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21ka Experimental Design

Summary table

The changes listed in this table are with respect to the PI configuration.

PMIP3	Minimum solution
Orbital parameters	[ ecc = 0.018994 ] - [ obl = 22.949° ] - [ peri-180° = 114.42° ]
Date of vernal equinox	March 21 at Noon
Trace gases	[ CO2 = 185 ppm ] - [ CH4 = 350 ppb ] - [ N2O = 200 ppb ] - [ CFC = 0 ] - [ O3 = same as in PI ]
Aerosols	Same as in CMIP5 PI (see Dust forcing note below)
Solar constant	Same as in CMIP5 PI
Vegetation	Same as in CMIP5 PI
Ice sheets	Ice sheet extent and related changes in topography provided (see below)
Topography and coastlines	Land-sea mask and topography changes provided (see below)	Minimum changes (see note below)
Ocean bathymetry	Up to groups depending on the flexibility of the model
River outflow	Modified according to a river pathway map (provided, see below)	Same as in CMIP5 PI
Ice sheet mass balance	Add excess LGM freshwater to ocean (see below)	Same as in CMIP5 PI
Mean ocean salinity	+1PSU everywhere (to be added once at the beginning of the simulation)
Mean atmospheric surface pressure	Global average equal to PI value (see below)

Boundary conditions

Insolation

Note that insolation should follows PMIP requirements. Please check it carefuly using the following tables (LGM insolation tables)

Check carefully the date of the Vernal equinox, because it has implications to compare the paleo and PI seasonal cycles

Ice-sheet

The ice sheet provided for PMIP3/CMIP5 LGM experiments is a blended product obtained by averaging three different ice sheets:

ICE6G provided by Dick Peltier

MOCA provided by Lev Tarasov

ANU provided by Kurt Lambeck

A short description and references for the different ice sheets is provided under the link below: LGM icesheet description

This solution was proposed by PMIP bureau in light of a community checking. Given the uncertainties that still exist on the reconstruction of the ice-sheet, resulting from uncertainties in datation for the data used for global or regional constraints, climate intput from ice core temperature reconstructions or climate model simulations, etc... it sounds reasonable to consider that the average is a best estimate of the LGM ice-sheet.

The ice sheet should be implemented following PMIP2 protocol by considering the CHANGE in surface elevation (over ice sheets and land due to sea level lowering) in order that all models will be perturbed (forced) in the same way. This change in elevation would be added by each modeling group to the topography used in their control run.

Data files are available on the Files for PMIP3 experiments page.

Land-sea mask

The land-sea mask is provided by the ICE6G model, except for Antarctica where you should deduce it from the averaged ice-sheet.

Data files are available on the Files for PMIP3 experiments page.

The minimum change is to close the Bering Strait by changing the land-sea mask or imposing rero flux on water and sea-ice across the Bering Strait

River outflow

The river pathways and basins should be at least adjusted so that fresh water is conserved at the Earth's surface: care should be taken that rivers reach the ocean (due to the lower sea level at LGM, some river mouths have to be displaced towards the coast).

You can use the river routing provided by Lev Tarasov to change the routing scheme in your code, so that the river pathways is consistent with the presence of the ice-sheet.

Data files are available on the Files for PMIP3 experiments page.

Ice-sheet mass balance

It is advised to ensure a closed fresh water balance at the Earth's surface: snow accumulating on the ice-sheets should be redistributed to the oceans, either globally or in the adjacent oceans (see PMIP2 recommendation mass balance recommendation).

Vegetation

The vegetation should be treated as in the CMIP5 PI experiment. The reason is that in CMIP5 we test the version of the model used for future climate projections. Since OA and ESM models will be considered, depending on the model used the vegetation will be

prescribed to PI (which means both vegetation types and LAI are prescribed )

Prescribed to PI with interactive LAI (models with interactive carbon cycle, but no vegetation dynamics)

Computed by the model (models with dynamic vegetation)

For Earth System Models with interactive carbon cycle

The simulations should be forced by the prescribed LGM CO2 concentrations. Please use the same protocol as in CMIP5 to store the diagnostic carbon fluxes and the variables needed for PCMIP (see pcmip Project and CMIP5)

Dust forcing

Some ESM have interative aerosols. In that case compute dust and associated forcing online, as in PI. If this is not the case then the recommendation is to keep dust and aerosols as in PI (I.E no change for 21 ka).

Initial conditions

The surface pressure field must be adjusted to the change in surface elevation over the continents. This can be done:

either by gradually changing the surface elevation in order to avoid generating gravity waves,

or by adjusting the initial pressure field to the LGM surface elevation.

If you choose the second option, you must be careful to conserve atmospheric mass.

The spin up procedure is up to group, following CMIP5 approach or from a previous cold state

Note several groups share the same ocean models for which an initial state can be provided from PMIP2 experiments.

If you need an initial state from a other group, you can contact Olivier.marti@lsce.ipsl.fr

Groups with high resolution model for which it is too difficult to run long simulations should contact Olivier.marti@lsce.ipsl.fr to find the best alternative solution.

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