Model Assessment Projects

Physics

GFDL climate model suite (CM2-O)

The suite of model experiments is based on GFDL climate model components. They represent a consistent hierarchy of models with different ocean sea-ice resolutions (see table).

The model suite comprises the GFDL CM2-1deg and the CM2.5 and CM2.6 high-resolution coupled models.

Model name CM2-1deg CM2.5 CM2.6
ocean horizontal resolution 0.25° 0.1°
grid cell size at the equator 111 km 28 km 11 km
grid cell size at 60° 56 km 14 km 5.5 km
eddy parameterization submeso-& mesoscale submesoscale submesoscale

 

Biogeochemistry

CM2-O-miniBLING Simulations

The hierarchy of climate models contain the biogeochemical ocean model miniBLING. The resulting CM2-O-miniBLING model suite is used to investigate the response of the circulation and carbon cycle to climate change.

Two simulations were run for each model:

  • a preindustrial control simulation over 200 years
  • an idealized sensitivity to climate change over 80 years.

 

CM2-O-miniBLING simulationsFig 1. Schematic of the atmospheric CO₂ scenario imposed in the set of two simulations run with the CM2-O-miniBLING suite

 

Mini-BLING model schematicFig 2. Mini-BLING model schematic. Courtesy of E. Galbraith.

 

The miniBLING module simulates global biogeochemistry with a minimal number of tracers (PO4, O2 and DIC). This extreme reduction of tracer number provides an order-of-magnitude decrease in computational overhead compared to typical Earth System models. Despite the small number of tracers, the model represents the most well-established nonlinear dependencies of biogeochemical cycling on environmental state by using empirical parameterizations developed for GFDL's TOPAZ and BLING models. These include an implicit size-based grazing relationship in order to determine the export fraction, iron-light colimitation using a fixed dissolved iron climatology, and alkalinity as an observationally-constrained function of local sea surface salinity. Although experimental, the simulation provides an internally-consistent, fully interactive representation of biogeochemical cycling at the global scale.

The numerous online diagnostics performed during the simulations allow study of the processes at play in this response.

In particular, the differing ocean resolutions enable comparison of the role of mesoscale eddies when resolved (as in CM2.6) or parameterized (as in CM2-1deg). Note that current climate models participating in the IPCC use parameterizations to represent the role of mesoscale eddies. Assessing how those parameterizations do compared to fully resolved eddies is an important research topic considering the conjectured crucial role that eddies play in the Southern Ocean circulation and carbon cycle responses to climate change.

 

Fig 3. Sea-to-air CO₂ fluxes in the CM2-O-miniBLING suite (in mol/m²/yr). The difference (third row) between the sensitivity (second row) and the control simulation (first row) gives the anthropogenic CO₂ flux that arises from  the increase in atmospheric CO₂ concentration as well as changes in the ocean and atmosphere circulations under climate change.