Opening sub-ice shelf cavities and exploring their impact on dense water Production and Export in NEMO global ocean models

Project Information

OPEN

Grant agreement ID: 898058

DOI

10.3030/898058

Project terminated on 19 August 2022

EC signature date 6 April 2020

Start date 1 October 2020

End date 30 September 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 196 707,84

EU contribution

€ 196 707,84

Coordinated by

SORBONNE UNIVERSITE
France

Periodic Reporting for period 1 - OPEN (Opening sub-ice shelf cavities and exploring their impact on dense water Production and Export in NEMO global ocean models)

Reporting period: 2020-10-01 to 2022-09-30

Summary of the context and overall objectives of the project

Summary:

Antarctic Bottom Water (AABW) is the densest and deepest water-mass in the world, formed via interactions between the atmosphere, ice sheets, sea ice and ocean. In winter in Antarctica surface waters lose their buoyancy, circulate beneath ice shelves and cascade down the continental slope to the deep ocean. AABW then circulates around the world’s oceans as the lower branch of the global ocean conveyor-belt. Perturbations in AABW properties (e.g. warming due to climate change and/or freshening from melting ice sheets) would thus have drastic consequences for global ocean properties, ecosystems and climate. The formation of the dense parent waters of AABW depends on a host of complex ice-ocean-bathymetry interactions within ice shelf cavities. At present, however, none of the Coupled Model Intercomparison Project (CMIP6) models whose projections inform the Intergovernmental Panel on Climate Change (IPCC), explicitly simulate the circulation within ice shelf cavities. Their projections neglect the feedback effects between the ice shelf and the ocean under various emission scenarios. This has resulted in a low confidence being attributed to certain projections for the Southern Ocean. Obtaining a better understanding of these processes and including sub-ice shelf cavities in global models is presently regarded as a priority. As a first step to addressing this challenge, Project OPEN has worked on opening sub-ice shelf cavity circulation under the three largest cold core ice shelves of Antarctica: Filchner Ronne, Ross and Larsen C. To do so we have used the latest version (v4.2) of the Nucleus for European Modeling of the Ocean model (NEMO) which is the ocean model for 6 of the climate groups participating in CMIP. Given the global importance of AABW, obtaining a better understanding of the dynamics of its source regions, and improving model capacity to simulate these processes is a scientific top priority.

Objectives:

Update the NEMO 1° global ocean configuration to open the large cold sub-ice shelf cavities responsible for dense water formation (namely Filchner-Ronne, Ross and Larsen C). We thereby allow for explicit ocean circulation within these cold cavities and keep the smaller Antarctic sub-ice shelf cavities closed for the time being.

Use an adapted version of the Ice Shelf-Ocean Model Inter-comparison Experiment (ISOMIP+), named ISOMIP+K as a tool for sensitivity testing to initial conditions, various coefficients and numerical schemes. ISOMIP+K is essentially a slice of the global ocean model for each of the ice shelves of interest, and so can be used for testing but is much quicker (and cheaper) to run than the global model.

Use observational data, reanalysis products and high resolution model outputs to validate NEMO global ocean configuration output in the Weddell and Ross Seas.

Explore the role of sub-ice shelf cavities and the various key processes taking place on the continental shelf in determining the nature, structure and properties of dense water formation.

Improve the capacity of NEMO to simulate critical dense water formation processes so as to provide new and improved configurations - fit for purpose for the next generation of coupled-climate models

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Summary: Results have shown that the NEMO global ocean 1° configuration has sufficient resolution at high latitudes to represent realistic melt rates and melt rate patterns of these three large ice shelves. Explicitly simulating circulation inside the cavities has also improved the realism of circulation on the continental shelf, acting to spread out and transform the hyper saline water that otherwise builds up on the continental shelf. Direct comparison of model output with in-situ observations across the ice shelf front illustrates the great improvement in representation of water mass properties and distribution and gives us confidence in the use of this configuration for the next generation of climate models.

Steps:

1) Use an idealized model to perform sensitivity tests and provide initial conditions for under the large ice shelves

2) Prepare the configuration set-up and run the latest version of NEMO (4.2) for 2 CORE forcing cycles, with and without open sub-ice shelf cavities.

3) Validate the model output via comparison with World Ocean Atlas maps, in-situ Conductivity-Temperature-Depth (CTD) measurements, Mixed Layer Depth Atlases and other high resolution model results.

Outreach and Dissemination:

Paper in progress: Hutchinson, K., J. Deshaye, C. Éthé, C. Rousset, C de Lavergne, M. Vancoppenolle, N. Jourdain and P. Mathiot. Improving the realism of NEMO for climate applications by explicitly simulating the large sub-ice shelf seas

In-person presentation at the EGU General Assembly 2022 titled: Increasing complexity of NEMO for climate applications by explicitly simulating the large sub-ice shelf seas

Virtual presentation at the EGU General Assembly 2021 titled: Navigating the challenges of explicitly including ocean-ice shelf interactions in a global ocean model using an adapted ISOMIP+ configuration as a fit-for-purpose tool.

Filmed, directed and edited Youtube video titled: Ocean Modelling: Sub-Ice Shelf Cavities
link: https://youtu.be/X8ie74cQfQg

Collaborative filming and editing with the Royal Meteorological Society on a video titled: Our Climate and the Ocean.
link: https://youtu.be/hv3R0aMai00

Wrote a chapter for a book published by the Royal Meteorological Society and British National History Museum titled: "Weather: A Force of Nature’"

Popular science article for the French "One Ocean Summit" titled: L’océan sous les glaces de l’Antarctique
link: https://www.meretmarine.com/fr/content/oos-polaire-locean-sous-les-glaces-de-lantarctique?xtor=EPR-56-20120118[Newsletter_V2_Drupal]-20220207-[_2]

Created the NEMO Weddell Sea Demonstrator, a workable example for beginners: 10.5281/zenodo.6817000

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Main scientific and/or technological achievements:

- New method for obtaining model initial conditions under the ice shelves is proposed.
- New configuration of eORCA1 with large sub-ice shelf cavities open was developed and is now being considered for use in the next generation of the Coupled Model Intercomparison Project (CMIP).
- Direct comparison with in-situ CTD transects in Antarctica is now being considered a valuable step in model testing and validation.

Potential users and climate applications:

The results from this study will be incorporated into the NEMO ocean model configuration eORCA1. If the eORCA1 configuration of NEMO developed by Project OPEN is chosen for the the next phase of CMIP (7th phase currently being discussed) then this configuration will be used to forecast ocean conditions in various emission scenarios which are summarized and presented in the reports from the Intergovernmental Panel on Climate Change (IPCC). This is currently a topic under discussion and the configuration is being explored for use in the IPSL coupled climate model.

Future work:
Work is in currently in progress to include the effect of tides within the cavities. We then plan to open the ice shelf cavities where the global model grid resolution is sufficient to do so, and employ a parameterization for the small ice shelves and for the inlets of large cavities that are too small to be resolved.

Graphic showing sub-ice shelf cavity circulation