The Laki Eruption – studying Weather and Climate during the Little Ice Age with Paleo-Reanalysis

The assimilation of early instrumental, documentary, and proxy data into model simulations allows the study of multivariate climate variability from monthly to centennial time scales. The strength of our paleo-reanalysis ModE-RA (Modern Era Reanalysis) lies specifically in the period of the Little Ice Age because the number of assimilated values per year increases from hundreds in the 17th century to thousands in the 18th century to tens of thousands in the 19th century. In addition, recent efforts of weather reconstruction based on early instrumental data even allow for European reconstructions at daily time scales back into the 18th century. Here, we present a case study of the global climate and European weather anomalies following the Laki eruption in 1783. Most reports have been limited to the European domain and described an unexpectedly warm summer of 1783 and extremely cold winters in the three following years. Our weather reconstruction and ModE-RA support recent model simulations which suggested atmospheric blocking to be the cause of the unexpected warm anomalies in Europe. However, the entire summer of 1783 was not hot, but only a relatively short period in June and July. On the northern hemisphere scale, we find an aerosol-induced cooling. African and Indian Monsoon rainfall is reduced due to a weaker land-sea temperature gradient in line with the response to strong tropical eruptions and an interhemispheric temperature contrast in line with the response to strong extratropical eruptions. In contrast to recent simulations of the Laki eruption, ModE-RA shows a clear boreal winter warming at high latitudes, slightly dampening the hemispheric-scale cooling signal. In the future, monthly paleo-reanalysis or even daily weather reconstructions could be used to drive models of Little Ice Age glacier dynamics.