Surface climate response to the size and season of northern hemisphere, high latitude, effusive volcanic eruptions

Effusive volcanic eruptions are known to impact climate through the emission of sulphur dioxide and subsequent formation of sulphate aerosols. These aerosols affect radiative transfer in the atmosphere, both directly by scattering sunlight and indirectly through aerosol-cloud interactions. By scattering sunlight, the direct aerosol effect leads to surface cooling. Changes in cloud properties as a result aerosol-cloud interactions, on the other hand, lead to both reflection of sunlight and trapping of outgoing thermal emissions from the ground. Clouds, therefore, have the potential to either cause surface warming or cooling, depending on factors such as the cloud response to the volcanic aerosols and the availability of sunlight. We perform a series of simulations using the Community Earth System Model with the Community Atmosphere Model (CESM2-CAM6) to simulate the climate impacts of northern hemisphere, high latitude, effusive volcanic eruptions. We construct a standard eruption scenario, using the 2014-15 Holuhraun eruption in Iceland as a reference. The Holuhraun eruption released up to 9.6 Tg SO2 over a period of six months, from September 2014 to February 2015, with the emission rate gradually decreasing over time. We apply several different magnitude scalings to this standard scenario and vary the timing of the eruption. This allows us to analyse the climate response as a function of both the eruption size and season. For eruptions starting in winter, we find significant surface warming in our simulations as a result of trapping of outgoing thermal emissions in the absence of sunlight. This warming is mainly confined to the Arctic but also appears over parts of northern Eurasia and North-America, albeit to a lesser extent. This is consistent with our previous work on the Holuhraun eruption where we found evidence for winter surface warming over the Greenland Sea as a result of that eruption, both in model simulations and observations. Conversely, we find surface cooling during summertime eruptions. The spatial distribution of the cooling pattern is different from the winter warming as the cooling predominantly occurs over the Eurasian and North-American continents and is hardly visible in the Arctic. Furthermore, based on preliminary results, whereas the Arctic winter warming is mainly due to aerosol-cloud interactions, the continental summer cooling stems mostly from the direct aerosol effect. Our results indicate a non-linear relationship between the surface air temperature response and the eruption size.