Three models, one earth system

Research background

The Ross Sea sits at the heart of Antarctica's climate system. As one of the coldest places on earth, its interactions between ocean, atmosphere, and sea ice shape both local ecosystems and the global climate system. 

It's also at the heart of climate research being led by Alexandra Gossart and Andrew Pauling, researchers at Victoria University of Wellington, and University of Canterbury researcher Alena Malyarenko. 

Their work focuses on improving climate projections of the Ross Sea. In particular, they're planning to run simulations with both recent sea ice conditions and future policy-relevant scenarios. They are building on an existing coupled atmosphere-ocean-sea ice model of this region by coupling a more sophisticated sea ice model into the existing model framework.

Figure 1: Sea ice concentration modelled in the Ross Sea region of Antarctica. Image courtesy of Alexandra Gossart and Alena Malyarenko.

Project challenges

State-of-the-art parallel numerical models for simulating atmospheric, oceanic, and sea-ice processes have been developed over the past several decades. Each model independently advances a system of equations in time, requiring appropriate initial and boundary conditions.

For these three models to operate seamlessly as a coupled system, their execution step must be carefully choreographed. Each component advances a specified number of time steps before exchanging key quantities—such as fluxes, temperature, pressure, and salinity—with other components.

This coordination is managed by a coupler: a dedicated software layer designed to handle the exchange of fields between parallel, distributed model components. The coupler must understand how each exchange field is partitioned across processors and perform any necessary regridding when importing data from components that operate on different computational grids.

Figure 2: Pictured above is a schemetic representation of a coupled model. Each component has its own set of MPI processors. The ESMF/NUOPC coupler extends over the atmosphere, ocean and ice components, and is responsible for exchanging data between components. Image courtesy of Andrew Pauling.

What was done

REANNZ Research Software Engineers Alex Pletzer and Chris Zweck collaborated with Alexandra, Alena, and Andrew to develop coupling software based on the Earth System Modeling Framework (ESMF).

ESMF provides a comprehensive suite of Fortran interfaces and utilities specifically designed to couple independently developed model components into a unified, parallel Earth system application.

Main outcomes

A parallel, containerised program was developed to demonstrate the coupling of three components representing the atmosphere, ocean, and sea ice. Each component runs concurrently within its own MPI communicator, enabling scalable execution across distributed computing resources.

The framework is designed to be extensible, allowing additional exchange fields and coupling pathways to be incorporated as needed. This prototype provides a robust foundation for the development of a fully coupled WRF–MITgcm–ice system.

Figure 3: The image above shows a mock field that belongs to the atmosphere component (left) and is accessed by the ice component (right). Each component has its own domain decomposition and grid resolution. The coupler takes care of regridding the field across disjoint processors. Image courtesy of Andrew Pauling.

Researcher feedback

This Consultancy project has been very helpful in starting to set up this new coupled model. Working through the toy examples developed by Alex Pletzer and understanding each step in the coupling process has already proved valuable in coupling the real sea ice model in the existing framework.

- Andrew Pauling, Research Fellow, Antarctic Research Centre, Victoria University of Wellington

Image credit (featured at top of page): Francisco Ardini / ©PNRA / CC-BY-SA 4.0

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