Abstract
Abstract. Increasing the temperature of the tropical cold-point region through heating by volcanic aerosols results in increases in the entry value of stratospheric water vapor (SWV) and subsequent changes in the atmospheric energy budget. We analyze tropical volcanic eruptions of different 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] %. However, 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 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 m−2, ranging between [2.5, 4] % 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 aerosol optical depth (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 of 12 % per Kelvin increase in mean cold-point temperature. For yearly averages, power functions are fitted to the cold-point warming and SWV forcing with increasing AOD.
Highlights
It has been established that the entry of water vapor into the stratosphere is largely controlled by the temperature of the tropical tropopause (e.g., Brewer, 1949; Mote et al, 1996; Fueglistaler et al, 2009; Dessler et al, 2014)
The annual cycle of the tropical tropopause tem5 peratures accounts for a variation of the spheric water vapor (SWV) content of ±1.4 ppmv around the mean background at 110 hPa in Stratospheric Aerosol and Gas Experiment II (SAGE II) data of the early 1990s (Mote et al, 1996) and ±1.0–1.3 ppmv based on the SPARC Data Initiative multiinstrument mean (SDI MIM) at 100 hPa in 2005–2010 (Davis 10 et al, 2017)
The SWV anomaly caused by the 2.5 Tg S eruption is comparable to changes caused by the quasi-biennial oscillation (QBO), which are [0.16– 0.32] ppmv according to the regression analysis by Dessler 30 et al (2013), whereas the 10 Tg S has an impact stronger than the changes in the Brewer–Dobson circulation (BDC)
Summary
It has been established that the entry of water vapor into the stratosphere is largely controlled by the temperature of the tropical tropopause (e.g., Brewer, 1949; Mote et al, 1996; Fueglistaler et al, 2009; Dessler et al, 2014). Following up on the discussion of the long-term increasing trend in strato- 45 spheric water vapor (SWV) observed during the 1980s and 1990s, it was proposed that volcanic eruptions could be influencing the SWV budget (e.g., Rosenlof et al, 2001; Joshi and Shine, 2003). Two processes are considered: the direct volcanic injection from the volcanic plume and an indi- 50 rect volcanic mechanism due to an increase of the tropopause temperature, referred to hereafter as the indirect volcanic pathway. The magnitude of the SWV increase and the contribution from the different entry mechanisms are still unclear
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