Decreases in stratospheric water vapor after 2001: Links to changes in the tropical tropopause and the Brewer‐Dobson circulation

Abstract

Time series of stratospheric water vapor measurements by satellites and balloons show persistent low values beginning in 2001. Temperature observations show that the tropical tropopause has been anomalously cold during this period, and the observed water vapor changes (approximately −0.4 ppmv) are consistent with the temperature decreases (approximately −1 K). The cold anomalies occur in the tropics over a narrow vertical layer near 15–20 km. There have been corresponding changes in the tropical ozone profile over the same period, with ∼10% reductions over a similar narrow layer near the tropopause. The variations in temperature and ozone appear coupled, and the spatial patterns of the changes since 2001 are consistent with an increase in the mean tropical upwelling (Brewer‐Dobson) circulation. Estimates of tropical upwelling derived from eddy statistics (“downward control”) show coherence with interannual temperature changes, including a consistent increase after 2001. Part of the temperature changes may also be explained as a radiative response to the observed ozone decreases. The results paint a consistent picture of enhanced tropical upwelling after 2001, resulting in colder temperatures, lower water vapor and lower ozone near the tropical tropopause. The recent low values significantly influence estimates of decadal‐scale trends in temperature and ozone near the tropical tropopause.