Ocean Model: The dynamical ocean component of IGSM 2.3 uses the MITgcm (MIT General Circulation Model), a state-of-the art z-coordinate ocean circulation model developed by the MIT Climate Modeling Initiative. In principle, we have the capability to run the ocean model at any conceivable resolution — the MITgcm was designed to study ocean and climate processes on scales ranging from kilometers to the globe — but in practice (i.e., much of our research involves running large ensembles of simulations, necessitating a relatively coarse model resolution), we run our model in one of two modes:
- 4° x 4° x 15 vertical levels (from 50m at the surface to 690m at depth)
- 2.5° x 2° x 22 vertical levels (from 10m at the surface to 765m at depth)
Both models include realistic, albeit crude depictions of the Earth's bathymetry, including the Arctic Ocean. In present form, both versions are hydrostatic and Boussinesq, with the effect of mesoscale eddies on isopycnals parameterized using the Gent-McWilliams scheme. Our model with 22 vertical levels resolves the ocean's mixed layer using the K-Profile Parameterization (KPP) scheme of Large et al. (1994), and thus is particularly suited to for process studies of ocean heat uptake. Other noteworthy parameterizations used in the models include enhanced horizontal mixed layer stirring in regions of sharp fronts, and a crude representation of enhanced tidal mixing near bottom topography. In some runs we have also explored the impact of increasing tropical cyclones due to climate change, using a simple parameterization described in Korty et al. (2008).
In our work, we are able to obtain ocean states characterized with different "heat uptake efficiency" by varying the background diapycnal diffusivity, as described in Dalan et al. (2005). Given the limited observational coverage of the global oceans, both in time and space, the ocean's ability to buffer atmospheric temperature increases is neither well constrained nor well understood, and thus exploring this uncertainty across a spread of ocean models is a major goal.
Atmosphere/Ocean Coupling: Coupling with the atmosphere occurs at every ocean tracer time step, which is either 4 or 8 hours (the model is run asynchronously, with a momentum time step of 8 minutes). The model can be set up to use fluxes directly from the atmospheric model, or can employ flux adjustments of heat, momentum and/or fresh water as necessary to ensure that a realistic present-day climate is obtained.
Sea-Ice: The ocean model includes a thermodynamic sea ice model, which is coupled to the atmosphere once per hour. The sea-ice model solves equations for conservation of enthalpy, and is based on CICE, and Winton (2000).The model has three layers (two ice layers and a snow layer) and computes ice concentration (the percentage of area covered by ice for a given grid cell) and ice thickness. A realistic treatment of sea-ice brine content is included in the model.
Additional information describing the ocean model and the coupling to the zonally-averaged atmosphere is provided in Dutkiewicz et al., 2005 (Joint Program 122).