EGU2020-12987
https://doi.org/10.5194/egusphere-egu2020-12987
EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Exploring and reducing biases in sub-ice-shelf melt rates in an Earth system model

Xylar Asay-Davis1, Carolyn Begeman1, Darin Comeau1, Matthew Hoffman1, Wuyin Lin2, Mark Petersen1, Stephen Price1, Andrew Roberts1, Milena Veneziani1, and Jonathan Wolfe1
Xylar Asay-Davis et al.
  • 1Los Alamos National Laboratory, Los Alamos, NM, USA (xylar@lanl.gov)
  • 2Brookhaven National Laboratory, Upton, NY, USA

Sub-ice-shelf melting plays a critical role in the dynamics of the Antarctic Ice  Sheet and also feeds back on the regional climate, transforming ocean properties (e.g., affecting deep-water production and sea-ice formation).  A full understanding of these processes, as well as the ability to project their response to a changing climate, requires Earth System Models (ESMs) that include coupling with ice-sheet processes.  However, biases in regional Antarctic climate can be amplified through sub-ice-shelf melting, and biased melt rates can have significant adverse effects on ice-sheet model initialization and evolution.  In preparation for inclusion of dynamic ice sheets in ESMs, this presentation discusses our recent experience in understanding the causes of biases in ocean properties on the Antarctic continental shelf and their relationship to ice-shelf melting.  Differences in model behavior across configurations and simulations using the Energy Exascale Earth System Model (E3SM) demonstrates a sensitivity of melt rates to climate. We assess the sensitivity of those melt rates to changes in the region’s climate, including freshening on the continental shelf and shoaling of the thermocline. We also show that ice-shelf meltwater feeds back onto the climate, for example, by affecting melting under neighboring ice shelves, sometimes dramatically so.  We demonstrate that significant reductions in melt-rate biases can be achieved through modifications to ocean model mixing parameterizations. This work charts a path forward for configuring ESMs to produce realistic Antarctic melt rates.

How to cite: Asay-Davis, X., Begeman, C., Comeau, D., Hoffman, M., Lin, W., Petersen, M., Price, S., Roberts, A., Veneziani, M., and Wolfe, J.: Exploring and reducing biases in sub-ice-shelf melt rates in an Earth system model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12987, https://doi.org/10.5194/egusphere-egu2020-12987, 2020

Display materials

Display file

Comments on the display material

AC: Author Comment | CC: Community Comment | Report abuse

Display material version 1 – uploaded on 26 Apr 2020
  • CC1: Comment on EGU2020-12987, Hartmut Hellmer, 08 May 2020

    Hi Xylar,

    I personally consider your talk as one of the highlights of the session CR5.7. Your simulation now is the third  - second Kaitlin's, using ROMS - showing the noticable increase of FRIS basal mass loss. Unfortunately, you don't show bottom temps, but I assume all the heat is getting into the cavity via the Filchner Trough. Actually, the Filchner Sill is the very sensitive point of the system where density structure AND thermocline depth control the onshore/in-trough flow of WDW. Therefore, two aspects of the atmospheric forcing play a role:

    (1) A warmer atmosphere causing (a) less sea ice formation on the wide continental shelf -> (b) less dense HSSW flows into the FRIS cavity -> (c) less dense ISW (via the Gade line) out -> (d) reduced strength of the density barrier at the Filchner sill.

    (2) The winds off Coats Land controlling (a) the shoreward Ekman transport -> (b) depth of the thermocline.

    It would be interesting to compare your atmospheric forcing with ours causing the onshore flow in the scenario simulations. I could imagine that the atmos forcing in your ESM somehow doesn't refelct pre-industrial and 20th-centruy conditions. The differences might be subtle, but as the structure at the sill is very sensitive it might be crucial.

    And, it's a runaway system. Once to get the enhanced melt onto the continental shelf, you cannot stabalize the front at the shelf break, which might even support onshore flow of warm water further to the west.

    It would be really interesting for us to see your atmos forcing, because in the framework of the EU-project TiPACCS we promised to find out the range of atmos temp? precip? wind? forcing to either keep the southern Weddell Sea continental shelf cold or make it warm.

    And related to one of the questions - you might not find such a sensitivity again elsewhere around Antarctica because the Filchner Trough, strechting from the shelf break to the GL, is unique which makes the sill the Achilles' heal of FRIS.

    Looking forward to seeing your reply!

    Take good care - Hartmut 

    • AC1: Reply to CC1, Xylar Asay-Davis, 08 May 2020

      Hi Hartmut,

      Thanks very much for your comment, both here and in the chat!

      > Unfortunately, you don't show bottom temps, but I assume all the heat is getting into the cavity via the Filchner Trough. 

      I don't feel comfortable publicly posting all the analysis for this run but here is the bottom temperature averaged over a period where we see the melt instability.  The warm water is definitely entering through the Filchner Trough.

      > ... two aspects of the atmospheric forcing play a role:

      > (1) A warmer atmosphere causing (a) less sea ice formation on the wide continental shelf -> (b) less dense HSSW flows into the FRIS cavity -> (c) less dense ISW (via the Gade line) out -> (d) reduced strength of the density barrier at the Filchner sill.

      > (2) The winds off Coats Land controlling (a) the shoreward Ekman transport -> (b) depth of the thermocline.

      This is what we were expecting as well.  But both runs with 2D GM and 3D GM coefficients experience the cascade you outline in (1), with the only difference being how stratified the density changes are (quite stratified with 2D GM, much less so with 3D GM).  The sea floor density is actually *lower* in the run where we don't see warm WDW getting into the Flichner Trough, though the surface is denser (stratification is quite low).

      Our runs are preindustrial control runs so there is natural variability but no overall difference in trends of winds that we could find, so it's not clear how (2) can play a significant role.

      More shortly...