Was there a volcanic induced long lasting cooling over the Northern Hemisphere in the mid 6th–7th century?

• Main Authors: Dijk, Evelien, Jungclaus, Johann, Lorenz, Stephan, Timmreck, Claudia, Krüger, Kirstin
• Format: Text
• Language: English
• Published: 2021
• Subjects: North Atlantic; North Atlantic oscillation; Sea ice
• Online Access: https://doi.org/10.5194/cp-2021-49; https://cp.copernicus.org/preprints/cp-2021-49/

The climate in the Northern Hemisphere (NH) of the mid-6th century was one of the coldest during the last two millennia. The onset of this cold period is attributed to the volcanic double eruption event in 536 and 540 Common Era (CE) based on multiple paleo-proxies. Recently, there has been a debate about how long lasting and cold this volcanic induced cold period actually was. To better understand this, we analyze new transient simulations over the Common Era and enhance the representation of mid 6th to 7th century climate by additional ensemble simulations covering 520–680 CE. We use the Max Planck Institute Earth System Model and apply external forcing as recommended in the Paleo Model Intercomparison Project, Phase 4. After the four large eruptions in 536, 540, 574, and 626 CE, a significant surface climate response up to 20 years is simulated. The Northern Hemisphere 2 m air temperature, and precipitation decreases up to 2 K, and 0.2 mm day −1 , respectively, and sea ice area increases up to 1.5 x 10 12 m 2 . The global ocean heat content decreases drastically by 1.5 x 10 23 Jm −1 , which is significant for 30–40 years, and does not totally recover during the entire study period. The surface maps reveal atmospheric circulation changes with a hemispheric dipole pattern and land see contrast in the first two years after the eruptions. Poleward of ∼ 45° N higher sea level pressure and a decrease in hydrological variables occur, accompanied by a land see contrast, with decreased values over land and an increase in values over the ocean, which is especially pronounced for evaporation during boreal summer. During boreal winter, a positive North Atlantic Oscillation develops in the first year after (three out of the four) large eruptions. Analysing underlying mechanisms in the North Atlantic reveals that complex interaction between sea-ice expansion, changes in barotropic streamfunction and meridional overturning circulation leads to a reduction in the ocean heat transport, which then further enhances sea ice expansion impacting NH surface climate up to 20 years. Temperature records reconstructed from tree-rings in the NH agree well with the model simulations and show a similar ∼20 year cooling after the eruptions. A century of surface cooling starting in the mid-6th century, as shown from local tree-ring records from the Alps and Altai, does not occur in our volcanic climate model simulations, nor in the NH tree-ring compilation.