Climate Change Research

Climate Change Research

Assessment of the Northward Displacement of the Northern Tropical Belt Edge toward Iran Based on Tropopause Indices

Document Type : Original Article

Authors
Seyed Hossein Mirmousavi 1
Halaleh Fahimi 2
1 Department of Geography, Faculty of Humanities, University of Zanjan, Zanjan
2 Postdoctoral Researcher, University of Zanjan, Zanjan
10.30488/ccr.2026.587558.1343
Abstract
The poleward expansion of the tropical belt is considered one of the most prominent responses of the climate system to global warming; however, its regional implications for Southwest Asia and Iran remain insufficiently understood. This study investigates the displacement of the northern edge of the tropical belt over Southwest Asia during 1980–2024 at monthly and seasonal scales, with particular emphasis on its spatial changes toward Iran. Using ERA5 data, the tropical belt edge was identified based on two tropopause-related indices, namely the Tropopause Gradient Latitude (TGL) and the 15-km Tropopause Height (TP15), and the composite HCE index was calculated as the mean of these two estimates. The Structural Convergence Index (SCI) was also used to quantify the coherence between the dynamical and thermodynamic boundaries. The results showed that, during 2001–2024, the northern tropical belt edge shifted to higher latitudes in all seasons compared with 1980–2000. The largest mean seasonal displacement occurred in summer, reaching 3.25° of latitude (approximately 361 km), whereas the smallest displacement was observed in winter, at 0.87° (approximately 97 km). The maximum structural convergence (84.8%) occurred in winter, while the minimum value (21.5%) was observed in summer. The greatest penetration toward Iran occurred in summer, accounting for 44% of the total annual penetration, when the tropical belt edge extended into the central and northern latitudes of the country, reaching approximately 40°N. The results also indicated that the most pronounced expansion was concentrated between June and September and within the latitudinal band of 42°–47°N. Statistical tests confirmed a significant difference in the position of the northern tropical belt edge between the two study periods. Overall, the findings showed that the displacement of the northern tropical belt edge over Southwest Asia follows a distinct seasonal pattern, with its greatest advance toward Iranian latitudes occurring during summer. In the present study, the simultaneous application of tropopause indices and the quantification of seasonal and spatial variations in the tropical belt edge provided a more integrated assessment of the expansion of this system toward Iran
Keywords
Tropical Belt Expansion
Hadley Cell
Tropopause
Structural Convergence
Southwest Asia
Iran Climate
Subjects
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