D spatiotemporal distribution, vertical characteristics and long-term trends of Northern Hemisphere within 10°-50°N. Results show that TFs predominantly concentrate along the subtropical westerly jet within 20°-40°N, with high-frequency regions shifting westward from winter to summer. Seasonally, the frequency peaks in winter, exceeding 15% over East Asia, followed by spring, while two high-frequency centers persist in summer. Over the Northern Hemisphere, the zonally averaged high-frequency band shifts north then south with the interannual cycle, exhibiting an overall northward protrusion. While shallow folds occur most frequently, deep folds exhibit more pronounced spatial and seasonal variability. Intrusion height decreases rapidly with latitude, forming a distinct meridional gradient with strong variability north of 30°N. Shallow folds are approximately symmetric around the climatological tropopause, whereas deeper folds descend below it with expanded vertical extent and pronounced isentropic tilting near 30°N. The total frequency increased significantly from 1979 to 2025, accelerating after 2000, while intrusions steadily deepened by 1.41 hPa per 10 yr, especially in spring and summer, reflecting a sustained enhancement of fold activity that influences STE processes and weather systems. Overall, given their rapid evolution and link to extreme weather, the long-term dataset with three-dimensional structures provides a robust climatological foundation for investigating their role under a changing climate.