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1
Qiu, Bo, Shuiming Chen, Patrice Klein, Hideharu Sasaki, and Yoshikazu Sasai. "Seasonal Mesoscale and Submesoscale Eddy Variability along the North Pacific Subtropical Countercurrent." Journal of Physical Oceanography 44, no. 12 (September 2, 2014): 3079–98. http://dx.doi.org/10.1175/jpo-d-14-0071.1.
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Abstract Located at the center of the western North Pacific Subtropical Gyre, the Subtropical Countercurrent (STCC) is not only abundant in mesoscale eddies, but also exhibits prominent submesoscale eddy features. Output from a ° high-resolution OGCM simulation and a gridded satellite altimetry product are analyzed to contrast the seasonal STCC variability in the mesoscale versus submesoscale ranges. Resolving the eddy scales of >150 km, the altimetry product reveals that the STCC eddy kinetic energy and rms vorticity have a seasonal maximum in May and April, respectively, a weak positive vorticity skewness without seasonal dependence, and an inverse (forward) kinetic energy cascade for wavelengths larger (shorter) than 250 km. In contrast, the submesoscale-resolving OGCM simulation detects that the STCC eddy kinetic energy and rms vorticity both appear in March, a large positive vorticity skewness with strong seasonality, and an intense inverse kinetic energy cascade whose short-wave cutoff migrates seasonally between the 35- and 100-km wavelengths. Using a 2.5-layer, reduced-gravity model with an embedded surface density gradient, the authors show that these differences are due to the seasonal evolution of two concurring baroclinic instabilities. Extracting its energy from the surface density gradient, the frontal instability has a growth time scale of O(7) days, a dominant wavelength of O(50) km, and is responsible for the surface-intensified submesoscale eddy signals. The interior baroclinic instability, on the other hand, extracts energy from the vertically sheared STCC system. It has a slow growth time scale of O(40) days, a dominant wavelength of O(250) km, and, together with the kinetic energy cascaded upscale from the submesoscales, determines the mesoscale eddy modulations.
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Whitt, Daniel B., and John R. Taylor. "Energetic Submesoscales Maintain Strong Mixed Layer Stratification during an Autumn Storm." Journal of Physical Oceanography 47, no. 10 (October 2017): 2419–27. http://dx.doi.org/10.1175/jpo-d-17-0130.1.
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AbstractAtmospheric storms are an important driver of changes in upper-ocean stratification and small-scale (1–100 m) turbulence. Yet, the modifying effects of submesoscale (0.1–10 km) motions in the ocean mixed layer on stratification and small-scale turbulence during a storm are not well understood. Here, large-eddy simulations are used to study the coupled response of submesoscale and small-scale turbulence to the passage of an idealized autumn storm, with a wind stress representative of a storm observed in the North Atlantic above the Porcupine Abyssal Plain. Because of a relatively shallow mixed layer and a strong downfront wind, existing scaling theory predicts that submesoscales should be unable to restratify the mixed layer during the storm. In contrast, the simulations reveal a persistent and strong mean stratification in the mixed layer both during and after the storm. In addition, the mean dissipation rate remains elevated throughout the mixed layer during the storm, despite the strong mean stratification. These results are attributed to strong spatial variability in stratification and small-scale turbulence at the submesoscale and have important implications for sampling and modeling submesoscales and their effects on stratification and turbulence in the upper ocean.
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Sasaki, Hideharu, Bo Qiu, Patrice Klein, Yoshikazu Sasai, and Masami Nonaka. "Interannual to Decadal Variations of Submesoscale Motions around the North Pacific Subtropical Countercurrent." Fluids 5, no. 3 (July 17, 2020): 116. http://dx.doi.org/10.3390/fluids5030116.
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The outputs from a submesoscale permitting hindcast simulation from 1990 to 2016 are used to investigate the interannual to decadal variations of submesoscale motions. The region we focus on is the subtropical Northwestern Pacific including the subtropical countercurrent. The submesoscale kinetic energy (KE) is characterized by strong interannual and decadal variability, displaying larger magnitudes in 1996, 2003, and 2015, and smaller magnitudes in 1999, 2009, 2010, and 2016. These variations are partially explained by those of the available potential energy (APE) release at submesoscale driven by mixed layer instability in winter. Indeed, this APE release depends on the mixed layer depth and horizontal buoyancy gradient, both of them modulated with the Pacific Decadal Oscillation (PDO). As a result of the inverse KE cascade, the submesoscale KE variability possibly leads to interannual to decadal variations of the mesoscale KE (eddy KE (EKE)). These results show that submesoscale motions are a possible pathway to explain the impact associated with the PDO on the decadal EKE variability. The winter APE release estimated from the Argo float observations varies synchronously with that in the simulation on the interannual time scales, which suggests the observation capability to diagnose the submesoscale KE variability.
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Drushka, Kyla, William E. Asher, Janet Sprintall, Sarah T. Gille, and Clifford Hoang. "Global Patterns of Submesoscale Surface Salinity Variability." Journal of Physical Oceanography 49, no. 7 (July 2019): 1669–85. http://dx.doi.org/10.1175/jpo-d-19-0018.1.
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AbstractSurface salinity variability on O(1–10) km lateral scales (the submesoscale) generates density variability and thus has implications for submesoscale dynamics. Satellite salinity measurements represent a spatial average over horizontal scales of approximately 40–100 km but are compared to point measurements for validation, so submesoscale salinity variability also complicates validation of satellite salinities. Here, we combine several databases of historical thermosalinograph (TSG) measurements made from ships to globally characterize surface submesoscale salinity, temperature, and density variability. In river plumes; regions affected by ice melt or upwelling; and the Gulf Stream, South Atlantic, and Agulhas Currents, submesoscale surface salinity variability is large. In these regions, horizontal salinity variability appears to explain some of the differences between surface salinities from the Aquarius and SMOS satellites and salinities measured with Argo floats. In other words, apparent satellite errors in highly variable regions in fact arise because Argo point measurements do not represent spatially averaged satellite data. Salinity dominates over temperature in generating submesoscale surface density variability throughout the tropical rainbands, in river plumes, and in polar regions. Horizontal density fronts on 10-km scales tend to be compensated (salinity and temperature have opposing effects on density) throughout most of the global oceans, with the exception of the south Indian and southwest Pacific Oceans between 20° and 30°S, where fronts tend to be anticompensated.
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Uchida, Takaya, Bruno Deremble, and Thierry Penduff. "The Seasonal Variability of the Ocean Energy Cycle from a Quasi-Geostrophic Double Gyre Ensemble." Fluids 6, no. 6 (June 2, 2021): 206. http://dx.doi.org/10.3390/fluids6060206.
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With the advent of submesoscale O(1km) permitting basin-scale ocean simulations, the seasonality of mesoscale O(50km) eddies with kinetic energies peaking in summer has been commonly attributed to submesoscale eddies feeding back onto the mesoscale via an inverse energy cascade under the constraint of stratification and Earth’s rotation. In contrast, by running a 101-member, seasonally forced, three-layer quasi-geostrophic (QG) ensemble configured to represent an idealized double-gyre system of the subtropical and subpolar basin, we find that the mesoscale kinetic energy shows a seasonality consistent with the summer peak without resolving the submesoscales; by definition, a QG model only resolves small Rossby and Froude number dynamics (O(Ro)≪1,O(Fr)≪1) while submesoscale dynamics are associated with O(Ro)∼1,O(Fr)≳1. Here, by quantifying the Lorenz cycle of the mean and eddy energy, defined as the ensemble mean and fluctuations about the mean, respectively, we propose a different mechanism from the inverse energy cascade. During summer, when the Western Boundary Current is stabilized and strengthened due to increased stratification, stronger mesoscale eddies are shed from the separated jet. Conversely, the opposite occurs during the winter; the separated jet destablizes and results in overall lower mean and eddy kinetic energies despite the domain being more susceptible to baroclinic instability from weaker stratification.
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Adams, Katherine A., Philip Hosegood, John R. Taylor, Jean-Baptiste Sallée, Scott Bachman, Ricardo Torres, and Megan Stamper. "Frontal Circulation and Submesoscale Variability during the Formation of a Southern Ocean Mesoscale Eddy." Journal of Physical Oceanography 47, no. 7 (July 2017): 1737–53. http://dx.doi.org/10.1175/jpo-d-16-0266.1.
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AbstractObservations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, O(1–10) km, variability along the periphery of a mesoscale O(10–100) km meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar Fronts converge and modifications of Subantarctic Mode Water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current dataset. Upwelling (downwelling) rates of O(100) m day−1 are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in temperature–salinity (T–S) space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along σθ = 27–27.2 kg m−3 have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously.
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Dever, Mathieu, Mara Freilich, J. Thomas Farrar, Benjamin Hodges, Tom Lanagan, Andrew J. Baron, and Amala Mahadevan. "EcoCTD for Profiling Oceanic Physical–Biological Properties from an Underway Ship." Journal of Atmospheric and Oceanic Technology 37, no. 5 (May 2020): 825–40. http://dx.doi.org/10.1175/jtech-d-19-0145.1.
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AbstractThe study of ocean dynamics and biophysical variability at submesoscales of O(1) km and O(1) h raises several observational challenges. To address these by underway sampling, we recently developed a towed profiler called the EcoCTD, capable of concurrently measuring both hydrographic and bio-optical properties such as oxygen, chlorophyll fluorescence, and optical backscatter. The EcoCTD presents an attractive alternative to currently used towed platforms due to its light footprint, versatility in the field, and ease of deployment and recovery without cranes or heavy-duty winches. We demonstrate its use for gathering high-quality data at submesoscale spatiotemporal resolution. A dataset of bio-optical and hydrographic properties, collected with the EcoCTD during field trials in 2018, highlights its scientific potential for the study of physical–biological interactions at submesoscales.
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Erickson, Zachary K., Andrew F. Thompson, Jörn Callies, Xiaolong Yu, Alberto Naveira Garabato, and Patrice Klein. "The Vertical Structure of Open-Ocean Submesoscale Variability during a Full Seasonal Cycle." Journal of Physical Oceanography 50, no. 1 (January 2020): 145–60. http://dx.doi.org/10.1175/jpo-d-19-0030.1.
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AbstractSubmesoscale dynamics are typically intensified at boundaries and assumed to weaken below the mixed layer in the open ocean. Here, we assess both the seasonality and the vertical distribution of submesoscale motions in an open-ocean region of the northeast Atlantic. Second-order structure functions, or variance in properties separated by distance, are calculated from submesoscale-resolving ocean glider and mooring observations, as well as a 1/48° numerical ocean model. This dataset combines a temporal coverage that extends through a full seasonal cycle, a horizontal resolution that captures spatial scales as small as 1 km, and vertical sampling that provides near-continuous coverage over the upper 1000 m. While kinetic and potential energies undergo a seasonal cycle, being largest during the winter, structure function slopes, influenced by dynamical characteristics, do not exhibit a strong seasonality. Furthermore, structure function slopes show weak vertical variations; there is not a strong change in properties across the base of the mixed layer. Additionally, we compare the observations to output from a high-resolution numerical model. The model does not represent variability associated with superinertial motions and does not capture an observed reduction in submesoscale kinetic energy that occurs throughout the water column in spring. Overall, these results suggest that the transfer of mixed layer submesoscale variability down to depths below the traditionally defined mixed layer is important throughout the weakly stratified subpolar mode waters.
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Viglione, Giuliana A., Andrew F. Thompson, M. Mar Flexas, Janet Sprintall, and Sebastiaan Swart. "Abrupt Transitions in Submesoscale Structure in Southern Drake Passage: Glider Observations and Model Results." Journal of Physical Oceanography 48, no. 9 (September 2018): 2011–27. http://dx.doi.org/10.1175/jpo-d-17-0192.1.
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AbstractEnhanced vertical velocities associated with submesoscale motions may rapidly modify mixed layer depths and increase exchange between the mixed layer and the ocean interior. These dynamics are of particular importance in the Southern Ocean, where the ventilation of many density classes occurs. Here we present results from an observational field program in southern Drake Passage, a region preconditioned for submesoscale instability owing to its strong mesoscale eddy field, persistent fronts, strong down-front winds, and weak vertical stratification. Two gliders sampled from December 2014 through March 2015 upstream and downstream of the Shackleton Fracture Zone (SFZ). The acquired time series of mixed layer depths and buoyancy gradients enabled calculations of potential vorticity and classifications of submesoscale instabilities. The regions flanking the SFZ displayed remarkably different characteristics despite similar surface forcing. Mixed layer depths were nearly twice as deep, and horizontal buoyancy gradients were larger downstream of the SFZ. Upstream of the SFZ, submesoscale variability was confined to the edges of topographically steered fronts, whereas downstream these motions were more broadly distributed. Comparisons to a one-dimensional (1D) mixing model demonstrate the role of submesoscale instabilities in generating mixed layer variance. Numerical output from a submesoscale-resolving simulation indicates that submesoscale instabilities are crucial for correctly reproducing upper-ocean stratification. These results show that bathymetry can play a key role in generating dynamically distinct submesoscale characteristics over short spatial scales and that submesoscale motions can be locally active during summer months.
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Capet, X., J. C. McWilliams, M. J. Molemaker, and A. F. Shchepetkin. "Mesoscale to Submesoscale Transition in the California Current System. Part I: Flow Structure, Eddy Flux, and Observational Tests." Journal of Physical Oceanography 38, no. 1 (January 1, 2008): 29–43. http://dx.doi.org/10.1175/2007jpo3671.1.
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Abstract In computational simulations of an idealized subtropical eastern boundary upwelling current system, similar to the California Current, a submesoscale transition occurs in the eddy variability as the horizontal grid scale is reduced to O(1) km. This first paper (in a series of three) describes the transition in terms of the emergent flow structure and the associated time-averaged eddy fluxes. In addition to the mesoscale eddies that arise from a primary instability of the alongshore, wind-driven currents, significant energy is transferred into submesoscale fronts and vortices in the upper ocean. The submesoscale arises through surface frontogenesis growing off upwelled cold filaments that are pulled offshore and strained in between the mesoscale eddy centers. In turn, some submesoscale fronts become unstable and develop submesoscale meanders and fragment into roll-up vortices. Associated with this phenomenon are a large vertical vorticity and Rossby number, a large vertical velocity, relatively flat horizontal spectra (contrary to the prevailing view of mesoscale dynamics), a large vertical buoyancy flux acting to restratify the upper ocean, a submesoscale energy conversion from potential to kinetic, a significant spatial and temporal intermittency in the upper ocean, and material exchanges between the surface boundary layer and pycnocline. Comparison with available observations indicates that submesoscale fronts and instabilities occur widely in the upper ocean, with characteristics similar to the simulations.
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Zheng, Shaojun, Yan Du, Jiaxun Li, and Xuhua Cheng. "Eddy characteristics in the South Indian Ocean as inferred from surface drifter." Ocean Science Discussions 11, no. 6 (December 11, 2014): 2879–905. http://dx.doi.org/10.5194/osd-11-2879-2014.
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Abstract. Using a geometric eddy identification method, cyclonic and anticyclonic eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have been statistically investigated based on 2082 surface drifters from 1979 to 2013. 19252 eddies are identified with 60% anticyclonic eddies. For the submesoscale eddies (radius r < 10 km), the ratio of cyclonic eddies (3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, number of anticyclonic and cyclonic eddies with radius r ≥ 10 km is almost equal. Mesoscale and submesoscale eddies show different spatial distribution. Eddies with radius r ≥ 100 km mainly appear in a band along 25° S, in Mozambique Channel, and Agulhas Current, characterized by large eddy kinetic energy. The submesoscale anticyclonic eddies are densely distributed in the subtropical basin in the central SIO. The number of mesoscale eddies shows statistically significant seasonal variability, reaching a maximum in October and then minimum in February.
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Zheng, S., Y. Du, J. Li, and X. Cheng. "Eddy characteristics in the South Indian Ocean as inferred from surface drifters." Ocean Science 11, no. 3 (May 12, 2015): 361–71. http://dx.doi.org/10.5194/os-11-361-2015.
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Abstract. Using a geometric eddy identification method, cyclonic and anticyclonic eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have been statistically investigated based on 2082 surface drifters from 1979 to 2013. A total of 19 252 eddies are identified, 60% of them anticyclonic eddies. For the submesoscale eddies (radius r<10 km), the ratio of cyclonic eddies (3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, the number of anticyclonic and cyclonic eddies with radius r≥10 km is almost equal. Mesoscale and submesoscale eddies show different spatial distributions. Eddies with radius r≥100 km mainly appear in the Leeuwin Current, a band along 25° S, Mozambique Channel, and Agulhas Current, areas characterized by large eddy kinetic energy. The submesoscale anticyclonic eddies are densely distributed in the subtropical basin in the central SIO. The number of mesoscale eddies shows statistically significant seasonal variability, reaching a maximum in October and minimum in February.
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Choi, Jun, Annalisa Bracco, Roy Barkan, Alexander F. Shchepetkin, James C. McWilliams, and Jeroen M. Molemaker. "Submesoscale Dynamics in the Northern Gulf of Mexico. Part III: Lagrangian Implications." Journal of Physical Oceanography 47, no. 9 (September 2017): 2361–76. http://dx.doi.org/10.1175/jpo-d-17-0036.1.
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AbstractFour numerical simulations are used to characterize the impact of submesoscale circulations on surface Lagrangian statistics in the northern Gulf of Mexico over 2 months, February and August, representative of winter and summer. The role of resolution and riverine forcing is explored focusing on surface waters in regions where the water column is deeper than 50 m. Whenever submesoscale circulations are present, the probability density functions (PDFs) of dynamical quantities such as vorticity and horizontal velocity divergence for Eulerian and Lagrangian fields differ, with particles preferentially mapping areas of elevated negative divergence and positive vorticity. The stronger the submesoscale circulations are, the more skewed the Lagrangian distributions become, with greater differences between Eulerian and Lagrangian PDFs. In winter, Lagrangian distributions are modestly impacted by the presence of the riverine outflow, while increasing the model resolution from submesoscale permitting to submesoscale resolving has a more profound impact. In summer, the presence of riverine-induced buoyancy gradients is the key to the development of submesoscale circulations and different Eulerian and Lagrangian PDFs. Finite-size Lyapunov exponents (FSLEs) are used to characterize lateral mixing rates. Whenever submesoscale circulations are resolved and riverine outflow is included, FSLEs slopes are broadly consistent with local stirring. Simulated slopes are close to −0.5 and support a velocity field where the ageostrophic and frontogenetic components contribute stirring at scales between about 5 and 7 times the model resolution and 100 km. The robustness of Lagrangian statistics is further discussed in terms of their spatial and temporal variability and of the number of particles available.
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Oscroft, Sarah, Adam M. Sykulski, and Jeffrey J. Early. "Separating Mesoscale and Submesoscale Flows from Clustered Drifter Trajectories." Fluids 6, no. 1 (December 31, 2020): 14. http://dx.doi.org/10.3390/fluids6010014.
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Drifters deployed in close proximity collectively provide a unique observational data set with which to separate mesoscale and submesoscale flows. In this paper we provide a principled approach for doing so by fitting observed velocities to a local Taylor expansion of the velocity flow field. We demonstrate how to estimate mesoscale and submesoscale quantities that evolve slowly over time, as well as their associated statistical uncertainty. We show that in practice the mesoscale component of our model can explain much first and second-moment variability in drifter velocities, especially at low frequencies. This results in much lower and more meaningful measures of submesoscale diffusivity, which would otherwise be contaminated by unresolved mesoscale flow. We quantify these effects theoretically via computing Lagrangian frequency spectra, and demonstrate the usefulness of our methodology through simulations as well as with real observations from the LatMix deployment of drifters. The outcome of this method is a full Lagrangian decomposition of each drifter trajectory into three components that represent the background, mesoscale, and submesoscale flow.
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Sun, Daoxun, Annalisa Bracco, Roy Barkan, Maristella Berta, Daniel Dauhajre, M. Jeroen Molemaker, Jun Choi, Guangpeng Liu, Annalisa Griffa, and James C. McWilliams. "Diurnal Cycling of Submesoscale Dynamics: Lagrangian Implications in Drifter Observations and Model Simulations of the Northern Gulf of Mexico." Journal of Physical Oceanography 50, no. 6 (June 2020): 1605–23. http://dx.doi.org/10.1175/jpo-d-19-0241.1.
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AbstractThe diurnal cycling of submesoscale circulations in vorticity, divergence, and strain is investigated using drifter data collected as part of the Lagrangian Submesoscale Experiment (LASER) experiment, which took place in the northern Gulf of Mexico during winter 2016, and ROMS simulations at different resolutions and degree of realism. The first observational evidence of a submesoscale diurnal cycle is presented. The cycling is detected in the LASER data during periods of weak winds, whereas the signal is obscured during strong wind events. Results from ROMS in the most realistic setup and in sensitivity runs with idealized wind patterns demonstrate that wind bursts disrupt the submesoscale diurnal cycle, independently of the time of day at which they happen. The observed and simulated submesoscale diurnal cycle supports the existence of a shift of approximately 1–3 h between the occurrence of divergence and vorticity maxima, broadly in agreement with theoretical predictions. The amplitude of the modeled signal, on the other hand, always underestimates the observed one, suggesting that even a horizontal resolution of 500 m is insufficient to capture the strength of the observed variability in submesoscale circulations. The paper also presents an evaluation of how well the diurnal cycle can be detected as function of the number of Lagrangian particles. If more than 2000 particle triplets are considered, the diurnal cycle is well captured, but for a number of triplets comparable to that of the LASER analysis, the reconstructed diurnal cycling displays high levels of noise both in the model and in the observations.
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Dauhajre, Daniel P., James C. McWilliams, and Yusuke Uchiyama. "Submesoscale Coherent Structures on the Continental Shelf." Journal of Physical Oceanography 47, no. 12 (December 2017): 2949–76. http://dx.doi.org/10.1175/jpo-d-16-0270.1.
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AbstractDiscovery and analysis of submesoscale variability O(0.3–30) km on the continental shelf is made possible by a high-resolution (Δx = 75 m) Regional Oceanic Modeling System (ROMS) simulation of the Southern California Bight (SCB). This variability is manifest in ubiquitous yet ephemeral coherent structures: fronts, filaments, and vortices. Similar to their open-ocean counterparts, fronts and filaments on the shelf are identified by their strong vertical velocity, surface convergence, cyclonic vorticity, and horizontal density gradient. Life cycles of these features typically last 3–5 days, with the formation dominated by a horizontal advective tendency that increases density and velocity gradients (i.e., frontogenesis). The shape of the coastline and depth of the water column both influence the abundance and spatial orientation of shallow-water fronts and filaments. Closer to shore, fronts and filaments often align themselves parallel to isobaths, and headlands often act as sites of intense vorticity generation through bottom stress. A quasi-steady, approximate momentum balance among rotation, pressure gradient, and vertical mixing—known as turbulent thermal wind (TTW)—often is valid in the strong secondary circulations local to fronts and filaments. However, front and filament circulations subject to strong diurnal variation in surface heating and vertical mixing are inconsistent with steady-state TTW balance. The secondary circulations can induce ephemeral material trapping and substantial vertical heat fluxes on the shelf.
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Timmermans, Mary-Louise, Sylvia Cole, and John Toole. "Horizontal Density Structure and Restratification of the Arctic Ocean Surface Layer." Journal of Physical Oceanography 42, no. 4 (April 1, 2012): 659–68. http://dx.doi.org/10.1175/jpo-d-11-0125.1.
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Abstract Ice-tethered profiler (ITP) measurements from the Arctic Ocean’s Canada Basin indicate an ocean surface layer beneath sea ice with significant horizontal density structure on scales of hundreds of kilometers to the order 1 km submesoscale. The observed horizontal gradients in density are dynamically important in that they are associated with restratification of the surface ocean when dense water flows under light water. Such restratification is prevalent in wintertime and competes with convective mixing upon buoyancy forcing (e.g., ice growth and brine rejection) and shear-driven mixing when the ice moves relative to the ocean. Frontal structure and estimates of the balanced Richardson number point to the likelihood of dynamical restratification by isopycnal tilt and submesoscale baroclinic instability. Based on the evidence here, it is likely that submesoscale processes play an important role in setting surface-layer properties and lateral density variability in the Arctic Ocean.
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Castro, Sandra, William Emery, Gary Wick, and William Tandy. "Submesoscale Sea Surface Temperature Variability from UAV and Satellite Measurements." Remote Sensing 9, no. 11 (October 25, 2017): 1089. http://dx.doi.org/10.3390/rs9111089.
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Shao, Mingming, David G. Ortiz‐Suslow, Brian K. Haus, Björn Lund, Neil J. Williams, Tamay M. Özgökmen, Nathan J. M. Laxague, Jochen Horstmann, and Jody M. Klymak. "The Variability of Winds and Fluxes Observed Near Submesoscale Fronts." Journal of Geophysical Research: Oceans 124, no. 11 (November 2019): 7756–80. http://dx.doi.org/10.1029/2019jc015236.
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Wenegrat, Jacob O., Leif N. Thomas, Jonathan Gula, and James C. McWilliams. "Effects of the Submesoscale on the Potential Vorticity Budget of Ocean Mode Waters." Journal of Physical Oceanography 48, no. 9 (September 2018): 2141–65. http://dx.doi.org/10.1175/jpo-d-17-0219.1.
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AbstractNonconservative processes change the potential vorticity (PV) of the upper ocean and, later, through the subduction of surface waters into the interior, affect the general ocean circulation. Here we focus on how boundary layer turbulence, in the presence of submesoscale horizontal buoyancy gradients, generates a source of potential vorticity at the ocean surface through a balance known as the turbulent thermal wind. This source of PV injection at the submesoscale can be of similar magnitude to PV fluxes from the wind and surface buoyancy fluxes, and hence can lead to a net injection of PV onto outcropped isopycnals even during periods of surface buoyancy loss. The significance of these dynamics is illustrated using a high-resolution realistic model of the North Atlantic Subtropical Mode Water (Eighteen Degree Water), where it is demonstrated that injection of PV at the submesoscale reduces the rate of mode water PV removal by a factor of ~2 and shortens the annual period of mode water formation by ~3 weeks, relative to air–sea fluxes alone. Submesoscale processes thus provide a direct link between small-scale boundary layer turbulence and the gyre-scale circulation, through their effect on mode water formation, with implications for understanding the variability and biogeochemical properties of ocean mode waters globally.
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Shay, Lynn K., Tom N. Lee, Elizabeth J. Williams, Hans C. Graber, and Claes G. H. Rooth. "Effects of low-frequency current variability on near-inertial submesoscale vortices." Journal of Geophysical Research: Oceans 103, no. C9 (August 15, 1998): 18691–714. http://dx.doi.org/10.1029/98jc01007.
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Gebremichael, Mekonnen, Enrique R. Vivoni, Christopher J. Watts, and Julio C. Rodríguez. "Submesoscale Spatiotemporal Variability of North American Monsoon Rainfall over Complex Terrain." Journal of Climate 20, no. 9 (May 1, 2007): 1751–73. http://dx.doi.org/10.1175/jcli4093.1.
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Abstract The authors analyze information from rain gauges, geostationary infrared satellites, and low earth orbiting radar in order to describe and characterize the submesoscale (<75 km) spatial pattern and temporal dynamics of rainfall in a 50 km × 75 km study area located in Sonora, Mexico, in the periphery of the North American monsoon system core region. The temporal domain spans from 1 July to 31 August 2004, corresponding to one monsoon season. Results reveal that rainfall in the study region is characterized by high spatial and temporal variability, strong diurnal cycles in both frequency and intensity with maxima in the evening hours, and multiscaling behavior in both temporal and spatial fields. The scaling parameters of the spatial rainfall fields exhibit dependence on the rainfall rate at the synoptic scale. The rainfall intensity exhibits a slightly stronger diurnal cycle compared to the rainfall frequency, and the maximum lag time between the two diurnal peaks is within 2.4 h, with earlier peaks observed for rainfall intensity. The time of maximum cold cloud occurrence does not vary with the infrared threshold temperature used (215–235 K), while the amplitude of the diurnal cycle varies in such a way that deep convective cells have stronger diurnal cycles. Furthermore, the results indicate that the diurnal cycle of cold cloud occurrence can be used as a surrogate for some basic features of the diurnal cycle of rainfall. The spatial pattern and temporal dynamics of rainfall are modulated by topographic features and large-scale features (circulation and moisture fields as related to geographical location). As compared to valley areas, mountainous areas are characterized by an earlier diurnal peak, an earlier date of maximum precipitation, closely clustered rainy hours, frequent yet small rainfall events, and less dependence of precipitation accumulation on elevation. As compared to the northern section of the study area, the southern section is characterized by strong convective systems that peak late diurnally. The results of this study are important for understanding the physical processes involved, improving the representation of submesoscale variability in models, downscaling rainfall data from coarse meteorological models to smaller hydrological scales, and interpreting and validating remote sensing rainfall estimates.
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Li, Jianing, Jihai Dong, Qingxuan Yang, and Xu Zhang. "Spatial-temporal variability of submesoscale currents in the South China Sea." Journal of Oceanology and Limnology 37, no. 2 (November 19, 2018): 474–85. http://dx.doi.org/10.1007/s00343-019-8077-1.
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Grados, Daniel, Ronan Fablet, Michael Ballón, Nicolas Bez, Ramiro Castillo, Ainhoa Lezama-Ochoa, and Arnaud Bertrand. "Multiscale characterization of spatial relationships among oxycline depth, macrozooplankton, and forage fish off Peru using geostatistics, principal coordinates of neighbour matrices (PCNMs), and wavelets." Canadian Journal of Fisheries and Aquatic Sciences 69, no. 4 (April 2012): 740–54. http://dx.doi.org/10.1139/f2012-017.
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Upwelling ecosystems are particularly heterogeneous and present intense mesoscale (tens of kilometres) and submesoscale (hundreds of metres to kilometres) activity that are expected to drive the distribution of the organisms and thus their interactions. Here we addressed the impact of the physical forcing in the northern Humboldt Current system off Peru, which is characterized by the presence of an intense and shallow oxygen minimum zone and used the variability of the depth of the oxycline as a proxy of the physical forcing that impacts the epipelagic communities. We analyzed simultaneous high-resolution acoustic observations of the oxycline depth, the biomass in macrozooplankton, and the biomass in pelagic fish. Three complementary methodologies were considered: (i) geostatistical methods and correlation tests, (ii) principal coordinates of neighbour matrices, and (iii) wavelet analysis. Our results highlight the relevance of a multimethod framework to characterize the multiscale relationships between marine ecosystem components. We also provided evidence that the submesoscale-to-mesoscale variability of the oxycline depth drives the distribution of macrozooplankton, which further structures the distribution of forage fish in a bottom-up cascade.
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Karimova, S. "OBSERVING SURFACE CIRCULATION OF THE WESTERN MEDITERRANEAN BASIN WITH SATELLITE IMAGERY." ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences XLII-3/W2 (November 16, 2017): 97–104. http://dx.doi.org/10.5194/isprs-archives-xlii-3-w2-97-2017.
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In this article, the benefits of using satellite imagery of different types (namely thermal infrared, visible-range, and synthetic aperture radar (SAR) images) for observing surface circulation of marine basins are being discussed. As a region of interest, we use the Western Mediterranean Basin. At first, the areas with sharpest thermal and chlorophyll-a gradients within the region of interest were defined on a seasonal base using the data provided by Aqua Moderate Resolution Imaging Spectrometer (MODIS). After that, mesoscale eddies were detected using different sea surface temperature (SST) products and, finally, submesoscale vortices were observed with Envisat Advanced SAR imagery. Thus found locations of eddies were compared with locations of the sharpest fronts discovered in the first part of the study, which showed that the biggest, mostly anticyclonic, eddies tended to correspond to locations of main surface currents; smaller cyclonic eddies were mostly attributed to thermal fronts, while submesoscale eddies were distributed quite homogeneous. The observations performed in that way revealed quite prominent basin-, meso- and submesoscale eddy activity in the region of interest. Additionally, significant seasonal variability in the type of surface water stirring was noted. Thus, the maximum of both meso- and submesoscale eddy activity seems to happen during the warm season, while during winter, presumably due to low Richardson numbers typical for the upper water layer, the turbulent features are still undeveloped and of the smaller spatial scale than during the warm period of year.
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Dauhajre, Daniel P., and James C. McWilliams. "Diurnal Evolution of Submesoscale Front and Filament Circulations." Journal of Physical Oceanography 48, no. 10 (October 2018): 2343–61. http://dx.doi.org/10.1175/jpo-d-18-0143.1.
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AbstractThe local circulation of submesoscale fronts and filaments can be partly understood through a horizontal momentum balance of Coriolis, a horizontal pressure gradient, and vertical diffusivity in a turbulent boundary layer, known as the turbulent thermal wind balance (TTW or T2W). T2W often reproduces the instantaneous relative vorticity and divergence of submesoscale circulations in open-ocean and shelf settings. However, a diurnal cycle in submesoscale vorticity and divergence is characterized by a non-T2W phasing: a maximum in divergence magnitude lags the maximum in vertical diffusivity (with vorticity lagging divergence). Here, an idealized model is used to solve the transient turbulent thermal wind (T3W) equations and to investigate the diurnal evolution of front and filament circulation in a 2D plane. Relative to a steady-state circulation, transient evolution can cause both instantaneous strengthening and a weaker diurnal average of the secondary circulation. The primary mechanisms controlling the diurnal variability exist in a 1D Ekman layer that imprints onto the 2D circulation. In midlatitudes, acceleration during separate phases of the diurnal cycle (from night to day and from day to night) is dominated by distinct inertial oscillation and vertically diffusive mechanisms, respectively. However, the manifestation of these dual accelerations is sensitive to latitude, boundary layer depth, and the strength of the forcing. A simple 1D model predicts the diurnal phasing of submesoscale divergence and vorticity in realistic primitive equation simulations of the southwestern Pacific and coastal California.
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Chassignet, Eric P., and Xiaobiao Xu. "Impact of Horizontal Resolution (1/12° to 1/50°) on Gulf Stream Separation, Penetration, and Variability." Journal of Physical Oceanography 47, no. 8 (August 2017): 1999–2021. http://dx.doi.org/10.1175/jpo-d-17-0031.1.
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AbstractThe impact of horizontal resolution (1/12° to 1/50°; 6 to 1.5 km at midlatitudes) on Gulf Stream separation, penetration, and variability is quantified in a series of identical North Atlantic experiments. The questions the authors seek to address are twofold: 1) Is the realism of the modeled solution increased as resolution is increased? 2) How robust is the modeled mesoscale and submesoscale eddy activity as a function of grid spacing and how representative is it of interior quasigeostrophic (QG) or surface quasigeostrophic (SQG) turbulence? This study shows that (i) the representation of Gulf Stream penetration and associated recirculating gyres shifts from unrealistic to realistic when the resolution is increased to 1/50° and when the nonlinear effects of the submesoscale eddies intensifies the midlatitude jet and increases its penetration eastward, (ii) the penetration into the deep ocean drastically increases with resolution and closely resembles the observations, and (iii) surface power spectra in the 70–250-km mesoscale range are independent of the horizontal resolution and of the latitude and are representative of 2D QG and SQG turbulence.
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Yan, Xiaomei, Dujuan Kang, Enrique N. Curchitser, Xiaohui Liu, Chongguang Pang, and Linlin Zhang. "Seasonal Variability of Eddy Kinetic Energy along the Kuroshio Current." Journal of Physical Oceanography 53, no. 7 (July 2023): 1731–52. http://dx.doi.org/10.1175/jpo-d-22-0155.1.
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Abstract The seasonal variability of the eddy kinetic energy (EKE) along the Kuroshio Current (KC) is examined using outputs from an eddy-resolving (1/10°) ocean model. Using a theoretical framework for climatological monthly mean EKE, the mechanisms governing the seasonal cycle of upper-ocean EKE are investigated. East of Taiwan, the EKE shows two comparable peaks in spring and summer in the surface layer; only the spring one is evident in the subsurface layer. The seasonality is determined by mixed barotropic (BTI) and baroclinic (BCI) instabilities. Northeast of Taiwan, the EKE is also elevated during spring–summer but with a sole peak in summer, which is dominated by the meridional EKE advection by the KC. In the middle part of the KC in the East China Sea, the mesoscale (>150 km) EKE (EKEMS) is relatively strong during spring–summer, whereas the submesoscale (50–150 km) EKE (EKESM) is significantly enhanced during winter–spring. The seasonal cycles of EKEMS and EKESM are primarily controlled by the external forcing and BCI, respectively. In particular, the higher EKEMS level in summer is mainly due to the increased wind work. West of the Tokara Strait, the EKE exhibits a prominent peak in winter and has its minimum in summer, which is regulated by the BCI. As the submesoscale signals are partially resolved by the model, further studies with higher-resolution simulations and observations are needed for a better understanding of the EKESM seasonality and its contribution to the seasonally modulating EKEMS along the KC.
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Vazquez-Cuervo, Jorge, Chelle Gentemann, Wenqing Tang, Dustin Carroll, Hong Zhang, Dimitris Menemenlis, Jose Gomez-Valdes, Marouan Bouali, and Michael Steele. "Using Saildrones to Validate Arctic Sea-Surface Salinity from the SMAP Satellite and from Ocean Models." Remote Sensing 13, no. 5 (February 24, 2021): 831. http://dx.doi.org/10.3390/rs13050831.
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The Arctic Ocean is one of the most important and challenging regions to observe—it experiences the largest changes from climate warming, and at the same time is one of the most difficult to sample because of sea ice and extreme cold temperatures. Two NASA-sponsored deployments of the Saildrone vehicle provided a unique opportunity for validating sea-surface salinity (SSS) derived from three separate products that use data from the Soil Moisture Active Passive (SMAP) satellite. To examine possible issues in resolving mesoscale-to-submesoscale variability, comparisons were also made with two versions of the Estimating the Circulation and Climate of the Ocean (ECCO) model (Carroll, D; Menmenlis, D; Zhang, H.). The results indicate that the three SMAP products resolve the runoff signal associated with the Yukon River, with high correlation between SMAP products and Saildrone SSS. Spectral slopes, overall, replicate the −2.0 slopes associated with mesoscale-submesoscale variability. Statistically significant spatial coherences exist for all products, with peaks close to 100 km. Based on these encouraging results, future research should focus on improving derivations of satellite-derived SSS in the Arctic Ocean and integrating model results to complement remote sensing observations.
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Touret, Richard X., Guangpeng Liu, Annalisa Bracco, and Karim G. Sabra. "Influence of the vertical resolution when simulating ocean sound speed variability in mesoscale eddies." Journal of the Acoustical Society of America 152, no. 4 (October 2022): A44. http://dx.doi.org/10.1121/10.0015485.
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Vertical resolution affects the representation of ocean sound speed according to a suite of regional simulations of the De Soto Canyon circulation in the Gulf of Mexico. Simulations have identical horizontal resolution of 0.5 km, partially resolving submesoscale dynamics, and increasing vertical resolution from 30 (i.e., comparable to what commonly used in mesoscale permitting or resolving hindcast and forecast products such as HYCOM) to 200 terrain-following layers. Simulations with 30- and 70-layers underestimate the ageostrophic contributions in and around the eddies below the mixed-layer and do not reproduce the sharp vorticity and density variations associated with the mesoscale circulations compared to the 140- and 200-layers runs. The ocean sound speed (based on the classical MacKenzie formula) was found to be far more variable when the submesoscale, ageostrophic circulations are captured also in their vertical structure and vertical contributions to the density field. Hence, the results of this study indicate that to better predict the influence of the submesocale oceanic circulation on ocean sound speed variability, model simulations should consider enhancing both horizontal and vertical resolution to resolve at least the first 3 baroclinic modes. To do so, more than 100 vertical layers were found to be needed in this study.
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Divinsky, B. V., S. B. Kuklev, A. G. Zatsepin, and B. V. Chubarenko. "Simulation of submesoscale variability of currents in the Black Sea coastal zone." Oceanology 55, no. 6 (November 2015): 814–19. http://dx.doi.org/10.1134/s000143701506003x.
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Manville, George, Thomas G. Bell, Jane P. Mulcahy, Rafel Simó, Martí Galí, Anoop S. Mahajan, Shrivardhan Hulswar, and Paul R. Halloran. "Global analysis of the controls on seawater dimethylsulfide spatial variability." Biogeosciences 20, no. 9 (May 16, 2023): 1813–28. http://dx.doi.org/10.5194/bg-20-1813-2023.
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Abstract. Dimethylsulfide (DMS) emitted from the ocean makes a significant global contribution to natural marine aerosol and cloud condensation nuclei and, therefore, our planet's climate. Oceanic DMS concentrations show large spatiotemporal variability, but observations are sparse, so products describing global DMS distribution rely on interpolation or modelling. Understanding the mechanisms driving DMS variability, especially at local scales, is required to reduce uncertainty in large-scale DMS estimates. We present a study of mesoscale and submesoscale (< 100 km) seawater DMS variability that takes advantage of the recent expansion in high-frequency seawater DMS observations and uses all available data to investigate the typical distances over which DMS varies in all major ocean basins. These DMS spatial variability length scales (VLSs) are uncorrelated with DMS concentrations. The DMS concentrations and VLSs can therefore be used separately to help identify mechanisms underpinning DMS variability. When data are grouped by sampling campaigns, almost 80 % of the DMS VLS can be explained using the VLSs of sea surface height anomalies, density, and chlorophyll a. Our global analysis suggests that both physical and biogeochemical processes play an equally important role in controlling DMS variability, which is in contrast with previous results based on data from the low to mid-latitudes. The explanatory power of sea surface height anomalies indicates the importance of mesoscale eddies in driving DMS variability, previously unrecognised at a global scale and in agreement with recent regional studies. DMS VLS differs regionally, including surprisingly high-frequency variability in low-latitude waters. Our results independently confirm that relationships used in the literature to parameterise DMS at large scales appear to be considering the right variables. However, regional DMS VLS contrasts highlight that important driving mechanisms remain elusive. The role of submesoscale features should be resolved or accounted for in DMS process models and parameterisations. Future attempts to map DMS distributions should consider the length scale of variability.
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du Plessis, Marcel, Sebastiaan Swart, Isabelle J. Ansorge, Amala Mahadevan, and Andrew F. Thompson. "Southern Ocean Seasonal Restratification Delayed by Submesoscale Wind–Front Interactions." Journal of Physical Oceanography 49, no. 4 (April 2019): 1035–53. http://dx.doi.org/10.1175/jpo-d-18-0136.1.
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AbstractOcean stratification and the vertical extent of the mixed layer influence the rate at which the ocean and atmosphere exchange properties. This process has direct impacts for anthropogenic heat and carbon uptake in the Southern Ocean. Submesoscale instabilities that evolve over space (1–10 km) and time (from hours to days) scales directly influence mixed layer variability and are ubiquitous in the Southern Ocean. Mixed layer eddies contribute to mixed layer restratification, while down-front winds, enhanced by strong synoptic storms, can erode stratification by a cross-frontal Ekman buoyancy flux. This study investigates the role of these submesoscale processes on the subseasonal and interannual variability of the mixed layer stratification using four years of high-resolution glider data in the Southern Ocean. An increase of stratification from winter to summer occurs due to a seasonal warming of the mixed layer. However, we observe transient decreases in stratification lasting from days to weeks, which can arrest the seasonal restratification by up to two months after surface heat flux becomes positive. This leads to interannual differences in the timing of seasonal restratification by up to 36 days. Parameterizing the Ekman buoyancy flux in a one-dimensional mixed layer model reduces the magnitude of stratification compared to when the model is run using heat and freshwater fluxes alone. Importantly, the reduced stratification occurs during the spring restratification period, thereby holding important implications for mixed layer dynamics in climate models as well as physical–biological coupling in the Southern Ocean.
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Hamze-Ziabari, Seyed Mahmood, Mehrshad Foroughan, Ulrich Lemmin, and David Andrew Barry. "Monitoring Mesoscale to Submesoscale Processes in Large Lakes with Sentinel-1 SAR Imagery: The Case of Lake Geneva." Remote Sensing 14, no. 19 (October 6, 2022): 4967. http://dx.doi.org/10.3390/rs14194967.
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As in oceans, large-scale coherent circulations such as gyres and eddies are ubiquitous features in large lakes that are subject to the Coriolis force. They play a crucial role in the horizontal and vertical distribution of biological, chemical and physical parameters that can affect water quality. In order to make coherent circulation patterns evident, representative field measurements of near-surface currents have to be taken. This, unfortunately, is difficult due to the high spatial and temporal variability of gyres/eddies. As a result, few complete field observations of coherent circulation in oceans/lakes have been reported. With the advent of high-resolution satellite imagery, the potential to unravel and improve the understanding of mesoscale and submesoscale processes has substantially increased. Features in the satellite images, however, must be verified by field measurements and numerical simulations. In the present study, Sentinel-1 SAR satellite imagery was used to detect gyres/eddies in a large lake (Lake Geneva). Comparing SAR images with realistic high-resolution numerical model results and in situ observations allowed for identification of distinct signatures of mesoscale gyres, which can be revealed through submesoscale current patterns. Under low wind conditions, cyclonic gyres manifest themselves in SAR images either through biogenic slicks that are entrained in submesoscale and mesoscale currents, or by pelagic upwelling that appears as smooth, dark elliptical areas in their centers. This unique combination of simultaneous SAR imagery, three-dimensional numerical simulations and field observations confirmed that SAR imagery can provide valuable insights into the spatial scales of thus far unresolved mesoscale and submesoscale processes in a lake. Understanding these processes is required for developing effective lake management concepts.
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Kunze, E., J. M. Klymak, R. C. Lien, R. Ferrari, C. M. Lee, M. A. Sundermeyer, and L. Goodman. "Submesoscale Water-Mass Spectra in the Sargasso Sea." Journal of Physical Oceanography 45, no. 5 (May 2015): 1325–38. http://dx.doi.org/10.1175/jpo-d-14-0108.1.
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AbstractSubmesoscale stirring contributes to the cascade of tracer variance from large to small scales. Multiple nested surveys in the summer Sargasso Sea with tow-yo and autonomous platforms captured submesoscale water-mass variability in the seasonal pycnocline at 20–60-m depths. To filter out internal waves that dominate dynamic signals on these scales, spectra for salinity anomalies on isopycnals were formed. Salinity-gradient spectra are approximately flat with slopes of −0.2 ± 0.2 over horizontal wavelengths of 0.03–10 km. While the two to three realizations presented here might be biased, more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1 km. A review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out (i) quasigeostrophic eddy stirring, (ii) atmospheric forcing through a relict submesoscale winter mixed layer structure or nocturnal mixed layer deepening, (iii) a downscale vortical-mode cascade, and (iv) horizontal diffusion because of shear dispersion of diapycnal mixing. Internal-wave horizontal strain appears to be able to explain horizontal wavenumbers of 0.1–7 cycles per kilometer (cpkm) but not the highest resolved wavenumbers (7–30 cpkm). Submesoscale subduction cannot be ruled out at these depths, though previous observations observe a flat spectrum well below subduction depths, so this seems unlikely. Primitive equation numerical modeling suggests that nonquasigeostrophic subinertial horizontal stirring can produce a flat spectrum. The last need not be limited to mode-one interior or surface Rossby wavenumbers of quasigeostrophic theory but may have a broaderband spectrum extending to smaller horizontal scales associated with frontogenesis and frontal instabilities as well as internal waves.
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Chen, Yinchao, Qian P. Li, and Jiancheng Yu. "Submesoscale variability of subsurface chlorophyll-a across eddy-driven fronts by glider observations." Progress in Oceanography 209 (December 2022): 102905. http://dx.doi.org/10.1016/j.pocean.2022.102905.
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Lee, Eun Ae, and Sung Yong Kim. "Regional Variability and Turbulent Characteristics of the Satellite‐sensed Submesoscale Surface Chlorophyll Concentrations." Journal of Geophysical Research: Oceans 123, no. 6 (June 2018): 4250–79. http://dx.doi.org/10.1029/2017jc013732.
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Hall, Kashawn, Alton Daley, Shanice Whitehall, Sanola Sandiford, and Chelle L. Gentemann. "Validating Salinity from SMAP and HYCOM Data with Saildrone Data during EUREC4A-OA/ATOMIC." Remote Sensing 14, no. 14 (July 13, 2022): 3375. http://dx.doi.org/10.3390/rs14143375.
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The 2020 ‘Elucidating the role of clouds-circulation coupling in climate-Ocean-Atmosphere’ (EUREC4A-OA) and the ‘Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign’ (ATOMIC) campaigns focused on improving our understanding of the interaction between clouds, convection and circulation and their function in our changing climate. The campaign utilized many data collection technologies, some of which are relatively new. In this study, we used saildrone uncrewed surface vehicles, one of the newer cutting edge technologies available for marine data collection, to validate Level 2 and Level 3 Soil Moisture Active Passive (SMAP) satellite and Hybrid Coordinate Ocean Model (HYCOM) sea surface salinity (SSS) products in the Western Tropical Atlantic. The saildrones observed fine-scale salinity variability not present in the lower-spatial resolution satellite and model products. In regions that lacked significant small-scale salinity variability, the satellite and model salinities performed well. However, SMAP Remote Sensing Systems (RSS) 70 km generally outperformed its counterparts outside of areas with submesoscale SSS variation, whereas RSS 40 km performed better within freshening events such as a fresh tongue. HYCOM failed to detect the fresh tongue. These results will allow researchers to make informed decisions regarding the most ideal product and its drawbacks for their applications in this region and aid in the improvement of mesoscale and submesoscale SSS products, which can lead to the refinement of numerical weather prediction (NWP) and climate models.
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Capó, Esther, James C. McWilliams, Evan Mason, and Alejandro Orfila. "Intermittent Frontogenesis in the Alboran Sea." Journal of Physical Oceanography 51, no. 5 (May 2021): 1417–39. http://dx.doi.org/10.1175/jpo-d-20-0277.1.
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AbstractWe present a phenomenological description and dynamical analysis of the Alboran fronts using a realistic simulation at submesoscale resolution. The study is focused on east Alboran fronts emerging within relatively strong flows that separate from the Spanish coast into the basin interior. Despite modest lateral shifting associated with the position of the Alboran anticyclonic gyres and variations in intensity, these fronts present a similar structure and dynamical configuration as the climatological Almeria–Oran front. The statistical analysis of our solution shows that strained-induced frontogenesis is a recurrent submesoscale mechanism associated with these fronts, and the process is assessed in terms of the advective Lagrangian frontogenetic tendencies associated with buoyancy and velocity horizontal gradients. Intermittency in their strength and patterns is indicative of high variability in the occurrence of active frontogenesis in association with the secondary (overturning) circulation across the frontal gradient. As a result, we find many episodes with strong surface fronts that do not have much associated downwelling. Frontogenesis and the associated secondary circulation are further explored during two particular frontal events, both showing strong downwelling of (1) cm s−1 extending down into the pycnocline. A frontogenetic contribution of turbulent vertical momentum mixing to the secondary circulation is identified in the easternmost region during the cold season, when the dynamics are strongly influenced by the intrusion of the salty Northern Current. The background vertical velocity fields observed during the analyzed events indicate other currents in the submesoscale range, including tidal and topographic internal waves.
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Uchida, Takaya, Julien Le Sommer, Charles Stern, Ryan P. Abernathey, Chris Holdgraf, Aurélie Albert, Laurent Brodeau, et al. "Cloud-based framework for inter-comparing submesoscale-permitting realistic ocean models." Geoscientific Model Development 15, no. 14 (July 27, 2022): 5829–56. http://dx.doi.org/10.5194/gmd-15-5829-2022.
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Abstract. 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.
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Wijesekera, Hemantha W., Emily Shroyer, Amit Tandon, M. Ravichandran, Debasis Sengupta, S. U. P. Jinadasa, Harindra J. S. Fernando, et al. "ASIRI: An Ocean–Atmosphere Initiative for Bay of Bengal." Bulletin of the American Meteorological Society 97, no. 10 (October 1, 2016): 1859–84. http://dx.doi.org/10.1175/bams-d-14-00197.1.
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Abstract Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.
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Gough, Matt K., Francisco J. Beron-Vera, María J. Olascoaga, Julio Sheinbaum, Julien Jouanno, and Rodrigo Duran. "Persistent Lagrangian Transport Patterns in the Northwestern Gulf of Mexico." Journal of Physical Oceanography 49, no. 2 (February 2019): 353–67. http://dx.doi.org/10.1175/jpo-d-17-0207.1.
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AbstractPersistent Lagrangian transport patterns at the ocean surface are revealed from climatological Lagrangian coherent structures (cLCSs) computed from daily climatological surface current velocities in the northwestern Gulf of Mexico (NWGoM). The climatological currents are computed from daily velocities produced by an 18-yr-long free-running submesoscale-permitting Nucleus for European Modelling of the Ocean (NEMO) simulation of the Gulf of Mexico. Despite the intense submesoscale variability produced by the model along the shelf break, which is found to be consistent with observations and previous studies, a persistent mesoscale attracting barrier between the NWGoM shelf and the deep ocean is effectively identified by a hook-like pattern associated with persistent strongly attracting cLCSs. Simulated tracer and satellite-tracked drifters originating over the shelf tend to be trapped there by the hook-like pattern as they spread cyclonically. Tracers and drifters originating beyond the shelf tend to be initially attracted to the hook-like pattern as they spread anticyclonically and eventually over the deep ocean. The findings have important implications for the mitigation of contaminant accidents such as oil spills.
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Marechal, Gwendal, and Charly de Marez. "Variability of surface gravity wave field over a realistic cyclonic eddy." Ocean Science 18, no. 5 (September 7, 2022): 1275–92. http://dx.doi.org/10.5194/os-18-1275-2022.
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Abstract. Recent remote sensing measurements and numerical studies have shown that surface gravity waves interact strongly with small-scale open ocean currents. Through these interactions, the significant wave height, the wave frequency, and the wave direction are modified. In the present paper, we investigate the interactions of surface gravity waves with a large and isolated realistic cyclonic eddy. This eddy is subject to instabilities, leading to the generation of specific features at both the mesoscale and submesoscale ranges. We use the WAVEWATCH III numerical framework to force surface gravity waves in the eddy before and after its destabilization. In the wave simulations the source terms are deactivated, and waves are initialized with different wave intrinsic frequencies. The study of these simulations illustrates how waves respond to the numerous kinds of instabilities in the large cyclonic eddy from a few hundred to a few tens of kilometres. Our findings show that the spatial variability of the wave direction, the mean period, and the significant wave height is very sensitive to the presence of submesoscale structures resulting from the eddy destabilization. The intrinsic frequency of the incident waves is key in the change of the wave direction resulting from the current-induced refraction and in the location, from the boundary where waves are generated, of the maximum values of significant wave height. However, for a given current forcing, the maximum values of the significant wave height are similar regardless of the frequency of the incident waves. In this idealized study it has been shown that the spatial gradients of wave parameters are sharper for simulations forced with the destabilized eddy. Because the signature of currents on waves encodes important information of currents, our findings suggest that the vertical vorticity of the current could be statistically estimated from the significant wave height gradients down to a very fine spatial scale. Furthermore, this paper shows the necessity to include currents in parametric models of sea-state bias; using a coarse-resolution eddy field may severely underestimate the sea-state-induced noise in radar altimeter measurements.
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Flexas, Mar M., Martina I. Troesch, Steve Chien, Andrew F. Thompson, Selina Chu, Andrew Branch, John D. Farrara, and Yi Chao. "Autonomous Sampling of Ocean Submesoscale Fronts with Ocean Gliders and Numerical Model Forecasting." Journal of Atmospheric and Oceanic Technology 35, no. 3 (March 2018): 503–21. http://dx.doi.org/10.1175/jtech-d-17-0037.1.
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ABSTRACTSubmesoscale fronts arising from mesoscale stirring are ubiquitous in the ocean and have a strong impact on upper-ocean dynamics. This work presents a method for optimizing the sampling of ocean fronts with autonomous vehicles at meso- and submesoscales, based on a combination of numerical forecast and autonomous planning. This method uses a 48-h forecast from a real-time high-resolution data-assimilative primitive equation ocean model, feature detection techniques, and a planner that controls the observing platform. The method is tested in Monterey Bay, off the coast of California, during a 9-day experiment focused on sampling subsurface thermohaline-compensated structures using a Seaglider as the ocean observing platform. Based on model estimations, the sampling “gain,” defined as the magnitude of isopycnal tracer variability sampled, is 50% larger in the feature-chasing case with respect to a non-feature-tracking scenario. The ability of the model to reproduce, in space and time, thermohaline submesoscale features is evaluated by quantitatively comparing the model and glider results. The model reproduces the vertical (~50–200 m thick) and lateral (~5–20 km) scales of subsurface subducting fronts and near-bottom features observed in the glider data. The differences between model and glider data are, in part, attributed to the selected glider optimal interpolation parameters and to uncertainties in the forecasting of the location of the structures. This method can be exported to any place in the ocean where high-resolution data-assimilative model output is available, and it allows for the incorporation of multiple observing platforms.
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Korotkina, O. A., P. O. Zavialov, and A. A. Osadchiev. "Submesoscale variability of the current and wind fields in the coastal region of Sochi." Oceanology 51, no. 5 (October 2011): 745–54. http://dx.doi.org/10.1134/s0001437011050109.
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Wang, Tao, Roy Barkan, James C. McWilliams, and M. Jeroen Molemaker. "Structure of Submesoscale Fronts of the Mississippi River Plume." Journal of Physical Oceanography 51, no. 4 (April 2021): 1113–31. http://dx.doi.org/10.1175/jpo-d-20-0191.1.
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AbstractSubmesoscale currents (SMCs), in the forms of fronts, filaments, and vortices, are studied using a high-resolution (~150 m) Regional Oceanic Modeling System (ROMS) simulation in the Mississippi River plume system. Fronts and filaments are identified by large horizontal velocity and buoyancy gradients, surface convergence, and cyclonic vertical vorticity with along-coast fronts and along-plume-edge filaments notably evident. Frontogenesis and arrest/destruction are two fundamental phases in the life cycle of fronts and filaments. In the Mississippi River plume region, the horizontal advective tendency induced by confluence and convergence plays a primary role in frontogenesis. Confluent currents sharpen preexisting horizontal buoyancy gradients and initiate frontogenesis. Once the fronts and filaments are formed and the Rossby number reaches O(1), they further evolve frontogenetically mainly by convergent secondary circulations, which can be maintained by different cross-front momentum balance regimes. Confluent motions and preexisting horizontal buoyancy gradients depend on the interaction between wind-induced Ekman transport and the spreading plume water. Consequently, the direction of wind has a significant effect on the temporal variability of SMCs, with more active SMCs generated during a coastally downwelling-favorable wind and fewer SMCs during an upwelling-favorable wind. Submesoscale instabilities (~1–3 km) play a primary role in the arrest and fragmentation of most fronts and filaments. These instabilities propagate along the fronts and filaments, and their energy conversion is a mixed barotropic–baroclinic type with horizontal-shear instabilities dominating.
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Gil, Julio. "Macro and mesoscale physical patterns in the Bay of Biscay." Journal of the Marine Biological Association of the United Kingdom 88, no. 2 (March 2008): 217–25. http://dx.doi.org/10.1017/s0025315408000490.
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The study area for this work includes all the southern edge of the Bay of Biscay, from the north-west Iberian Peninsula to the southern half of the French shelf. The principal aim of this article is to provide a complete overview of the physical oceanography of the area, mainly in its mesoscale aspects, of which there are few published studies, and the implications for early fish life history stages. The results showed the existence of two space and temporal scales for most of the physical processes that occur in the Bay of Biscay, a macroscale for seasonal time periods and a meso and submesoscale for the periods between seasons. The importance of local phenomena, such as upwelling or the variability of the Poleward Current, was observed. The interaction of both scales on these physical processes is discussed and the need for sampling at the submesoscale level to determine the distribution of ichthyoplankton is considered. Moreover, the mesoscale physical oceanography study is essential to improve the knowledge of interactions between strategies and environmental conditions that result in a significant mortality reduction in fish early stages.
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Pidcock, Rosalind, Meric Srokosz, John Allen, Mark Hartman, Stuart Painter, Matt Mowlem, David Hydes, and Adrian Martin. "A Novel Integration of an Ultraviolet Nitrate Sensor On Board a Towed Vehicle for Mapping Open-Ocean Submesoscale Nitrate Variability." Journal of Atmospheric and Oceanic Technology 27, no. 8 (August 1, 2010): 1410–16. http://dx.doi.org/10.1175/2010jtecho780.1.
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Abstract Initial results from a deployment of the SUV-6 ultraviolet spectrophotometer, integrated with the SeaSoar towed vehicle, are presented. The innovative, combined system measures nitrate concentration at high spatial resolution (4 m vertically, 5 km horizontally), high sensitivity (0.2 μM), and concomitantly with temperature, salinity, and dissolved oxygen. The authors demonstrate that this approach constitutes a powerful new tool for quantifying the role of mesoscale and submesoscale vertical nutrient fluxes to the euphotic zone, using measurements from a high-resolution survey of an eddy dipole in the Iceland Basin during the summer of 2007.
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Ramachandran, Sanjiv, Amit Tandon, Jennifer Mackinnon, Andrew J. Lucas, Robert Pinkel, Amy F. Waterhouse, Jonathan Nash, et al. "Submesoscale Processes at Shallow Salinity Fronts in the Bay of Bengal: Observations during the Winter Monsoon." Journal of Physical Oceanography 48, no. 3 (March 2018): 479–509. http://dx.doi.org/10.1175/jpo-d-16-0283.1.
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AbstractLateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often ≤10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1–10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (inertial instability); and (v) strong contribution from differential sheared advection at O(1) km scales to the growth rate of the depth-averaged stratification. The findings here show one-dimensional vertical processes alone cannot explain the vertical stratification and its lateral variability over O(1–10) km scales at the radiator survey.
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Shcherbina, Andrey Y., Miles A. Sundermeyer, Eric Kunze, Eric D’Asaro, Gualtiero Badin, Daniel Birch, Anne-Marie E. G. Brunner-Suzuki, et al. "The LatMix Summer Campaign: Submesoscale Stirring in the Upper Ocean." Bulletin of the American Meteorological Society 96, no. 8 (August 1, 2015): 1257–79. http://dx.doi.org/10.1175/bams-d-14-00015.1.
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Abstract Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.