Variability and long-term changes in tropical cold-point temperature

The tropical tropopause layer (TTL) serves as a crucial boundary for air exchange between the troposphere and stratosphere, influencing the chemical composition and radiative balance of the lower stratosphere. Specifically, the cold-point tropopause, where air parcels undergo final dehydration, plays a key role in determining stratospheric water vapor content, which has significant implications for the global energy budget.

Our research utilizes Global Navigation Satellite System – Radio Occultation (GNSS-RO) and radiosonde data to investigate long-term changes in cold-point temperature and their impact on water vapor trends. We present evidence of a shift from pre-2000 cooling to post-2000 warming in TTL and lower stratospheric temperatures. Between 2002 and 2023, the cold point exhibits significant warming trends, reaching up to 0.7 K per decade during boreal winter and spring, with pronounced longitudinal asymmetries. These trends are strongest over the Atlantic and weakest over the central Pacific and are anti-correlated with upper tropospheric temperature trends. Our analysis shows a decrease in the seasonal cycle of cold-point temperature by ∼7%, driving a corresponding reduction of 6% in the seasonal cycle of water vapor at 100 hPa. This decrease of the water vapor seasonal cycle is transported upwards weakening the amplitude of the well-known stratospheric tape recorder signal.

Our findings are reproduced by reanalysis data (ERA5, JRA-55, MERRA-2), which accurately capture the spatial and seasonal variations in temperature trends. The reanalyses also highlight an important connection between TTL temperatures and tropical upwelling with a pre-2000 increase in tropical upwelling consistent with observed cold-point cooling and a post-2000 decrease in upwelling consistent with observed cold-point warming.