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Abstract

Volcanic eruptions increase the stratospheric water vapour (SWV) entry via long wave heating through the aerosol layer in the cold point region, and this additional SWV alters the atmospheric energy budget. We analyze tropical volcanic eruptions of different eruption strengths with sulfur (S) injections ranging from 2.5 Tg S up to 40 Tg S using EVAens, the 100-member ensemble of the Max Planck Institute – Earth System Model in its low resolution configuration (MPI-ESM-LR) with artificial volcanic forcing generated by the Easy Volcanic Aerosol (EVA) tool. Significant increases in SWV are found for the mean over all ensemble members from 2.5 Tg S onward ranging between [5,160] %, while for single ensemble members the standard deviation between the control run members (0 Tg S) is larger than SWV increase of single ensemble members for the eruption strengths up to 20 Tg S. A historical simulation using observation based forcing files of the Mt. Pinatubo eruption, which was estimated to have emitted (7.5 ± 2.5) Tg S, returns SWV increases slightly higher than the 10 Tg S EVAens simulations due to differences in the aerosol profile shape. An additional amplification of the tape recorder signal is also apparent, which is not present in the 10 Tg S run. These differences underline that it is not only the eruption volume, but also the aerosol layer shape and location with respect to the cold point that have to be considered for post-eruption SWV increases. The additional tropical clear sky SWV forcing for the different eruption strengths amounts to [0.02, 0.65] W/m2, ranging between [2.5, 4] percent of the aerosol radiative forcing in the 10 Tg S scenario. The monthly cold point temperature increases leading to the SWV increase are not linear with respect to AOD nor is the corresponding SWV forcing, among others, due to hysteresis effects, seasonal dependencies, aerosol profile heights, and feedbacks. However, knowledge of the cold point temperature increase allows for an estimation of SWV increases with a 12 % increase per Kelvin increase in mean cold point temperature, and yearly averages show an approximately linear behaviour in the cold point warming and SWV forcing with respect to the AOD