A unified view of the tropical tropopause layer over India from long-term radiosonde and ozonesonde observations

Abstract

The tropical tropopause layer (TTL) regulates the exchange of air, moisture, and trace gases between the troposphere and stratosphere, yet its long-term structure and variability over tropical regions remain poorly constrained by in situ observations. TTL is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere with vertical range extends ~ 12–21 km. Unlike the tropopause represents a single boundary defined by one specific condition, the TTL is a complex transition zone where dynamical, chemical, thermo-dynamical, and radiative processes interact, therefore, cannot be fully relying on a single definition of the tropopause. This study presents a comprehensive characterization of the TTL and tropopause over Gadanki, India (13.5° N, 79.2° E), using nearly two decades (2006–2025) of high-vertical-resolution radiosonde and ozonesonde measurements. Multiple complementary diagnostics are applied to identify thermal, radiative, chemical, humidity-based, and stability-based tropopause definitions, enabling a unified assessment of TTL boundaries, sub-layer structure, and long-term trends. The results reveal a robust and seasonally coherent vertical hierarchy of TTL diagnostics, confirming that the TTL is best represented as an extended transition zone rather than a single tropopause level. Seasonal variability of TTL strongly exceeds its diurnal variability, underscoring the dominant role of large-scale radiative-dynamical control. Monsoon convection primarily modulates TTL depth from below by lifting convective, radiative, and humidity-based lower boundaries, while the upper TTL boundary remains relatively stable and radiatively anchored. The middle TTL sub-layer consistently dominates the total TTL thickness (~ 6–8 km) and exhibits the largest variability, highlighting its central role in dehydration and tracer transport. Long-term trends indicate cooling (~ − 0.5 K decade −1 ) at the cold-point tropopause, decreasing TTL thickness (~ − 0.2 km decade⁻ 1 ) driven by upward shifts of the lower and middle boundaries, and a tightening of the vertical moisture gradient across the TTL. Further, this study also examines the detailed variability of vertical profiles temperature, ozone and humidity and their tropopauses during convective events. Composite analyses further demonstrate that persistent deep convection weakens (~ 0.5 km) thermal, chemical, and hygrometric gradients, reducing the coherence among different tropopause definitions. These findings provide one of the longest in situ TTL climatologies over the Indian region and emphasize the importance of explicitly accounting for convective influence in tropical tropopause diagnostics and climate assessments.