Long-term Changes in Global Tropopause Characteristics from GNSS-RO Observations

The tropopause is an important boundary (or transition layer) for many studies in the atmospheric sciences and is an indicator of global climate change. Numerous characteristics of the tropopause have been shown to exhibit significant long-term change over the most recent 40+ year period, including height, temperature, and the occurrence of multiple tropopauses. However, prior observational studies rely mostly upon suitable records of radiosondes, which are only available over land and vary in coverage globally. Observation-based model reanalyses have been used in many studies to provide global coverage but resulting assessments of tropopause characteristics are not always consistent with observational analyses. In recent decades, the emergence of global navigational satellite system (GNSS) radio occultation (RO) atmospheric profiles provides an observational record with global coverage and fine vertical resolution necessary for tropopause analysis. The GNSS-RO data record is now approaching the period length necessary for robust assessment of long-term changes (trends). In this study, we leverage a continuous record of nearly 25 years of GNSS-RO data and apply two universal tropopause definitions, the WMO temperature lapse-rate tropopause (LRT) and the potential temperature gradient tropopause (PTGT), to evaluate global tropopause characteristics and their long-term changes. We find widespread increases in multiple tropopause frequency in the midlatitudes, consistent with several recent radiosonde and reanalysis studies. We also find regionally varying changes in tropopause height and temperature, which in some cases imply changes in the width of the tropics. Results are generally insensitive to the choice of LRT or PTGT definition. Implications of the diagnosed changes and their relationships to drivers of climate variability will be discussed.