| Summary: | The early 19th century was the coldest period over the past 500 years, when strong tropical volcanic events and a solar minimum coincided. The 1809 unidentified eruption and the 1815 Tambora eruption happened consecutively during the Dalton minimum of solar irradiance; however, the relative role of the two forcing (volcano and solar) agents is still unclear. In this study, we examine the effects from combinations of one volcanic with two different solar forcing reconstructions (SATIRE and PMOD) suggested in the protocol for the past1000 experiment of the Paleoclimate Modelling Intercomparison Project - Phase 4 (PMIP4) by simulating the early 19th century climates with the Max Planck Institute Earth System Model (MPI-ESM1-2-LR). From 20-member ensemble simulations we find that the volcano- and solar-induced surface cooling is in general additive, regardless of combining or separating the forcing agents. The two solar reconstructions (SATIRE and PMOD) contribute on average ~0.05 K/month and ~0.15 K/month surface air cooling, respectively, indicating a limited solar contribution to the early 19th century cold period. The volcanic events provide the main cooling contributions, inducing a surface cooling peak of ~0.82 K for the 1809 event and ~1.35 K for Tambora. After the Tambora eruption, besides the northern extratropical oceans, the cooling in most regions reduces largely within 5 years when a global cooling of ~0.34 K is reached, along with the diminution of volcanic aerosol. In the northern extratropical oceans, the cooling reduces only slowly with a constant rate until 1830, which is related to the reduction of seasonality and the increased Arctic sea ice extent. Also, this albedo feedback of Arctic sea ice is found to be the main contributor to the Arctic amplification of the cooling signal. Several non-additive responses to solar and volcanic forcing happen on regional scales. When combining the two forcings, additional cold water propagates to the northern extra-tropics from the additional solar cooling in ...
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