CCSM3 simulation of climatic impact by volcanism during mid-late 13th century

Consecutive volcanic eruptions during the second half of 13th century were hypothesized to have initiated the transition from the Medieval Warm Period to the Little Ice Age. To test this hypothesis, we study the climatic impact on the Northern Hemisphere high-latitudes by the historical volcanism with a focus on its potential dependency on the state of Arctic sea ice cover, using the Community Climate System Model Version 3 (CCSM3). Our results suggest the instantaneous volcanic climatic impact is a fairly robust phenomenon with a spatial pattern that is insensitive to the Arctic sea ice state; whereas the long-term integrated impact is susceptible to the Arctic state and contingent on a responsive overturning circulation in the Northern North Atlantic and the Arctic Ocean, which could get weaker transporting less heat poleward and thus help sustain the Arctic surface cooling. Further experiments employing the Community Atmosphere Model (CAM) stand-alone version show the volcanic impact via atmospheric physical and dynamic processes is short-lived up to a couple of years, in consistent with previous studies, while the longer-term persistence can be achieved through coupling with the underlying Ocean and Cryosphere. When the Atmosphere-Ocean-Cryosphere is interactive, nonlinearity occurs with the short-term climate response to volcanism in the sense that a winter warming pattern across northern Eurasia is found in the case of weaker volcanism, but a general cooling pattern as the volcanic forcing gets stronger. Previous studies argued for dominancy by atmosphere dynamic warming effect in the former case and direct radiative cooling effect in the latter. However, our CAM stand-alone experiments with fixed sea surface temperature and sea ice cover yield warming across northern Eurasia regardless of the strength of volcanism, hinting at an ocean/sea ice control of continental climate. Further attribution efforts suggest the expanding Arctic sea ice resultant from strong volcanism acts to enhance the warming in northern Eurasia, while the ocean surface cooling induces predominant negative North Atlantic Oscillation (NAO)-like change in atmospheric circulation and therefore dictates a largely cooling pattern.