A numerical investigation of solitary internal waves with trapped cores formed via shoaling

The formation of solitary internal waves with trapped cores via shoaling is investigated numerically. For density fields for which the buoyancy frequency increases monotonically towards the surface, sufficiently large solitary waves break as they shoal and form solitary-like waves with trapped fluid cores. Properties of large-amplitude waves are shown to be sensitive to the near-surface stratification. For the monotonic stratifications considered, waves with open streamlines are limited in amplitude by the breaking limit (maximum horizontal velocity equals wave propagation speed). When an exponential density stratification is modified to include a thin surface mixed layer, wave amplitudes are limited by the conjugate flow limit, in which case waves become long and horizontally uniform in the centre. The maximum horizontal velocity in the limiting wave is much less than the wave's propagation speed and as a consequence, waves with trapped cores are not formed in the presence of the surface mixed layer.

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Shoaling solitary internal waves: on a criterion for the formation of waves with trapped cores

Journal of Fluid Mechanics ◽

10.1017/s0022112002003269 ◽

2003 ◽

Vol 478 ◽

pp. 81-100 ◽

Cited By ~ 65

Author(s):

KEVIN G. LAMB

Keyword(s):

Internal Waves ◽

Solitary Waves ◽

Large Amplitude ◽

Propagation Speed ◽

Background Current ◽

Near Surface ◽

Numerical Evidence ◽

Flow Limit ◽

Limiting Behaviour ◽

Solitary Internal Waves

Shoaling solitary internal waves are ubiquitous features in the coastal regions of the world's oceans where waves with a core of recirculating fluid (trapped cores) can provide an effective transport mechanism. Here, numerical evidence is presented which suggests that there is a close connection between the limiting behaviour of large-amplitude solitary waves and the formation of such waves via shoaling. For some background states, large-amplitude waves are broad, having a nearly horizontal flow in their centre. The flow in the centre of such waves is called a conjugate flow. For other background states, large-amplitude waves can reach the breaking limit, at which the maximum current in the wave is equal to the wave's propagation speed. The presence of a background current with near-surface vorticity of the same sign as that induced by the wave can change the limiting behaviour from the conjugate-flow limit to the breaking limit. Numerical evidence is presented here which suggests that if large solitary waves cannot reach the breaking limit in the shallow water, that is if the background flow has a conjugate flow, then waves with trapped cores will not be formed via shoaling. It is also shown that, due to a change in the limiting behaviour of large waves, an appropriate background current can enable the formation of waves with trapped cores in stratifications for which such waves are not formed in the absence of a background current.

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Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf

Journal of Physical Oceanography ◽

10.1175/jpo2821.1 ◽

2005 ◽

Vol 35 (12) ◽

pp. 2425-2443 ◽

Cited By ~ 55

Author(s):

J. A. MacKinnon ◽

M. C. Gregg

Keyword(s):

New England ◽

Internal Waves ◽

Mixed Layer ◽

Wave Interaction ◽

Heat Fluxes ◽

Surface Mixed Layer ◽

Turbulent Dissipation ◽

Surface Heat Fluxes ◽

Bottom Boundary Layers ◽

Diapycnal Diffusivity

Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.

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Pelagic primary production in the Coral and southern Solomon Seas

Marine and Freshwater Research ◽

10.1071/mf9960695 ◽

1996 ◽

Vol 47 (5) ◽

pp. 695 ◽

Cited By ~ 20

Author(s):

MJ Furnas ◽

AW Mitchell

Keyword(s):

Primary Production ◽

Mixed Layer ◽

Standing Crop ◽

Central Basin ◽

Strong Wind ◽

Surface Mixed Layer ◽

Barrier Reef ◽

Near Surface ◽

Western Margin ◽

Coral Sea

Phytoplankton primary production was measured around the periphery of the Coral Sea during October 1985 and in the boundary current systems bordering the northern Australian Great Barrier Reef (GBR) and Papuan Barrier Reef (PBR) during October 1985 and June-July 1988. Under strong wind conditions (mean winds 8-12 m s-1), the north-western Papuan Barrier Reef region was characterized by a shallow surface mixed layer, shallow nutriclines (25-75 m) and shallow subsurface chlorophyll maxima. Under low wind stress conditions (mean winds <5 m s-1), the southern and western Coral Sea were also characterized by a shallow surface mixed layer and stable underlying density profiles but deep (>I00 m) nutriclines and deep (60-125 m) subsurface chlorophyll and primary production maxima. Regardless of location, most primary production occurred above the 20% mid-day isolume surface. Phytoplankton standing crop and primary production in all regions were dominated by picoplankton (<2 μm size fraction). Very high primary production rates (1-3 g C m-2 day-1) were measured at a number of stations adjacent to the western margin of the PBR and within the central basin of the Louisiade Archipelago. Evidence for upwelling along the western margin of the PBR was observed under both north-easterly (normal to the reef axis) and south-easterly (parallel to the reef axis) wind regimes; however, surface outcropping of upwelled water did not occur. Oceanic primary production in the Coral Sea is estimated to be between 100 and 200 g C m-2 year-1. Primary production in and around the Louisiade Archipelago appears to be on the order of 200-300 g C m-2 year-1. Near-surface chlorophyll standing crop was generally better correlated with near-surface primary production than was total chlorophyll with total areal primary production.

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Halocarbon emissions and sources in the equatorial Atlantic Cold Tongue

Biogeosciences Discussions ◽

10.5194/bgd-12-5559-2015 ◽

2015 ◽

Vol 12 (7) ◽

pp. 5559-5608 ◽

Cited By ~ 3

Author(s):

H. Hepach ◽

B. Quack ◽

S. Raimund ◽

T. Fischer ◽

E. L. Atlas ◽

...

Keyword(s):

Surface Water ◽

Mixed Layer ◽

Global Radiation ◽

Surface Mixed Layer ◽

Cold Tongue ◽

Equatorial Atlantic ◽

Tropical Ocean ◽

Near Surface ◽

Biological Source ◽

Atlantic Cold Tongue

Abstract. Halocarbons from oceanic sources contribute to halogens in the troposphere, and can be transported into the stratosphere where they take part in ozone depletion. This paper presents distribution and sources in the equatorial Atlantic from June and July 2011 of the four compounds bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I) and diiodomethane (CH2I2). Enhanced biological production during the Atlantic Cold Tongue (ACT) season, indicated by phytoplankton pigment concentrations, led to elevated concentrations of CHBr3 of up to 44.7 pmol L−1 and up to 9.2 pmol L−1 for CH2Br2 in surface water, which is comparable to other tropical upwelling systems. While both compounds correlated very well with each other in the surface water,CH2Br2 was often more elevated in greater depth than CHBr3, which showed maxima in the vicinity of the deep chlorophyll maximum. The deeper maximum of CH2Br2 indicates an additional source in comparison to CHBr3 or a slower degradation of CH2Br2. Concentrations of CH3I of up to 12.8 pmol L−1 in the surface water were measured. In contrary to expectations of a predominantly photochemical source in the tropical ocean, its distribution was mostly in agreement with biological parameters, indicating a~biological source. CH2I2 was very low in the near surface water with maximum concentrations of only 3.7 pmol L−1, and the observed anticorrelation with global radiation was likely due to its strong photolysis. CH2I2 showed distinct maxima in deeper waters similar to CH2Br2. For the first time, diapycnal fluxes of the four halocarbons from the upper thermocline into and out of the mixed layer were determined. These fluxes were low in comparison to the halocarbon sea-to-air fluxes. This indicates that despite the observed maximum concentrations at depth, production in the surface mixed layer is the main oceanic source for all four compounds and has an influence on emissions into the atmosphere. The calculated production rates of the compounds yield 34 (CHBr3), 10 (CH2Br2), 21 (CH3I) and 384 (CH2I2) pmol m−3 h−1 in the whole mixed layer.

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Halocarbon emissions and sources in the equatorial Atlantic Cold Tongue

Biogeosciences ◽

10.5194/bg-12-6369-2015 ◽

2015 ◽

Vol 12 (21) ◽

pp. 6369-6387 ◽

Cited By ~ 7

Author(s):

H. Hepach ◽

B. Quack ◽

S. Raimund ◽

T. Fischer ◽

E. L. Atlas ◽

...

Keyword(s):

Surface Water ◽

Mixed Layer ◽

Surface Mixed Layer ◽

Cold Tongue ◽

Equatorial Atlantic ◽

Tropical Ocean ◽

Near Surface ◽

Biological Source ◽

First Time ◽

Atlantic Cold Tongue

Abstract. Halocarbons from oceanic sources contribute to halogens in the troposphere, and can be transported into the stratosphere where they take part in ozone depletion. This paper presents distribution and sources in the equatorial Atlantic from June and July 2011 of the four compounds bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I) and diiodomethane (CH2I2). Enhanced biological production during the Atlantic Cold Tongue (ACT) season, indicated by phytoplankton pigment concentrations, led to elevated concentrations of CHBr3 of up to 44.7 and up to 9.2 pmol L−1 for CH2Br2 in surface water, which is comparable to other tropical upwelling systems. While both compounds correlated very well with each other in the surface water, CH2Br2 was often more elevated in greater depth than CHBr3, which showed maxima in the vicinity of the deep chlorophyll maximum. The deeper maximum of CH2Br2 indicates an additional source in comparison to CHBr3 or a slower degradation of CH2Br2. Concentrations of CH3I of up to 12.8 pmol L−1 in the surface water were measured. In contrary to expectations of a predominantly photochemical source in the tropical ocean, its distribution was mostly in agreement with biological parameters, indicating a biological source. CH2I2 was very low in the near surface water with maximum concentrations of only 3.7 pmol L−1. CH2I2 showed distinct maxima in deeper waters similar to CH2Br2. For the first time, diapycnal fluxes of the four halocarbons from the upper thermocline into and out of the mixed layer were determined. These fluxes were low in comparison to the halocarbon sea-to-air fluxes. This indicates that despite the observed maximum concentrations at depth, production in the surface mixed layer is the main oceanic source for all four compounds and one of the main driving factors of their emissions into the atmosphere in the ACT-region. The calculated production rates of the compounds in the mixed layer are 34 ± 65 pmol m−3 h−1 for CHBr3, 10 ± 12 pmol m−3 h−1 for CH2Br2, 21 ± 24 pmol m−3 h−1 for CH3I and 384 ± 318 pmol m−3 h−1 for CH2I2 determined from 13 depth profiles.

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Large-Eddy Simulation of Deep-Cycle Turbulence in an Equatorial Undercurrent Model

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-016.1 ◽

2013 ◽

Vol 43 (11) ◽

pp. 2490-2502 ◽

Cited By ~ 18

Author(s):

Hieu T. Pham ◽

Sutanu Sarkar ◽

Kraig B. Winters

Keyword(s):

Large Eddy Simulation ◽

Mixed Layer ◽

Buoyancy Frequency ◽

Surface Mixed Layer ◽

Equatorial Undercurrent ◽

Eddy Simulation ◽

Large Eddy ◽

Model Components ◽

Surface Buoyancy ◽

Les Model

Abstract Dynamical processes leading to deep-cycle turbulence in the Equatorial Undercurrent (EUC) are investigated using a high-resolution large-eddy simulation (LES) model. Components of the model include a background flow similar to the observed EUC, a steady westward wind stress, and a diurnal surface buoyancy flux. An LES of a 3-night period shows the presence of narrowband isopycnal oscillations near the local buoyancy frequency N as well as nightly bursts of deep-cycle turbulence at depths well below the surface mixed layer, the two phenomena that have been widely noted in observations. The deep cycle of turbulence is initiated when the surface heating in the evening relaxes, allowing a region with enhanced shear and a gradient Richardson number Rig less than 0.2 to form below the surface mixed layer. The region with enhanced shear moves downward into the EUC and is accompanied by shear instabilities and bursts of turbulence. The dissipation rate during the turbulence bursts is elevated by up to three orders of magnitude. Each burst is preceded by westward-propagating oscillations having a frequency of 0.004–0.005 Hz and a wavelength of 314–960 m. The Rig that was marginally stable in this region decreases to less than 0.2 prior to the bursts. A downward turbulent flux of momentum increases the shear at depth and reduces Rig. Evolution of the deep-cycle turbulence includes Kelvin–Helmholtz-like billows as well as vortices that penetrate downward and are stretched by the EUC shear.

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Acoustic mapping of mixed layer depth

10.5194/os-2017-103 ◽

2018 ◽

Cited By ~ 2

Author(s):

Christian Stranne ◽

Larry Mayer ◽

Martin Jakobsson ◽

Elizabeth Weidner ◽

Kevin Jerram ◽

...

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Chemical Components ◽

Surface Mixed Layer ◽

Central Arctic Ocean ◽

Near Surface ◽

Direct Observations ◽

Spatial Coverage ◽

Major Factors ◽

Acoustic Mapping

Abstract. The ocean surface mixed layer is a nearly universal feature of the world oceans. The depth of the mixed layer (MLD) influences the exchange of heat and gases between the atmosphere and the ocean and constitutes one of the major factors controlling ocean primary production as it affects the vertical distribution of biological and chemical components in near-surface waters. Direct observations of the MLD are traditionally made by means of conductivity, temperature and depth (CTD) casts. However, CTD instrument deployment limits the observation of temporal and spatial variability of the MLD. Here, we present an alternative method where acoustic mapping of the MLD is done remotely by means of commercially available ship-mounted echosounders. The method is shown to be highly accurate when the MLD is well defined and biological scattering does not dominate the acoustic returns. These prerequisites are often met in the open ocean and it is shown that the method is successful in 95 % of data collected in the central Arctic Ocean. The primary advantages of acoustically mapping the MLD over CTD measurements are: (1) considerably higher temporal and horizontal resolutions and (2) potentially larger spatial coverage.

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Cold-Water Events and Dissipation in the Mixed Layer of a Lake

Journal of Physical Oceanography ◽

10.1175/jpo2946.1 ◽

2006 ◽

Vol 36 (10) ◽

pp. 1928-1939 ◽

Cited By ~ 7

Author(s):

B. Ozen ◽

S. A. Thorpe ◽

U. Lemmin ◽

T. R. Osborn

Keyword(s):

Kinetic Energy ◽

Turbulent Kinetic Energy ◽

Mixed Layer ◽

Large Scale ◽

Cold Water ◽

Velocity Shear ◽

Surface Mixed Layer ◽

Near Surface ◽

Law Of The Wall ◽

Upward Direction

Abstract Measurements of temperature, velocity, and microscale velocity shear were made from the research submarine F. A. Forel in the near-surface mixed layer of Lake Geneva under conditions of moderate winds of 6–8 m s−1 and of net heating at the water surface. The submarine carried arrays of thermistors and a turbulence package, including airfoil shear probes. The rate of dissipation of turbulent kinetic energy per unit mass, estimated from the variance of the shear, is found to be lognormally distributed and to vary with depth roughly in accordance with the law of the wall at the measurement depths, 15–20 times the significant wave height. Measurements revealed large-scale structures, coherent over the 2.38-m vertical extent sampled by a vertical array of thermistors, consisting of filaments tilted in the wind direction. They are typically about 1.5 m wide, decreasing in width in the upward direction, and are horizontally separated by about 25 m in the downwind direction. Originating in the upper thermocline, they are characterized in the mixed layer by their relatively low temperature and low rates of dissipation of turbulent kinetic energy and by an upward vertical velocity of a few centimeters per second.

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Tropical Cells and a Secondary Circulation near the Northern Front of the Equatorial Pacific Cold Tongue*

Journal of Physical Oceanography ◽

10.1175/2010jpo4366.1 ◽

2010 ◽

Vol 40 (9) ◽

pp. 2091-2106 ◽

Cited By ~ 12

Author(s):

Renellys C. Perez ◽

Meghan F. Cronin ◽

William S. Kessler

Keyword(s):

Mixed Layer ◽

Equatorial Pacific ◽

Secondary Circulation ◽

Subsurface Water ◽

Surface Mixed Layer ◽

Cold Tongue ◽

Near Surface ◽

Instability Waves ◽

Tropical Instability Waves ◽

The Mean

Abstract Shipboard measurements and a model are used to describe the mean structure of meridional–vertical tropical cells (TCs) in the central equatorial Pacific and a secondary circulation associated with the northern front of the cold tongue. The shape of the front is convoluted by the passage of tropical instability waves (TIWs). When velocities are averaged in a coordinate system centered on the instantaneous position of the northern front, the measurements show a near-surface minimum in northward flow north of the surface front (convergent flow near the front). This convergence and inferred downwelling extend below the surface mixed layer, tilt poleward with depth, and are meridionally bounded by regions of divergence and upwelling. Similarly, the model shows that, on average, surface cold tongue water moves northward toward the frontal region and dives below tilted front, whereas subsurface water north of the front moves southward toward the front, upwells, and then moves northward in the surface mixed layer. The model is used to demonstrate that this mean quasi-adiabatic secondary circulation is not a frozen field that migrates with the front but is instead highly dependent on the phase of the TIWs: southward-upwelling flow on the warm side of the front tends to occur when the front is displaced southward, whereas northward-downwelling flow on the cold side of the front occurs when the front is displaced northward. Consequently, when averaged in geographic coordinates, the observed and simulated TCs appear to be equatorially asymmetric and show little trace of a secondary circulation near the mean front.

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