Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?

Abstract A generic shortcoming of constant‐depth (or “z‐coordinate”) ocean models such as MOM5 and Nucleus for European Models of the Ocean (NEMO) is a tendency for the advection scheme to produce unphysical numerical diapycnal mixing, which may exceed the explicitly parameterized mixing based on ob...

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Autores principales:	Alex Megann, David Storkey
Formato:	article
Lenguaje:	EN
Publicado:	American Geophysical Union (AGU) 2021
Materias:	climate numerical mixing ocean model viscosity Physical geography GB3-5030 Oceanography GC1-1581
Acceso en línea:	https://doaj.org/article/5254279668d2467190cb59ef544e8468
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spelling	oai:doaj.org-article:5254279668d2467190cb59ef544e84682021-11-24T08:11:42ZExploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?1942-246610.1029/2020MS002263https://doaj.org/article/5254279668d2467190cb59ef544e84682021-05-01T00:00:00Zhttps://doi.org/10.1029/2020MS002263https://doaj.org/toc/1942-2466Abstract A generic shortcoming of constant‐depth (or “z‐coordinate”) ocean models such as MOM5 and Nucleus for European Models of the Ocean (NEMO) is a tendency for the advection scheme to produce unphysical numerical diapycnal mixing, which may exceed the explicitly parameterized mixing based on observed physical processes. Megann (2018, https://doi.org/10.1016/j.ocemod.2017.11.001) estimated the effective diapycnal diffusivity in the Global Ocean Version 5.0 (GO5.0) 0.25° global implementation of the NEMO model and showed that this was up to 10 times the explicit diffusivity used in the model's mixing scheme and argued that this was at least partly caused by large transient vertical velocities on length scales comparable to the horizontal grid scale. The current UK global NEMO configuration GO6, as used in the Global Coupled Model version 3.1 (GC3.1) and UK Earth System Model (UKESM1), is integrated in forced mode at 0.25° resolution with a range of viscosity parameterizations. In the present study, the effective diffusivity is evaluated for each integration and compared with the explicit value from the model mixing scheme, as well as with that in the control (using the default viscosity). It is shown that there is a strong correspondence between lower viscosity and enhanced numerical mixing and that larger viscosities lead to a marked reduction in the unrealistic internal temperature drift seen in the control configuration, without incurring excessive damping of the large‐scale circulation, mixed layer depths, or sea ice cover. The results presented here will inform the choices made in global ocean configurations used in climate and Earth System models following the sixth Coupled Model Intercomparison Project (CMIP6).Alex MegannDavid StorkeyAmerican Geophysical Union (AGU)articleclimatenumerical mixingocean modelviscosityPhysical geographyGB3-5030OceanographyGC1-1581ENJournal of Advances in Modeling Earth Systems, Vol 13, Iss 5, Pp n/a-n/a (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	climate numerical mixing ocean model viscosity Physical geography GB3-5030 Oceanography GC1-1581
spellingShingle	climate numerical mixing ocean model viscosity Physical geography GB3-5030 Oceanography GC1-1581 Alex Megann David Storkey Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?
description	Abstract A generic shortcoming of constant‐depth (or “z‐coordinate”) ocean models such as MOM5 and Nucleus for European Models of the Ocean (NEMO) is a tendency for the advection scheme to produce unphysical numerical diapycnal mixing, which may exceed the explicitly parameterized mixing based on observed physical processes. Megann (2018, https://doi.org/10.1016/j.ocemod.2017.11.001) estimated the effective diapycnal diffusivity in the Global Ocean Version 5.0 (GO5.0) 0.25° global implementation of the NEMO model and showed that this was up to 10 times the explicit diffusivity used in the model's mixing scheme and argued that this was at least partly caused by large transient vertical velocities on length scales comparable to the horizontal grid scale. The current UK global NEMO configuration GO6, as used in the Global Coupled Model version 3.1 (GC3.1) and UK Earth System Model (UKESM1), is integrated in forced mode at 0.25° resolution with a range of viscosity parameterizations. In the present study, the effective diffusivity is evaluated for each integration and compared with the explicit value from the model mixing scheme, as well as with that in the control (using the default viscosity). It is shown that there is a strong correspondence between lower viscosity and enhanced numerical mixing and that larger viscosities lead to a marked reduction in the unrealistic internal temperature drift seen in the control configuration, without incurring excessive damping of the large‐scale circulation, mixed layer depths, or sea ice cover. The results presented here will inform the choices made in global ocean configurations used in climate and Earth System models following the sixth Coupled Model Intercomparison Project (CMIP6).
format	article
author	Alex Megann David Storkey
author_facet	Alex Megann David Storkey
author_sort	Alex Megann
title	Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?
title_short	Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?
title_full	Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?
title_fullStr	Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?
title_full_unstemmed	Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?
title_sort	exploring viscosity space in an eddy‐permitting global ocean model: is viscosity a useful control for numerical mixing?
publisher	American Geophysical Union (AGU)
publishDate	2021
url	https://doaj.org/article/5254279668d2467190cb59ef544e8468
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Exploring Viscosity Space in an Eddy‐Permitting Global Ocean Model: Is Viscosity a Useful Control for Numerical Mixing?

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