Tropical upwelling in observations and reanalyses

Tropical upwelling transports air masses across the tropical tropopause into the lower stratosphere and constitutes the ascending branch of the global mean stratospheric circulation. The strength of the tropical upwelling influences the thermal characteristics and chemical composition of the lower stratosphere and the transition region between troposphere and stratosphere, the tropical tropopause layer (TTL). Given the lack of direct measurements and the small magnitude of vertical velocities, the variability and long-term changes of tropical upwelling are difficult to determine and poorly constrained in meteorological analysis data.

Here we use water vapor measurements from the MLS (Microwave Limb Sounder) instrument to determine interannual variations and long-term changes in tropical upwelling in the lower stratosphere for 2005-2023. Our upwelling estimates represent an effective vertical transport velocity and provide an estimate of the speed of the vertical branch of the stratospheric circulation. We show that interannual variations of the tropical upwelling are anti-correlated with TTL temperatures derived from Global Navigation Satellite System – Radio Occultation (GNSS-RO) measurements with warmer (colder) temperatures coinciding with years of less (more) upwelling. A regression analysis results in a negative upwelling trend of  consistent with positive temperature trends in the TTL. Upwelling is also found to be anti-correlated with independent time series of ozone and other gases in the lower stratosphere.

We compare the observational upwelling estimates to residual vertical velocity from four reanalysis and find very good agreement of the interannual variability between all data sets. The reanalysis eddy and momentum fluxes are used to investigate the impact of extratropical waves on tropical upwelling. Our analysis shows that a large fraction of the interannual variability in tropical upwelling is associated with waves propagating meridionally into the subtropical stratosphere.