HadCM3LC, Hadley Center

HadCM3LC is the low resolution version of the coupled atmosphere-ocean GCM HadCM3 developped at the Hadley Center and described by Gordon et al. (1999). It has a stable control climatology and does not use flux adjustment. Both the atmosphere and ocean components ohf HadCM3-LC have horizontal resolution of 2.5 degrees (N-S) by 3.75 degrees (E-W), then 20 and 19 vertical levels in the ocean and atmosphere respectively.

Horizontal mixing of tracers uses a version of the Gent and McWilliams (1990) adiabatic diffusion scheme with a variable thickness diffusion parameterisation (Wright 1997; Visbeck et al. 1997). Near-surface vertical mixing is parameterised partly by a Kraus-Turner mixed layer scheme for tracers (Kraus and Turner 1967). Below the upper layers the vertical diffusivity is an increasing function of depth only. Convective adjustment is modified in the region of the Denmark Straits and Iceland-Scotland ridge better to represent down-slope mixing of the overflow water, which is allowed to find its proper level of neutral byoyancy rather than mixing vertically with surronding water masses (based on Roether et al. (1994)). Mediterranean water is partially mixed with Atlantic water across the Strait of Gibraltar as a simple representation of water mass exchange since the channel is not resolved in the model.

The sea ice model uses a simple thermodynamic scheme including leads and snow-cover. Ice is advected by the surface ocean current, with convergence prevented when the depth exceeds 4 m (Cattle and Crossley 1995). There is no explicit representation of iceberg calving, so a prescribed water flux is returned to the ocean at a rate calibrated to balance the net snowfall accumulation on the ice sheets, geographically distributed within regions where icebergs are found. In order to avoid a global average salinity drift, surface water fluxes are converted to surface salinity fluxes using a constant reference salinity of 35 PSU.

HadCM3-LC runs the HadOCC ocean biogeochemistry sub-model (Palmer & Totterdell, 2001). HadOCC is a simple Nutrient-Phytoplankton-Wooplankton-Detritus ecosystem model using nitrogen as the currency but with coupled flows of carbon and alkalinity. Phytoplankton growth is limited by the external availability of nutrients, temperature, and light. Detritus is the only model compartment which sinks, and so is the mechanism for most of the downward transport of carbon. The sinking rate is 10m a day, with increased remineralisation at shallower depths. As well as the organic carbon flux, the export of biogenic calcium carbonate is also represented as an instantaneous redistribution on alkalinity and carbon at depth. 0.01 units of calcium carbonate are produced for each unit of primary productivity, and is remineralised equally throughout the water coumn below 2500m.

The terrestrial carbon cycle is represented by the TRIFFD model (Top-down Representation of Interactive Foliage and Flora Dynamics). TRIFFD contains five plant functional types and five soil carbon pols (Cox et al, 1999). Vegetation change is driven by carbon fluxes, with photosythesis and respiration dependent on the climate and atmospheric CO2 concentrations. In all of the OCMIP5 simulations the terrestrial vegetation fractions were held constant.

All HadCM3-LC simulations were undertaken following the CMIP4 protocol. The model was spun-up in an ocean-only configuration for 2000 year with unchanging pre-industrial forcings and atmospheric CO2 fixed at 286ppm. The ocean and atmosphere sub-models were then re-coupled and spun-up for a further 100 years before starting the time-varying emission simulations.