Difference between revisions of "Slab ocean model"

Revision as of 16:30, 6 October 2022

A brief description of the slab ocean model

The slab ocean model is from Codron [2012]. This model uses the same horizontal grid as the GCM and is composed of two layers. The first layer (50 m depth) represents the surface mixed layer, where the exchanges with the atmosphere take place. The second layer (150 m depth) represents the deep ocean. The transport of heat by the ocean circulation is given by two components. First, the impact of subgrid-scale eddies is represented by horizontal diffusion, with a uniform diffusivity in both layers. Then, the mean wind-driven circulation is computed by calculating the Ekman mass fluxes in the surface layer from the surface wind stress and taking an opposite return flow at depth. These mass fluxes are then used to advect the ocean temperature horizontally. In the case of divergent horizontal mass fluxes, the upwelling or downwelling mass flux is obtained by continuity. This simplified model reproduces the global meridional oceanic heat transport quite closely compared to a full GCM.

The oceanic model also computes the formation of oceanic ice. Sea ice forms when the ocean temperature falls below –1.8°C and melts when its temperature rises above freezing. The changes in ice extent and thickness are computed based on energy conservation, keeping the ocean temperature at –1.8°C as long as ice is present. A layer of snow can be present above the ice. The surface albedo is then that of snow, or for bare ice: A = A$$^\rm{max}_\rm{ice}$$ – (A$$^\rm{max}_\rm{ice}$$ – A$$^\rm{min}_\rm{ice}$$ ) exp(–h$$_\rm{ice}$$/h$$^0_\rm{ice}$$) with A the albedo, A$$^\rm{max}_\rm{ice}$$ = 0.65 the maximal albedo, A$$^\rm{min}_\rm{ice}$$ =0.2 the minimal albedo, hice the ice thickness (in m) and h$$^0_\rm{ice}$$ = 0.5 m. The albedo over the ice-free ocean is taken to be equal to 0.07. The value for the maximal sea ice albedo we used (i.e., 0.65) is classical for GCMs.

Variables of the slab ocean model which can be written as outputs in diagfi.nc

• tslab1/tslab2: temperature of the two ocean layers (K)
• pctsrf_sic: fraction of sea ice
• sea_ice: mass of sea ice (kg/m$$^2$$)
• tsea_ice: temperature of the sea ice surface (K)
• dt_hdiff1/dt_hdiff2: heating of the ocean layers by diffusion (K/s)
• dt_ekman1/dt_ekman2: heating of the ocean layers by Ekman transport (K/s)

Technical aspects

The slab ocean model is activated in callphys.def with: ok_slab_ocean = .true.

The sea ice model is activated in callphys.def with: ok_slab_sic = .true.

The horizontal heat transport is activated in callphys.def with: ok_slab_heat_transp = .true. The horizontal heat transport cannot be activated in parallel at the moment.

Values of A$$^{max}_{ice}$$ and A$$^{min}_{ice}$$ are fixed in slab_ice_h.F90

FOR SIDDHARTH: ADD NOTE ON SURFACE TYPES (RNAT)