Article

Using a resolution function to regulate parameterizations of oceanic mesoscale eddy effects

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Abstract

Mesoscale eddies play a substantial role in the dynamics of the ocean, but the dominant length-scale of these eddies varies greatly with latitude, stratification and ocean depth. Global numerical ocean models with spatial resolutions ranging from 1° down to just a few kilometers include both regions where the dominant eddy scales are well resolved and regions where the model’s resolution is too coarse for the eddies to form, and hence eddy effects need to be parameterized. However, common parameterizations of eddy effects via a Laplacian diffusion of the height of isopycnal surfaces (a Gent–McWilliams diffusivity) are much more effective at suppressing resolved eddies than in replicating their effects. A variant of the Phillips model of baroclinic instability illustrates how eddy effects might be represented in ocean models. The ratio of the first baroclinic deformation radius to the horizontal grid spacing indicates where an ocean model could explicitly simulate eddy effects; a function of this ratio can be used to specify where eddy effects are parameterized and where they are explicitly modeled. One viable approach is to abruptly disable all the eddy parameterizations once the deformation radius is adequately resolved; at the discontinuity where the parameterization is disabled, isopycnal heights are locally flattened on the one side while eddies grow rapidly off of the enhanced slopes on the other side, such that the total parameterized and eddy fluxes vary continuously at the discontinuity in the diffusivity. This approach should work well with various specifications for the magnitude of the eddy diffusivities.

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... A common problem an ocean modeller is facing when he/ she deals with primitive equations is the numerical discretization in space and time. As described in Hallberg (2013), numerical ocean models need to represent the effects of mesoscale eddies, which are the typical horizontal scales of less than 100 km and timescales in the order of a month. When defining the spatial grid for the numerical integration of the primitive equations, it is important to account for the ratio of a model's grid spacing to the deformation radius, defined as: ...
... Once the mesh is defined, we move to the final step related to the primitive equations discretization by using numerical methods, which consist in transforming the mathematical model into an algebraic, nonlinear system of equations for the mesh-related unknown quantities. The horizontal resolution needed to resolve the first baroclinic deformation radius with two grid points, based on a 1/8º model on a Mercator grid on Jan 1 after one year of spinup from climatology (from Hallberg, 2013). ...
... However, in any of the two cases, numerical stability is dictated by the smallest grid element, which substantially increases the computational problem. An additional difficulty is that sub-grid parameterizations have to be valid throughout the domain, whatever the grid size and eddy resolution regime are (Hallberg, 2013). In the structured grid case, block structured refinement techniques enable to circumvent some of the aforementioned difficulties by allowing a stepwise change (over a given grid patch) of the space and time resolutions (by integer factors, Figure 5.5B). ...
Chapter
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The architecture of ocean monitoring and forecasting systems
... Eddies thus play an important role in regulating climate on regional and global scales and on timescales of weeks to centuries. Mesoscale eddies form on spatial scales near the baroclinic Rossby deformation radius (Smith and Vallis, 2002;Arbic and Flierl, 2004;Thompson and Young, 2007;Hallberg, 2013). The deformation radius varies regionally between 10-100 km horizontally (Chelton et al., 1998). ...
... As the horizontal grid spacing of climate models is refined, such that the grid box size becomes comparable to the deformation scale, a regime commonly referred to as the "gray zone" is reached. A gray zone is present in virtually all eddying simulations with large meridional extent and continental slopes (Hallberg, 2013). In this regime, some eddies are being partially resolved, but the resolution does not allow for their effects on the large-scale current and stratification to be fully accounted for. ...
... Here, we present an idealized model to capture the essence of mesoscale eddy dynamics at varying horizontal grid resolutions, investigate the effect of mesoscale eddies on the large-scale dynamics, and provide a framework for testing and evaluating eddy parameterizations. The model allows for a clean and extensive analysis of the dynamics and energetics of the flow as a function of horizontal resolution, which is often limited in primitive-equation and diabatic global models due to computational resources (Hewitt et al., 2020;McClean et al., 2011). We introduce a model configuration -referred to as NeverWorld2 (NW2), which is an extension of the Southern Hemisphere-only NeverWorld configuration presented in Khani et al. (2019) and Jansen et al. (2019). ...
Article
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We describe an idealized primitive-equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the Equator and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally include diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence.
... We also ran simulations of the two-layer stacked shallow water equations (34),(35) using the ocean general circulation model MOM6 [44,45]. We simulated an ocean current [46] that is baroclinically unstable [47,48], resulting in mesoscale eddy generation and geostrophic turbulence, which we visualize in Fig. 7. Details of the simulation setup can be found in [46]. The equations solved in MOM6 are ...
... We also ran simulations of the two-layer stacked shallow water equations (34),(35) using the ocean general circulation model MOM6 [44,45]. We simulated an ocean current [46] that is baroclinically unstable [47,48], resulting in mesoscale eddy generation and geostrophic turbulence, which we visualize in Fig. 7. Details of the simulation setup can be found in [46]. The equations solved in MOM6 are ...
... η 3/2 are the pressure of the layers 1 and 2 normalized by ρ 0 , the density of the top layer. ∆ρ is the density difference between the two layers, h m is the layer thickness, K h is the interface height diffusivity coefficient, and γ is a rate that is proportional to the mass flux between the layers [46]. K h allows for the dissipation of available potential energy [50] and γ forces the interface height between the two layers to reference zonal mean by Ekman pumping in the real ocean [48]. ...
Preprint
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We highlight the differing roles of vorticity and strain in the transport of coarse-grained scalars at length-scales larger than $\ell$ by smaller scale (subscale) turbulence. %subscale flux/stress which appear in the evolution of coarse-grained (resolved) scalars/momentum account for the effect of (subgrid) scales smaller than the coarse-graining length $\ell$. We use the first term in a multiscale gradient expansion due to Eyink \cite{Eyink06a}, which exhibits excellent correlation with the exact subscale physics when the partitioning length $\ell$ is any scale smaller than that of the spectral peak. We show that unlike subscale strain, which acts as an anisotropic diffusion/anti-diffusion tensor, subscale vorticity's contribution is solely a conservative advection of coarse-grained quantities by an eddy-induced non-divergent velocity, $\bv_*$, that is proportional to the curl of vorticity. Therefore, material (Lagrangian) advection of coarse-grained quantities is accomplished not by the coarse-grained flow velocity, $\OL\bu_\ell$, but by the effective velocity, $\OL\bu_\ell+\bv_*$, the physics of which may improve commonly used LES models.
... Eddies thus play an important role in regulating climate on regional and global scales and on timescales of weeks to centuries. Mesoscale eddies form on spatial scales near the baroclinic Rossby deformation radius (Smith and Vallis, 2002;Arbic and Flierl, 2004;25 Thompson and Young, 2007;Hallberg, 2013). The deformation radius varies regionally between 10-100 km horizontally (Chelton et al., 1998). ...
... 45 As the horizontal grid spacing of climate models is refined, such that the grid box size becomes comparable to the defor-mation scale, a regime commonly referred to as the "grey zone" is reached. In this regime, some eddies are being partially resolved but the resolution does not allow for their effects on the large scale current and stratification to be fully accounted for (Hallberg, 2013). In particular, the inverse kinetic energy cascade (or backscatter) and the barotrozipation of the flow remain too weak in both idealized (Jansen and Held, 2014) and global models (Kjellsson and Zanna, 2017). ...
... Eddies thus play an important role in regulating climate on regional and global scales and on timescales of weeks to centuries. Mesoscale eddies form on spatial scales near the baroclinic Rossby deformation radius (Smith and Vallis, 2002;Arbic and Flierl, 2004;25 Thompson and Young, 2007;Hallberg, 2013). The deformation radius varies regionally between 10-100 km horizontally (Chelton et al., 1998). ...
... 45 As the horizontal grid spacing of climate models is refined, such that the grid box size becomes comparable to the defor-mation scale, a regime commonly referred to as the "grey zone" is reached. In this regime, some eddies are being partially resolved but the resolution does not allow for their effects on the large scale current and stratification to be fully accounted for (Hallberg, 2013). In particular, the inverse kinetic energy cascade (or backscatter) and the barotrozipation of the flow remain too weak in both idealized (Jansen and Held, 2014) and global models (Kjellsson and Zanna, 2017). ...
Preprint
We describe an idealized primitive equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans, and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the equator, and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally have diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest, and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence.
... We chose to develop our method using a state estimate because such products offer (1) uniform coverage in latitude, longitude, and time as well as (2) relatively high fidelity with respect to observations. We chose B-SOSE, in particular, because it represents the Southern Ocean using a spatial resolution of 1/6 • , which is eddypermitting in the latitude range of the ACC, enabling a realistic representation of mesoscale eddy structure (Hallberg, 2013). We expect that training our model on the physics-only SOSE would produce similar results, although we did not attempt that here. ...
... River runoff comes from the product of Dai and Trenberth (2002) augmented with an estimate of Antarctic freshwater input from iceberg and ice sheet melting (Hammond and Jones, 2016). It does not include mesoscale eddy parameterisation, as this particular configuration falls into the horizontal resolution range wherein mesoscale parameterisation may actually worsen the representation of the mesoscale (Hallberg, 2013). Because we are interested in quantifying physical, large-scale fronts, we only used monthly mean temperature and salinity data. ...
Article
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Oceanographic fronts are transitions between thermohaline structures with different characteristics. Such transitions are ubiquitous, and their locations and properties affect how the ocean operates as part of the global climate system. In the Southern Ocean, fronts have classically been defined using a small number of continuous, circumpolar features in sea surface height or dynamic height. Modern observational and theoretical developments are challenging and expanding this traditional framework to accommodate a more complex view of fronts. Here, we present a complementary new approach for calculating fronts using an unsupervised classification method called Gaussian mixture modelling (GMM) and a novel inter-class parameter called the I-metric. The I-metric approach produces a probabilistic view of front location, emphasising the fact that the boundaries between water masses are not uniformly sharp across the entire Southern Ocean. The I-metric approach uses thermohaline information from a range of depth levels, making it more general than approaches that only use near-surface properties. We train the GMM using an observationally constrained state estimate in order to have more uniform spatial and temporal data coverage. The probabilistic boundaries defined by the I-metric roughly coincide with several classically defined fronts, offering a novel view of this structure. The I-metric fronts appear to be relatively sharp in the open ocean and somewhat diffuse near large topographic features, possibly highlighting the importance of topographically induced mixing. For comparison with a more localised method, we also use an edge detection approach for identifying fronts. We find a strong correlation between the edge field of the leading principal component and the zonal velocity; the edge detection method highlights the presence of jets, which are supported by thermal wind balance. This more localised method highlights the complex, multiscale structure of Southern Ocean fronts, complementing and contrasting with the more domain-wide view offered by the I-metric. The Sobel edge detection method may be useful for defining and tracking smaller-scale fronts and jets in model or reanalysis data. The I-metric approach may prove to be a useful method for inter-model comparison, as it uses the thermohaline structure of those models instead of tracking somewhat ad hoc values of sea surface height and/or dynamic height, which can vary considerably between models. In addition, the general I-metric approach allows front definitions to shift with changing temperature and salinity structures, which may be useful for characterising fronts in a changing climate.
... However, small-scale features in ocean circulation, which are not resolved at the grid spacing of most coupled climate models (∼1°), including submesoscale and mesoscale eddies and narrow coastal boundary currents, play a fundamental role in SO circulation and feedbacks between the open ocean and the Antarctic shelf (Dinniman et al., 2016;Goddard et al., 2017;Stewart & Thompson, 2015;Stewart et al., 2019). At the latitudes where the Antarctic continental slope is located, the horizontal grid spacing required to resolve mesoscale eddies is generally finer than 5 km and even much finer than 1 km in regions on the shelf (Hallberg, 2013). Experiments imposing individual wind or meltwater perturbations that have been performed using fine resolution models have been limited to global ocean sea-ice models (Lago & England, 2019;Moorman et al., 2020;Spence et al., 2014Spence et al., , 2017Waugh et al., 2019Waugh et al., , 2021 or high-resolution sector models (Snow et al., 2016). ...
... At 0.25° grid spacing, transient eddies are present in the simulation in the tropical and subtropical oceans, yet incompletely resolved at higher latitudes. The baroclinic Rossby deformation radius is ∼4 km near the Antarctic shelf (Hallberg, 2013), thus the CM4 grid spacing is insufficient to resolve the mesoscale eddy field on and near the Antarctic shelf. The baroclinicly induced submesoscale eddy restratification parameterization is based on a front length of 500 m in CM4. ...
Article
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We use two coupled climate models, GFDL‐CM4 and GFDL‐ESM4, to investigate the physical response of the Southern Ocean to changes in surface wind stress, Antarctic meltwater, and the combined forcing of the two in a pre‐industrial control simulation. The meltwater cools the ocean surface in all regions except the Weddell Sea, where the wind stress warms the near‐surface layer. The limited sensitivity of the Weddell Sea surface layer to the meltwater is due to the spatial distribution of the meltwater fluxes, regional bathymetry, and large‐scale circulation patterns. The meltwater forcing dominates the Antarctic shelf response and the models yield strikingly different responses along West Antarctica. The disagreement is attributable to the mean‐state representation and meltwater‐driven acceleration of the Antarctic Slope Current (ASC). In CM4, the meltwater is efficiently trapped on the shelf by a well resolved, strong, and accelerating ASC which isolates the West Antarctic shelf from warm offshore waters, leading to strong subsurface cooling. In ESM4, a weaker and diffuse ASC allows more meltwater to escape to the open ocean, the West Antarctic shelf does not become isolated, and instead strong subsurface warming occurs. The CM4 results suggest a possible negative feedback mechanism that acts to limit future melting, while the ESM4 results suggest a possible positive feedback mechanism that acts to accelerate melt. Our results demonstrate the strong influence the ASC has on governing changes along the shelf, highlighting the importance of coupling interactive ice sheet models to ocean models that can resolve these dynamical processes.
... These processes are parameterized or muted in typical CMIP5/CMIP6 coarse-resolution climate models (about 100 km resolution in the atmosphere and ocean), representing one of the main causes of uncertainty in climate projection. The high-resolution climate models can resolve mesoscale eddies in most of the ocean (apart from shallow bathymetry and polar regions) and better simulate the synoptic processes in the atmosphere (Delworth et al., 2012;Hallberg, 2013), provide a better representation of key processes (Griffies et al., 2015), and potentially offer more robust projections and predictions of climate variability and change (Haarsma et al., 2016;Hewitt et al., 2017). High-resolution models show significant improvements in simulating the mean state in the atmosphere and ocean, more realistic small-scale phenomena, and a better representation of extreme events such as heatwaves and floods. ...
... FGOALS-f3-H employs the eddy-rich ocean component model LICOM3.0, with 0.1° horizontal resolution, resolving the first Rossby radius of deformation over most of the ocean (Hallberg, 2013) and generating mesoscale eddies through barotropic and baroclinic instabilities. Therefore, FGOALS-f3-H enables a more nonlinear solution and, thus, a better representation of western boundary currents like Gulf Stream and Kuroshio. ...
Article
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Following the High-Resolution Model Intercomparison Project (HighResMIP) Tier 2 protocol under the Coupled Model Intercomparison Project Phase 6 (CMIP6), three numerical experiments are conducted with the Chinese Academy of Sciences Flexible Global Ocean-Atmosphere-Land System Model, version f3-H (CAS FGOALS-f3-H), and a 101-year (1950–2050) global high-resolution simulation dataset is presented in this study. The basic configuration of the FGOALS-f3-H model and numerical experiments design are briefly described, and then the historical simulation is validated. Forced by observed radiative agents from 1950 to 2014, the coupled model essentially reproduces the observed long-term trends of temperature, precipitation, and sea ice extent, as well as the large-scale pattern of temperature and precipitation. With an approximate 0.25° horizontal resolution in the atmosphere and 0.1° in the ocean, the coupled models also simulate energetic western boundary currents and the Antarctic Circulation Current (ACC), reasonable characteristics of extreme precipitation, and realistic frontal scale air-sea interaction. The dataset and supporting detailed information have been published in the Earth System Grid Federation (ESGF, https://esgf-node.llnl.gov/projects/cmip6/ ).
... 1/2 • reaches 4.5-6.5 km with the refinement to 1/10 • applied. Still, this must be considered only eddy-permitting poleward of 90 about 50 • latitude (Smith et al., 2000;Hallberg, 2013). In contrast, with a default 1/2 • ocean grid spacing the standard FOCI ocean clearly is non-eddying and the effect of mesoscale eddies is parameterized following Gent and Mcwilliams (1990) applying a space invariant eddy induced velocity coefficient (rn_aeiv_0) of 1000 m 2 /s. ...
Preprint
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Increasing Greenland Ice Sheet–melting is anticipated to impact watermass transformation in the subpolar North Atlantic and ultimately the meridional overturning circulation. Complex ocean and climate models are widely applied to predict magnitude and timing of related impacts under projected future climate. We discuss the role of the ocean mean state, subpolar gyre circulation, mesoscale eddies and atmospheric coupling in shaping the response of the subpolar North Atlantic Ocean to enhanced Greenland runoff. In a suite of eight dedicated 60 to 100-year long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated, with regular and significantly enlarged Greenland runoff, we find (1) a major impact by the interactive atmosphere in enabling a compensating temperature feedback, (2) a non-negligible influence by the ocean mean state biased towards greater stability in the coupled simulations, both of which making the Atlantic Merdional Overturning Circulation less susceptible to the freshwater perturbation applied, and (3) a more even spreading of the runoff tracer in the subpolar North Atlantic and enhanced inter-gyre exchange with the subtropics in the strongly eddying simulations. Overall, our experiments demonstrate the important role of mesoscale ocean dynamics and atmosphere feedbacks in projections of the climate system response to enhanced Greenland Ice Sheet–melting and hence underline the necessity to advance scale-aware eddy parameterizations for next-generation climate models.
... Due to expensive downscaling numerical simulations, the spatial resolution of the global ocean models is eddy-permitting at high latitudes and in marginal seas. Thus, we need regional eddy-resolving numerical simulations, in particular at high latitudes, where there are obstacles for the global ocean modeling and in situ observations of mesoscale dynamics induced by the significantly decreasing spatial size of mesoscale eddies in contrast to the mid-and low latitudes [20,21]. ...
Article
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The driving mechanisms of mesoscale processes and associated heat transport in the Japan/East Sea (JES) from 1990 to 2010 were examined using eddy-resolving ocean model simulations. The simulated circulation showed correctly reproduced JES major basin-scale currents and mesoscale dynamics features. We show that mesoscale eddies can deepen isotherms/isohalines up to several hundred meters and transport warm and low salinity waters along the western and eastern JES boundaries. The analysis of eddy kinetic energy (EKE) showed that the mesoscale dynamics reaches a maximum intensity in the upper 300 m layer. Throughout the year, the EKE maximum is observed in the southeastern JES, and a pronounced seasonal variability is observed in the southwestern and northwestern JES. The comparison of the EKE budget components confirmed that various mechanisms can be responsible for the generation of mesoscale dynamics during the year. From winter to spring, the baroclinic instability of basin-scale currents is the leading mechanism of the JES mesoscale dynamics’ generation. In summer, the leading role in the generation of the mesoscale dynamics is played by the barotropic instability of basin-scale currents, which are responsible for the emergence of mesoscale eddies, and in autumn, the leading role is played by instabilities and the eddy wind work. We show that the meridional heat transport (MHT) is mainly polewards. Furthermore, we reveal two paths of eddy heat transport across the Subpolar Front: along the western and eastern (along 138∘ E) JES boundaries. Near the Tsugaru Strait, we describe the detected intensive westward eddy heat transport reaching its maximum in the first half of the year and decreasing to the minimum by summer.
... We ran two additional cases of the sub-grid model with the resolution of ∼ 19.5 km and ∼ 39 km (256 and 128 grid points respectively) keeping the parameters identical to the mesoscale-resolving run except for numerical viscosity. As noted earlier, the first deformation radius is around 25 km, so the two resolutions can be considered mesoscale permitting (Hallberg 2013). The biharmonic viscosities were 4 = (6.25, ...
Article
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Mesoscale eddies, although being on the scales of O (20–100km), have a dispro-portionate role in shaping the mean stratification, which varies on the scale of O (1000km). With the increase in computational power, we are now able to partially resolve the eddies in basin-scale and global ocean simulations, a model resolution often referred to as mesoscale permitting. It is well-known, however, that due to grid-scale numerical viscosity, mesoscale-permitting simulations have less energetic eddies and consequently weaker eddy feedback onto the mean flow. In this study, we run a quasi-geostrophic model at mesoscale-resolving resolution in a double gyre configuration and formulate a deterministic closure for the eddy rectification term of potential vorticity (PV), namely, the eddy PV flux divergence. Our closure successfully reproduces the spatial patterns and magnitude of eddy kinetic and potential energy diagnosed from the mesoscale-resolving model. One novel point about our approach is that we account for non-local eddy feedbacks onto the mean flow by solving the ‘sub-grid’ eddy PV equation prognostically in addition to the mean PV.
... This technique permits ocean eddies over the whole core of the ACC when compared to the local Rossby radius of deformation 43 by concentrating grid points in that area (Fig. 2g, i). Over the ACC, the HR grid is thus similar in resolution to a 1/10°ocean model that is typically termed "eddyrich" over mid-latitudes, but stops being eddy-resolving in polar regions 43 (Supplementary Figs. 4 and 5). The HighResMIP experimental design involves complementary experiments at lower ocean resolution similar to the ocean resolution used in CMIP5 (AWI-CM-LR, Supplementary Fig. 4 and 5), without retuning the model to isolate the sole impact of resolution 44 . ...
Article
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Despite global warming and Arctic sea-ice loss, on average the Antarctic sea-ice extent has not declined since 1979 when satellite data became available. In contrast, climate model simulations tend to exhibit strong negative sea-ice trends for the same period. This Antarctic sea-ice paradox leads to low confidence in 21st-century sea-ice projections. Here we present multi-resolution climate change projections that account for Southern Ocean mesoscale eddies. The high-resolution configuration simulates stable September Antarctic sea-ice extent that is not projected to decline until the mid-21st century. We argue that one reason for this finding is a more realistic ocean circulation that increases the equatorward heat transport response to global warming. As a result, the ocean becomes more efficient at moderating the anthropogenic warming around Antarctica and hence at delaying sea-ice decline. Our study suggests that explicitly simulating Southern Ocean eddies is necessary for providing Antarctic sea-ice projections with higher confidence.
... As realistic ocean simulations with kilometric horizontal resolution have emerged (e.g., Rocha et al., 2016;Brodeau 20 et al., 2020;Gula et al., 2021;Ajayi et al., 2021), such a framework has become cumbersome with tera-and peta-bytes of data needed to be transferred and stored as ghost copies. Nevertheless, a real demand exists for collaboration to intercompare models to examine their fidelity and quantify robust features of submeso-and meso-scale turbulence (the former on the horizontal spatial scales of O(10 km) and latter on O(100 km); here on referred to jointly as (sub)mesoscale; Hallberg, 2013;McWilliams, 2016;Lévy et al., 2018;Uchida et al., 2019;Dong et al., 2020). The Ocean Model Intercomparison Project 25 (OMIP), for example, has been successful in diagnosing systematic biases in non-eddying and mesoscale-permitting ocean models used for global climate simulations (Griffies et al., 2016;Chassignet et al., 2020). ...
Preprint
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With the increase in computational power, ocean models with kilometer-scale resolution have emerged over the last decade. These models have been used for quantifying the energetic exchanges between spatial scales, informing the design of eddy parametrizations and preparing observing networks. The increase in resolution, however, has drastically increased the size of model outputs, making it difficult to transfer and analyze the data. Nonetheless, it is of primary importance to assess more systematically the realism of these models. Here, we showcase a cloud-based analysis framework proposed by the Pangeo Project that aims to tackle such distribution and analysis challenges. We analyze the output of eight submesoscale-permitting simulations, all on the cloud, for a crossover region of the upcoming Surface Water and Ocean Topography (SWOT) altimeter mission near the Gulf Stream separation. The models used in this study are run with the NEMO, CROCO, MITgcm, HYCOM, FESOM and FIO-COM code bases. The cloud-based analysis framework: i) minimizes the cost of duplicating and storing ghost copies of data, and ii) allows for seamless sharing of analysis results amongst collaborators. We describe the framework and provide example analyses (e.g., sea-surface height variability, submesoscale vertical buoyancy fluxes, and comparison to predictions from the mixed-layer instability parametrization). Basin-to-global scale, submesoscale-permitting models are still at their early stage of development; their cost and carbon footprints are also rather large. It would, therefore, benefit the community to document the different model configurations for future best practices. We also argue that an emphasis on data analysis strategies would be crucial for improving the models themselves.
... The ocean domain extends from 70 • S to 59 • S and from 129 • E to 151 • E. The grid is derived from the eORCA1 global tripolar grid which was refined to a resolution of 1∕24 • . The grid spacing ranges from 1.8 to 2.3 km, which is enough to resolve eddies away from the continental shelves (Hallberg 2013). The vertical discretization consists of 75 levels of increasing thickness from top to bottom (from 1 to 200 m). ...
Article
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Heat and momentum exchanges at the Southern Ocean surface are crucial for the Earth’s Climate, but the importance of the small-scale spatial variability of these surface fluxes is poorly understood. Here, we explore how small-scale heterogeneities of the surface conditions due in particular to ocean eddies affect the atmosphere–sea ice–ocean interactions off Adélie Land, in East Antarctica. To this end, we use a high-resolution regional atmosphere–sea ice–ocean coupled model based on the NEMO-LIM and MAR models. We explore how the atmosphere responds to small-scale heterogeneity of the ocean or sea ice surface conditions, how eddies affect the sea ice and atmosphere, and how the eddy-driven surface fluxes impact the heat, freshwater, and momentum budget of the ocean. The atmosphere is found to be more sensitive to small-scale surface temperature gradients above the ice-covered than above the ice-free ocean. Sea ice concentration is found to be weaker above anticyclonic than cyclonic eddies due to increased sea ice melting or freezing (0.8 cm/day) partly compensated by sea ice convergence or divergence. The imprint of ice-free eddies on the atmosphere is weak, but in the presence of sea ice, air warming (+ 0.3 $$^{\circ }$$ ∘ C) and wind intensification (+ 0.1 m/s) are found above anticyclonic eddies, while cyclonic eddies have the opposite effects. Removing the interactions of eddies with the sea ice or atmosphere does not affect the total sea ice volume, but increases the ocean kinetic energy by 8% and weakens northward advection of sea ice, leading to a 15% decrease in freshwater flux north of 62.5 $$^{\circ }$$ ∘ S and weaker ocean restratification.
... The AR5 noted problems with the simulation of clouds in this region which were later attributed to a lack of supercooled liquid clouds (Bodas-Salcedo et al., 2016). Mesoscale ocean processes are critical to maintaining the Southern Ocean stratification and response to wind forcing (Marshall and Radko, 2003;Hallberg and Gnanadesikan, 2006), and their explicit representation requires even higher ocean resolution (Hallberg, 2013). Similarly, atmospheric convection remains unresolved even in the highest-resolution climate models participating in HighResMIP. ...
Conference Paper
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The evidence for human influence on recent climate change strengthened from the IPCC Second Assessment Report to the IPCC Fifth Assessment Report, and is now even stronger in this assessment. The IPCC Second Assessment Report (1995) concluded ‘the balance of evidence suggests that there is a discernible human influence on global climate’. In subsequent assessments (TAR, 2001; AR4, 2007 and AR5, 2013), the evidence for human influence on the climate system was found to have progressively strengthened. AR5 concluded that human influence on the climate system is clear, evident from increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and physical understanding of the climate system. This chapter updates the assessment of human influence on the climate system for large-scale indicators of climate change, synthesizing information from paleo records, observations and climate models. It also provides the primary evaluation of large-scale indicators of climate change in this report, that is complemented by fitness-for-purpose evaluation in subsequent chapters.
... Intensifying Southern Ocean eddy fields will have a significant local impact on biological productivity, ecosystem structure, and carbon uptake, both directly and via submesoscale processes. At typical CMIP5 ESM resolutions, it is only in the tropics that mesoscale eddies are adequately resolved to explicitly model their effects (Hallberg, 2013), while submesoscale eddies are not resolved anywhere, so eddy effects need to be parameterized in ESMs. Despite great progress over the past 30 years in parameterizing eddy effects, uncertainties in these parameterizations and how eddies will respond to novel conditions continue to contribute to uncertainties in projections of oceanic climate change (medium confidence). ...
... At 65 • S, the model's horizontal grid spacing is roughly equivalent to 0.2 • longitude by 0.1 • latitude. This horizontal resolution is "eddy-permitting" as it does not fully resolve the fastestgrowing linear modes of baroclinic instability, which in this configuration would require a horizontal grid spacing of 2 km or less (Hallberg 2013;LaCasce and Groeskamp 2020). As discussed in Section 3, these idealized simulations nevertheless generate a rich mesoscale eddy field and reproduce key, large-scale hydrographic features that have been observed across the Southern Ocean. ...
Article
The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally-averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally-oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre-ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing.
... At 658S, the model's horizontal grid spacing is roughly equivalent to 0.28 longitude 3 0.18 latitude. This horizontal resolution is "eddy permitting" as it does not fully resolve the fastest-growing linear modes of baroclinic instability, which in this configuration would require a horizontal grid spacing of 2 km or less (Hallberg 2013; LaCasce and Groeskamp 2020). As discussed in section 3, these idealized simulations nevertheless generate a rich mesoscale eddy field and reproduce key, large-scale hydrographic features that have been observed across the Southern Ocean. ...
Article
The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre–ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing. Significance Statement This study explores the impact of seafloor bathymetry on the dynamics of subpolar gyres in the Southern Ocean. The subpolar gyres are major circulation features that connect the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. This work provides deeper insight for how the submarine ridges that exist along the northern periphery of these gyres shape the vertical distribution of tracers and overturning circulation in these regions. These findings highlight an underappreciated yet fundamentally important topographical constraint on the three-dimensional cycling of heat and carbon in the Southern Ocean—processes that have far-reaching implications for the global climate. Future work should explore how the presence of these ridges affect the time-evolving response of the Southern Ocean to changes in surface conditions.
... The physical processes taking place in the oceans occupy a wide range of spatial and time scales. The ocean circulation is highly complex, in which physical processes at large scales are transferred to smaller scales, resulting in mesoscale and 15 sub-mesoscale structures, or eddies (Hallberg, 2013). ...
Preprint
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This paper presents the MPI-based parallelization of the three-dimensional hydrodynamic model SHYFEM (System of HydrodYnamic Finite Element Modules). The original sequential version of the code was parallelized in order to reduce the execution time of high-resolution configurations using state-of-the-art HPC systems. A distributed memory approach was used, based on the message passing interface (MPI). Optimized numerical libraries were used to partition the unstructured grid (with a focus on load balancing) and to solve the sparse linear system of equations in parallel in the case of semi-to-fully implicit time stepping. The parallel implementation of the model was validated by comparing the outputs with those obtained from the sequential version. The performance assessment demonstrates a good level of scalability with a realistic configuration used as benchmark.
... There is a general improvement at the highest 0. 1 • configuration when solving several features of the ocean: those relevant for this study are the representation Southern Ocean water masses, the overturning circulation and the characteristics of the circulation on the Antarctic continental shelf and slope (Kiss et al., 2020;Moorman et al., 2020;Morrison et al., 2020). The Rossby radius of deformation in the Weddell Gyre region is less than 10 km, so only the 0. 1 • resolution simulation is close to eddy-resolving (Chelton et al., 1998;Hallberg, 2013). ...
Article
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Plain Language Summary The Weddell Gyre, located east of the Antarctic Peninsula, is one of the largest features of the ocean circulation of the Southern Hemisphere. It is adjacent to an important site of bottom water formation, a process that sequesters carbon and heat from the atmosphere and sets the density of the deep ocean, therefore making the region important for global climate. However, extensive sea ice cover throughout the year has historically prevented continuous observations. Several unique features of the gyre, such as open boundaries and intense surface buoyancy fluxes, make the identification of its forcing mechanisms difficult. A deeper understanding of the dynamics in this remote region will shed light on the role of the gyre in our present climate, and help us understand its potential evolution with climate change. We use a high resolution numerical model which shows that the Weddell Gyre undergoes large seasonal and interannual changes. We find that the gyre spins up during winter and slows down during summer, and that strong/weak events in our model simulation are correlated with the strength of the regional easterly winds close to the Antarctic continent. These strong/weak events affect sea ice cover, water mass characteristics and bottom water production.
... than in the surrounding deep ocean which requests an even higher resolution to resolve mesoscale processes. Over the northwestern European continental shelf, a resolution of at least 1/50 ° is required for ocean models to be eddy resolving, while models at 1/12 ° are eddy resolving in the deep part of the Atlantic domain in IBI (Hallberg, 2013). The RCM is therefore eddy-resolving in the deep Atlantic part of the domain, while the GCM is only eddy-permitting. ...
Preprint
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Projections of coastal sea level (SL) changes are of great interest for coastal risk assessment and decision-making. SL projections are typically produced using global climate models (GCMs) which cannot fully resolve SL changes at the coast due to their coarse resolution and lack of representation of some relevant processes. To overcome these limitations and refine projections at regional scales, GCMs can be dynamically downscaled through the implementation of a high-resolution regional climate model (RCM). In this study, we developed the IBI-CCS regional ocean model based on a 1/12 ° north-eastern Atlantic NEMO ocean model configuration to dynamically downscale CNRM-CM6-1-HR, a GCM with a ¼ ° resolution ocean model component developed for the Coupled Model Intercomparison Project 6th Phase (CMIP6) by the Centre National de Recherches Météorologiques (CNRM). For a more complete representation of processes driving coastal SL changes, tides and atmospheric surface pressure forcing are explicitly resolved in IBI-CCS in addition to the ocean general circulation. To limit the propagation of climate drifts and biases from the GCM into the regional simulations, several corrections are applied to the GCM fields used to force the RCM. The regional simulations are performed over the 1950 to 2100 period for two climate change scenarios (SSP1-2.6 and SSP5-8.5). To validate the dynamical downscaling method, the RCM and GCM simulations are compared to reanalyses and observations over the 1993–2014 period for a selection of ocean variables including SL. Results indicate that large-scale performances of IBI-CCS are better than those of the GCM thanks to the corrections applied to the RCM. Extreme SLs are also satisfactorily represented in the IBI-CCS historical simulation. Comparison of the RCM and GCM 21st century projections show a limited impact of increased resolution (1/4° to 1/12°) on SL changes. Overall, bias corrections have a moderate impact on projected coastal SL changes projections, except in the Mediterranean Sea where GCM biases were substantial.
... Increasing resolution can mitigate model bias (that are not well handled in data assimilation) and remove the need for representativity error (Janjić et al., 2018) when assimilating high-resolution observations. On the other hand, increasing resolution can move the model into a gray zone of mixed parametrised/resolved resolution in particular for ocean models (Hallberg, 2013) that are difficult to handle. Furthermore, there is an inverse cascade in the kinetic energy spectrum that lowers predictability for the small scale processes (Sandery and Sakov, 2017). ...
Preprint
Increasing the resolution of a model can improve the performance of a data assimilation system: first because model field are in better agreement with high resolution observations, then the corrections are better sustained and, with ensemble data assimilation, the forecast error covariances are improved. However, resolution increase is associated with a cubical increase of the computational costs. Here we are testing an approach inspired from images super-resolution techniques and called "Super-resolution data assimilation" (SRDA). Starting from a low-resolution forecast, a neural network (NN) emulates a high-resolution field that is then used to assimilate high-resolution observations. We apply the SRDA to a quasi-geostrophic model representing simplified surface ocean dynamics, with a model resolution up to four times lower than the reference high-resolution and we use the Ensemble Kalman Filter data assimilation method. We show that SRDA outperforms the low-resolution data assimilation approach and a SRDA version with cubic spline interpolation instead of NN. The NN's ability to anticipate the systematic differences between low and high resolution model dynamics explains the enhanced performance, for example by correcting the difference of propagation speed of eddies. Increasing the computational cost by 55\% above the LR data assimilation system (using a 25-members ensemble), the SRDA reduces the errors by 40\% making the performance very close to the HR system (16\% larger, compared to 92\% larger for the LR EnKF). The reliability of the ensemble system is not degraded by SRDA.
... Although sub-mesoscale processes, not presently included in the circulation model, are known to have an impact on mesoscale fronts and vortexes (Haza et al., 2012;Sasaki et al., 2014), this resolution is sufficient to sustain turbulence generated by the model itself at the first baroclinic wavelength or coming from assimilation procedure increments. In this sense, the Mediterranean region is fully resolved both at spatial (Hallberg, 2013) and temporal scale, i.e., eddies can be followed in time with daily sub-sampling while their typical lifetime ranges between several days and few months. ...
Article
The mesoscale variability in the Mediterranean Sea is investigated through eddy detection techniques. The analysis is performed over 24 years (1993–2016) considering the three-dimensional (3D) fields from an ocean re-analysis of the Mediterranean Sea (MED-REA). The objective is to achieve a fit-for-purpose assessment of the 3D mesoscale eddy field. In particular, we focus on the contribution of eddy-driven anomalies to ocean dynamics and thermodynamics. The accuracy of the method used to disclose the 3D eddy contributions is assessed against pointwise in-situ measurements and observation-based data sets. Eddy lifetimes ≥ 2 weeks are representative of the 3D mesoscale field in the basin, showing a high probability (> 60 % ) of occurrence in the areas of the main quasi-stationary mesoscale features. The results show a dependence of the eddy size and thickness on polarity and lifetime: anticyclonic eddies (ACE) are significantly deeper than cyclonic eddies (CE), and their size tends to increase in long-lived structures which also show a seasonal variability. Mesoscale eddies result to be a significant contribution to the ocean dynamics in the Mediterranean Sea, as they account for a large portion of the sea-surface height variability at temporal scales longer than 1 month and for the kinetic energy (50–60 % ) both at the surface and at depth. Looking at the contributions to ocean thermodynamics, the results exhibit the existence of typical warm (cold) cores associated with ACEs (CEs) with exceptions in the Levantine basin (e.g., Shikmona gyre) where a structure close to a mode-water ACE eddy persists with a positive salinity anomaly. In this area, eddy-induced temperature anomalies can be affected by a strong summer stratification in the surface water, displaying an opposite sign of the anomaly whether looking at the surface or at depth. The results show also that temperature anomalies driven by long-lived eddies (≥ 4 weeks) can affect up to 15–25 % of the monthly variability of the upper ocean heat content in the Mediterranean basin.
... The present BS-REA hydrodynamic model is configured for the Black Sea region (the Azov Sea is not included) and it is based on NEMO v3.6 implicit free-surface implementation (Madec and The Nemo team, 2016), with a horizontal resolution of 1/27 • in the zonal direction and 1/36 • in the meridional direction, and 31 unevenly spaced vertical z-levels. This horizontal spatial resolution is chosen in order to have the same cartesian resolution in latitudinal and longitudinal directions, around 3 km at the model domain latitudes, which is conformed to an eddy-resolving scale; the Rossby radius of deformation in the Black Sea is approximately 20 km (Hallberg, 2013). The BS-REA horizontal spatial domain is shown in Figure 1. ...
Article
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Ocean reanalyses are becoming increasingly important to reconstruct and provide an overview of the ocean state from the past to the present-day. In this article, we present a Black Sea reanalysis covering the whole satellite altimetry era. In the scope of the Copernicus Marine Environment Monitoring Service, the Black Sea reanalysis system is produced using an advanced variational data assimilation method to combine the best available observations with a state-of-the-art ocean general circulation model. The hydrodynamical model is based on Nucleus for European Modeling of the Ocean, implemented for the Black Sea domain with a horizontal resolution of 1/27°× 1/36°, and 31 unevenly distributed vertical levels. The model is forced by the ECMWF ERA5 atmospheric reanalysis and climatological precipitation, whereas the sea surface temperature is relaxed to daily objective analysis fields. The model is online coupled to OceanVar, a 3D-Var ocean data assimilation scheme, to assimilate sea level anomaly along-track observations and in situ vertical profiles of temperature and salinity. Temperature fields present a continuous warming in the layer between 25 and 150 m, where the Black Sea Cold Intermediate Layer resides. This is an important signal of the Black Sea response to climate change. Sea surface temperature shows a basin-wide positive bias and the root mean square difference can reach 0.75°C along the Turkish coast in summer. The overall surface dynamic topography is well reproduced as well as the reanalysis can represent the main Black Sea circulation such as the Rim Current and the quasi-permanent anticyclonic Sevastopol and Batumi eddies. The system produces very accurate estimates of temperature, salinity and sea level which makes it suitable for understanding the Black Sea physical state in the last decades. Nevertheless, in order to improve the quality of the Black Sea reanalysis, new developments in ocean modeling and data assimilation are still important, and sustaining the Black Sea ocean observing system is crucial.
... Figure from(Hallberg, 2013). Estimation (based on a 1/8 • Mercator grid) of the horizontal resolution needed to resolve the first baroclinic deformation radius with two grid points. ...
Thesis
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Anthropogenic climate change is one of the greatest challenges for humanity today. How does the Earth system react to the atmospheric greenhouse gas increase that has never happened before with such speed in Earth's history? So far, climate models project an increase of the global average temperature, sea level and the number of `extremely' warm days in the coming decades and centuries. Although it is clear that the Earth is warming, uncertainty remains in the progression of those developments. These climate models are inherently wrong, but are they useful? Climate models give a proper representation of present-day climate, which we know from validations with observations. These models, which contain fundamental physical processes and have been developed over the past decades, cannot be validated in the increased greenhouse-climate of the future. We can however, compare the models with observations of warmer climates in the past, which are similar to the future climate, to get an understanding of these type of `extreme' climates. Although (unfortunately) no direct observations are available for the past hundred millions of years, we do find indirect evidence about the past climate (for example fossils or ice cores). These so-called proxies provide an archive of past climate and can be used to compare with climate model simulations. As a result, the combination of models and proxies of past climate can be used to get a better understanding of how a future climate, which is warmer than today, may look like. A primary part of the Earth's archive to reconstruct past climates is provided by marine sediments, consisting of (fossil remains from) microplankton. The microplankton species in the bottom sediments originated from a location close to the ocean surface before they started sinking to the bottom. Hence, microplankton at the ocean bottom is representative of the ocean surface environment. It is often assumed that these planktonic species sunk vertically downwards. However, the microplankton is transported laterally by ocean currents during its sinking journey. In this thesis, we investigate how sedimentary distributions of microplankton can be explained. We determine how sinking microplankton is advected by ocean currents, which may have great implications for the interpretation of sedimentary microplankton data. For example, subtropical and (sub)polar microplankton species alternate in sediment cores near Antarctica from 34 million years ago until the present-day. If subtropical microplankton species are found near Antarctica in a specific time period, two hypotheses can be tested: (a) Antarctica had a subtropical climate, or (b) Antarctica was not subtropical, but the microplankton were transported laterally by ocean currents and originated from another region with a subtropical climate. We study microplankton particles at the ocean bottom, which got there after a sinking journey, and determine their origin at the ocean surface back again. The ultimate goal is to bridge a gap between the models, which represent the global climate, and the measurements, representing the climate at specific geographic locations. As such, we study past climates back again, to get an idea how we get there in the future.
... Higher dissipation in LLC540 must stem from the sixfold increase in horizontal resolution, which can have various effects. First, grid spacing of 1 ⁄6 • is sufficiently small to resolve the first baroclinic Rossby radius in the deep ocean within latitudes |φ| < 40 • ; cf. Figure 1 in Hallberg (2013). Transient eddy features are therefore admitted in the LLC540 simulation and tend to drain energy from the large-scale fields important to OAM quantities. ...
Article
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We revisit the problem of modeling the ocean's contribution to rapid, non-tidal Earth rotation variations at periods of 2–120 days. Estimates of oceanic angular momentum (OAM, 2007–2011) are drawn from a suite of established circulation models and new numerical simulations, whose finest configuration is on a 1/6° grid. We show that the OAM product by the Earth System Modeling Group at GeoForschungsZentrum Potsdam has spurious short period variance in its equatorial motion terms, rendering the series a poor choice for describing oceanic signals in polar motion on time scales of less than ~2 weeks. Accounting for OAM in rotation budgets from other models typically reduces the variance of atmosphere-corrected geodetic excitation by ~54% for deconvolved polar motion and by ~60% for length-of-day. Use of OAM from the 1/6° model does provide for an additional reduction in residual variance such that the combined oceanic–atmospheric effect explains as much as 84% of the polar motion excitation at periods < 120 days. Employing statistical analysis and bottom pressure changes from daily Gravity Recovery and Climate Experiment solutions, we highlight the tendency of ocean models run at a 1° grid spacing to misrepresent topographically constrained dynamics in some deep basins of the Southern Ocean, which has adverse effects on OAM estimates taken along the 90° meridian. Higher model resolution thus emerges as a sensible target for improving the oceanic component in broader efforts of Earth system modeling for geodetic purposes.
... The shelf break (defined by the 200 m isobath) is indicated in yellow. the deep part of the Atlantic domain in IBI (Hallberg, 2013). The RCM is therefore eddy resolving in the deep Atlantic part of the domain, while the GCM is only eddy permitting. ...
Article
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Projections of coastal sea level (SL) changes are of great interest for coastal risk assessment and decision making. SL projections are typically produced using global climate models (GCMs), which cannot fully resolve SL changes at the coast due to their coarse resolution and lack of representation of some relevant processes (tides, atmospheric surface pressure forcing, waves). To overcome these limitations and refine projections at regional scales, GCMs can be dynamically downscaled through the implementation of a high-resolution regional climate model (RCM). In this study, we developed the IBI-CCS (Iberian–Biscay–Ireland Climate Change Scenarios) regional ocean model based on a 1/12∘ northeastern Atlantic Nucleus for European Modelling of the Ocean (NEMO) model configuration to dynamically downscale CNRM-CM6-1-HR, a GCM with a 1/4∘ resolution ocean model component participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) by the Centre National de Recherches Météorologiques (CNRM). For a more complete representation of the processes driving coastal SL changes, tides and atmospheric surface pressure forcing are explicitly resolved in IBI-CCS in addition to the ocean general circulation. To limit the propagation of climate drifts and biases from the GCM into the regional simulations, several corrections are applied to the GCM fields used to force the RCM. The regional simulations are performed over the 1950 to 2100 period for two climate change scenarios (SSP1-2.6 and SSP5-8.5). To validate the dynamical downscaling method, the RCM and GCM simulations are compared to reanalyses and observations over the 1993–2014 period for a selection of ocean variables including SL. Results indicate that large-scale performance of IBI-CCS is better than that of the GCM thanks to the corrections applied to the RCM. Extreme SLs are also satisfactorily represented in the IBI-CCS historical simulation. Comparison of the RCM and GCM 21st century projections shows a limited impact of increased resolution (1/4 to 1/12∘) on SL changes. Overall, bias corrections have a moderate impact on projected coastal SL changes, except in the Mediterranean Sea, where GCM biases were substantial.
... VIKING20X is an updated configuration of VIKING20, which has been shown to reproduce dynamics like the North Atlantic Current or the Deep Western Boundary Current in the North Atlantic well (Mertens et al., 2014;Breckenfelder et al., 2017;Handmann et al., 2018). Previous studies have shown that VIKING20X successfully resolves mesoscale eddies (Rieck et al., 2019;Biastoch et al., 2021) although continental shelves ideally require even higher resolution to explicitly resolve eddies (Hallberg, 2013), that is two grid points within the Rossby radius. ...
Article
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Marine Heatwaves (MHWs) are ocean extreme events, characterized by anomalously high temperatures, which can have significant ecological impacts. The Northeast U.S. continental shelf is of great economical importance as it is home to a highly productive ecosystem. Local warming rates exceed the global average and the region experienced multiple MHWs in the last decade with severe consequences for regional fisheries. Due to the lack of subsurface observations, the depth-extent of MHWs is not well-known, which hampers the assessment of impacts on pelagic and benthic ecosystems. This study utilizes a global ocean circulation model with a high-resolution (1/20°) nest in the Atlantic to investigate the depth structure of MHWs and associated drivers on the Northeast U.S. continental shelf. It is shown that MHWs exhibit varying spatial extents, with some only occurring at depth. The highest intensities are found around 100 m depth with temperatures exceeding the climatological mean by up to 7°C, while surface intensities are typically smaller (around 3°C). Distinct vertical structures are associated with different spatial MHW patterns and drivers. Investigation of the co-variability of temperature and salinity reveals that over 80% of MHWs at depth (>50 m) coincide with extreme salinity anomalies. Two case studies provide insight into opposing MHW patterns at the surface and at depth, being forced by anomalous air-sea heat fluxes and Gulf Stream warm core ring interaction, respectively. The results highlight the importance of local ocean dynamics and the need to realistically represent them in climate models.
... The advantage of including the high-res simulation, albeit in a light version, is that we can better demonstrate the benefit of a regional grid refinement for the representation of relevant hydrodynamic features in the coastal ocean that provide the background conditions for the biogeochemical processes. In particular at the upper end of the resolution range (mesh size 10 km), we reach or come close to the first baroclinic radius of deformation in many shelf seas and ocean-shelf transition zones, thus incorporating mesoscale activity more extensively than in the low-res simulation (Hallberg, 2013;Hewitt et al., 2017). Representing the mesoscale explicitly was shown to tangibly improve the simulated mean ocean state as well as the temporal variability (Hewitt et al., 2020). ...
Article
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We present the first global ocean‐biogeochemistry model that uses a telescoping high resolution for an improved representation of coastal carbon dynamics: ICON‐Coast. Based on the unstructured triangular grid topology of the model, we globally apply a grid refinement in the land‐ocean transition zone to better resolve the complex circulation of shallow shelves and marginal seas as well as ocean‐shelf exchange. Moreover, we incorporate tidal currents including bottom drag effects, and extend the parameterizations of the model's biogeochemistry component to account explicitly for key shelf‐specific carbon transformation processes. These comprise sediment resuspension, temperature‐dependent remineralization in the water column and sediment, riverine matter fluxes from land including terrestrial organic carbon, and variable sinking speed of aggregated particulate matter. The combination of regional grid refinement and enhanced process representation enables for the first time a seamless incorporation of the global coastal ocean in model‐based Earth system research. In particular, ICON‐Coast encompasses all coastal areas around the globe within a single, consistent ocean‐biogeochemistry model, thus naturally accounting for two‐way coupling of ocean‐shelf feedback mechanisms at the global scale. The high quality of the model results as well as the efficiency in computational cost and storage requirements proves this strategy a pioneering approach for global high‐resolution modeling. We conclude that ICON‐Coast represents a new tool to deepen our mechanistic understanding of the role of the land‐ocean transition zone in the global carbon cycle, and to narrow related uncertainties in global future projections.
... There are also pragmatic challenges with increasing resolution. First, increasing resolution of ocean models moves them into a gray zone of mixed parameterized/resolved, Hallberg (2013), that are difficult to handle. Second, the inverse cascade of kinetic energy spectrum lowers the predictability of small-scale processes, Sandery and Sakov (2017). ...
Article
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Increasing model resolution can improve the performance of a data assimilation system because it reduces model error, the system can more optimally use high-resolution observations, and with an ensemble data assimilation method the forecast error covariances are improved. However, increasing the resolution scales with a cubical increase of the computational costs. A method that can more effectively improve performance is introduced here. The novel approach called “Super-resolution data assimilation” (SRDA) is inspired from super-resolution image processing techniques and brought to the data assimilation context. Starting from a low-resolution forecast, a neural network (NN) emulates the fields to high-resolution, assimilates high-resolution observations, and scales it back up to the original resolution for running the next model step. The SRDA is tested with a quasi-geostrophic model in an idealized twin experiment for configurations where the model resolution is twice and four times lower than the reference solution from which pseudo-observations are extracted. The assimilation is performed with an Ensemble Kalman Filter. We show that SRDA outperforms both the low-resolution data assimilation approach and a version of SRDA with cubic spline interpolation instead of NN. The NN’s ability to anticipate the systematic differences between low- and high-resolution model dynamics explains the enhanced performance, in particular by correcting the difference of propagation speed of eddies. With a 25-member ensemble at low resolution, the SRDA computational overhead is 55% and the errors reduce by 40%, making the performance very close to that of the high-resolution system (52% of error reduction) that increases the cost by 800%. The reliability of the ensemble system is not degraded by SRDA.
... The physical processes taking place in the oceans occupy a wide range of spatial and timescales. The ocean circulation is highly complex, in which physical processes at large scales are transferred to smaller scales, resulting in mesoscale and sub-mesoscale structures, or eddies (Hallberg, 2013). ...
Article
Full-text available
This paper presents the message passing interface (MPI)-based parallelization of the three-dimensional hydrodynamic model SHYFEM (System of HydrodYnamic Finite Element Modules). The original sequential version of the code was parallelized in order to reduce the execution time of high-resolution configurations using state-of-the-art high-performance computing (HPC) systems. A distributed memory approach was used, based on the MPI. Optimized numerical libraries were used to partition the unstructured grid (with a focus on load balancing) and to solve the sparse linear system of equations in parallel in the case of semi-to-fully implicit time stepping. The parallel implementation of the model was validated by comparing the outputs with those obtained from the sequential version. The performance assessment demonstrates a good level of scalability with a realistic configuration used as benchmark.
... As realistic ocean simulations with kilometric horizontal resolution have emerged (e.g., Rocha et al., 2016;Schubert et al., 2019;Brodeau et al., 2020;Gula et al., 2021;Ajayi et al., 2021), such a framework has become cumbersome, with terabytes and petabytes of data needed to be transferred and stored as ghost copies. Nevertheless, a real demand exists for collaboration to inter-compare models to examine their fidelity and quantify robust features of submesoscale and mesoscale turbulence (the former on the horizontal spatial scales of O(10 km) and the latter on O(100 km), from here on referred to jointly as (sub)mesoscale; Hallberg, 2013;McWilliams, 2016;Lévy et al., 2018;Uchida et al., 2019;Dong et al., 2020). The Ocean Model Intercomparison Project (OMIP), for example, has been successful in diagnosing systematic biases in non-eddying and mesoscalepermitting ocean models used for global climate simulations (Griffies et al., 2016;Chassignet et al., 2020). ...
Article
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With the increase in computational power, ocean models with kilometer-scale resolution have emerged over the last decade. These models have been used for quantifying the energetic exchanges between spatial scales, informing the design of eddy parametrizations, and preparing observing networks. The increase in resolution, however, has drastically increased the size of model outputs, making it difficult to transfer and analyze the data. It remains, nonetheless, of primary importance to assess more systematically the realism of these models. Here, we showcase a cloud-based analysis framework proposed by the Pangeo project that aims to tackle such distribution and analysis challenges. We analyze the output of eight submesoscale-permitting simulations, all on the cloud, for a crossover region of the upcoming Surface Water and Ocean Topography (SWOT) altimeter mission near the Gulf Stream separation. The cloud-based analysis framework (i) minimizes the cost of duplicating and storing ghost copies of data and (ii) allows for seamless sharing of analysis results amongst collaborators. We describe the framework and provide example analyses (e.g., sea-surface height variability, submesoscale vertical buoyancy fluxes, and comparison to predictions from the mixed-layer instability parametrization). Basin- to global-scale, submesoscale-permitting models are still at their early stage of development; their cost and carbon footprints are also rather large. It would, therefore, benefit the community to document the different model configurations for future best practices. We also argue that an emphasis on data analysis strategies would be crucial for improving the models themselves.
... As the computing power has been increasing rapidly, about one order higher with each five years, the state-of-art total computing ability of the modern global ocean numerical models is becoming enormously high, leading to the recent achievements of global high resolution ocean models. The definition of "high resolution" of current stage global ocean models may refer to these with horizontal resolution range from 1 to 5 km, which are well beyond the mesoscale resolving 30 threshold in most of open ocean (Hallberg, 2013). Further improved resolution has significant impact on the simulated https://doi. ...
Preprint
Model resolution and the included physical processes are two of the most important factors that determine the realism of the ocean model simulations. In this study, a new global surface wave-tide-circulation coupled ocean model FIO- COM32 with resolution of 1/32° × 1/32° is developed and validated. Promotion of the horizontal resolution from 1/10° to 1/32° leads to significant improvements of the simulations of surface eddy kinetic energy (EKE), fine structures of sub- mesoscale to mesoscale movements and the accuracy of simulated global tide. The non-breaking surface wave-induced mixing (Bv) is proved to be an important contributor that improves the agreement of the simulated summer mixed layer depth (MLD) of the model and the Argo observations even with high horizontal resolution of 1/32°, the mean error of the simulated mid-latitude summer MLD is reduced from -4.8 m in numerical experiment without Bv to -0.6 m in experiment with Bv. With the global tide is included, the global distributions of internal tide can be explicitly simulated in this new model and is comparable to the satellite observations. Comparisons using Jason3 along-track sea surface height (SSH) wave-number spectral slopes of mesoscale ranges show that internal tide induced SSH undulations is a key factor contributing to the substantially improved agreement of model and satellite observations in the low latitude and low EKE regions. For ocean model community, surface wave, tidal current and ocean circulation have been separating into different streams for more than half century. It should be the time to merge these streams for new generation ocean model development.
... The model used in this study is a high-resolution global ocean and sea-ice model, ACCESS-OM2-01 (Kiss et al., 2020), which has 75 vertical levels and 0.1°spatial resolution. The model is hence eddy-resolving at Southern Ocean latitudes (Hallberg, 2013). ACCESS-OM2-01 uses ocean numerics and configurations based on the Modular Ocean Model version 5 (MOM5) (Griffies, 2012). ...
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... Studies using idealized ocean models, with finer resolution, show that mesoscale eddies and tides can also affect the heat transport and stratification around the Antarctic slope front (Flexas et al., 2015;Stewart and Thompson, 2013;Nøst et al., 2011). In order to get more accurate simulations of the Antarctic coastal environment in the future, the horizontal resolution of the models needs to be increased to higher than 1/50° to resolve the first baroclinic deformation radius (Hallberg, 2013). Iceberg calving and basal melting of ice shelves, which are not included in our model, can also affect the temperature and salinity of ASBW (Rye et al., 2014;Hellmer, 2004). ...
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The Southern Hemisphere (SH) westerly winds have intensified and shifted poleward since the 1970s and this trend is projected to sustain under future anthropogenic forcing. The influences of intensified SH westerlies on the Antarctic coastal waters are still not clear. The variability of Antarctic Continental Shelf Bottom Water (ASBW) temperature is crucial for ice shelf basal melting and hence ice shelf mass balance in Antarctica. In order to understand the impacts of SH westerlies on the variability of ASBW temperature, atmospheric forcing in 1992 with weak westerlies and in 1998 with strong westerlies are used to drive a high-resolution ocean-sea ice general circulation model, MITgcm-ECCO2. Our simulated results show that under the atmospheric forcing in 1998, the ASBW becomes warmer in most regions around Antarctica except the coastal region between 60°-150°W, than for the case under atmospheric forcing in 1992. The warming of ASBW around Antarctica is due to the intense shoaling and warming of CDW induced by enhanced Ekman pumping as well as strengthened subpolar gyres. The strengthened subpolar gyres favor the transportation of warm water to the coast of Antarctica. The cooling of ASBW along the coast of the western Antarctic Peninsula is caused by stronger coastal currents, which bring colder water downstream from the northwest flank of the Weddell Sea.
... This explains the lack of agreement between the model and observations in highly dynamic regions, where events are shorter, stochastic and greatly influenced by anticyclonic eddy propagation (Supplementary Figure 1). This type of MHW event is not well-resolved and is difficult to predict (Hallberg, 2013;Pilo et al., 2019;Hayashida et al., 2020). Driven by rising ocean surface temperatures (Frölicher et al., 2018;Darmaraki et al., 2019;Oliver, 2019;Marin et al., 2021) and the recent surge of scientific interest, most major MHW events studied occurred in the last few years. ...
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... Similarly, the representation of oceanic fronts, eddies and currents is markedly different depending on the type of model that is used to represent them, whether turbulent mixing by eddies is resolved or parameterized. The ocean depth varies widely between pelagic zones and the abyss, and consequently the Rossby deformation radius -the length scale representing when mesoscale eddy mixing is significant -varies from O(100) km in the Equatorial open ocean, to O(1) km or less near coastlines and toward the poles (53). Attempts to build "scale-aware" parameterization of eddy mixing (e.g., 54,55), must contend with these variations. ...
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Traditional general circulation models, or GCMs -- i.e. 3D dynamical models with unresolved terms represented in equations with tunable parameters -- have been a mainstay of climate research for several decades, and some of the pioneering studies have recently been recognized by a Nobel prize in Physics. Yet, there is considerable debate around their continuing role in the future. Frequently mentioned as limitations of GCMs are the structural error and uncertainty across models with different representations of unresolved scales; and the fact that the models are tuned to reproduce certain aspects of the observed Earth. We consider these shortcomings in the context of a future generation of models that may address these issues through substantially higher resolution and detail, or through the use of machine learning techniques to match them better to observations, theory, and process models. It is our contention that calibration, far from being a weakness of models, is an essential element in the simulation of complex systems, and contributes to our understanding of their inner workings. Models can be calibrated to reveal both fine-scale detail, or the global response to external perturbations. New methods enable us to articulate and improve the connections between the different levels of abstract representation of climate processes, and our understanding resides in an entire hierarchy of models where GCMs will continue to play a central role for the foreseeable future..
... As even coarse-resolution models can resolve some eddy activity in the tropics where the associated length scales are larger, we also include a grid-scaling factor, not considered by Groeskamp et al. (2020), that reduces κ n where eddy-mixing may be resolved following Hallberg (2013), ...
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Abstract Motivated by recent advances in mapping mesoscale eddy tracer mixing in the ocean we evaluate the sensitivity of a coarse‐resolution global ocean model to a spatially variable neutral diffusion coefficient κn(x, y, z). We gradually introduce physically motivated models for the horizontal (mixing length theory) and vertical (surface mode theory) structure of κn along with suppression of mixing by mean flows. Each structural feature influences the ocean's hydrography and circulation to varying extents, with the suppression of mixing by mean flows being the most important factor and the vertical structure being relatively unimportant. When utilizing the full theory (experiment “FULL”) the interhemispheric overturning cell is strengthened by 2 Sv at 26°N (a ∼20% increase), bringing it into better agreement with observations. Zonal mean tracer biases are also reduced in FULL. Neutral diffusion impacts circulation through surface temperature‐induced changes in surface buoyancy fluxes and nonlinear equation of state effects. Surface buoyancy forcing anomalies are largest in the Southern Ocean where a decreased neutral diffusivity in FULL leads to surface cooling and enhanced dense‐to‐light surface water mass transformation, reinforced by reductions in cabbeling and thermobaricity. The increased water mass transformation leads to enhanced midlatitude stratification and interhemispheric overturning. The spatial structure for κn in FULL is important as it enhances the interhemispheric cell without degrading the Antarctic bottom water cell, unlike a spatially uniform reduction in κn. These results highlight the sensitivity of modeled circulation to κn and motivate the use of physics‐based models for its structure.
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Plain Language Summary Over the recent past the oxygen content of the global ocean has been declining, with consequences for marine ecosystems. Ocean eddies (rotating currents of water on the order ≈100 km diameter) have been identified as vehicles of extremely low oxygen concentrations but are understudied in the context of biogeochemical extreme events in the ocean. This investigation is the first of its kind to use a four‐dimensional data set (time, latitude, longitude, depth) to create a regional scale census of eddies and their oxygen conditions across the period 1992–2018 as they travel offshore from typically low oxygen waters of the near‐coastal Atlantic and Pacific, carrying and modifying low oxygen signals into otherwise more oxygenated ocean regions. We track eddies associated with low oxygen conditions and assess how much they contribute to low oxygen extreme events. In some places eddies contribute to more than half of the simulated low oxygen extreme events, signaling the need to further explore the role that eddies play in marine extreme events.
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Chapter
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A subgrid-scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non-eddy-resolving ocean circulation models. The mixing is applied in isopycnal coordinates to isopycnal layer thickness, or inverse density gradient, as well as to passive scalars, temperature and salinity. The transformation of these mixing forms to physical coordinates is also presented.
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The authors use data collected by a line of tall current meter moorings deployed across the axis of the Kuroshio Extension (KE) jet at the location of maximum time-mean eddy kinetic energy to characterize the mean jet structure, the eddy variability, and the nature of eddy-mean flow interactions observed during the Kuroshio Extension System Study (KESS). A picture of the 2-yr record mean jet structure is presented in both geographical and stream coordinates, revealing important contrasts in jet strength, width, vertical structure, and flanking recirculation structure. Eddy variability observed is discussed in the context of some of its various sources: jet meandering, rings, waves, and jet instability. Finally, various scenarios for eddy-mean flow interaction consistent with the observations are explored. It is shown that the observed cross-jet distributions of Reynolds stresses at the KESS location are consistent with wave radiation away from the jet, with the sense of the eddy feedback effect on the mean consistent with eddy driving of the observed recirculations. The authors consider these results in the context of a broader description of eddy-mean flow interactions in the larger KE region using KESS data in combination with in situ measurements from past programs in the region and satellite altimetry. This demonstrates important consistencies in the along-stream development of time-mean and eddy properties in the KE with features of an idealized model of a western boundary current (WBC) jet used to understand the nature and importance of eddy-mean flow interactions in WBC jet systems.
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This chapter summarizes our present knowledge of Gulf Stream separation in numerical ocean models. High horizontal resolution ocean numerical models are now capable of simulating quite realistically the separation and path of the Gulf Stream, and significant advances have been made in the last decade in our understanding of western boundary current separation. However, the Gulf Stream separation in numerical models continues to be a challenge because it remains very sensitive to the choices made for subgrid scale parameterizations.
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It is shown that the effects of mesoscale eddies on tracer transports can be parameterized in a large-scale model by additional advection and diffusion of tracers. Thus, tracers are advected by the effective transport velocity, which is the sum of the large-scale velocity and the eddy-induced transport velocity. The density and continuity equations are the familiar equations for adiabatic, Boussinesq, and incompressible flow with the effective transport velocity replacing the large-scale velocity. One of the main points of this paper is to show how simple the parameterization of Gent and McWilliams appears when interpreted in terms of the effective transport velocity. This was not done in their original 1990 paper. It is also shown that, with the Gent and McWilliams parameterization, potential vorticity in the planetary geostrophic model satisfies an equation close to that for tracers. The analogy of this parameterization with vertical mixing of momentum is then described. The effect of the Gent ...
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The Modeling Eddies in the Southern Ocean (MESO) project uses numerical sensitivity studies to examine the role played by Southern Ocean winds and eddies in determining the density structure of the global ocean and the magnitude and structure of the global overturning circulation. A hemispheric isopy- cnal-coordinate ocean model (which avoids numerical diapycnal diffusion) with realistic geometry is run with idealized forcing at a range of resolutions from coarse (2°) to eddy-permitting (1Ú6°). A comparison of coarse resolutions with fine resolutions indicates that explicit eddies affect both the structure of the over- turning and the response of the overturning to wind stress changes. While the presence of resolved eddies does not greatly affect the prevailing qualitative picture of the ocean circulation, it alters the overturning cells involving the Southern Ocean transformation of dense deep waters and light waters of subtropical origin into intermediate waters. With resolved eddies, the surface-to-intermediate water cell extends farther southward by hundreds of kilometers and the deep-to-intermediate cell draws on comparatively lighter deep waters. The overturning response to changes in the winds is also sensitive to the presence of eddies. In noneddying simulations, changing the Ekman transport produces comparable changes in the overturning, much of it involving transformation of deep waters and resembling the mean circulation. In the eddy- permitting simulations, a significant fraction of the Ekman transport changes are compensated by eddy- induced transport drawing from lighter waters than does the mean overturning. This significant difference calls into question the ability of coarse-resolution ocean models to accurately capture the impact of changes in the Southern Ocean on the global ocean circulation.
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Ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean. In this paper a parameterization for the restratification driven by finite-amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal. The streamfunction is proportional to the product of the horizontal density gradient, the mixed layer depth squared, and the inertial period. Hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence. In this paper the parameterization is theoretically motivated, confirmed to perform well for a wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. It is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification. Two companion papers discuss the numerical implementation and the climate impacts of this parameterization.
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Fluid dynamics is fundamental to our understanding of the atmosphere and oceans. Although many of the same principles of fluid dynamics apply to both the atmosphere and oceans, textbooks tend to concentrate on the atmosphere, the ocean, or the theory of geophysical fluid dynamics (GFD). This textbook provides a comprehensive unified treatment of atmospheric and oceanic fluid dynamics. The book introduces the fundamentals of geophysical fluid dynamics, including rotation and stratification, vorticity and potential vorticity, and scaling and approximations. It discusses baroclinic and barotropic instabilities, wave-mean flow interactions and turbulence, and the general circulation of the atmosphere and ocean. Student problems and exercises are included at the end of each chapter. Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation will be an invaluable graduate textbook on advanced courses in GFD, meteorology, atmospheric science and oceanography, and an excellent review volume for researchers. Additional resources are available at www.cambridge.org/9780521849692. Includes end of chapter review questions to aid understanding Unified and comprehensive treatment of both atmospheric and oceanic fluid dynamics Covers many modern topics and provides up to date knowledge
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