ISMIP6-Projections-Antarctica

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Overview

NOTE: THIS PAGE IS UNDER CONSTRUCTION. WHEN IT IS FINISHED, THIS STATEMENT WILL BE REMOVED

This page describes the experimental protocol for the ISMIP6 projections that target the upcoming IPCC AR6 assessment. Due to the delay in CMIP6 climate simulations, the initial set of ISMIP6 simulations are based on CMIP5 projections. As CMIP6 model output become available, ISMIP6 will include simulations based on these models.

The experimental framework was revised in September 2018 during an ISMIP6 workshop held in Sassenheim (NL). The protocol, summarized in Fig 1, allows for:

• Sampling CMIP scenarios: main focus is on the high emission RCP8.5, but ice sheet evolution in response to low emission RCP2.6 is also investigated.

• Sampling CMIP models: 6 AOGCMs have been selected from the CMIP5 model ensemble. The AOGCMs were identified based on the following steps: 1) present plausible climates near Antarctica (evaluated by model biases over the historical period), and 2) sample a diversity of forcing (evaluated by differences in projections and code similarities).

• Sampling ice sheet model uncertainty: "standard" and "open" experiments. The "standard" experiments are based on parameterizations developed by the ocean and atmospheric focus groups, while "open" experiments utilize the parameterizations already in use by respective ice sheet models.

• Sampling forcing uncertainty: the standard experiments include "high", "mid" and "low" parameters.

• Experiment ranking: This experimental framework results in 12 to 72 experiments, and a control run. Not every ice sheet model will be able to carry out the full set of experiments. The experiments are therefore ranked, groups are encouraged to work shown the list and complete as many experiments as possible. This approach is based on Shannon et al. (2013): it ensures that all groups do a subset of identical experiments, while it also allows faster models to explore the experiment space more fully.

Greenland overview.png Figure 1: Overview of the Antarctic experimental framework TO DO: CHANGE TO BETTER QUALITY FIGURE

Initial state and experiment duration

Groups can reuse their initMIP initial state, or start from a new initial state. To facilitate analysis of the sea level projections resulting from the ISMIP6 suite of ice sheet model simulations, ISMIP6 uses January 1995 to December 2014 as reference period. The experiments start on January 2015 and end in December 2100. The cut off date is constrained by the availability of forcing.

Control run

The control run is needed to evaluate model drift. As in the initMIP setup, the control run is obtained by running the model forward without any anomaly forcing, such that whatever surface mass balance (SMB) was used in the initialization technique would continue unchanged.

Atmospheric forcing: SMB and temperature anomalies

ISMIP6 will provide surface forcing datasets for the Antarctic ice sheet (AIS) based on CMIP global climate model (GCM) simulations. Two basic approaches are possible: using GCM output directly, or re-interpreting the GCM climates through higher-resolution regional climate models (RCMs). The later allows to capture narrow regions at the periphery of the ice sheet with large SMB gradients, which are not captured by CMIP5 GCMs, and is the technique used for the Greenland ice sheet. For the Antarctic CMIP5 based projections, RCMs will not be used. Many of the GCMs that have indicated participation in ISMIP6 use multiple elevation classes to downscale SMB to finer grid resolution. Once these models have completed the CMIP6 projections, our goal is to include additional ISMIP6 projections using SMB downscaled via elevation class.

As a further complication, experience with initMIP shows that there is a wide variation in ice-sheet profiles, extents and forcing requirements across the ISMs that will be used. For the forcing datasets to be usable, they must be able to be used flexibly, and not be tied to a single ice-sheet shape that may have existed in a GCM or RCM. The revised experimental framework described here takes into account the wide variation in ice-sheet profiles.

Forcing will consist of anomalies of surface mass balance and surface temperature. SMB is needed by ISMs to compute mass changes at the surface, and surface temperature (i.e., the ice temperature at the base of the snow, as distinct from the 2-m air temperature or skin temperature) is used by many ISMs as an upper boundary condition. SMB will be given in units of kg m-2 s-1, and surface temperature in units of deg C. The units of SMB_anomaly are (meter ice equivalent/year) with an assumed density of 910 kg/m^3 and 31556926 s/yr. The following remarks refer mostly to SMB, but the same comments would generally apply to surface temperature as well.

The SMB and its anomalies will be provided on the ISMIP6 standard ice sheet grid for Antarctica. ISMs will then horizontally interpolate the anomaly forcing from the standard grid to their native grids. Before applying SMB anomalies, ISMs will need to be initialized by applying a baseline SMB (either a time series or a climatology). ISMIP6 will provide SMB climatologies for the reference period (January 1995 to December 2014) from the same models computing the anomalies. ISMs can use these these climatologies for spin-up, if desired, but are free to use their own preferred SMB forcing. Let SMB_ref(x,y) denote the SMB used to initialize the ISM. If a time-dependent SMB is used for spin-up, then SMB_ref(x,y) is the average over the reference period.

ISMIP6 provides yearly averaged surface mass balance anomalies, aSMB(x,y,t), along with its components (precipitation, evaporation and runoff). During the run, SMB is computed as:

SMB(x,y,t) = SMB_ref(x,y) + aSMB(x,y,t).

The datasets can be obtained via the ISMIP6 ftp server (email ismip6@gmail.com to obtain the login information).

ADD DATA PATH WHEN DATA IS ON THE SERVER

Modeling groups should use the 8km version to conservatively interpolate to their model native grid (see Appendix 1, below). Files of higher/lower resolution (1km, 2km, 4km, 16km, and 32km) are provided for groups using the output grid as “native grid”.

To DO: check units for SMB and Temp from the AOGCM. are these the same as the output in the GMD paper? Ans: GMD paper/initMIP data request say temp = kelvin, SMB= kg m-2 s-1. Unit of SMB_anomaly for initMIP m ice equivalent / year

Oceanic forcing: temperature, salinity, thermal forcing and melt rate parameterization

ISMIP6 provides datasets of extrapolated ocean "ambient" temperature (T), salinity (S) and thermal forcing (TF) from 1850-2100 that are appropriate for present and future ice-shelf cavities. These datasets originate from CMIP models and have been extrapolated under ice shelves, using rules that account for sills and troughs. The yearly anomalies are with respect to the reference climatology, which is also provided. The datasets are on the 8km ISMIP6 Antarctic grid (add link to grid info). More information on how the datasets were produced is available in the presentations and webinar that can be retrieved from: ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Ocean_Forcing/Antarctica or at https://github.com/xylar/ismip6-ocean-forcing


ISMIP6 standard approach was developed by the Antarctic ocean focus group, and consist of two approaches for the parameterization of basal melt. These are described in ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Ocean_Forcing/Antarctica/parameterizations/melt_parameterization_ISMIP6.pdf

The first approach, a non-local quadratic melting parameterization, is the preferred method for ISMIP6 simulations. However, an alternative (and easier to implement) takes the form of a local quadratic melting parameterization. Example codes for both parameterizations can be found in the "parameterizations" directory. In addition to the annual forcing datasets needed for use with the parameterizations, parameters needed to sample the uncertainty in the melt evaluations are also provided.


TO DO: Think of a pretty figure

Antarctic fracture

Surface melting can trigger ice shelf collapse (for example, the Larsen B ice shelf in the Antarctic Peninsula). This mechanism is separate from cliff-collapse, but is a precursor to cliff-collapse. Although the mechanisms for Larsen B-style ice shelf collapse are still poorly understood, ISMIP6 provides dataset of surface melt obtained from CMIP near surface air temperature (tas) following the method described in Trusel et al. (2015). These annual surface melt have been bias-corrected, and are available on the 4, 5, 8, 10, 16, 20, 32, 40, and 64 km ISMIP6 grids from: give ftp link

Implementation of Larsen B-style collapse not only requires an indication of the timing of the collapse (when surface melt has reached a certain threshold), but also ice flow regime that is favorable to collapse. DO WE HAVE ANY GUIDANCE????

Requirements for the standard experiments

• Participants can and are encouraged to contribute with different models and/or initialisation methods

• Models have to be able to prescribe a given SMB anomaly

• Adjustment of SMB due to geometric changes in forward experiments is encouraged.

• Bedrock adjustment in forward experiment is allowed.

• The choice of model input data is unconstrained to allow participants the use of their preferred model setup without modification. Modelers without preferred data set choice can have a look at the ISMIP6 Datasets page for possible options.

• To allow for analysis, any modeling choice needs to be well documented. Please follow the guidance for model output described in XXXX

Requirements for the open experiments

IS THIS NEEDED? DO WE HAVE ANYTHING DIFFERENT TO ADD COMPAIRED TO THE STANDARD EXP?

Experiment Ranking

With the help of the atmosphere and ocean focus groups, 6 CMIP5 AOGCMs have been selected for ISMIP6 standalone ice sheet model projections. The table below lists the initial number of experiments based on the first three AOGCMs: MIROC_ESM_CHEM, NORESM1 and CCSM4. This table is the minimum contribution expected from ISMIP6 models. Groups that have their own methods for implementing ocean forcing, are encouraged to do the suite with "open" experiments. All groups are encouraged to contribute to the "standard" experiments. Depending on the results of experiments 3 and 7, which consider RCP2.6, additional AOGCMs may be suggested with RCP2.6 for models that are able to do many simulations, but these would be a lower priority than the completing the set with the 6 AOGCMs with RCP8.5.

Note: As of Jan 21, Xylar has processed the Ocean dataset for the 3 models, Luke has processed the surface melt based on his parameterization for the 3 models, Erika is preparing the SMB and surface temp for the 3 models. NORESM required computing runoff based on daily values, as there was a bug for the annual and monthly values


Expt RCP AOGCM Std/open Forcing Unc Note
0 N/A N/A Control N/A Model drift evaluation
1 8.5 AOGCM1 Open Medium Expected high SLR
2 8.5 AOGCM2 Open Medium Expected low SLR
3* 2.6 AOGCM2 Open Medium Expected high SLR
4 8.5 AOGCM3 Open Medium Expected mid SLR
5 8.5 AOGCM1 Standard Medium Expected high SLR
6 8.5 AOGCM2 Standard Medium Expected low SLR
7* 2.6 AOGCM2 Standard Medium Expected high SLR
8 8.5 AOGCM3 Standard Medium Expected mid SLR
9 8.5 AOGCM1 Standard High Forcing Uncertainty
10 8.5 AOGCM1 Standard Low Forcing Uncertainty

References

Franco et al. (2012) FINIR

Goelzer, H., Nowicki, S., Edwards, T., Beckley, M., Abe-Ouchi, A., Aschwanden, A., Calov, R., Gagliardini, O., Gillet-Chaulet, F., Golledge, N. R., Gregory, J., Greve, R., Humbert, A., Huybrechts, P., Kennedy, J. H., Larour, E., Lipscomb, W. H., Le clec'h, S., Lee, V., Morlighem, M., Pattyn, F., Payne, A. J., Rodehacke, C., Rückamp, M., Saito, F., Schlegel, N., Seroussi, H., Shepherd, A., Sun, S., van de Wal, R., and Ziemen, F. A. (2018). Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison, The Cryosphere, 12, 1433-1460, doi:10.5194/tc-12-1433-2018.


Le clec’h et al. (2017) FINIR

Shannon, S.R., Payne A.J., Bartholomew I.D., Van Den Broeke M.R., Edwards T.L., Fettweis X., Gagliardini O., Gillet-Chaulet F., Goelzer H., Hoffman M.J., Huybrechts P. (2013) Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise, Proceedings of the National Academy of Sciences, 110(35):14156-61.