Centennial-scale climate change from decadally-paced explosive volcanism: a coupled sea ice-ocean mechanism
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Northern Hemisphere summer cooling through the Holocene is largely driven by the steady decrease in summer insolation tied to the precession of the equinoxes. However, centennial-scale climate departures, such as the Little Ice Age, must be caused by other forcings, most likely explosive volcanism and changes in solar irradiance. Stratospheric volcanic aerosols have the stronger forcing, but their short residence time likely precludes a lasting climate impact from a single eruption. Decadally paced explosive volcanism may produce a greater climate impact because the long response time of ocean surface waters allows for a cumulative decrease in sea-surface temperatures that exceeds that of any single eruption. Here we use a global climate model to evaluate the potential long-term climate impacts from four decadally paced large tropical eruptions. Direct forcing results in a rapid expansion of Arctic Ocean sea ice that persists throughout the eruption period. The expanded sea ice increases the flux of sea ice exported to the northern North Atlantic long enough that it reduces the convective warming of surface waters in the subpolar North Atlantic. In two of our four simulations the cooler surface waters being advected into the Arctic Ocean reduced the rate of basal sea-ice melt in the Atlantic sector of the Arctic Ocean, allowing sea ice to remain in an expanded state for > 100 model years after volcanic aerosols were removed from the stratosphere. In these simulations the coupled sea ice-ocean mechanism maintains the strong positive feedbacks of an expanded Arctic Ocean sea ice cover, allowing the initial cooling related to the direct effect of volcanic aerosols to be perpetuated, potentially resulting in a centennial-scale or longer change of state in Arctic climate. The fact that the sea ice-ocean mechanism was not established in two of our four simulations suggests that a long-term sea ice response to volcanic forcing is sensitive to the stability of the seawater column, wind, and ocean currents in the North Atlantic during the eruptions.
KeywordsArctic Ocean Volcanic Aerosol Explosive Volcanism Initial Climate State Meridional Overturn Circulation Response
This work is funded by the VAST (Volcanism in the Arctic SysTem) project, sponsored by the U.S. National Science Foundation (ARC 0714074) and the Icelandic Science Foundation (RANNIS Grant of Excellence 070272012). Climate modeling at NCAR is sponsored by the National Science Foundation through UCAR. We thank Caspar Ammann, Darren Larsen, Thor Thordarson, Alan Robock and Ray Bradley for constructive suggestions during the course of this work. We also thank Cecilia Bitz for her scripts and all reviewers for their thoughtful comments.
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