Evaluation of the carbon cycle components in the Norwegian Earth System Model (NorESM)

The recently developed Norwegian Earth System Model (NorESM) is employed for simulations contributing to the CMIP5 (Coupled Model Intercomparison Project phase 5) experiments and the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC-AR5). In this manuscript, we focus on evaluating the ocean and land carbon cycle components of the NorESM, based on the control and historical simulations. Many of the observed large scale ocean biogeochemical features are reproduced satisfactorily by the NorESM. When compared to the climatological estimates from the World Ocean Atlas (WOA), the model simulated temperature, salinity, oxygen, and phosphate distributions agree reasonably well in both the surface layer and deep water structure. However, the model simulates a relatively strong overturning circulation strength that leads to noticeable model-data bias, especially within the North Atlantic Deep Water (NADW). This strong overturning circulation slightly distorts the structure of the biogeochemical tracers at depth. Advancements in simulating the oceanic mixed layer depth with respect to the previous generation model particularly improve the surface tracer distribution as well as the upper ocean biogeochemical processes, particularly in the Southern Ocean. Consequently, near surface ocean processes such as biological production and air-sea gas exchange, are in good agreement with climatological observations. NorESM reproduces the general pattern of land-vegetation gross primary productivity (GPP) when compared to the observationally-based values derived from the FLUXNET network of eddy covariance towers. Globally, the NorESM simulated annual mean GPP and terrestrial respiration are 129.8 and 106.6 Pg C yr⁻¹, slightly larger than observed of 119.4 ± 5.9 and 96.4 ± 6.0 Pg C yr⁻¹. The latitudinal distribution of GPP fluxes simulated by NorESM shows a GPP overestimation of 10% in the tropics and a substantial underestimation of GPP at high latitudes.