Abstract
AbstractResults are presented and compared for the Community Earth System Model version 2 (CESM2) simulations of the middle Holocene (MH, 6 ka) and Last Interglacial (LIG, 127 ka). These simulations are designated as Tier 1 experiments (midHoloceneandlig127k) for the Coupled Model Intercomparison Project phase 6 (CMIP6) and the Paleoclimate Modeling Intercomparison Project phase 4 (PMIP4). They use the low‐top, standard 1° version of CESM2 contributing to CMIP6 DECK, historical, and future projection simulations, and to other modeling intercomparison projects. ThemidHoloceneandlig127kprovide the opportunity to examine the responses in CESM2 to the orbitally induced changes in the seasonal and latitudinal distribution of insolation. The insolation anomalies result in summer warming over the Northern Hemisphere continents, reduced Arctic summer minimum sea ice, and increased areal extent of the North African monsoon. The Arctic remains warm throughout the year. These changes are greater in thelig127kthanmidHolocenesimulation. Other notable changes are reduction of the Niño3.4 variability and Drake Passage transport and a small increase in the Atlantic Meridional Overturning Circulation from thepiControltomidHolocenetolig127ksimulation. Comparisons to paleo‐data and to simulations from previous model versions are discussed. Possible reasons for mismatches with the paleo‐observations are proposed, including missing processes in CESM2, simplifications in the CMIP6 protocols for these experiments, and dating and calibration uncertainties in the data reconstructions.
Highlights
The midHolocene is considered Tier 1 within CMIP6 and one of two paleoclimate experiments, with the Last Glacial Maximum (LGM), that serves as an entry card to PMIP4 along with a CMIP6 DECK preindustrial (PI) experiment as a paired control (Eyring et al, 2016; Kageyama et al, 2018)
The positive Northern Hemisphere (NH) summer insolation anomalies at 6 ka force warming greater than 2 °C over the NH continents in June–July–August (JJA) in the midHolocene simulation, with some regions at mid‐latitudes over Eurasia experiencing surface air temperature increases of more than 4 °C (Figure 3(f))
The nature of the orbital parameters, with the positive insolation anomalies from April to September extending into the Southern Hemisphere (SH), results in considerable warming over South America, Australia, and southern Africa as well as in the Atlantic and Indian
Summary
Hemisphere summer insolation versus the preindustrial, with greater anomalies at 127 ka. Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, USA, 2Department of Environmental and Earth Sciences, University of Milano‐Bicocca, Milan, Italy, 3Department of Geography, University of Oregon, Eugene, OR, USA, 4Department of Earth and Atmosphere Sciences, Cornell University, Ithaca, NY, USA, 5U.S. Geological Survey, Corvallis, OR, USA
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