Understanding global hydroclimatic effects of explosive volcanic eruptions

Abstract

In this thesis, the effects of explosive volcanic eruptions on the global hydroclimate is investigated by using observations and multi-model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Community Earth System Model Last Millennium Ensemble (CESM-LME). Understanding weather and climate changes from external forcing is important because it has devastating effects on human society and the environment. In particular, achieving reliable prediction and projection of hydroclimate change is a major challenge for the climate modeling community because large uncertainties remain in both the sign and magnitude of rainfall change from external forcing. In this context, important effects of volcanic eruptions on the hydroclimate has been reported; however, no comprehensive studies have been reported on these influences on the global hydroclimate including extreme responses, uncertainties among models, and the associated mechanisms. Thus, the objective of this thesis is to provide a profound explanation of the influence of volcanic eruptions on the global hydroclimate. This thesis consists of four parts, with each part examining this influence in global monsoon regions. In the first part, extreme temperature and precipitation responses over the global land to five historical explosive tropical volcanic eruptions that occurred since the 1880s are analyzed by using CMIP5 multi-model simulations as well as observations. The results indicate that these tropical volcanic eruptions induce robust extreme temperature cooling and precipitation decreases over the global land and continental monsoon regions, respectively. Moisture budget analysis reveals that most of the precipitation decrease over the monsoon regions is explained by evaporation decrease as well as atmospheric circulation weakening and humidity drying. In particular, the dynamic effect is found to have a large influence on intermodel uncertainty in mean and extreme precipitation responses. These model-based results are largely supported by observational analysis. In the second part, the causes of intermodel uncertainty of CMIP5 in the hydrological responses to volcanic eruptions are explored. Following explosive tropical volcanic eruptions, most of the CMIP5 models are able to simulate El Niño, but significant differences occur in its amplitude. It is found that volcanically induced El Niño affects monsoon precipitation by altering dynamical circulations. The diverse El Niño response among models is found to be largely responsible for the intermodel uncertainty in precipitation responses. In addition, large intermodel uncertainties in El Niño responses are in part caused by different strengths of the volcanic forcing implemented among models. This difference in turn causes significant differences in the precipitation decrease over the Maritime Continent, weakened vertical motion, the corresponding anomalous westerly winds over the western to central Pacific, and El Niño amplitude. Further, the initial states of the tropical Pacific warm water volume (WWV) prior to volcanic eruptions are also found to have a crucial role in the El Niño magnitude differences among models regardless of the imposed volcanic aerosol forcing. In the third part, the influence of volcanic eruptions on the daily precipitation characteristics over the global monsoon regions is assessed on the basis of observations and CMIP5 multi-model simulations. The observations show consistent decreases in light, medium, and heavy precipitation intensities after the Mount Pinatubo eruption. The frequency also significantly decreases above the 70th percentile precipitation, while light precipitation becomes more frequent. The observed changes are reasonably captured by CMIP5 multi-models following five historical volcanic eruptions, in which large intermodel spreads are shown in its intensity. The intermodel difference in the daily precipitation decrease is influenced largely by the larger uncertainty in dynamical atmospheric circulation changes for precipitation below the 99th percentile. Variation in dynamical circulation is also found to be related to the response of El Niño to volcanoes, with a large influence shown on the decrease in daily precipitation. Composite analysis for the daily precipitation and moisture budget of each percentile reveals that heavy rainfall decreases originate mainly from a reduction in vertical moisture advection, which is caused by reduced local atmospheric humidity and weakened vertical motion. However, the response in vertical motion is relatively uncertain, which contributes to uncertain responses in heavy precipitation. In the last part, the role of latitude in volcanic eruption on the global hydroclimate is assessed for volcanoes that erupted during the last millennium by using CESM-LME simulations. After high-latitude eruptions, decreases in solar radiation and surface temperature over the hemisphere of the eruption are greater than those of the other hemisphere, which induces an inter-hemispheric gradient. Owing to the thermal gradient, the Intertropical Convergence Zone (ITCZ) shifts away to the opposite hemisphere after high-latitude eruptions, which causes drying in mean and extreme precipitation, particularly over the hemisphere of the eruption. On the contrary, tropical volcanic eruptions induce largely symmetric solar radiation and surface temperature decreases, causing monsoon drying over both hemispheres owing to the dynamic circulation weakening and humidity reduction. It is shown that the different eruption latitudes induce significant El Niño-like warming over the eastern Pacific through different mechanisms such as southward ITCZ shifting for Northern volcanoes, delayed oscillation from initial La Niña for Southern volcanoes, and Maritime Continent drying for tropical volcanoes. Volcanically induced El Niño is found to largely amplify the responses of summer monsoon drying to volcanic eruptions. In short, this thesis provides new and comprehensive evidence for the influence of volcanic eruptions on global hydroclimate characteristics including extreme precipitation, daily precipitation distribution, and feedback processes through El Niño development. The results obtained on the response and intermodel diversity of global hydrological cycle changes to volcanic forcing have important implications for our understanding of climate system responses to external forcings and evaluation of climate model performance as well as projections of adverse effects from the geoengineering scheme based on solar radiation management.