Sensitivity to glacial forcing in the CCSM4

• Published in: Journal of Climate
• Other Authors: Brady, Esther (author), Otto-Bliesner, Bette (author), Kay, Jennifer (author), Rosenbloom, Nan (author)
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Meteorological Society 2013
• Subjects: Antarctic; Pacific; Antarc*; North Atlantic Deep Water; North Atlantic
• Online Access: http://nldr.library.ucar.edu/repository/collections/OSGC-000-000-015-454; https://doi.org/10.1175/JCLI-D-11-00416.1

Results are presented from the Community Climate System Model, version 4 (CCSM4), simulation of the Last Glacial Maximum (LGM) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) at the standard 1° resolution, the same resolution as the majority of the CCSM4 CMIP5 long-term simulations for the historical and future projection scenarios. The forcings and boundary conditions for this simulation follow the protocols of the Paleoclimate Modeling Intercomparison Project, version 3 (PMIP3). Two additional CCSM4 CO₂ sensitivity simulations, in which the concentrations are abruptly changed at the start of the simulation to the low 185 ppm LGM concentrations (LGMCO₂) and to a quadrupling of the preindustrial concentration (4×CO₂), are also analyzed. For the full LGM simulation, the estimated equilibrium cooling of the global mean annual surface temperature is 5.5°C with an estimated radiative forcing of -6.2 W m⁻². The radiative forcing includes the effects of the reduced LGM greenhouse gases, ice sheets, continental distribution with sea level lowered by approximately 120 m from the present, and orbital parameters, but not changes to atmospheric aerosols or vegetation biogeography. The LGM simulation has an equilibrium climate sensitivity (ECS) of 3.1(±0.3)°C, comparable to the CCSM4 4×CO₂ result. The LGMCO₂ simulation shows a greater ECS of 4.2°C. Other responses found at the LGM in CCSM4 include a global precipitation rate decrease at a rate of ~2% °C−1, similar to climate change simulations in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4); a strengthening of the Atlantic meridional overturning circulation (AMOC) with a shoaling of North Atlantic Deep Water and a filling of the deep basin up to sill depth with Antarctic Bottom Water; and an enhanced seasonal cycle accompanied by reduced ENSO variability in the eastern Pacific Ocean’s SSTs.