Project OPEN | Katherine Hutchinson Oceanography

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

What is bottom water?

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 Meridional Overturning Circulation (MOC). 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.

Do we model this?

At present, none of the Coupled Model Intercomparison Project (CMIP6) models whose projections inform the IPCC explicitly simulate the circulation within ice shelf cavities. Their projections therefore neglect the feedback effects between the ice shelf and the ocean under various emission scenarios. The Nucleus for European Modeling of the Ocean model (NEMO) is the ocean model for 6 of the climate groups participating in CMIP. In most NEMO-based climate models, a “wall” is located at the mouth of the ice shelf cavities and the absence of explicit circulation under ice shelves is addressed via an input of freshwater at this wall . Dense water in these models is consequently formed rather unrealistically via deep convection offshore. This means that the feedback of ocean conditions onto the ice shelf and dense water formation on the continental shelf is a missing puzzle piece in the models that are used to help inform climate policy and adaptation strategies. As a result, obtaining a better understanding of these processes and including sub-ice shelf cavities in global models is presently regarded as a priority.

What to do:

Project OPEN aims to better understand the key processes controlling dense water production and export in Antarctica, both in observations and in numerical models. Once these key indicators have been developed, we can assess the realism of AABW in NEMO (ocean component) configurations used in the CMIP6 climate models, as well as the most recent state-of-the-art version of NEMO. In parallel to developing these metrics, we plan to work on improving the capability of the current NEMO configuration for the next generation of climate models so as to better simulate sub-ice shelf cavity circulation, dense water production and AABW export.

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 would 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.
Identify key indicators describing dense water production and export in Antarctica. Use observational data, reanalysis products and high resolution model outputs to develop these indicators and compare with output from NEMO global ocean configurations.
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

Getting started using ISOMIP+K:

Before explicitly simulating sub-ice shelf cavity circulation in the NEMO global 1° ocean model (named eORCA1), some choices need to be made regarding various parameters such as initial conditions, parameterizations and mixing underneath the ice shelves. To test model sensitivity to these choices and the feedback effects of biases, large super-computing costs associated with running a global configuration are incurred. To address this, we adapted a NEMO configuration of the Ice Shelf-Ocean Model Intercomparison Project (ISOMIP) to essentially be a slice of the global ocean model (eORCA1) in the location of our ice shelves of interest. The picture to the right shows the bathymetry for the configuration for Filchner-Ronne Ice Shelf. The model gird is regular, but matches the mean grid size from eORCA1, the bathymetry matches eORCA1 and the simulation has 75 vertical layers just like eORCA1. The reason for not using the default ISOMIP+ setup is that this configuration was not appropriate for modelling the deep, cold Antarctic cavities responsible for forming the dense parent waters of AABW.

The adapted version is named ISOMIP+K.

As opposed to regional high resolution Southern Ocean configurations, the ISOMIP+K configurations are designed so that the lessons learnt are directly transferable to a global ocean configuration where each choice made is backed-up by extensive, yet affordable, testing.

Below are some videos of the evolution of ice shelf melt (left) and a cross section of temperature (right) of 8 years of ISOMIP+K for Filcher-Ronne Ice Shelf.