Interaction between upper-ocean submesoscale currents and convective turbulence

2021 ◽  
Keyword(s):  
Potential Energy ◽  
Gravitational Instability ◽  
Baroclinic Instability ◽  
Upper Ocean ◽  
Surface Cooling ◽  
Convective Turbulence ◽  
Velocity Gradients ◽  
Submesoscale Currents ◽  
Symmetric Instability ◽  
Energy Pathways

Abstract The interaction between upper-ocean submesoscale fronts evolving with coherent features, such as vortex filaments and eddies, and finescale convective turbulence generated by surface cooling of varying magnitude is investigated. While convection is energized by gravitational instability, predominantly at the finescale (FS), which feeds off the potential energy that is input through cooling, the submesoscale (SMS) is energized at larger scales by the release of available potential energy stored in the front. Here, we decompose the flow into FS and SMS fields explicitly to investigate the energy pathways and the strong interaction between them. Overall, the SMS is energized due to surface cooling. The frontogenetic tendency at the submesoscale increases, which counters the enhanced horizontal diffusion by convection-induced turbulence. Downwelling/upwelling strengthens, and the peak SMS vertical buoyancy flux increases as surface cooling is increased. Furthermore, the production of FS energy by SMS velocity gradients is significant, up to half of the production by convection. Examination of potential vorticity reveals that surface cooling promotes higher levels of secondary symmetric instability, which coexists with the persistent baroclinic instability.

scholarly journals Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

2016 ◽  
Vol 46 (4) ◽  
pp. 1285-1307 ◽  
Author(s):  
Andrew F. Thompson ◽  
Ayah Lazar ◽  
Christian Buckingham ◽  
Alberto C. Naveira Garabato ◽  
Gillian M. Damerell ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Seasonal Cycle ◽  
Gravitational Instability ◽  
Baroclinic Instability ◽  
Open Ocean ◽  
Global Ocean ◽  
Surface Cooling ◽  
Near Surface ◽  
Submesoscale Processes ◽  
Symmetric Instability

AbstractThe importance of submesoscale instabilities, particularly mixed layer baroclinic instability and symmetric instability, on upper-ocean mixing and energetics is well documented in regions of strong, persistent fronts such as the Kuroshio and the Gulf Stream. Less attention has been devoted to studying submesoscale flows in the open ocean, far from long-term, mean geostrophic fronts, characteristic of a large proportion of the global ocean. This study presents a year-long, submesoscale-resolving time series of near-surface buoyancy gradients, potential vorticity, and instability characteristics, collected by ocean gliders, that provides insight into open-ocean submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 km2 region in the subtropical northeast Atlantic, measuring temperature, salinity, and pressure along 292 short (~20 km) hydrographic sections. Glider observations show a seasonal cycle in near-surface stratification. Throughout the fall (September–November), the mixed layer deepens, predominantly through gravitational instability, indicating that surface cooling dominates submesoscale restratification processes. During winter (December–March), mixed layer depths are more variable, and estimates of the balanced Richardson number, which measures the relative importance of lateral and vertical buoyancy gradients, depict conditions favorable to symmetric instability. The importance of mixed layer instabilities on the restratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the open-ocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter, and emphasize that accurate parameterizations of submesoscale processes are needed throughout the ocean.

Effects of three-dimensionality on instability and turbulence in a frontal zone

2015 ◽  
Vol 784 ◽  
pp. 252-273 ◽  
Author(s):  
Eric Arobone ◽  
Sutanu Sarkar
Keyword(s):  
Stability Analysis ◽  
Potential Energy ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Frontal Zone ◽  
Gravitational Instability ◽  
Three Dimensional ◽  
Computational Domain ◽  
Coriolis Parameter ◽  
Symmetric Instability

Linear stability analysis and direct numerical simulation are used to investigate the evolution of a symmetrically unstable uniform frontal zone. Simulations in a three-dimensional computational domain capable of resolving near-symmetric currents develop strong nonlinearities without the emergence of pure symmetric instability. Linear stability analysis demonstrates that for $ft>1$ ( $f$ is the Coriolis parameter and $t$ denotes time) the flow generates strongly asymmetric structures which become nearly symmetric when $ft\gg 1$. Unlike the currents generated during pure symmetric instability, near-symmetric instability generates currents that do not align with isopycnals. This greatly modifies their energetics and evolution, leading to regions of the flow that are unstable to gravitational instability and energized by the reservoir of available potential energy. A high-resolution simulation demonstrates the flow evolution from near-symmetric currents to secondary shear-convective instabilities and finally, through tertiary instabilities, to fully three-dimensional turbulence. The effect of this sequence of instabilities is quantified through velocity and vorticity statistics as well as budgets for turbulent kinetic and potential energy. It is not until $ft\sim 10$ that the energy source for fluctuations is primarily shear, in contrast to the purely symmetric instability which draws its energy exclusively from shear production.

scholarly journals Accumulation and Subduction of Buoyant Material at Submesoscale Fronts

2018 ◽  
Vol 48 (6) ◽  
pp. 1233-1241 ◽  
Author(s):  
John R. Taylor
Keyword(s):  
Mixed Layer ◽  
Baroclinic Instability ◽  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Surface Cooling ◽  
Additional Source ◽  
Tracer Concentration ◽  
Vertical Diffusivity ◽  
Large Eddy ◽  
Submesoscale Currents

AbstractThe influence of submesoscale currents on the distribution and subduction of passive, buoyant tracers in the mixed layer is examined using large-eddy simulations. Submesoscale eddies are generated through an ageostrophic baroclinic instability associated with a background horizontal buoyancy gradient. The simulations also include various levels of surface cooling, which provides an additional source of three-dimensional turbulence. Submesoscales compete against turbulent convection and restratify the mixed layer while generating strong turbulence along a submesoscale front. Buoyant tracers accumulate at the surface along the submesoscale front where they are subducted down into the water column. The presence of submesoscales strongly modifies the vertical tracer flux, even in the presence of strong convective forcing. The correlation between high tracer concentration and strong downwelling enhances the vertical diffusivity for buoyant tracers.

Study of the Mechanisms of Vortex Variability in the Lofoten Basin Based on the Energy Analysis

2021 ◽  
Vol 37 (3) ◽  
Author(s):  
V. S. Travkin ◽  
◽  
T. V. Belonenko ◽  
Keyword(s):  
Kinetic Energy ◽  
Potential Energy ◽  
Baroclinic Instability ◽  
Positive Trend ◽  
Barotropic Instability ◽  
Available Potential Energy ◽  
Conversion Rates ◽  
Order Of Magnitude ◽  
The Mean ◽  
Mean Kinetic Energy

Purpose. The Lofoten Basin is one of the most energetic zones of the World Ocean characterized by high activity of mesoscale eddies. The study is aimed at analyzing different components of general energy in the basin, namely the mean kinetic and vortex kinetic energy calculated using the integral of the volume of available potential and kinetic energy of the Lofoten Vortex, as well as variability of these characteristics. Methods and Results. GLORYS12V1 reanalysis data for the period 2010–2018 were used. The mean kinetic energy and the eddy kinetic one were analyzed; and as for the Lofoten Vortex, its volume available potential and kinetic energy were studied. The mesoscale activity of eddies in winter is higher than in summer. Evolution of the available potential energy and kinetic energy of the Lofoten Vortex up to the 1000 m horizon was studied. It is shown that the vortex available potential energy exceeds the kinetic one by an order of magnitude, and there is a positive trend with the coefficient 0,23⋅1015 J/year. It was found that in the Lofoten Basin, the intermediate layer from 600 to 900 m made the largest contribution to the potential energy, whereas the 0–400 m layer – to kinetic energy. The conversion rates of the mean kinetic energy into the vortex kinetic one and the mean available potential energy into the vortex available potential one (barotropic and baroclinic instability) were analyzed. It is shown that the first type of transformation dominates in summer, while the second one is characterized by its increase in winter. Conclusions. The vertical profile shows that the kinetic energy of eddies in winter is higher than in summer. The available potential energy of a vortex is by an order of magnitude greater than the kinetic energy. An increase in the available potential energy is confirmed by a significant positive trend and by a decrease in the vortex Burger number. The graphs of the barotropic instability conversion rate demonstrate the multidirectional flows in the vortex zone with the dipole structure observed in a winter period, and the tripole one – in summer. The barotropic instability highest intensity is observed in summer. The baroclinic instability is characterized by intensification of the regime in winter that is associated with weakening of stratification in this period owing to winter convection.

scholarly journals Spatially Broad Observations of Internal Waves in the Upper Ocean at the Hawaiian Ridge

2006 ◽  
Vol 36 (6) ◽  
pp. 1085-1103 ◽  
Author(s):  
Joseph P. Martin ◽  
Daniel L. Rudnick ◽  
Robert Pinkel
Keyword(s):  
Numerical Model ◽  
Energy Density ◽  
Potential Energy ◽  
Internal Waves ◽  
Upper Ocean ◽  
Mean Square ◽  
The South ◽  
South Side ◽  
Topographic Slope ◽  
Hawaiian Ridge

Abstract The density and current structure at the Hawaiian Ridge was observed using SeaSoar and Doppler sonar during a survey extending from Oahu to Brooks Banks. Across- and along-ridge changes in internal wave statistics in the upper ocean within 200 km of the ridge are investigated. Internal waves with trough-to-crest amplitude as large as 60 m and horizontal wavelength of about 50 km are observed repeatedly in across-ridge sections of potential density. Within 150 km of the ridge, kinetic and potential energy density exceed open-ocean values with maxima about 10 times Garrett–Munk levels. In the Kauai Channel (KC), the kinetic energy density is largest along an M2 internal tide ray. The ray originates at the northern edge of the ridge peak at a large across-ridge change in topographic slope and terminates at the ocean surface about 30–40 km south of the ridge peak. Kinetic and potential energy density are larger on the south side of the ridge at KC, the side with larger topographic slope. Energy density is also larger on the south side of the ridge at KC in numerical model results and on the side of steeper topographic slope in analytical model results. Along the ridge, the largest observed values of mean-square shear and mean-square slope of isopycnal depth are collocated with the largest energy density in numerical model results. Mean-square shear and mean-square slope increase with decreasing bottom depth and with increasing M2 barotropic tidal forcing.

scholarly journals Intensification of Upper-Ocean Submesoscale Turbulence through Charney Baroclinic Instability

2016 ◽  
Vol 46 (11) ◽  
pp. 3365-3384 ◽  
Author(s):  
Xavier Capet ◽  
Guillaume Roullet ◽  
Patrice Klein ◽  
Guillaume Maze
Keyword(s):  
Kinetic Energy ◽  
Gulf Stream ◽  
Energy Input ◽  
Baroclinic Instability ◽  
Upper Ocean ◽  
Mode Water ◽  
Near Surface ◽  
Turbulent Statistics ◽  
Frontal Activity ◽  
Southern Flank

AbstractThis study focuses on the description of an oceanic variant of the Charney baroclinic instability, arising from the joint presence of (i) an equatorward buoyancy gradient that extends from the surface into the ocean interior and (ii) reduced subsurface stratification, for example, as produced by wintertime convection or subduction. This study analyzes forced dissipative simulations with and without Charney baroclinic instability (C-BCI). In the former, C-BCI strengthens near-surface frontal activity with important consequences in terms of turbulent statistics: increased variance of vertical vorticity and velocity and increased vertical turbulent fluxes. Energetic consequences are explored. Despite the atypical enhancement of submesoscale activity in the simulation subjected to C-BCI, and contrary to several recent studies, the downscale energy flux at the submesoscale en route to dissipation remains modest in the flow energetic equilibration. In particular, it is modest vis à vis the global energy input to the system, the eddy kinetic energy input through conversion of available potential energy, and the classical inverse cascade of kinetic energy. Linear stability analysis suggests that the southern flank of the Gulf Stream may be conducive to oceanic Charney baroclinic instability in spring, following mode water formation and upper-ocean destratification.

scholarly journals Impact of Sea Spray and Sea Surface Roughness on the Upper Ocean Response to Super Typhoon Haitang (2005)

2021 ◽  
Author(s):  
Lianxin Zhang ◽  
Xuefeng Zhang ◽  
William Perrie ◽  
Changlong Guan ◽  
Bo Dan ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Ocean Waves ◽  
Sea Surface ◽  
Sea Spray ◽  
Upper Ocean ◽  
Surface Cooling ◽  
Super Typhoon ◽  
Sea Surface Roughness ◽  
Momentum Fluxes ◽  
The Right

AbstractA coupled ocean-wave-sea spray model system is used to investigate the impacts of sea spray and sea surface roughness on the response of the upper ocean to the passage of the super typhoon Haitang. Sea spray mediated heat and momentum fluxes are derived from an improved version of Fairall’s heat fluxes formulation (Zhang et al., 2017) and Andreas’s sea spray-mediated momentum flux models. For winds ranging from low to extremely high speeds, a new parameterization scheme for the sea surface roughness is developed, in which the effects of wave state and sea spray are introduced. In this formulation, the drag coefficient has minimal values over the right quadrant of the typhoon track, along which the typhoon-generated waves are longer, smoother, and older, compared to other quadrants. Using traditional interfacial air-sea turbulent (sensible, latent, and momentum) fluxes, the sea surface cooling response to typhoon Haitang is overestimated by 1 °C, which can be compensated by the effects of sea spray and ocean waves on the right side of the storm. Inclusion of sea spray-mediated turbulent fluxes and sea surface roughness, modulated by ocean waves, gives enhanced cooling along the left edges of the cooling area by 0.2 °C, consistent with the upper ocean temperature observations.

scholarly journals GRAVITY HEATS THE UNIVERSE

2009 ◽  
Vol 18 (14) ◽  
pp. 2201-2207
Author(s):  
ADAM MOSS ◽  
DOUGLAS SCOTT
Keyword(s):  
Kinetic Energy ◽  
Potential Energy ◽  
Energy Conservation ◽  
Cosmic Microwave Background ◽  
Initial Conditions ◽  
Gravitational Instability ◽  
Simplified Model ◽  
Microwave Background ◽  
Average Temperature ◽  
The Universe

Structures in the Universe grew through gravitational instability from very smooth initial conditions. Energy conservation requires that the growing negative potential energy of these structures be balanced by an increase in kinetic energy. A fraction of this is converted into heat in the collisional gas of the intergalactic medium. Using a toy model of gravitational heating, we attempt to link the growth of structure in the Universe with the average temperature of this gas. We find that the gas is rapidly heated from collapsing structures at around z ~ 10, reaching a temperature > 106 K today, depending on some assumptions of our simplified model. Before that there was a cold era from z ~ 100 to ~10 in which the matter temperature was below that of the cosmic microwave background.

scholarly journals Marginal Sea Overflows and the Upper Ocean Interaction

2009 ◽  
Vol 39 (2) ◽  
pp. 387-403 ◽  
Author(s):  
Shinichiro Kida ◽  
Jiayan Yang ◽  
James F. Price
Keyword(s):  
Ocean Circulation ◽  
Potential Vorticity ◽  
Baroclinic Instability ◽  
Upper Ocean ◽  
Marginal Sea ◽  
The Mediterranean ◽  
The Difference ◽  
Set Up ◽  
Numerical Model Experiments ◽  
Gyre Circulation

Abstract Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume. The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more. Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentum flux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone. Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger.

Experiments with baroclinic vortex pairs in a rotating fluid

1986 ◽  
Vol 173 ◽  
pp. 501-518 ◽  
Author(s):  
R. W. Griffiths ◽  
E. J. Hopfinger
Keyword(s):  
Velocity Field ◽  
Potential Energy ◽  
Baroclinic Instability ◽  
Point Vortices ◽  
Density Stratification ◽  
Rotating Fluid ◽  
Vortex Pairs ◽  
Layer Density

When vortices are generated in one layer of a rotating, two-layer density stratification, the velocity field of each vortex is strongly baroclinic within a distance of order one Rossby radius from its centre. In this system there are two classes of vortex pairs: those pairs (consisting of vortices of opposite signs) for which the vortices are in the same layer, and those for which the vortices are in opposite layers. We pay particular attention to a laboratory demonstration of the properties of the latter class. These vortex pairs have the ability to transport density (or heat) in the horizontal, and provide a means for describing the release of potential energy by baroclinic instability. We also observe that interactions of real vortices and real vortex pairs differ from those computed for point vortices.

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