A 31 year analysis of long-term variability of atmospheric refractivity and gradients from tropics to poles

Atmospheric refractivity and its vertical changes play an important role in how radio waves travel, but detailed studies across different climate regions are still limited. This study provides a long-term analysis of refractivity, its vertical gradients, and k-factor changes across five major climate zones: tropical, arid, temperate, subarctic, and polar. Using several decades of atmospheric data, we analyse vertical structures, seasonal patterns, year-to-year changes, and extreme conditions, and relate them to physical processes in the atmosphere that affect radio-wave behaviour. The results show that tropical regions have the strongest negative refractivity gradients near the surface (– 54.6 N/km) and the highest occurrence of super-refraction, mainly due to the combined effects of high humidity and temperature, especially during wet-season convection. In contrast, arid and polar regions are mainly influenced by temperature, showing more frequent sub-refraction and weaker seasonal changes. Temperate and subarctic regions show mixed characteristics, influenced by both moisture and temperature layering. In addition, we provide a direct evidence for the influence of large-scale climate variability, particularly El Niño, on refractivity gradients. Specifically, we examined the relationship between the refractivity gradient and the Niño 3.4 Index, and a clear correspondence between the temporal variations of the refractivity gradient and the Niño 3.4 Index is observed, providing independent support for our interpretation and strengthening the link between ENSO-related climate variability and refractivity gradient changes. We also demonstrate the effect of surface-pressure masking on refractivity gradient estimates. These findings highlight the need for climate-specific refractivity models and provide useful guidance for improving radio-wave prediction and communication system design in different environmental settings.