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  • 1
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 479-509, doi:10.1175/JPO-D-16-0283.1.
    Description: Lateral 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.
    Description: S. Ramachandran acknowledges support from the National Science Foundation through award OCE 1558849 and the U.S. Office of Naval Research, Grants N00014-13-1-0456 and N00014-17- 1-2355. A. Tandon acknowledges support from the U.S. Office of Naval Research, Grants N00014-13-1-0456 and N00014-17-1-2355. J. T. Farrar and R. A. Weller were supported by the U.S. Office of Naval Research, Grant N00014-13-1-0453, to collect the UCTD data and process theUCTD and shipboard meteorological data. J. Nash, J. Mackinnon, and A. F. Waterhouse acknowledge support from the U. S. Office of Naval Research, Grants N00014-13-1-0503 and N00014-14-1-0455. E. Shroyer acknowledges support from the U. S. Office of Naval Research, Grants N00014-14-10236 and N00014-15- 12634. A. Mahadevan acknowledges support fromthe U. S. Office of Naval Research, Grant N00014-13-10451. A. J. Lucas and R. Pinkel acknowledge support from the U. S. Office of Naval Research, Grant N00014-13-1-0489.
    Description: 2018-08-26
    Keywords: Indian Ocean ; Baroclinic flows ; Potential vorticity ; Fronts ; Monsoons ; Oceanic mixed layer
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
    Publication Date: 2022-05-25
    Description: © The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ocean Modelling 105 (2016): 44-59, doi:10.1016/j.ocemod.2016.07.002.
    Description: We study the production and dissipation of the eddy kinetic energy (EKE) in a submesoscale eddy field forced with downfront winds using the Process Study Ocean Model (PSOM) with a horizontal grid resolution of 0.5 km. We simulate an idealized 100 m deep mixed-layer front initially in geostrophic balance with a jet in a domain that permits eddies within a range of O(1km–100 km). The vertical eddy viscosities and the dissipation are parameterized using four different subgrid vertical mixing parameterizations: the k−ϵ,k−ϵ, the KPP, and two different constant eddy viscosity and diffusivity profiles with a magnitude of O(10−2m2s−1) in the mixed layer. Our study shows that strong vertical eddy viscosities near the surface reduce the parameterized dissipation, whereas strong vertical eddy diffusivities reduce the lateral buoyancy gradients and consequently the rate of restratification by mixed-layer instabilities (MLI). Our simulations show that near the surface, the spatial variability of the dissipation along the periphery of the eddies depends on the relative alignment of the ageostrophic and geostrophic shear. Analysis of the resolved EKE budgets in the frontal region from the simulations show important similarities between the vertical structure of the EKE budget produced by the k−ϵk−ϵ and KPP parameterizations, and earlier LES studies. Such an agreement is absent in the simulations using constant eddy-viscosity parameterizations.
    Description: This research was supported by the Office of Naval Research Grant (N00014-09-1-0196).
    Description: 2018-07-16
    Keywords: Submesoscale ; Mixed layer ; Dissipation ; Eddies ; Restratification ; Vertical mixing
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 8495–8511, doi:10.1002/2014JC010211.
    Description: Oceanic frontal instabilities are of importance for the vertical exchange of properties in the ocean. Submesoscale, O(1) Rossby number, dynamics are particularly relevant for inducing the vertical (and lateral) flux of buoyancy and tracers in the mixed layer, but how these couple with the stratified pycnocline is less clear. Observations show surface fronts often persist beneath the mixed layer. Here we use idealized, three-dimensional model simulations to show how surface fronts that extend deeper into the pycnocline invoke enhanced vertical fluxes through the coupling of submesoscale and mesoscale instabilities. We contrast simulations in which the front is restricted to the mixed layer with those in which it extends deeper. For the deeper fronts, we examine the effect of density stratification on the vertical coupling. Our results show deep fronts can dynamically couple the mixed layer and pycnocline on time scales that increase with the peak stratification beneath the mixed layer. Eddies in the interior generate skew fluxes of buoyancy and tracer oriented along isopycnals, thus providing an adiabatic pathway for the interior to interact with the mixed layer at fronts. The vertical enhancement of tracer fluxes through the mesoscale-submesoscale coupling described here is thus relevant to the vertical supply of nutrients for phytoplankton in the ocean. A further implication for wind-forced fronts is that the vertical structure of the stream function characterizing the exchange between the interior and the mixed layer exhibits significant qualitative differences compared to a linear combination of existing parameterizations of submesoscale eddies in the mixed layer and mesoscale eddies in the interior. The discrepancies are most severe within the mixed layer suggesting a potential role for Ekman-layer dynamics absent in existing submesoscale parameterizations.
    Description: S.R. and A.T. acknowledge financial support from the National Science Foundation (NSF OCE-0928138) and the Office of Naval Research (ONR N00014-09-1-0196, ONR N00014-12-1-0101). A.M. acknowledges funding from the National Science Foundation (NSF OCE-0928617) and the Office of Naval Research (ONR N00014-12-1-0101).
    Description: 2015-06-11
    Keywords: Submesoscale ; Mixed layer ; Meso-submeso coupling ; Deep fronts
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 4
    Publication Date: 2023-03-11
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tarry, D., Ruiz, S., Johnston, T., Poulain, P., Özgökmen, T., Centurioni, L., Berta, M., Esposito, G., Farrar, J., Mahadevan, A., & Pascual, A. Drifter observations reveal intense vertical velocity in a surface ocean front. Geophysical Research Letters, 49(18), (2022): e2022GL098969, https://doi.org/10.1029/2022gl098969.
    Description: Measuring vertical motions represent a challenge as they are typically 3–4 orders of magnitude smaller than the horizontal velocities. Here, we show that surface vertical velocities are intensified at submesoscales and are dominated by high frequency variability. We use drifter observations to calculate divergence and vertical velocities in the upper 15 m of the water column at two different horizontal scales. The drifters, deployed at the edge of a mesoscale eddy in the Alboran Sea, show an area of strong convergence (urn:x-wiley:00948276:media:grl64766:grl64766-math-0001(f)) associated with vertical velocities of −100 m day−1. This study shows that a multilayered-drifter array can be an effective tool for estimating vertical velocity near the ocean surface.
    Description: This research was supported by the Office of Naval Research (ONR) Departmental Research Initiative CALYPSO under program officers Terri Paluszkiewicz and Scott Harper. The authors' ONR Grant No. are as follows: DT, SR, AM, and AP N000141613130, TMSJ N000146101612470, PP N000141812418, TO N000141812138, LRC N000141712517, and N00014191269, MB and GE N000141812782 and N000141812039, and JTF N000141812431.
    Keywords: Drifters ; Vertical velocity ; Submesoscale ; Kinematic properties ; Fronts ; Alboran Sea
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
    Publication Date: 2022-10-26
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(5), (2021): e2020JC017042, https://doi.org/10.1029/2020JC017042.
    Description: In frontal zones, water masses that are tens of kilometers in extent with origins in the mixed layer can be identified in the pycnocline for days to months. Here, we explore the pathways and mechanisms of subduction, the process by which water from the surface mixed layer makes its way into the pycnocline, using a submesoscale-resolving numerical model of a mesoscale front. By identifying Lagrangian trajectories of water parcels that exit the mixed layer, we study the evolution of dynamical properties from a statistical standpoint. Velocity- and buoyancy-gradients increase as water parcels experience both mesoscale (geostrophic) and submesoscale (ageostrophic) frontogenesis and subduct beneath the mixed layer into the stratified pycnocline along isopycnals that outcrop in the mixed layer. Subduction is transient and occurs in coherent regions along the front, the spatial and temporal scales of which influence the scales of the subducted water masses in the pycnocline. An examination of specific subduction events reveals a range of submesoscale features that support subduction. Contrary to the forced submesoscale processes that sequester low potential vorticity (PV) anomalies in the interior, we find that PV can be elevated in subducting water masses. The rate of subduction is of similar magnitude to previous studies (∼100 m/year), but the Lagrangian evolution of properties on water parcels and pathways that are unraveled in this study emphasize the role of submesoscale dynamics coupled with mesoscale frontogenesis.
    Description: This research was funded by the ONR CALYPSO DRI grant N00014-16-1-3130. MAF was partially funded by a Martin Fellowship from MIT.
    Keywords: Frontal dynamics ; Mixed layer ; Process study ; Submesoscale ; Vertical velocity | Lagrangian
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
    Publication Date: 2022-11-10
    Description: © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in [citation], doi:[doi]. Gommenginger, C., Chapron, B., Hogg, A., Buckingham, C., Fox-Kemper, B., Eriksson, L., Soulat, F., Ubelmann, C., Ocampo-Torres, F., Nardelli, B. B., Griffin, D., Lopez-Dekker, P., Knudsen, P., Andersen, O., Stenseng, L., Stapleton, N., Perrie, W., Violante-Carvalho, N., Schulz-Stellenfleth, J., Woolf, D., Isern-Fontanet, J., Ardhuin, F., Klein, P., Mouche, A., Pascual, A., Capet, X., Hauser, D., Stoffelen, A., Morrow, R., Aouf, L., Breivik, O., Fu, L., Johannessen, J. A., Aksenov, Y., Bricheno, L., Hirschi, J., Martin, A. C. H., Martin, A. P., Nurser, G., Polton, J., Wolf, J., Johnsens, H., Soloviev, A., Jacobs, G. A., Collard, F., Groom, S., Kudryavtsev, V., Wilkin, J., Navarro, V., Babanin, A., Martin, M., Siddorn, J., Saulter, A., Rippeth, T., Emery, B., Maximenko, N., Romeiser, R., Graber, H., Azcarate, A. A., Hughes, C. W., Vandemark, D., da Silva, J., Van Leeuwen, P. J., Naveira-Garabato, A., Gemmrich, J., Mahadevan, A., Marquez, J., Munro, Y., Doody, S., & Burbidge, G. SEASTAR: A mission to study ocean submesoscale dynamics and small-scale atmosphere-ocean processes in coastal, shelf and polar seas. Frontiers in Marine Science, 6, (2019):457, doi:10.3389/fmars.2019.00457.
    Description: High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean mixing and ocean vertical transports, with advanced numerical models and in situ observations highlighting fundamental changes in dynamics when scales reach 1 km. Numerous scientific publications highlight the global impact of small oceanic scales on marine ecosystems, operational forecasts and long-term climate projections through strong ageostrophic circulations, large vertical ocean velocities and mixed layer re-stratification. Small-scale processes particularly dominate in coastal, shelf and polar seas where they mediate important exchanges between land, ocean, atmosphere and the cryosphere, e.g., freshwater, pollutants. As numerical models continue to evolve toward finer spatial resolution and increasingly complex coupled atmosphere-wave-ice-ocean systems, modern observing capability lags behind, unable to deliver the high-resolution synoptic measurements of total currents, wind vectors and waves needed to advance understanding, develop better parameterizations and improve model validations, forecasts and projections. SEASTAR is a satellite mission concept that proposes to directly address this critical observational gap with synoptic two-dimensional imaging of total ocean surface current vectors and wind vectors at 1 km resolution and coincident directional wave spectra. Based on major recent advances in squinted along-track Synthetic Aperture Radar interferometry, SEASTAR is an innovative, mature concept with unique demonstrated capabilities, seeking to proceed toward spaceborne implementation within Europe and beyond.
    Description: CG and AM received funding from the United Kingdom Centre for Earth Observation Instrumentation SEASTAR+ project (Contract No. RP10G0435A02). PVL was supported by the European Research Council (ERC) CUNDA project 694509 under the European Union Horizon 2020 Research and Innovation Program.
    Keywords: Satellite ; Air sea interactions ; Upper ocean dynamics ; Submesoscale ; Coastal ; Marginal ice zone ; Radar ; Along-track interferometry
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 7
    Publication Date: 2022-10-26
    Description: Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(8), (2019): 5999-6014, doi: 10.1029/2019JC015034.
    Description: Oceanic fronts are dynamically active regions of the global ocean that support upwelling and downwelling with significant implications for phytoplankton production and export. However (on time scales urn:x-wiley:jgrc:media:jgrc23568:jgrc23568-math-0001 the inertial time scale), the vertical velocity is 103–104 times weaker than the horizontal velocity and is difficult to observe directly. Using intensive field observations in conjunction with a process study ocean model, we examine vertical motion and its effect on phytoplankton fluxes at multiple spatial horizontal scales in an oligotrophic region in the Western Mediterranean Sea. The mesoscale ageostrophic vertical velocity (∼10 m/day) inferred from our observations shapes the large‐scale phytoplankton distribution but does not explain the narrow (1–10 km wide) features of high chlorophyll content extending 40–60 m downward from the deep chlorophyll maximum. Using modeling, we show that downwelling submesoscale features concentrate 80% of the downward vertical flux of phytoplankton within just 15% of the horizontal area. These submesoscale spatial structures serve as conduits between the surface mixed layer and pycnocline and can contribute to exporting carbon from the sunlit surface layers to the ocean interior.
    Description: The AlborEx experiment was conducted in the framework of PERSEUS EU‐funded project (Grant 287600) and was led by the Spanish National Research Council (CSIC) and involved other national and international partners: Balearic Islands Coastal Observing and Forecasting System (SOCIB, Spain); Consiglio Nazionale delle Ricerche (CNR, Italy); Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS, Italy); and Woods Hole Oceanographic Institution (WHOI, ONR Grant N00014‐16‐1‐3130). Glider operations were partially funded by JERICO FP7 project. Part of this work has been carried out as part of the Copernicus Marine Environment Monitoring Service (CMEMS) MedSUB project. CMEMS is implemented by Mercator Ocean in the framework of a delegation agreement with the European Union. S. R. and A. P. acknowledge support from WHOI Subcontract A101339. Data available from authors: Ship CTDs, glider and VM‐ADCP data files are available in the SOCIB data catalog (https://doi.org/10.25704/z5y2-qpye); model data are available at IMEDEA data catalog https://ide.imedea.uib-csic.es/thredds/catalog/data/projects/alborex/catalog.html. We thank all the crew and participants on board R/V SOCIB for their collaboration and Marc Torner and the SOCIB glider Facility for their efficient cooperation. We also thank B. Mourre for numerical data from the Western Mediterranean Operational Model to initialize the Process Study Ocean Model. Figures were created using the cmocean colormaps package (Thyng et al., 2016).
    Keywords: Vertical motion ; Ocean front ; Mesoscale ; Submesoscale ; Transport ; Phytoplankton
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 8
    Publication Date: 2022-10-26
    Description: © The Authors, 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Pascual, A., Ruiz, S., Olita, A., Troupin, C., Claret, M., Casas, B., Mourre, B., Poulain, P. M., Tovar-Sanchez, A., Capet, A., Mason, E., Allen, J. T., Mahadevan, A., & Tintore, J. A multiplatform experiment to unravel meso- and submesoscale processes in an intense front (AlborEx). Frontiers in Marine Science, 4(39), (2017), doi:10.3389/fmars.2017.00039.
    Description: The challenges associated with meso- and submesoscale variability (between 1 and 100 km) require high-resolution observations and integrated approaches. Here we describe a major oceanographic experiment designed to capture the intense but transient vertical motions in an area characterized by strong fronts. Finescale processes were studied in the eastern Alboran Sea (Western Mediterranean) about 400 km east of the Strait of Gibraltar, a relatively sparsely sampled area. In-situ systems were coordinated with satellite data and numerical simulations to provide a full description of the physical and biogeochemical variability. Hydrographic data confirmed the presence of an intense salinity front formed by the confluence of Atlantic Waters, entering from Gibraltar, with the local Mediterranean waters. The drifters coherently followed the northeastern limb of an anticyclonic gyre. Near real time data from acoustic current meter data profiler showed consistent patterns with currents of up to 1 m/s in the southern part of the sampled domain. High-resolution glider data revealed submesoscale structures with tongues of chlorophyll-a and oxygen associated with the frontal zone. Numerical results show large vertical excursions of tracers that could explain the subducted tongues and filaments captured by ocean gliders. A unique aspect of AlborEx is the combination of high-resolution synoptic measurements of vessel-based measurements, autonomous sampling, remote sensing and modeling, enabling the evaluation of the underlying mechanisms responsible for the observed distributions and biogeochemical patchiness. The main findings point to the importance of fine-scale processes enhancing the vertical exchanges between the upper ocean and the ocean interior.
    Description: The AlborEx experiment was conducted in the framework of PERSEUS EU-funded project (Grant agreement no: 287600). The experiment was led by the Spanish National Research Council (CSIC) institution with strong involvement and cooperation from other national and international partners: Balearic Islands Coastal Observing and Forecasting System (SOCIB, Spain); Consiglio Nazionale delle Ricerche (CNR, Italy), McGill University (Canada); Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS, Italy) and Woods Hole Oceanographic Institution (WHOI, USA). Glider operations were partially funded by JERICO FP7 project. AP acknowledges support from the Spanish National Research Program (E-MOTION/CTM2012-31014 and PRE-SWOT/CTM2016-78607-P). SR and AP are also supported by the Copernicus Marine Environment Monitoring Service (CMEMS) MedSUB project. EM is supported by a post-doctoral grant from the Conselleria d'Educació, Cultura i Universitats del Govern de les Illes Balears (Mallorca, Spain) and the European Social Fund. AC is a FNRS researcher under the FNRS BENTHOX project (Convention T.1009.15). The altimeter products were produced by Ssalto/Duacs and distributed by CMEMS. The profiling floats and some drifters were contributed by the Argo-Italy program. The authors are in debt with A. Massanet, F. Margirier, M. Palmer, C. Castilla, P. Balaguer and for their efficient work and implication during the AlborEx cruise. We also thank M. Menna, G. Notarstefano and A. Bussani for their help with the drifter and float data processing and the production of some figures. This article was initiated during a research visit of the first two authors to Woods Hole Oceanographic Institution.
    Keywords: Mesoscale ; Submesoscale ; Ocean front ; Western Mediterranean ; Integrated multidisciplinary ocean observations ; Multiplatform ; Numerical simulations
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 9
    Publication Date: 2023-02-01
    Description: Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of the Atmospheric and Oceanic Technology 39(8), (2022): 1183-1198, https://doi.org/10.1175/jtech-d-21-0068.1.
    Description: Horizontal kinematic properties, such as vorticity, divergence, and lateral strain rate, are estimated from drifter clusters using three approaches. At submesoscale horizontal length scales O(1–10)km, kinematic properties become as large as planetary vorticity f, but challenging to observe because they evolve on short time scales O(hourstodays). By simulating surface drifters in a model flow field, we quantify the sources of uncertainty in the kinematic property calculations due to the deformation of cluster shape. Uncertainties arise primarily due to (i) violation of the linear estimation methods and (ii) aliasing of unresolved scales. Systematic uncertainties (iii) due to GPS errors, are secondary but can become as large as (i) and (ii) when aspect ratios are small. Ideal cluster parameters (number of drifters, length scale, and aspect ratio) are determined and error functions estimated empirically and theoretically. The most robust method—a two-dimensional, linear least squares fit—is applied to the first few days of a drifter dataset from the Bay of Bengal. Application of the length scale and aspect-ratio criteria minimizes errors (i) and (ii), and reduces the total number of clusters and so computational cost. The drifter-estimated kinematic properties map out a cyclonic mesoscale eddy with a surface, submesoscale fronts at its perimeter. Our analyses suggest methodological guidance for computing the two-dimensional kinematic properties in submesoscale flows, given the recently increasing quantity and quality of drifter observations, while also highlighting challenges and limitations.
    Description: This research was supported by the Office of Naval Research (ONR) Departmental Research Initiative ASIRI under Grant N00014-13-1-0451 (SE and AM) and Grant N00014-13-1-0477 (VH and LC). The authors thank the captain and crew of the R/V Roger Revelle, and Andrew Lucas with the Multiscale Ocean Dynamics group at the Scripps Institution for Oceanography for providing the FastCTD data collected in 2015, which was supported by ONR Grant N00014-13-1-0489, as well as Eric D’Asaro for helpful discussions and Lance Braasch for assistance with the drifter dataset. AM and SE further thank NSF (Grant OCE-I434788) and ONR (Grant N00014-16-1-2470) for support. VH and LC were additionally supported by ONR Grants N00014-15-1-2286, N00014-14-1-0183, N00014-19-1-26-91 and NOAA Global Drifter Program (GDP) Grant NA15OAR4320071.
    Description: 2023-02-01
    Keywords: Indian Ocean ; Eddies ; Frontogenesis/frontolysis ; Fronts ; Lagrangian circulation/transport ; Ocean circulation ; Ocean dynamics
    Repository Name: Woods Hole Open Access Server
    Type: Article
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