Abstract
Abstract. The second version of the coupled Norwegian Earth System Model (NorESM2) is presented and evaluated. NorESM2 is based on the second version of the Community Earth System Model (CESM2) and shares with CESM2 the computer code infrastructure and many Earth system model components. However, NorESM2 employs entirely different ocean and ocean biogeochemistry models. The atmosphere component of NorESM2 (CAM-Nor) includes a different module for aerosol physics and chemistry, including interactions with cloud and radiation; additionally, CAM-Nor includes improvements in the formulation of local dry and moist energy conservation, in local and global angular momentum conservation, and in the computations for deep convection and air–sea fluxes. The surface components of NorESM2 have minor changes in the albedo calculations and to land and sea-ice models. We present results from simulations with NorESM2 that were carried out for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Two versions of the model are used: one with lower (∼ 2∘) atmosphere–land resolution and one with medium (∼ 1∘) atmosphere–land resolution. The stability of the pre-industrial climate and the sensitivity of the model to abrupt and gradual quadrupling of CO2 are assessed, along with the ability of the model to simulate the historical climate under the CMIP6 forcings. Compared to observations and reanalyses, NorESM2 represents an improvement over previous versions of NorESM in most aspects. NorESM2 appears less sensitive to greenhouse gas forcing than its predecessors, with an estimated equilibrium climate sensitivity of 2.5 K in both resolutions on a 150-year time frame; however, this estimate increases with the time window and the climate sensitivity at equilibration is much higher. We also consider the model response to future scenarios as defined by selected Shared Socioeconomic Pathways (SSPs) from the Scenario Model Intercomparison Project defined under CMIP6. Under the four scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5), the warming in the period 2090–2099 compared to 1850–1879 reaches 1.3, 2.2, 3.0, and 3.9 K in NorESM2-LM, and 1.3, 2.1, 3.1, and 3.9 K in NorESM-MM, robustly similar in both resolutions. NorESM2-LM shows a rather satisfactory evolution of recent sea-ice area. In NorESM2-LM, an ice-free Arctic Ocean is only avoided in the SSP1-2.6 scenario.
Highlights
DECK consists of the following four baseline experiments: (1) the historical Atmospheric Model Intercomparison Project (AMIP) experiment; (2) the pre-industrial control experiment defined by estimated forcings from 1850, started from initial conditions obtained from a spinup with the same, constant forcings during which the coupled model climatology stabilises towards stationary statistics; (3) an experiment otherwise identical to piControl, except that the CO2 concentrations are set to 4 times the piControl concentrations, from piControl initial conditions; (4) an experiment otherwise identical to piControl but where the CO2 concentrations are gradually increased by 1 % per year starting from piControl concentrations and initial conditions (1pctCO2)
If 500 years are included in the analysis, Norwegian Earth System Model version 2 (NorESM2) shows a sustained warming similar to CESM2. This suggests that the actual equilibrium temperature response to a large GHG forcing in NorESM2 and CESM2 is not very different, but that the Gregory et al (2004) method based on the first 150 years does not give a good estimate of equilibrium climate sensitivity (ECS) for models
This paper presents and evaluates NorESM2 used for conducting experiments for CMIP6
Summary
The Norwegian Earth System Model version 2 (NorESM2) is the second generation of the coupled Earth system model (ESM) developed by the Norwegian Climate Center, and is the successor of NorESM1 (Bentsen et al, 2013; Iversen et al, 2013; Kirkevåg et al, 2013; Tjiputra et al, 2013) which was used in the fifth phase of the Coupled Model Intercomparison Project (CMIP5; Taylor et al, 2012) and for the evaluation of potential climate impacts between the 1.5 and 2 ◦C warming targets of “the 21st Conference of Parties” (COP21) (Graff et al, 2019). NorESM uses the Bergen Layered Ocean Model (BLOM; Bentsen, 2020) coupled with the isopycnic coordinate Hamburg Ocean Carbon Cycle (iHAMOCC) model for ocean biogeochemistry (Tjiputra et al, 2020) It uses a different atmospheric aerosol module (OsloAero6; Kirkevåg et al, 2018; Olivié, 2020). This paper gives a description of NorESM2 and a basic evaluation against observations of the simulation of the atmosphere, sea ice, and ocean in a small set of baseline longduration experiments with the new model It focuses on such aspects as the simulated climatology, stability, and internal variability, and on the response under historical and enhanced greenhouse gas scenario forcings.
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