Spatiotemporal Distribution of Tropopause Folds over the Arctic during Spring Season: A 24-Year Analysis (2000–2023)

Abstract. Springtime tropopause folds facilitate stratosphere–troposphere exchange and may influence Arctic tropospheric composition, yet their long-term variability and dynamical background over the Arctic remain insufficiently characterized. Here, we use ERA5 reanalysis data and a three-dimensional labeling algorithm to identify tropopause folds over the Arctic during March–May from 2000 to 2023, with particular focus on four monitoring sites: Alert, Barrow, Villum, and Zeppelin. The results show spatial heterogeneity and depth dependence. Total and shallow folds occur over relatively broad Arctic regions, with high-frequency centers around Greenland, the Greenland–Barents Sea region, and the North Pacific side of the Arctic. Medium and deep folds are less frequent and are more strongly confined to the lower-latitude Arctic periphery. This spatial structure is dynamically consistent with the 700–250 hPa layer-mean maximum Eady growth rate, which is larger along the Arctic margins than over the central Arctic. Station-based composites further indicate that deeper folds are associated with stronger 250 hPa geopotential height contrasts, larger horizontal GPH gradient magnitude (G_Z), and stronger wind speed characteristics. Temporally, total fold frequency shows interannual positive trends at all four stations during 2000–2023, with the strongest increases at Alert and Villum, mainly contributed by shallow folds. A monthly decline is also found from March to May, with May frequencies only about 20 %–30 % of March values, whereas diurnal variations are weak. These results provide a climatological framework for understanding Arctic springtime tropopause folds and their potential implications for Arctic tropospheric composition.