The Northern Hemisphere winter response to historic volcanic eruptions: How it looked like and how it may have looked like differently

In contrast to the radiative cooling that dominates atmospheric response to volcanoes in most regions, Northern Eurasia shows a warming signal when averaging the observed signal over several eruptions. Up to the current understanding this warming is likely caused by a positive NAO response leading to compensation of the radiative cooling through enhanced advection of mild air from the North Atlantic towards the continent. However, individual eruptions show remarkable differences when computing the response as the difference between the pre and post eruption states for each eruption separately. When only analyzing observations it is difficult to quantify the contributions of internal variability on the one hand and differences in the volcanic forcing on the other hand. Also the response mechanisms are potentially influenced by many different factors. e.g. the strength and the location of the eruption, the ocean state at the time of eruption and internal variability causing different pre-eruption states of the atmosphere. Therefore generalized statements on the volcanic response are difficult to make given the limited number of well-observed major eruptions. Ensemble climate model simulations can help to better understand the related processes by providing multiple realizations of the same historic eruptions and thereby providing a way to separate internal variability from forced signals. Here, we use ModE-Sim, a medium-size atmospheric model ensemble, and its companion dataset ModE-RA, a reanalysis product that uses ModE-Sim as an a-priori state before assimilating historic climate data from different sources. Our first results show that the commonly used practice of averaging about 15 observed eruptions may inherit high uncertainties when interpreting the volcanic winter response.