Preprints
https://doi.org/10.5194/gmd-2020-307
https://doi.org/10.5194/gmd-2020-307

Submitted as: development and technical paper 24 Nov 2020

Submitted as: development and technical paper | 24 Nov 2020

Review status: a revised version of this preprint is currently under review for the journal GMD.

A Schwarz iterative method to evaluate ocean- atmosphere coupling schemes. Implementation and diagnostics in IPSL-CM6-SW-VLR

Olivier Marti1, Sébastien Nguyen1,a, Pascale Braconnot1, Sophie Valcke2, Florian Lemarié3, and Eric Blayo3 Olivier Marti et al.
  • 1Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France
  • 2CECI, Université de Toulouse, CNRS, CERFACS, Toulouse, France
  • 3Univ. Grenoble Alpes, Inria, CNRS, Grenoble INP, LJK, 38000, Grenoble, France
  • anow at: Laboratoire d'Océanographie et du Climat: Expérimentations et Approches Numériques, Sorbonne Université/CNRS, France

Abstract. State-of-the-art Earth System models, like the ones used in the CMIP6 intercomparison project, suffer from temporal inconsistencies at the ocean-atmosphere interface. Indeed, the coupling algorithms generally implemented in those models do not allow for a correct phasing between the ocean and the atmosphere, and hence between their diurnal cycles. A possibility to remove these temporal inconsistencies is to use an iterative coupling algorithm based on Schwarz methods. Despite the fact that the computational cost is large compared to standard coupling methods, which makes the method impractical as is for production runs, Schwarz algorithms are useful to evaluate some of the errors made in state-of-the-art ocean-atmosphere coupled models (e.g. in the representation of the processes related to diurnal cycle), as illustrated by the present study. A new coupling scheme based on such iterative method has been implemented in the IPSL coupled model. Comparisons between coupled solutions obtained with this new scheme and the standard IPSL coupling scheme show large differences at sunrise and sunset, when the external forcing (insolation at top of atmosphere) has the fastest pace of change. At these times of the day, the difference between the two numerical solutions is often larger than 100 % of the solution, even with a small coupling time step, thus suggesting that significant errors are potentially made with current coupling methods. Most of those differences can be strongly reduced by making only two iterations of the Schwarz method which leads to a doubling of the computing cost. A thorough design of the first guess to initialize the iterative process may yield a solution that reduces the error with only one iteration. The present study focuses on the ocean-atmosphere interface, with no sea-ice. The problem with three components (ocean/sea-ice/atmosphere) remains to be investigated.

Olivier Marti et al.

 
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Status: final response (author comments only)
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Olivier Marti et al.

Data sets

A Schwarz iterative method to evaluate ocean- atmosphere coupling schemes. Implementation and diagnostics in IPSL-CM6-SW-VLR. GMD-2020-307 Olivier Marti, Sebastien Nguyen, Pascale Braconnot, Sophie Valcke, Florian Lemarié, and Eric Blayo https://doi.org/10.5281/zenodo.4273949

Model code and software

A Schwarz iterative method to evaluate ocean- atmosphere coupling schemes. Implementation and diagnostics in IPSL-CM6-SW-VLR. GMD-2020-307 Olivier Marti, Sebastien Nguyen, Pascale Braconnot, Sophie Valcke, Florian Lmarié, and Eric Blayo https://doi.org/10.5281/zenodo.4273949

Olivier Marti et al.

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Short summary
State-of-the-art Earth System models, like the ones used in the CMIP6 intercomparison project, suffer from temporal inconsistencies at the ocean-atmosphere interface. In this study, a mathematically consistent iterative Schwarz method is used as a reference. It's tremendous computational cost make it unusable for production runs. But it allow us to evaluate the error made when using legacy coupling schemes. The impact on the climate at longer time scale, from days to decades, is not evaluated.