Model implementation #5
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Consensus thoughts on the model implementation: Different approaches needed for models that fix discharge at PI levels (Type-1) vs. models that maintain ice sheet mass balance (Type-2). Consequences are possibly different for global mass balance and ocean freshwater sources. Commonalities: Suggestion that climatological fluxes by treated the same way as the anomalous flux - which may involve some new coding for existing models. This is not essential, but makes dealing with possibly negative FW anomalies easier (since the total discharge is never going to be negative). In historical period: Estimates of mass imbalance from multiple sources (see Bamber, Davison notes), including floating ice shelf variations and trends (more important in Antarctica). Possible use of ensemble mean calibrated output from Coulon et al (1950 onwards) as estimate of forced trend? Scenarios from 2022 - for the time being, derived from existing ISMIP6 or other single model efforts (Coulon et al, Edwards group etc., UKESM?). Reconciliation with historical obs will be needed. Secondary priority. Note that anomalous flux in scenarios can be of both signs (i.e. net gain of mass in Antarctica for instance). Implementation: Type 1 approach: These models will have imbalances in the historical runs due to changes in the SMB on the ice sheets - for instance an increase in precipitation due to rising water vapor and convergence over Antarctica. Adding the anomalous flux to the PIControl discharge will provide a more accurate discharge into the Southern Ocean, but the implicit mass balance on the ice sheets will be different to observed (or simulated by the ISM), and so the overall ocean salinity/sea level will not necessarily match observations. Type 2 approach: These models assume mass balance of the ice sheets, via a change in iceberg/runoff flux (however implemented) on either instantaneous or decadal scale. Assessment of the errors associated with each approach should be included in any paper. |
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For type-2 models that remain type-2 throughout projections, we could discuss what to do with the freshwater anomaly that compensates future precipitation anomalies over the ice sheet. If the aim is really to close a global budget, then the freshwater anomaly could be spread globally, not under the form of a physical glacial freshwater flux around the ice sheets (which has strong impacts on sea ice). Then we can prescribe historical freshwater fluxes on top of this in the way we discussed at this meeting, without adding too unrealistic freshwater near ice sheets. |
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This could be informed by an analysis of how large these fluxes actually are - it may well be that the net discharge with this approach is still within the uncertainties of the observed discharge - in which case it's probably not too bad. |
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Very naive question that might be off topic - sorry if so. Until the models have dynamic ice sheets implemented, do we as a group prefer type 1 or type 2? And does the introduction of a FW flux field change our preference? I think that if the models don't have a FW flux field imposed then maybe we might prefer type 2, in that at least the ice sheet is interacting with the rest of the climate and there will be an increasing discharge in forced runs. But as soon as there is a FW flux field imposed I think maybe we might prefer type 1. If there is a trend of increasing snowfall onto Antarctica and that causes a sea level drop/salinification in a model, well that makes physical sense, right, as part of the simulation? It might not match observations, but we'd like to know that because it will cause us to think carefully about the projection from that model. Is the above reasonable do we think? If we did have the chance to influence what the modelling centres do on this topic, actually the switch from type 1 to type 2 or vice versa might be an important outcome. If there is a majority agreement on some of this we should probably say so. |
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Hi all, Following all the discussions above, I made some graphics to wrap my head around the
I thought it might be interesting to share this to make things clearer about what is going on in the different models and what we want to do for the experiment. Please let me know if you have some comments, if you think that I made a mistake or mixed things up. I think the historical experiment is quite straightforward, However, where I am not too sure, it is on the implementation for the scenario case (CMIP6 forcings, ...), see my question marks. Here is document with the four cases:
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This is the discussion on how climate models can/should implement the input of freshwater fluxes into the ocean.
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