The Influence of Internal Climate Variability on Stratospheric Water Vapor Increases After Large-Magnitude Explosive Tropical Volcanic Eruptions

Substantial and prolonged enhancements in stratospheric water vapor (SWV) have occurred after large-magnitude explosive tropical volcanic eruptions, with modified tropopause entry caused by aerosol-absorptive heating. Here, we analyze the timing and longevity of heating-driven post-eruption SWV changes within CMIP6-VolMIP short-term climate-response experiments with the UK Earth System Model (UKESM1). We find aerosol-absorptive heating causes peak SWV increases of 17% (∼1 ppmv) and 10% (0.5 ppmv) at 100 and 50 hPa, at ∼18 and ∼23 months after a Pinatubo-like eruption, respectively. We track the temperature response in the tropical lower stratosphere and identify the main SWV increase occurs only after the descending aerosol heating reaches the tropopause, suggesting a key role for aerosol microphysical processes (sedimentation rate). We explore how El Niño–Southern Oscillation variability modulates this effect. Post-eruption SWV increases are ∼80% stronger for the La Nina phase compared to the ensemble mean. Tropical upwelling strongly mediates this effect.