Introduction

The Zagros Mountains is an important regional source of diabatic heating during summer. Their elevation, dry surface conditions, and strong solar radiation can produce substantial surface sensible heat flux, allowing the Zagros to function as an elevated heat source. Previous numerical studies have demonstrated that Zagros heating can influence summertime circulation over Iran and the Middle East, including low-level cyclonic circulation, upward motion, and a mid-tropospheric anticyclonic circulation. However, the physical pathway linking surface sensible heat flux to the three-dimensional atmospheric response is not sufficiently articulated in many synoptic climatological studies. Sensible heat flux does not directly generate a 500-hPa anticyclone. Surface heating first modifies near-surface temperature, virtual potential temperature, buoyancy, pressure gradients, and boundary-layer circulation, which can subsequently affect vertical motion, vorticity, atmospheric thickness, and geopotential height. The present study therefore examines the process-based relationship among surface sensible heat flux, thermal structure, buoyancy-related processes, vertical motion, relative vorticity, and the development of the mid-tropospheric anticyclonic circulation over the Zagros.



Data and Methods

The study focuses on the Zagros region (30–34°N and 48–54°E). Summer conditions during 1990–2021 were investigated using reanalysis-derived atmospheric and surface variables. Downward shortwave radiation and surface sensible heat flux were used to characterize surface energy input and sensible heating. Relative vorticity at 850, 800, 750, 700, and 500 hPa was examined to determine the vertical organization of cyclonic and anticyclonic circulation. Potential temperature was used to characterize the thermal structure of the lower and middle troposphere, while vertically integrated total-energy tendency provided a complementary indicator of atmospheric column energy evolution. Pressure vertical velocity (omega), geopotential height, 850–500-hPa thickness, and the meridional wind component were analyzed to diagnose vertical motion and the three-dimensional circulation structure. Two contrasting years, 1999 and 2003, were selected as representative dry and wet cases, respectively, and 10 July was used as a representative summer day for detailed process-oriented analysis. The analysis involved spatial comparisons, vertical cross sections, and joint interpretation of heating, potential temperature, omega, vorticity, thickness, geopotential height, and wind fields. The physical interpretation was based on established thermodynamic and dynamical relationships. The analysis treats the observed relationships as evidence of process consistency rather than direct proof of causality.



Results

The results show a spatially organized maximum of surface sensible heating over the elevated and relatively compact central Zagros during summer. The major spatial structure of heating is evident in both selected years, although the comparison of only two years cannot establish the statistical significance of interannual variability. Enhanced sensible heating coincides with a thermally modified lower atmospheric layer and an organized vertical-motion response. Negative omega, representing upward motion in pressure coordinates, is evident over the Zagros and extends toward the middle troposphere. At lower levels, particularly 850, 800, and 750 hPa, positive relative vorticity is concentrated over or near the central elevated region. This spatial correspondence is dynamically consistent with low-level convergence and stretching of atmospheric columns, although a complete vorticity budget was not calculated and individual contributions cannot therefore be quantified. With increasing altitude, the circulation changes from predominantly cyclonic at lower levels to anticyclonic in the middle troposphere. Around 500 hPa, the geopotential-height and wind fields indicate a pronounced anticyclonic circulation over the elevated Zagros region. The potential-temperature and thickness fields further indicate a vertically organized warm atmospheric column associated with the mountainous heat source.



Discussion

The results support a physically coherent sequence linking surface heating with regional atmospheric circulation. Enhanced solar radiation and sensible heat flux warm the air immediately above the Zagros. Where this energy is effectively transferred and mixed through the boundary layer, virtual potential temperature and buoyancy increase. The resulting Low-level convergence can subsequently contribute to column stretching. Because the Coriolis parameter is positive over the Zagros, convergence acting on positive absolute vorticity can increase positive relative vorticity. Thus, the observed low-level positive vorticity should not be interpreted as being directly produced by sensible heat flux; rather, it is part of the dynamical response associated with thermally induced circulation and convergence.

The 500-hPa anticyclone requires a distinct but connected interpretation. Surface sensible heating does not act directly at 500 hPa. Instead, heating alters the temperature and thickness of the atmospheric column, changes geopotential heights through hydrostatic adjustment, and interacts with vertical motion, background flow, and potential-vorticity redistribution. Over the elevated Zagros heat dome, vertical compression and associated changes in absolute vorticity can favor anticyclonic curvature. This interpretation is consistent with previous controlled numerical experiments demonstrating an important role for Zagros heating in the development of the regional mid-tropospheric high.

The circulation cannot, however, be attributed exclusively to thermal forcing. The Zagros Mountains exert both thermal and mechanical influences, while the regional circulation is embedded within larger-scale subtropical and monsoonal systems. Consequently, the present reanalysis-based results demonstrate consistency among the observed processes but cannot isolate the causal contribution of Zagros heating from topographic mechanical forcing or remote circulation. Controlled model sensitivity experiments, together with explicit divergence, vorticity-budget, and potential-vorticity analyses, would be required for quantitative attribution.



Conclusion

The study identifies a coherent three-dimensional summertime circulation structure over the Zagros. Enhanced surface sensible heat flux is associated with lower-tropospheric warming and conditions favorable for increased buoyancy, convergence, and upward motion. The associated convergence and column stretching are dynamically consistent with enhanced positive relative vorticity in the lower troposphere. At middle levels, the warm-column structure, increased thickness, hydrostatic adjustment, vertical compression, and redistribution of vorticity provide a physically plausible pathway toward the development of the 500-hPa anticyclonic circulation. The most defensible conceptual sequence is therefore: surface heating → boundary-layer warming and increased buoyancy → low-level convergence and ascent → column stretching and positive low-level vorticity → thermal and hydrostatic adjustment of the atmospheric column → mid-tropospheric anticyclonic circulation. The scientific contribution of the study lies in integrating surface energy flux, thermal structure, omega, vorticity, thickness, geopotential height, and wind fields within a unified synoptic-dynamical framework. Nevertheless, definitive attribution requires controlled numerical experiments capable of separating thermal forcing, mechanical topography, and large-scale circulation.