scholarly journals Seismic imaging of a thermohaline staircase in the western tropical North Atlantic

Ocean Science ◽  
2010 ◽  
Vol 6 (3) ◽  
pp. 621-631 ◽  
Author(s):  
I. Fer ◽  
P. Nandi ◽  
W. S. Holbrook ◽  
R. W. Schmitt ◽  
P. Páramo
Keyword(s):  
Internal Wave ◽  
North Atlantic ◽  
Seismic Data ◽  
Vertical Mixing ◽  
Global Ocean ◽  
Turbulent Dissipation ◽  
Seismic Image ◽  
Tropical North Atlantic ◽  
Order Of Magnitude ◽  
Wave Induced

Abstract. Multichannel seismic data acquired in the Lesser Antilles in the western tropical North Atlantic indicate that the seismic reflection method has imaged an oceanic thermohaline staircase. Synthetic acoustic modeling using measured density and sound speed profiles corroborates inferences from the seismic data. In a small portion of the seismic image, laterally coherent, uniform layers are present at depths ranging from 550–700 m and have a separation of ~20 m, with thicknesses increasing with depth. The reflection coefficient, a measure of the acoustic impedance contrasts across these reflective interfaces, is one order of magnitude greater than background noise. Hydrography sampled in previous surveys suggests that the layers are a permanent feature of the region. Spectral analysis of layer horizons in the thermohaline staircase indicates that internal wave activity is anomalously low, suggesting weak internal wave-induced turbulence. Results from two independent measurements, the application of a finescale parameterization to observed high-resolution velocity profiles and direct measurements of turbulent dissipation rate, confirm these low levels of turbulence. The lack of internal wave-induced turbulence may allow for the maintenance of the staircase or may be due to suppression by the double-diffusive convection within the staircase. Our observations show the potential for seismic oceanography to contribute to an improved understanding of occurrence rates and the geographical distribution of thermohaline staircases, and should thereby improve estimates of vertical mixing rates ascribable to salt fingering in the global ocean.

scholarly journals Seismic imaging of a thermohaline staircase in the western tropical North Atlantic

2010 ◽  
Vol 7 (1) ◽  
pp. 361-389
Author(s):  
I. Fer ◽  
P. Nandi ◽  
W. S. Holbrook ◽  
R. W. Schmitt ◽  
P. Páramo
Keyword(s):  
Internal Wave ◽  
North Atlantic ◽  
Vertical Mixing ◽  
Global Ocean ◽  
Turbulent Dissipation ◽  
Tropical North Atlantic ◽  
Seismic Reflection Method ◽  
Order Of Magnitude ◽  
Salt Fingering ◽  
Wave Induced

Abstract. Multichannel seismic data acquired in the Lesser Antilles in the western tropical North Atlantic indicate that the seismic reflection method has imaged an oceanic thermohaline staircase. Synthetic modeling of observed density and sound speed profiles corroborates inferences from the seismic imagery. Laterally coherent, uniform layers are present at depths ranging from 550–700 m and have a separation of ~20 m, with thicknesses increasing with depth. Reflection coefficient, a measure of the acoustic impedance contrasts, associated with the interfaces is one order of magnitude greater than the background levels. Hydrography sampled in previous surveys puts a constraint on the longevity of these layers in this area to within a maximum of three years. Spectral analysis of layer horizons in the thermohaline staircase indicates that internal wave activity is anomalously low, suggesting weak internal wave-induced turbulence and mixing. Results from two independent measurements, the application of a finescale parameterization to observed high-resolution velocity profiles and direct measurements of turbulent dissipation rate, confirm the low levels of turbulence and mixing. The lack of internal wave-induced mixing allows for the maintenance of the staircase. Our observations show the potential that seismic oceanography can contribute to an improved understanding of temporal occurrence rates, and the geographical distribution of thermohaline staircases and can improve current estimates of vertical mixing rates ascribable to salt fingering in the global ocean.

scholarly journals Inferences and Observations of Turbulent Dissipation and Mixing in the Upper Ocean at the Hawaiian Ridge

2007 ◽  
Vol 37 (3) ◽  
pp. 476-494 ◽  
Author(s):  
Joseph P. Martin ◽  
Daniel L. Rudnick
Keyword(s):  
Energy Density ◽  
Internal Wave ◽  
Dissipation Rate ◽  
Internal Tide ◽  
Eddy Diffusivity ◽  
Internal Tides ◽  
Upper Ocean ◽  
Turbulent Dissipation ◽  
Hawaiian Ridge ◽  
Wave Induced

Abstract The Hawaiian Ridge is one of the most energetic generators of internal tides in the pelagic ocean. The density and current structure of the upper ocean at the Hawaiian Ridge were observed using SeaSoar and Doppler sonar during a survey extending from Oahu to Brooks Banks and up to 200 km from the ridge peak. Survey observations are used to quantify spatial changes in internal-wave-induced turbulent dissipation and mixing. The turbulent dissipation rate of kinetic energy ɛ and diapycnal eddy diffusivity Kρ are inferred from an established parameterization using internal wave shear as input. At the Kauai Channel (KC) and French Frigate Shoals/Brooks Banks sites, ɛ and Kρ decay away from the ridge with maxima exceeding minima by 5 times. At both sites, average Kρ is everywhere greater than the canonical open-ocean value of 10−5 m2 s−1. Along the ridge, ɛ and Kρ vary by up to 100 times and are largest at sites of largest numerical model internal tide energy density. In the eastern KC, Kρ > 10−3 m2 s−1 is typical in a patch more than 200 m thick located above the path of an M2 internal tide ray. An upper limit on the dissipation rate from M2 internal tides to turbulence within 50 km of the Hawaiian Ridge is roughly estimated to be in the range of 4–9 GW. At KC, the depth-integrated internal wave energy density and dissipation rate are positively correlated. Potential density inversions occur near the main ridge axis at significant topographic features. Average Kρ is larger inside inversions.

scholarly journals A global viral oceanography database (gVOD)

2020 ◽  
Author(s):  
Le Xie ◽  
Wei Wei ◽  
Lanlan Cai ◽  
Xiaowei Chen ◽  
Yuhong Huang ◽  
...  
Keyword(s):  
North Atlantic ◽  
Deep Ocean ◽  
Marine Ecology ◽  
Global Ocean ◽  
Viral Production ◽  
Viral Abundance ◽  
The North ◽  
Southeast Pacific ◽  
Order Of Magnitude ◽  
Marine Biosphere

Abstract. Virioplankton is a key component of marine biosphere in maintaining diversity of microorganisms and stabilizing ecosystems. They also greatly contribute to nutrient recycles by releasing organic matter after lysis of hosts. In this study, we constructed the first global viral oceanography database (gVOD) by collecting 10 931 viral abundance (VA) data and 727 viral production (VP) data, along with host and other oceanographic parameters when available. Most VA data were obtained in the North Atlantic (32 %) and North Pacific Oceans (29 %), while the Southeast Pacific and Indian Oceans were quite under sampled. The VA in the global ocean was 1.17 (± 3.31) × 107 particles ml−1. The lytic and lysogenic VP in the global ocean was 9.87 (± 24.16) × 105 and 2.53 (± 8.64) × 105 particles ml−1 h−1, respectively. Average VA in coastal oceans was higher than that in surface open oceans [3.61 (± 6.30) × 107 versus 0.73 (± 1.24) × 107 particles ml−1], while average VP in coastal and surface open oceans was close. Vertically, VA, lytic and lysogenic VP deceased from surface to deep ocean by about one order of magnitude. The total number of viruses in the global ocean estimated by bin average and random forest methods was 1.4 × 1030 particles and 1.39 × 1030 particles, leading to an estimate of global ocean viral biomass at 32.3 and 32.2 Tg C, respectively. We expect that the gVOD will be a fundamental and very useful database for laboratory, field and modeling studies in marine ecology and biogeochemistry. The full gVOD database is stored at PANGAEA (a temporary link: https://www.pangaea.de/tok/19f9d7b496a00f57f491e639440708aee00b6a49).

scholarly journals Submesoscale Mixing Across the Mixed Layer in the Gulf of Mexico

2021 ◽  
Vol 8 ◽  
Author(s):  
Guangpeng Liu ◽  
Annalisa Bracco ◽  
Alexandra Sitar
Keyword(s):  
Gulf Of Mexico ◽  
Water Column ◽  
Mixed Layer ◽  
Vertical Mixing ◽  
Global Ocean ◽  
Relative Role ◽  
Loop Current ◽  
Turbulent Layer ◽  
Open Questions ◽  
Order Of Magnitude

Submesoscale circulations influence momentum, buoyancy and transport of biological tracers and pollutants within the upper turbulent layer. How much and how far into the water column this influence extends remain open questions in most of the global ocean. This work evaluates the behavior of neutrally buoyant particles advected in simulations of the northern Gulf of Mexico by analyzing the trajectories of Lagrangian particles released multiple times at the ocean surface and below the mixed layer. The relative role of meso- and submesoscale dynamics is quantified by comparing results in submesoscale permitting and mesoscale resolving simulations. Submesoscale circulations are responsible for greater vertical transport across fixed depth ranges and also across the mixed layer, both into it and away from it, in all seasons. The significance of the submesoscale-induced transport, however, is far greater in winter. In this season, a kernel density estimation and a detailed vertical mixing analysis are performed. It is found that in the large mesoscale Loop Current eddy, upwelling into the mixed layer is the major contributor to the vertical fluxes, despite its clockwise circulation. This is opposite to the behavior simulated in the mesoscale resolving case. In the “submesoscale soup,” away from the large mesoscale structures such as the Loop Current and its detached eddies, upwelling into the mixed layer is distributed more uniformly than downwelling motions from the surface across the base of the mixed layer. Maps of vertical diffusivity indicate that there is an order of magnitude difference among simulations. In the submesoscale permitting case values are distributed around 10–3 m2 s–1 in the upper water column in winter, in agreement with recent indirect estimates off the Chilean coast. Diffusivities are greater in the eastern portion of the Gulf, where the submesoscale circulations are more intense due to sustained density gradients supplied by the warmer and saltier Loop Current.

atmospheric dust studies on Barbados1

1968 ◽  
Vol 49 (6) ◽  
pp. 645-652 ◽  
Author(s):  
J. M. Prospero
Keyword(s):  
North Atlantic ◽  
North Atlantic Ocean ◽  
Meteorological Conditions ◽  
Atmospheric Dust ◽  
Seasonal Periodicity ◽  
Trade Winds ◽  
Considerable Evidence ◽  
Tropical North Atlantic ◽  
Order Of Magnitude ◽  
Dust Load

The concentration of dust in the trade winds passing over Barbados has been measured continuously since August 1965; the average dust load for the 25 month period ending in August 1967 is 2.5 μgm m−3. There is a marked seasonal periodicity in the transport with the average dust concentrations for the summer months being approximately an order of magnitude larger than those for the winter months. In addition, very sharp fluctuations in dust concentrations are observed on a day-to-day basis. There is considerable evidence that the dust originates in Africa and that the composition of the dust and the quantities transported are dependent on the meteorological conditions over Africa and the tropical North Atlantic Ocean. The future programs for atmospheric dust studies on Barbados will be directed towards the utilization of dust as a tracer for a variety of meteorological processes; a number of the more important of these investigations are discussed.

scholarly journals Stochastic Heterogeneity Mapping around a Mediterranean salt lens

Ocean Science ◽  
2010 ◽  
Vol 6 (1) ◽  
pp. 423-429 ◽  
Author(s):  
G. G. Buffett ◽  
C. A. Hurich ◽  
E. A. Vsemirnova ◽  
R. W. Hobbs ◽  
V. Sallarès ◽  
...  
Keyword(s):  
Internal Wave ◽  
Seismic Data ◽  
Seismic Reflection ◽  
Theoretical Work ◽  
Seismic Image ◽  
Stochastic Parameters ◽  
Band Limited ◽  
Wave Surfaces ◽  
Stochastic Heterogeneity ◽  
Mediterranean Water

Abstract. We present the first application of Stochastic Heterogeneity Mapping based on the band-limited von Kármán function to a seismic reflection stack of a Mediterranean water eddy (meddy), a large salt lens of Mediterranean water. This process extracts two stochastic parameters directly from the reflectivity field of the seismic data: the Hurst number, which ranges from 0 to 1, and the correlation length (scale length). Lower Hurst numbers represent a richer range of high wavenumbers and correspond to a broader range of heterogeneity in reflection events. The Hurst number estimate for the top of the meddy (0.39) compares well with recent theoretical work, which required values between 0.25 and 0.5 to model internal wave surfaces in open ocean conditions based on simulating a Garrett-Munk spectrum (GM76) slope of −2. The scale lengths obtained do not fit as well to seismic reflection events as those used in other studies to model internal waves. We suggest two explanations for this discrepancy: (1) due to the fact that the stochastic parameters are derived from the reflectivity field rather than the impedance field the estimated scale lengths may be underestimated, as has been reported; and (2) because the meddy seismic image is a two-dimensional slice of a complex and dynamic three-dimensional object, the derived scale lengths are biased to the direction of flow. Nonetheless, varying stochastic parameters, which correspond to different spectral slopes in the Garrett-Munk spectrum (horizontal wavenumber spectrum), can provide an estimate of different internal wave scales from seismic data alone. We hence introduce Stochastic Heterogeneity Mapping as a novel tool in physical oceanography.

scholarly journals Turbulence and Diapycnal Mixing in Drake Passage

2012 ◽  
Vol 42 (12) ◽  
pp. 2143-2152 ◽  
Author(s):  
L. St. Laurent ◽  
A. C. Naveira Garabato ◽  
J. R. Ledwell ◽  
A. M. Thurnherr ◽  
J. M. Toole ◽  
...  
Keyword(s):  
Drake Passage ◽  
Global Ocean ◽  
Mixing Enhancement ◽  
Turbulent Dissipation ◽  
Dissipation Rates ◽  
Direct Measurements ◽  
Order Of Magnitude ◽  
Frontal Zones ◽  
Circumpolar Current ◽  
The Antarctic

Abstract Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10−8 W kg−1 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 × 10−4 m2 s−1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10−10) W kg−1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.

scholarly journals Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

2016 ◽  
Vol 13 (19) ◽  
pp. 5633-5647 ◽  
Author(s):  
Björn Fiedler ◽  
Damian S. Grundle ◽  
Florian Schütte ◽  
Johannes Karstensen ◽  
Carolin R. Löscher ◽  
...  
Keyword(s):  
Organic Matter ◽  
North Atlantic ◽  
Utilization Rate ◽  
Surface Mixed Layer ◽  
Oxygen Utilization ◽  
Near Surface ◽  
Underwater Gliders ◽  
Tropical North Atlantic ◽  
Order Of Magnitude ◽  
Oxygen Utilization Rate

Abstract. The occurrence of mesoscale eddies that develop suboxic environments at shallow depth (about 40–100 m) has recently been reported for the eastern tropical North Atlantic (ETNA). Their hydrographic structure suggests that the water mass inside the eddy is well isolated from ambient waters supporting the development of severe near-surface oxygen deficits. So far, hydrographic and biogeochemical characterization of these eddies was limited to a few autonomous surveys, with the use of moorings, underwater gliders and profiling floats. In this study we present results from the first dedicated biogeochemical survey of one of these eddies conducted in March 2014 near the Cape Verde Ocean Observatory (CVOO). During the survey the eddy core showed oxygen concentrations as low as 5 µmol kg−1 with a pH of around 7.6 at approximately 100 m depth. Correspondingly, the aragonite saturation level dropped to 1 at the same depth, thereby creating unfavorable conditions for calcifying organisms. To our knowledge, such enhanced acidity within near-surface waters has never been reported before for the open Atlantic Ocean. Vertical distributions of particulate organic matter and dissolved organic matter (POM and DOM), generally showed elevated concentrations in the surface mixed layer (0–70 m), with DOM also accumulating beneath the oxygen minimum. With the use of reference data from the upwelling region where these eddies are formed, the oxygen utilization rate was calculated by determining oxygen consumption through the remineralization of organic matter. Inside the core, we found these rates were almost 1 order of magnitude higher (apparent oxygen utilization rate (aOUR); 0.26 µmol kg−1 day−1) than typical values for the open North Atlantic. Computed downward fluxes for particulate organic carbon (POC), were around 0.19 to 0.23 g C m−2 day−1 at 100 m depth, clearly exceeding fluxes typical for an oligotrophic open-ocean setting. The observations support the view that the oxygen-depleted eddies can be viewed as isolated, westwards propagating upwelling systems of their own, thereby represent re-occurring alien biogeochemical environments in the ETNA.

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