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

• Published in: Geophysical research letters 2023-10, Vol.50 (19), p.n/a
• Main Authors: Zhou, Xin, Mann, Graham W., Feng, Wuhu, Dhomse, Sandip S., Chipperfield, Martyn P.
• Format: Article
• Language: English
• Subjects: Absorptivity; aerosol heating; Aerosols; Chemical reactions; Climate; Climate variability; cold‐point temperature; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; ENSO; Eruptions; Heating; La Nina; Lower stratosphere; Mixing ratio; Ocean circulation; Perturbation; Southern Oscillation; Stratosphere; Stratospheric water vapor; tropical upwelling; Tropopause; Upwelling; Variability; volcanic eruption; Volcanic eruption effects; Volcanic eruptions; Water vapor; Water vapour
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL103076

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. Plain Language Summary Strong volcanic eruptions, such as the 1991 eruption of Mt Pinatubo, inject a large amount of SO2 directly into the stratosphere, thereby enhancing the stratospheric aerosol layer and causing a short‐term climatic perturbation. Another substantial part of the climatic influence is the change in stratospheric water vapor (SWV), which affects the chemical processes and the radiative budget of the atmosphere. Along with near‐instantaneous injection of water vapor into the stratosphere, volcanic eruptions can indirectly enhance the entry of water vapor into the stratosphere through aerosol‐induced tropopause heating. This work analyses Earth system model experiments designed to explore how volcanic impacts combine with internal climate variability. We find that peak SWV entry mixing ratios occur only within the second post‐eruption year, consistent with the substantially lagged timing of SWV increase seen in post‐Pinatubo satellite measurements. This analysis provides a new perspective on the temporal evolution of the observed post‐Pinatubo SWV increase and an improved quantification of its impacts. Key Points Aerosol‐induced absorptive‐heating increases stratospheric water vapor (SWV) by up to 17% at 18 months post‐eruption in a Pinatubo‐like experiment Analyzing simulations by El Niño–Southern Oscillation (ENSO) variability show an 80% larger peak SWV increase occurs if an eruption