Simulating oceanic mesoscale eddy dynamics: A comparison of novel parameterizations and energy diagnostics and their impact on the global ocean circulation
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In this study, we present a variety of parameterizations for simulating ocean eddy dynamics including novel viscous and kinetic energy backscatter closures. Their effect is analyzed using new diagnostics that allow for application on unstructured meshes.Ocean mesoscale eddy dynamics play a crucial role for large-scale ocean currents as well as for the variability in the ocean and climate. The interactions between eddies and the mean flow affect strength, position and variations of ocean currents. Mesoscale eddies have a substantial impact on oceanic heat transport and the coupling between the atmosphere and ocean. However, at so-called eddy-permitting model resolutions around ¼°, eddy kinetic energy and variability is often substantially underestimated due to excessive dissipation of energy. Despite ever-increasing model resolutions, eddy-permitting simulations will still be used in uncoupled and coupled climate and Earth system simulations for years to come.To improve the presentation of eddy dynamics in such resolution regimes, we present and systematically compare a set of viscous and kinetic energy backscatter parameterization with different complexity. These schemes are implemented in the unstructured grid, finite volume ocean model FESOM2 and tested in both idealized channel and global ocean simulations. We show that kinetic energy backscatter and adjusted viscosity parameterizations can alleviate some of the substantial eddy related biases, for example biases in sea surface height variability, mean currents and in water mass properties. We then further analyze the effect of these schemes on energy and dissipation spectra using new diagnostics that can be extended to the unstructured grid used by FESOM2. The rigorous intercomparison allows to make informed decisions on which schemes are the most suitable for a given application, considering the complexity of the schemes, their computational costs, their adaptability to various model resolutions and any simulation improvements related to a specific scheme. We will show that novel viscous and kinetic energy backscatter schemes outperform previously used, classical viscous closures. Furthermore, when compared to higher resolution simulations, they are computationally less expensive but achieve similar results.Keywords:
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Ocean dynamics
Sea-surface height
Large-Eddy Simulation
Warm core rings formed in the, Agulhas Retroflection transfer water from the Indian Ocean to the South Atlantic. In an attempt to measure the strength of this exchange, a combination of satellite altimeter and hydrographic data are used to examine Agulhas eddy paths and decay rates in the South Atlantic. Because the surface dynamic height of a warm core eddy is higher than surrounding waters, the rings are visible in satellite altimeter measurements. Over 20 Agulhas eddies have been tracked from maps of anomalous sea surface height (SSH) derived from the Geosat Exact Repeat Mission (ERM) dataset. The correlation (r2) of dynamic height referenced to 2000 dbar and anomaly SSH for one coincidentally sampled area is 97% within an Agulhas eddy, dropping to a fraction of that outside of it, indicating that the SSH anomaly signal is a reliable measure for strong features like Agulhas eddies. The sizes and distribution of the Agulhas eddies in the ERM record compare favorably with those in recent hydrographic records from the area. Individual eddy tracks from the ERM show the influence of topography, with slowed translation over area of steep relief. The eddies tracked take a generally WNW course across the South Atlantic, propelled by the mean flow and internal dynamics. While propagating westward, Agulhas eddies decay in amplitude with an e-folding distance of O(1700–3000 km) alongtrack. As they approach the western boundary of the South Atlantic, at 40°W, the eddies have O(10%) of their initial amplitude remaining. This study finds the residence time of an Agulhas eddy in the South Atlantic to be 3–4 years. On average, the authors find six eddies per year form by the retroflection that enter the South Atlantic. The 20 eddies tracked therefore represent 50%–60% of the population that would have been extant during the ERM. The Agulhas eddies appear to contribute a minimum of 5 × 106 m3 s−1 to the Indian-South Atlantic water mass transfer, with a corresponding energy flux on the order of 1017 J.
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In this letter we present a variation of the recently proposed “semi‐prognostic method” for use with ocean models. The new version has the advantage that model drift is effectively prevented, while at the same time the meso‐scale eddy field is free to evolve. We use the method to probe the importance of the eddy‐driven circulation in the northwest Atlantic Ocean. For the particular model we use here, it is shown that the eddies strongly reinforce the eastward Gulf Stream jet and the northern recirculation in the slope region, with over 50% of the total transport of this recirculation being directly eddy‐driven. The eddies also play a role in setting the temperature and salinity properties of the “northwest corner” southeast of Newfoundland.
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Abstract Coherent ocean vortices, or eddies, are usually tracked on the surface of the ocean. However, tracking subsurface eddies is important for a complete understanding of deep ocean circulation. In this study, we develop an algorithm designed for the detection of subsurface eddies in the Arabian Sea using Nucleus for European Modelling of the Ocean (NEMO) model simulations. We optimize each parameter of our algorithm to achieve favorable results when compared with an algorithm using sea surface height (SSH). When compared to similar methods, we find that using the rescaled isopycnal potential vorticity (PV) is best for subsurface eddy detection. We proceed to demonstrate that our new algorithm can detect eddies successfully between specific isopycnals, such as those that define the Red Sea Water (RSW). In doing so, we showcase how our method can be used to describe the properties of eddies within the RSW and even identify specific long-lived subsurface eddies. We conduct one such case study by discerning the structure of a completely subsurface RSW eddy near the Chagos Archipelago using Lagrangian particle tracking and PV diagnostics. We conclude that our rescaled PV method is an efficient tool for investigating eddy dynamics within the ocean’s interior, and publicly provide our optimization methodology as a way for other researchers to develop their own subsurface detection algorithms with optimized parameters for any spatiotemporal model domain. Significance Statement Eddies are a key part of ocean circulation both at the surface and in the subsurface. The purpose of our study was to design the first detection method comprehensively optimized for subsurface eddy detection from numerical simulations. We demonstrate that potential vorticity (PV) is the best field to use when algorithmically tracking eddies in subsurface water masses, using our new method to identify and track eddies in the Red Sea Water (RSW). Additionally, our method allows us to efficiently evaluate the dynamics of eddies through potential vorticity diagnostics, exemplified with a previously undescribed eddy near the Chagos Archipelago. Our methodology can be used by future researchers to study the eddy dynamics hidden within subsurface water masses around the world.
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The majority of the kinetic energy of ocean currents is contained in the mesoscale eddies. The pathways of ocean eddies, which are the “weather” of ocean circulation, are mapped from space using a decade‐long record of sea surface height measured by two simultaneously flying satellite radar altimeters. The speed and direction of the propagation of eddies in the South Atlantic Ocean are presented in the paper. The patterns of the eddy propagation velocity reveal the effects of the interaction between mean flow and eddies with strong influence of bottom topography. The information describes a unique property of the ocean general circulation and serves as a basis for testing ocean models.
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Ocean mesoscale eddies are ubiquituos in the global ocean. They are responsible of about 80% of the total eddy kinetic energy and are suggested to exert a significant impact on air-sea interactions, ocean large-scale circulation, weather and marine ecosystems. They have been qualified as "coherent" structures as they can leave for months if not years propagating in the ocean interior. As ocean observations are very sparse, they have been essentially characterized from satellite altimetry fields, which provides access to a limited number of surface characteristics of only those eddies having an imprint on sea surface height.  Observations of mesoscale eddies 3D structure, or even 2D vertical sections are rare.  On the other hand, accurate description of ocean eddies from high-resolution ocean numerical simuation are also limited. In general, they have been accoubted for via statistics, instead of individual descriptions as the latter is difficult as they move away from fixed positions. In this work we present a detailed study of ocean eddies (surface and subsurface intensified) sampled during 10 oceanographic cruises which have a sufficient horizontal spatial resolution of the vertical eddy sampling - 9 in the Atlantic Ocean (during experiments EUREC4A-OA, M124, MSM60, MSM74, M160, HM2016611, KB2017606, KB 2017618), and one in the Indian (during the Physindien 2011 experiment). Our study characterizes the eddy core and boundary in a generic way using diagnostics based on active (PV, oxygen) and passive (temperature, salinity) tracers. Despite the different resolutions of the eddy sampling in the 9 studied regions, we show that the 3D boundary of an eddy behaves like a frontal zone characterized by the Ertel PV where the water mass trapped in the eddy joins with the surrounding waters. Whatever the  origin and size of the eddy are, the core is homogeneous in properties with the anomaly maximum located at depth, which makes its altimetric characterization difficult. Moreover, these analyses provide a new metrix for defining the coherence of an ocean eddy, a concept that has been always ill-defined because of the elusive character and undersampling of these structures. 
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[1] Observational surveys have shown significant oceanic bottom water warming, but they are too spatially and temporally sporadic to quantify the deep ocean contribution to the present-day sea level rise (SLR). In this study, altimetry sea surface height (SSH), Gravity Recovery and Climate Experiment (GRACE) ocean mass, and in situ upper ocean (0–700 m) steric height have been assessed for their seasonal variability and trend maps. It is shown that neither the global mean nor the regional trends of altimetry SLR can be explained by the upper ocean steric height plus the GRACE ocean mass. A non-Boussinesq ocean general circulation model (OGCM), allowing the sea level to rise as a direct response to the heat added into the ocean, is then used to diagnose the deep ocean steric height. Constrained by sea surface temperature data and the top of atmosphere (TOA) radiation measurements, the model reproduces the observed upper ocean heat content well. Combining the modeled deep ocean steric height with observational upper ocean data gives the full depth steric height. Adding a GRACE-estimated mass trend, the data-model combination explains not only the altimetry global mean SLR but also its regional trends fairly well. The deep ocean warming is mostly prevalent in the Atlantic and Indian oceans, and along the Antarctic Circumpolar Current, suggesting a strong relation to the oceanic circulation and dynamics. Its comparison with available bottom water measurements shows reasonably good agreement, indicating that deep ocean warming below 700 m might have contributed 1.1 mm/yr to the global mean SLR or one-third of the altimeter-observed rate of 3.11 ± 0.6 mm/yr over 1993–2008.
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Mesoscale eddies in the Kuroshio recirculation region south of Japan have been investigated by using surface current data measured by an Acoustic Doppler Current Profiler (ADCP) installed on a regular ferry shuttling between Tokyo and Chichijima, Bonin Islands, and sea surface height anomaly derived from the TOPEX/POSEIDON altimeter. Many cyclonic and anticyclonic eddies were observed in the region. Spatial and temporal scales of the eddies were determined by lag-correlation analyses in space and time. The eddies are circular in shape with a diameter of 500 km and a temporal scale of 80 days. Typical maximum surface velocity and sea surface height anomaly associated with the eddies are 15–20 cm s−1 and 15 cm, respectively. The frequency of occurrence, temporal and spatial scales, and intensity are all nearly the same for the cyclonic and anticyclonic eddies, which are considered to be successive wave-like disturbances rather than solitary eddies. Phase speed of westward propagation of the eddies is estimated as 6.8 cm s−1, which is faster than a theoretical estimate based on the baroclinic first-mode Rossby wave with or without a mean current. The spatial distribution of sea surface height variations suggests that these eddies may be generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region, though further studies are needed to clarify the generation processes.
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