Abstract
Abstract. Volcanic eruptions can have a significant impact on the Earth's weather and climate system. Besides the subsequent tropospheric changes, the stratosphere is also influenced by large eruptions. Here changes in stratospheric water vapour after the two major volcanic eruptions of El Chichón in Mexico in 1982 and Mount Pinatubo on the Philippines in 1991 are investigated with chemistry–climate model simulations. This study is based on two simulations with specified dynamics of the European Centre for Medium-Range Weather Forecasts Hamburg – Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry (EMAC) model, performed within the Earth System Chemistry integrated Modelling (ESCiMo) project, of which only one includes the long-wave volcanic forcing through prescribed aerosol optical properties. The results show a significant increase in stratospheric water vapour induced by the eruptions, resulting from increased heating rates and the subsequent changes in stratospheric and tropopause temperatures in the tropics. The tropical vertical advection and the South Asian summer monsoon are identified as sources for the additional water vapour in the stratosphere. Additionally, volcanic influences on tropospheric water vapour and El Niño–Southern Oscillation (ENSO) are evident, if the long-wave forcing is strong enough. Our results are corroborated by additional sensitivity simulations of the Mount Pinatubo period with reduced nudging and reduced volcanic aerosol extinction.
Highlights
As the most important greenhouse gas in the troposphere, water vapour plays a key role in the climate feedback loop
As the volcanic aerosols were mostly injected near the Equator and their effects of increasing heating rates and temperatures are concentrated in the tropical region, we find the main stratospheric water vapour (SWV) perturbations in the tropical stratosphere as seen in Fig. 6 as a difference plot (VOL-NOVOL) for both volcanoes
The relative increases compared to the background values of NOVOL are up to 20 % for El Chichón at a height between 90 and 80 hPa occurring in the first winter after the eruption (Fig. 6)
Summary
As the most important greenhouse gas in the troposphere, water vapour plays a key role in the climate feedback loop. This feedback amplifies the greenhouse effect of CO2 by about 60 % (Forster et al, 2007). The abundance of stratospheric water vapour (SWV) is mainly controlled by the temperatures at the tropical tropopause (Mote et al, 1996). It is further subject to a high inter-annual and multi-decadal variability, mostly dominated by the quasi-biennial oscillation (QBO) and El Niño–Southern Oscillation (ENSO), which both affect tropical tropopause temperatures (Dessler et al, 2014b). Besides the two well-known phenomena, changes in the chemical balance (like higher methane oxidation rates through an increase of stratospheric chlorine, hydroxyl and ozone), changes in circulation patterns, and volcanic influences have to be taken into account (Forster et al, 2007; Stenke and Grewe, 2005)
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