This thesis investigates the spatio-temporal characteristics and the mechanisms of explosive volcanic eruptions’ hydrological effects in subregions of the Asian monsoon region (AMR). Based on two volcanic forcing indices, volcanic eruptions are classi ed according to their northern hemisphere volcanic aerosol injection (NHVAI). Comparisons on the hydrological effect of NHVAI in subregions of AMR are made between tree ring-based hydrological reconstruction data MADA, and the model data from CMIP5 and Last Millennium Ensemble project. MADA and the multi-model ensemble means agree on the drying effect of volcanic eruptions in the monsoon-dominated subregion, while discrepancies can be found in the westerlies-dominated subregion. A better agreement between proxy reconstruction and models is visible one year after the eruption rather than in the eruption year. Less proxy- model discrepancies are apparent in subregions with more available tree-ring chronologies. Results also indicate that the MMEM of CMIP5 is able to reproduce volcanic eruptions’ hydrological effects in southern Asian monsoon region.
Based on the multi-model ensemble mean of CMIP5, the mechanism of the hydrological effects of NHVAI is investigated. The analyses focus on the drying effect in the relatively wettest area (RWA) and the wetting effect in the relatively driest area (RDA). The radiative effect of NHVAI causes a reduction of land-ocean thermal contrast. This changes the transport of water vapor and the formation of atmospheric clouds in the region. The regional temperature and precipitation responses result from the radiative effect of atmospheric clouds. The reduced land-ocean thermal contrast causes a weakened Asian summer monsoon circulation. The transport of water vapor from the ocean to the land is reduced. This results in the drying effect in the RWA due to the reduced formation of atmospheric clouds and precipitation. On the other hand, more water vapor is transported to the RDA due to the change of atmospheric circulation. The vertical motion is also enhanced. This increases the formation of atmospheric clouds and precipitation, and results in the wetting effect in the RDA. Another volcanic classi cation with only southern hemisphere volcanic aerosol injection (SHVAI) is constructed in order to study the climate effect of SHVAI. Results show inversed climate responses to SHVAI compared to NHVAI. Especially in the RWA, the results suggest signi cant drying effect after NHVAI, but slight wetting effect after SHVAI.
In order to investigate the climate effects of volcanic eruptions at different latitudes, volcanic forcing generated from the Easy Volcanic Aerosol (EVA) is implemented into the full-coupled land-ocean general circulation model MPI-ESM-LR. The aim is to simulate a Pinatubo-like eruption at 30°N (NH), equator (EQ) and 30°S (SH). The results con rm the opposite effects on monsoon precipitation after the NH and SH volcanic eruptions. Especially in India, one nds warming and drying effects after the NH eruption, but cooling and wetting effects after the SH eruption. The climate responses to the EQ eruption are similar to that after the SH eruption.
A tendency towards an El Niño event is visible one to three years after the NH eruption. After the SH eruption, a tendency towards the negative phase of the ENSO can be found. One year after the EQ eruption, a tendency towards an El Niño event can be found. The ENSO moves to the negative phase three years after the EQ eruption.
In summary, the mechanism of the climate response in India can be described as follows: The radiative effect of stratospheric aerosol from the NH (SH) volcanic eruption causes decreased (increased) interhemispheric thermal contrast and land-ocean thermal contrast. It strengthens (weakens) the South Asian summer monsoon and forces a southward (north- ward) move of the Intertropical Convergence Zone (ITCZ). This decreases (increases) the water vapor transport from the ocean to the land, and leads to the depletion (formation) of atmospheric cloud and less (more) precipitation in India. The radiative effect of the atmospheric cloud and the physical feedback of the system result further in the warming (cooling) and drying (wetting) effects in India. Results in this study indicate an important role the dynamical response and the resultant change of atmospheric cloud plays on the regional climate responses to volcanic eruptions.
