Investigating climate responses to large volcanic eruptions in an ensemble of climate model simulations

This research investigated the climate response to large volcanic eruptions in the Community Earth System Model (CESM). To differentiate the climatic response to volcanic eruptions from the model's natural variability, we use the CESM Large Ensemble (CESM-LE), a 30-member ensemble of 20th Century simulations of the coupled climate model. We analyzed the climate following three large 20th Century eruptions: Mount Agung, El Chichón, and Mount Pinatubo. Each volcanic eruption is simulated by an idealized aerosol distribution so each eruption's forcing is identical across ensemble members. The three climate variables we focused on were: surface temperature, precipitation, and surface pressure. With these variables we investigated global and regional climate responses. Globally, the model exhibits the expected cooling that persists for several years following large eruptions. Associated with this, the model shows that the global hydrologic cycle slows and the cooler troposphere holds less water vapor resulting in a decreased surface pressure. On the regional scale, we investigate two responses to large volcanic eruptions that have been described observationally: Northern Hemisphere winter warming, and an enhanced likelihood for El Niño Southern Oscillation (ENSO) events. Investigating the northern hemisphere winters following the eruptions, some simulations produce anomalous warming over land that is similar to the observed warming, but just as many produce anomalous cooling. Comparing the distribution of temperature anomalies, we cannot distinguish the northern hemisphere winter response from the model's natural variability. A similar analysis for ENSO events, however, hints that warm events (i.e., El Niño) are more likely following large volcanic eruptions than is typical of the model's natural variability.