Global Changes in Water Vapor from Observations and Reanalysis Products

Understanding the long-term changes in the global water vapor content is critical for assessing natural vs. human-caused climate change. Despite the strong thermodynamical relationship between temperature and water vapor changes, substantial discrepancies exist between climate model simulations, reanalysis products, and observations.

In this work, we assess the consistencies and discrepancies of total column water vapor (TCWV) estimates from three reanalysis products (ERA5, MERRA-2, and JRA-55) and three observational techniques: satellite-borne microwave radiometers (MWR) over the ocean, GPS–Radio Occultation (GPS-RO) observations from low-orbiting satellites, and observations from a global network of ground-based GNSS receivers. All three reanalyses assimilate satellite microwave radiometer measurements (in the form of radiances) and GPS-RO data which are, therefore, not entirely independent, while ground-based GNSS measurements are not assimilated in either reanalysis and serve as a completely independent data set.

We examine the overall agreement in global trends and variability in the different data sets over the period from 1980 to the present and highlight some interesting features in the last few years connected with the recent persistent strong La Niña period (2020-2022) and the set-up of the current El Niño event. The relationship with other essential climate variables, such as tropospheric temperature and surface precipitation, is also discussed.