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Abstract:
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 climate. From 20-member ensemble simulations with the Max Planck Institute Earth System Model (MPI-ESM1.2-LR), 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, the cooling in most regions reduces largely within 5 years when a global cooling of ~0.34 K is reached, along with the reduction of volcanic forcing. 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, the 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. In the atmosphere, the polar vortex tends to strengthen when combining both volcano and solar forcing, even though the two forcing agents separately induce opposite responses. In the ocean, when combining the two forcings, additional surface cold water propagates to the northern extra-tropics from the additional solar cooling in the tropics, which results in regional cooling along the propagation. Overall, this study not only quantifies the surface responses from combinations of volcano and solar forcing, but also highlights the components that cannot be simply added from the responses of the individual forcing agents, indicating that a relatively small forcing agent (such as solar in early 19th century) can impact the response from the large forcing (such as the Tambora eruption) when considering regional climates.
