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Submesoscale Processes in the Ocean

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A special issue of Fluids (ISSN 2311-5521). This special issue belongs to the section "Geophysical and Environmental Fluid Mechanics".

Deadline for manuscript submissions: closed (30 November 2020) | Viewed by 26892

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Special Issue Editors

Prof. Jonathan Gula

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Guest Editor

Laboratoire d’Océanographie Physique et Spatiale, Université de Brest, CNRS, IRD, Ifremer, IUEM, 29238 Brest, France
Interests: submesoscale processes; submesoscale instabilities and mixing; flow–topography interactions; internal waves; mesoscale vortices; Gulf stream dynamics; North Atlantic circulation

Prof. James C. McWilliams

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Guest Editor

Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA
Interests: general circulation and its natural variability; mesoscale eddy processes; surface turbulent boundary layer; surface gravity waves; submesoscale currents

Prof. Leif N. Thomas

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Guest Editor

Department of Earth System Science, Stanford University, Stanford, CA 94305, USA
Interests: lateral mixing in the ocean by submesoscale flows; mode water formation in the Gulf Stream; frontal dynamics; wave–mean flow interactions; and submesoscale processes near the Equator

Special Issue Information

Dear Colleagues,

Submesoscale currents occur on lateral scales of 100 m–10 km in the ocean, mostly in the form of density fronts and filaments, vortices and topographic wakes at the surface and in the ocean’s interior. They are found worldwide, from the global ocean down to marginal seas, in the upper ocean as well as in the abyssal ocean. They have intermediate space and time scales between the quasi-geostrophic mesoscale eddies and the fully three-dimensional turbulence, which make direct observations of these processes an open challenge.

Submesoscale processes have order one Rossby and Richardson numbers and facilitate the transfer of energy between balanced motions and small-scale turbulence. They are also instrumental for the transport of heat, salt, and biogeochemical properties. The strong heterogeneities they generate at the ocean surface have important implications for modulating air–sea fluxes of energy and in structuring marine ecosystems. Submesoscale processes also drive significant vertical velocities that control exchanges between the surface layer and the ocean interior.

A broad range of observational, theoretical, and numerical studies are needed to achieve further understanding of their generation mechanisms, 3D dynamics, variability, impact on energy dissipation, mixing and transport of tracers, and interactions with biogeochemical variables, as well as to quantify and parameterize the upscale effects of submesoscale processes on large-scale circulation and global biogeochemical budgets.

In this Special Issue of Fluids, “Submesoscale Processes in the Ocean”, we welcome all new research contributions to the field.

Prof. Jonathan Gula
Prof. James C. McWilliams
Prof. Leif N. Thomas
Guest Editors

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Keywords

submesoscale currents
frontal and filamentary dynamics
vertical velocities
instabilities
turbulence
coherent vortices
physics/biology interactions
topographic interactions
ocean mixing
wave–mean flow interactions

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Published Papers (7 papers)

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Research

26 pages, 10318 KiB

Open AccessArticle

Wind-Forced Submesoscale Symmetric Instability around Deep Convection in the Northwestern Mediterranean Sea

by Anthony Bosse, Pierre Testor, Pierre Damien, Claude Estournel, Patrick Marsaleix, Laurent Mortier, Louis Prieur and Vincent Taillandier

Fluids 2021, 6(3), 123; https://doi.org/10.3390/fluids6030123 - 17 Mar 2021

Cited by 14 | Viewed by 3642

Abstract

During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100 m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export. Full article

(This article belongs to the Special Issue Submesoscale Processes in the Ocean)

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31 pages, 12249 KiB

Open AccessArticle

Filaments, Fronts and Eddies in the Cabo Frio Coastal Upwelling System, Brazil

by Paulo H. R. Calil, Nobuhiro Suzuki, Burkard Baschek and Ilson C. A. da Silveira

Fluids 2021, 6(2), 54; https://doi.org/10.3390/fluids6020054 - 25 Jan 2021

Cited by 20 | Viewed by 4338

Abstract

We investigate the dynamics of meso- and submesoscale features of the northern South Brazil Bight shelf region with a 500-m horizontal resolution regional model. We focus on the Cabo Frio upwelling center, where nutrient-rich, coastal waters are transported into the mid- and outer shelf, because of its importance for local and remote productivity. The Cabo Frio upwelling center undergoes an upwelling phase, from late September to March, and a relaxation phase, from April to early September. During the upwelling phase, an intense front around 200

km

long and 20

km

wide with horizontal temperature gradients as large as 8

^{\circ} C

over less than 10

km

develops. A surface-intensified frontal jet of 0.7

{ms}^{- 1}

in the upper 20 m and velocities of around 0.3

{ms}^{- 1}

reaching down to 65 m depth makes this front a preferential cross-shelf transport pathway. Large vertical mixing and vertical velocities are observed within the frontal region. The front is associated with strong cyclonic vorticity and strong variance in relative vorticity, frequently with

O

(1) Rossby numbers. The dynamical balance within the front is between the pressure gradient, Coriolis and vertical mixing terms, which are induced both by the winds, during the upwelling season, and by the geostrophic frontal jet. Therefore, the frontal dynamics may be largely described as sum of Ekman and turbulent thermal wind balances. During the upwelling phase, a mix of barotropic and baroclinic instabilities dominates in the upwelling center. However, these instabilities do not lead to the local formation of coherent eddies when the front is strong. In the relaxation phase, the front vanishes, and the water column becomes less stratified. The interaction between eastward coastal currents generated by sea level variability, coastal intrusions of the Brazil Current, and sporadic wind-driven, coastal upwelling events induce the formation of cyclonic eddies with diameters of, approximately, 20

km

. They are in gradient-wind balance and propagate along the 100-m isobath on the shelf. During this phase baroclinic instability dominates. Cold filaments with widths of 2

km

are formed due to straining and stretching of cold, coastal temperature anomalies. They last for a few days and are characterized by downwelling as large as 1

{cms}^{- 1}

. The turbulent thermal wind balance provides a good first order estimate of the dynamical balance within the filament, but vertical and horizontal advection are shown to be important. To our knowledge, this is the first account of these smaller scale features in the region. Because these meso- and submesoscale features on the shelf heavily affect the water properties crucial to productivity of the South Brazil Bight, it is important to take these features into account for a better understanding of the functioning of this ecosystem and its resilience to both direct human activities as well as to climate change. Full article

(This article belongs to the Special Issue Submesoscale Processes in the Ocean)

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36 pages, 7620 KiB

Open AccessArticle

From Mixing to the Large Scale Circulation: How the Inverse Cascade Is Involved in the Formation of the Subsurface Currents in the Gulf of Guinea

by Fernand Assene, Yves Morel, Audrey Delpech, Micael Aguedjou, Julien Jouanno, Sophie Cravatte, Frédéric Marin, Claire Ménesguen, Alexis Chaigneau, Isabelle Dadou, Gael Alory, Ryan Holmes, Bernard Bourlès and Ariane Koch-Larrouy

Fluids 2020, 5(3), 147; https://doi.org/10.3390/fluids5030147 - 29 Aug 2020

Cited by 9 | Viewed by 4109

Abstract

In this paper, we analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation), mesoscale vortices and the mean currents in the area. In the simulation, eastward currents, the South and North Equatorial Undercurrents (SEUC and NEUC respectively) and the Guinea Undercurrent (GUC), are shown to be linked to the westward currents located equatorward. We show that east of 20° W, both westward and eastward currents are associated with the spreading of PV tongues by mesoscale vortices. The Equatorial Undercurrent (EUC) brings salty waters into the Gulf of Guinea. Mixing diffuses the salty anomaly downward. Meridional advection, mixing and friction are involved in the formation of fluid parcels with PV anomalies in the lower part and below the pycnocline, north and south of the EUC, in the Gulf of Guinea. These parcels gradually merge and vertically align, forming nonlinear anticyclonic vortices that propagate westward, spreading and horizontally mixing their PV content by stirring filamentation and diffusion, up to 20° W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5° and 5° S or N. This mean PV field is associated with westward and eastward zonal jets flanking the EUC with the homogeneous PV tongues corresponding to the westward currents, and the strong PV gradient regions at their edges corresponding to the eastward currents. Mesoscale vortices strongly modulate the mean fields explaining the high spatial and temporal variability of the jets. Full article

(This article belongs to the Special Issue Submesoscale Processes in the Ocean)

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19 pages, 18260 KiB

Open AccessFeature PaperArticle

Submesoscale Dynamics in the Gulf of Aden and the Gulf of Oman

by Mathieu Morvan, Xavier Carton, Stéphanie Corréard and Rémy Baraille

Fluids 2020, 5(3), 146; https://doi.org/10.3390/fluids5030146 - 28 Aug 2020

Cited by 10 | Viewed by 3687

Abstract

We have investigated the surface and subsurface submesoscale dynamics in the Gulf of Aden and the Gulf of Oman. Our results are based on the analyses of regional numerical simulations performed with a primitive equation model (HYCOM) at submesoscale permitting horizontal resolution. A model zoom for each gulf was embedded in a regional mesoscale-resolving simulation. In the Gulf of Aden and the Gulf of Oman, the interactions of mesoscale structures and fronts instabilities form submesoscale eddies and filaments. Rotational energy spectra show that the Gulf of Aden has a higher ratio of submesoscale to mesocale energy than the Gulf of Oman. Fast waves (internal gravity waves, tidal waves, Kelvin waves) and slow waves (Rossby waves) were characterized via energy spectra of the divergent velocity. Local upwelling systems which shed cold filaments, coastal current instabilities at the surface, and baroclinic instability at capes in subsurface were identified as generators of submesocale structures. In particular, the Ras al Hamra and Ras al Hadd capes in the Gulf of Oman, and the Cape of Berbera in the Gulf of Aden, are loci of submesoscale eddy generation. To determine the instability mechanisms involved in these generations, we diagnosed the Ertel potential vorticity and the energy conversion terms: the horizontal and vertical Reynolds stresses and the vertical buoyancy flux. Finally, the impacts of the subsurface submesoscale eddy production at capes on the diffusion and fate of the Red Sea Water (in the Gulf of Aden) and the Persian Gulf Water (in the Gulf of Oman) are highlighted. Full article

(This article belongs to the Special Issue Submesoscale Processes in the Ocean)

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23 pages, 22148 KiB

Open AccessArticle

Altimetry-Based Diagnosis of Deep-Reaching Sub-Mesoscale Ocean Fronts

by Lia Siegelman, Patrice Klein, Andrew F. Thompson, Hector S. Torres and Dimitris Menemenlis

Fluids 2020, 5(3), 145; https://doi.org/10.3390/fluids5030145 - 28 Aug 2020

Cited by 17 | Viewed by 3693

Abstract

Recent studies demonstrate that energetic sub-mesoscale fronts (10–50 km width) extend in the ocean interior, driving large vertical velocities and associated fluxes. However, diagnosing the dynamics of these deep-reaching fronts from in situ observations remains challenging because of the lack of information on the 3-D structure of the horizontal velocity. Here, a realistic numerical simulation in the Antarctic Circumpolar Current (ACC) is used to study the dynamics of submesocale fronts in relation to velocity gradients, responsible for the formation of these fronts. Results highlight that the stirring properties of the flow at depth, which are related to the velocity gradients, can be inferred from finite-size Lyapunov exponent (FSLE) at the surface. Satellite altimetry observations of FSLE and velocity gradients are then used in combination with recent in situ observations collected by an elephant seal in the ACC to reconstruct frontal dynamics and their associated vertical velocities down to 500 m. The approach proposed here is well suited for the analysis of sub-mesoscale-resolving datasets and the design of future sub-mesoscale field campaigns. Full article

(This article belongs to the Special Issue Submesoscale Processes in the Ocean)

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15 pages, 7252 KiB

Open AccessArticle

Interannual to Decadal Variations of Submesoscale Motions around the North Pacific Subtropical Countercurrent

by Hideharu Sasaki, Bo Qiu, Patrice Klein, Yoshikazu Sasai and Masami Nonaka

Fluids 2020, 5(3), 116; https://doi.org/10.3390/fluids5030116 - 17 Jul 2020

Cited by 6 | Viewed by 2495

Abstract

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. Full article

(This article belongs to the Special Issue Submesoscale Processes in the Ocean)

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29 pages, 1004 KiB

Open AccessArticle

The Effects of Surface Wind Stress and Buoyancy Flux on the Evolution of a Front in a Turbulent Thermal Wind Balance

by Matthew N. Crowe and John R. Taylor

Fluids 2020, 5(2), 87; https://doi.org/10.3390/fluids5020087 - 1 Jun 2020

Cited by 5 | Viewed by 3417

Abstract

Here we consider the effects of surface buoyancy flux and wind stress on a front in turbulent thermal wind (TTW) balance using the framework of Crowe and Taylor (2018). The changes in the velocity and density profiles induced by the wind stress and buoyancy flux interact with the TTW and can qualitatively change the evolution of the front. In the absence of surface-forcing, Crowe and Taylor (2018) found that shear dispersion associated with the TTW circulation causes the frontal width to increase. In many cases, the flow induced by the surface-forcing enhances the spreading rate. However, if the wind stress drives a cross-front flow which opposes the frontal buoyancy gradient or the buoyancy flux drives an unstable stratification, it is possible to obtain an up-gradient cross-front buoyancy flux, which can act to sharpen the front. In certain conditions, an equilibrium state develops where the tendency for the TTW circulation to spread the front is balanced by the frontogenetic tendency of the surface forces. We use numerical solutions to a nonlinear diffusion equation in order to test these predictions. Finally, we describe the connection between surface-forcing and vertical mixing and discuss typical parameters for mid-ocean fronts. Full article

(This article belongs to the Special Issue Submesoscale Processes in the Ocean)

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