Log-Normal Turbulence Dissipation in Global Ocean Models

<p>Numerical mixing, the physically spurious diffusion of tracers due to the numerical discretization of advection, is known to contribute to biases in ocean circulation models. However, quantifying numerical mixing is non-trivial, with most studies utilizing specifically targeted experiments in idealized settings. Here, we present a precise method based on water-mass transformation for quantifying numerical mixing, including its spatial structure, that can be applied to any conserved variable in global general circulation ocean models. The method is applied to a suite of global MOM5 ocean-sea ice model simulations with differing grid spacings and sub-grid scale parameterizations. In all configurations numerical mixing drives across-isotherm heat transport of comparable magnitude to that associated with explicitly-parameterized mixing. Numerical mixing is prominent at warm temperatures in the tropical thermocline, where it is sensitive to the vertical diffusivity and resolution. At colder temperatures, numerical mixing is sensitive to the presence of explicit neutral diffusion, suggesting that much of the numerical mixing in these regions acts as a proxy for neutral diffusion when it is explicitly absent. Comparison of equivalent (with respect to vertical resolution and explicit mixing parameters) 1/4-degree and 1/10-degree horizontal resolution configurations shows only a modest enhancement in numerical mixing at the eddy-permitting 1/4-degree resolution. Our results provide a detailed view of numerical mixing in ocean models and pave the way for future improvements in numerical methods.</p>

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Statistical Comparisons of Temperature Variance and Kinetic Energy in Global Ocean Models and Observations: Results From Mesoscale to Internal Wave Frequencies

Journal of Geophysical Research Oceans ◽

10.1029/2019jc015306 ◽

2020 ◽

Vol 125 (5) ◽

Author(s):

Conrad A. Luecke ◽

Brian K. Arbic ◽

James G. Richman ◽

Jay F. Shriver ◽

Matthew H. Alford ◽

...

Keyword(s):

Kinetic Energy ◽

Internal Wave ◽

Global Ocean ◽

Temperature Variance ◽

Ocean Models

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Interactive comment on “Prospects for improving the representation of coastal and shelf seas in global ocean models” by Jason Holt et al.

10.5194/gmd-2016-145-rc2 ◽

2016 ◽

Author(s):

Roger Proctor

Keyword(s):

Global Ocean ◽

Shelf Seas ◽

Ocean Models

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Stochastic Subgrid-Scale Ocean Mixing: Impacts on Low-Frequency Variability

Journal of Climate ◽

10.1175/jcli-d-16-0539.1 ◽

2017 ◽

Vol 30 (13) ◽

pp. 4997-5019 ◽

Cited By ~ 16

Author(s):

Stephan Juricke ◽

Tim N. Palmer ◽

Laure Zanna

Keyword(s):

Southern Ocean ◽

Interannual Variability ◽

Time Scales ◽

High Frequency ◽

Low Frequency ◽

Global Ocean ◽

Small Scale ◽

Subgrid Scale ◽

Low Frequency Variability ◽

Ocean Models

In global ocean models, the representation of small-scale, high-frequency processes considerably influences the large-scale oceanic circulation and its low-frequency variability. This study investigates the impact of stochastic perturbation schemes based on three different subgrid-scale parameterizations in multidecadal ocean-only simulations with the ocean model NEMO at 1° resolution. The three parameterizations are an enhanced vertical diffusion scheme for unstable stratification, the Gent–McWilliams (GM) scheme, and a turbulent kinetic energy mixing scheme, all commonly used in state-of-the-art ocean models. The focus here is on changes in interannual variability caused by the comparatively high-frequency stochastic perturbations with subseasonal decorrelation time scales. These perturbations lead to significant improvements in the representation of low-frequency variability in the ocean, with the stochastic GM scheme showing the strongest impact. Interannual variability of the Southern Ocean eddy and Eulerian streamfunctions is increased by an order of magnitude and by 20%, respectively. Interannual sea surface height variability is increased by about 20%–25% as well, especially in the Southern Ocean and in the Kuroshio region, consistent with a strong underestimation of interannual variability in the model when compared to reanalysis and altimetry observations. These results suggest that enhancing subgrid-scale variability in ocean models can improve model variability and potentially its response to forcing on much longer time scales, while also providing an estimate of model uncertainty.

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Intrabasin comparison of surface radiocarbon levels in the Indian Ocean between coral records and three-dimensional global ocean models

Global Biogeochemical Cycles ◽

10.1029/2004gb002289 ◽

2005 ◽

Vol 19 (2) ◽

pp. n/a-n/a ◽

Cited By ~ 5

Author(s):

Nancy S. Grumet ◽

Philip B. Duffy ◽

Michael E. Wickett ◽

Ken Caldeira ◽

Robert B. Dunbar

Keyword(s):

Indian Ocean ◽

Three Dimensional ◽

Global Ocean ◽

The Indian Ocean ◽

Ocean Models

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Global ocean storage of anthropogenic carbon

Biogeosciences Discussions ◽

10.5194/bgd-9-8931-2012 ◽

2012 ◽

Vol 9 (7) ◽

pp. 8931-8988 ◽

Cited By ~ 13

Author(s):

S. Khatiwala ◽

T. Tanhua ◽

S. Mikaloff Fletcher ◽

M. Gerber ◽

S. C. Doney ◽

...

Keyword(s):

Ocean Circulation ◽

Global Scale ◽

Global Ocean ◽

Observational Methods ◽

Robust Estimates ◽

Anthropogenic Carbon ◽

Ocean Models ◽

Biogeochemical Parameters ◽

Range Of Values ◽

Made In

Abstract. The global ocean is a significant sink for anthropogenic carbon (Cant), absorbing roughly a third of human CO2 emitted over the industrial period. Robust estimates of the magnitude and variability of the storage and distribution of Cant in the ocean are therefore important for understanding the human impact on climate. In this synthesis we review observational and model-based estimates of the storage and transport of Cant in the ocean. We pay particular attention to the uncertainties and potential biases inherent in different inference schemes. On a global scale, three data based estimates of the distribution and inventory of Cant are now available. While the inventories are found to agree within their uncertainty, there are considerable differences in the spatial distribution. We also present a review of the progress made in the application of inverse and data-assimilation techniques which combine ocean interior estimates of Cant with numerical ocean circulation models. Such methods are especially useful for estimating the air-sea flux and interior transport of Cant, quantities that are otherwise difficult to observe directly. However, the results are found to be highly dependent on modeled circulation, with the spread due to different ocean models at least as large as that from the different observational methods used to estimate Cant. Our review also highlights the importance of repeat measurements of hydrographic and biogeochemical parameters to estimate the storage of Cant on decadal timescales in the presence of the variability in circulation that is neglected by other approaches. Data-based Cant estimates provide important constraints on ocean forward models, which exhibit both broad similarities and regional errors relative to the observational fields. A compilation of inventories of Cant gives us a "best" estimate of the global ocean inventory of anthropogenic carbon in 2010 of 155 Pg C with an uncertainty of ±20%. This estimate includes a broad range of values suggesting that a combination of approaches is necessary in order to achieve a robust quantification of the ocean sink of anthropogenic CO2.

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Scattering of internal tide energy to super-tidal frequencies in global HYCOM

10.5194/egusphere-egu21-13990 ◽

2021 ◽

Author(s):

Miguel Solano ◽

Maarten Buijsman

Keyword(s):

Internal Tide ◽

Tidal Energy ◽

Horizontal Resolution ◽

Internal Tides ◽

Numerical Dispersion ◽

Global Ocean ◽

Wave Interactions ◽

Continental Shelves ◽

Lee Wave ◽

Ocean Models

<p>Energy decay in realistically forced global ocean models has been mostly studied in the diurnal and semi-diurnal tidal bands and it is unclear how much of the tidal energy in these bands is scattered to higher frequencies. Global ocean models and satellite altimetry have shown that low-mode internal tides can propagate thousands of kilometers from their generation sites before being dissipated in the ocean interior but their pathway to dissipation is obscured due to lee-wave breaking at generation, wave-wave interactions, topographic scattering, shearing instabilities and shoaling on continental shelves. Internal tides from some generation sites, such as the Amazon shelf and the Nicobar and Andaman island chain, have large amounts of energy resulting in a steepening of the internal waves into solitary wave trains due to non-hydrostatic dispersion. In HYCOM, a hydrostatic model, this process is partially simulated by numerical dispersion. However, it is yet unknown how the dissipation of internal tides is affected by the numerical dispersion in hydrostatic models. In this study we use the method of vertical modes and rotary spectra to quantify the scattering of internal tides to higher-frequencies and analyze the dissipation processes in global HYCOM simulations with 4-km horizontal resolution.</p>

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