scholarly journals Hydrography, Circulation, and Mixing at the Calypso Deep (the Deepest Mediterranean Trough) during 2006–09

2016 ◽  
Vol 46 (4) ◽  
pp. 1255-1276 ◽  
Author(s):  
H. Kontoyiannis ◽  
V. Lykousis ◽  
V. Papadopoulos ◽  
S. Stavrakakis ◽  
E. G. Anassontzis ◽  
...  
Keyword(s):  
Bottom Topography ◽  
Vertical Mixing ◽  
Average Error ◽  
Surrounding Water ◽  
Local Maxima ◽  
Vertical Diffusivity ◽  
Station Grid ◽  
Vertical Scale ◽  
Bottom Depth ◽  
Lens Center

AbstractThe mass and flow fields from June 2006 to May 2009 in the Calypso Deep (bottom depth ~5.2 km) are investigated using eddy-resolving surface-to-bottom hydrography (station grid spacing ~0.2°) and two tall moorings yielding current-meter records at depths from 700 m to near bottom. A salty warm lens (excess core salinity and temperature are ~0.01 and 0.025°C relative to the surrounding water) of Cretan Deep Water with a core at ~3000 m and a horizontal (vertical) scale of ~50 km (1.5 km) is identified in June 2006 to be locked over the trough. The lens coincides with local maxima in dissolved oxygen. In October 2006 the salinity content of the lens and of all deeper layers is increased; the oxygen maxima are shifted to the bottom layers, indicating an episodic intrusion of higher-density ventilated Adriatic water. The circulation changes from anticyclonic at all depths in June 2006 to cyclonic below ~2.5 km in October 2006, whereas after January 2007 it is cyclonic at all instrumented depths. The measured currents are weak (mean speeds < 5 cm s−1) and persistent in direction, being mostly along the bottom topography at all current-meter depths. After October 2006, the lens erodes due to salt/heat loss caused predominantly by lateral (intrusive) mixing, which works from the outside toward the lens center. The horizontal diffusivity is on the order of ~10 m2 s−1, near the center of the lens, and ~102 to 103 m2 s−1, at its periphery, with an average error ~15 times the diffusivity value. In the deepest part of the trough and in periods of predominance of vertical mixing the vertical diffusivity at 4400 m is ~(4 ± 3) × 10−3 m2 s−1.

scholarly journals The Behavior of Jet Currents over a Continental Slope Topography with a Possible Application to the Northern Current

2005 ◽  
Vol 35 (5) ◽  
pp. 790-810 ◽  
Author(s):  
M. M. Flexas ◽  
G. J. F. van Heijst ◽  
R. R. Trieling
Keyword(s):  
High Speed ◽  
Bottom Topography ◽  
Laboratory Model ◽  
Topographic Effects ◽  
Topographic Slope ◽  
Eddy Formation ◽  
Slow Flows ◽  
Bottom Depth ◽  
Source Sink ◽  
Northern Current

Abstract The Northern Current is a slope current in the northwest Mediterranean that shows high mesoscale variability, generally associated with meander and eddy formation. A barotropic laboratory model of this current is used here to study the role of the bottom topography on the current variability. For this purpose, a source–sink setup in a cylindrical tank placed on a rotating table is used to generate an axisymmetric barotropic current. To study inviscid topographic effects, experiments are performed over a topographic slope and also over a constant-depth setup, the latter being used as a reference for the former. With the aim of obtaining a fully comprehensive view of the vorticity balance at play, the flow may be forced in either azimuthal direction, leading to a “westward” prograde current (similar to the Northern Current) or an “eastward” retrograde current. For slow flows, eastward and westward currents showed similar patterns, dominated by Kelvin–Helmholtz-type instabilities. For high-speed flows, eastward and westward currents showed very different behavior. In eastward currents, the variability is observed to concentrate toward the center of the jet and shows strong meandering formation. Westward currents, instead, showed major variability toward the edges of the jet, together with a strong variability over the uppermost slope, which has been associated here with a topographic Rossby wave trapped over the shelf break. The differences between eastward and westward jets are explained through the balance between shear-induced and topographically induced vorticity at play in each case. Moreover, a model of jets over a beta plane is successfully applied here, allowing its extension to any ambient potential vorticity gradient caused either by latitudinal or bottom depth changes.

scholarly journals On the Generation of Bottom-Trapped Internal Tides

2015 ◽  
Vol 45 (2) ◽  
pp. 526-545 ◽  
Author(s):  
Saeed Falahat ◽  
Jonas Nycander
Keyword(s):  
Energy Density ◽  
Energy Flux ◽  
General Circulation ◽  
Bottom Topography ◽  
Vertical Mixing ◽  
Internal Tides ◽  
The Arctic ◽  
Linear Wave ◽  
Tidal Constituent ◽  
Tidal Constituents

AbstractThe interaction of the barotropic tide with bottom topography when the tidal frequency ω is smaller than the Coriolis frequency f is examined. The resulting waves are called bottom-trapped internal tides. The energy density associated with these waves is computed using linear wave theory and vertical normal-mode decomposition in an ocean of finite depth. The global calculation of the modal energy density is performed for the semidiurnal M2 tidal constituent and the two major diurnal tidal constituents K1 and O1. An observationally based decay time scale of 3 days is then used to transform the energy density to energy flux in units of watts per square meter. The globally integrated energy fluxes are found to be 1.99 and 1.43 GW for the K1 and O1 tidal constituents, respectively. For the M2 tidal constituent, it is found to be 1.15 GW. The Pacific Ocean is found to be the most energetic basin for the bottom-trapped diurnal tides. Two regional estimates of the bottom-trapped energy flux are given for the Kuril Islands and the Arctic Ocean, in which the bottom-trapped waves play a role for the tidally induced vertical mixing. The results of this study can be incorporated into ocean general circulation models and coupled climate models to improve the parameterization of the vertical mixing induced by breaking of the internal tides.

scholarly journals The stratification maxima of the seasonally varying Surface layer in the Arctic Ocean’s Beaufort Gyre

2021 ◽  
Author(s):  
◽  
Peter A. Roemer
Keyword(s):  
Mixed Layer ◽  
Vertical Mixing ◽  
Heat Energy ◽  
The Arctic ◽  
Small Scale ◽  
Geographic Variability ◽  
Vertical Diffusivity ◽  
Beaufort Gyre ◽  
The Arctic Ocean ◽  
Initial Evolution

The Beaufort Gyre region of the Arctic Ocean is strongly stratified at the base of the wintertime mixed layer, which impedes the vertical transport of heat, energy, and other tracers. Ice-Tethered Profiler observations during 2004-2018 were used to characterize and investigate the seasonal and interannual variability of the strength, depth, density, and thickness of this highly stratified layer at the base of the mixed layer. This includes investigating the remnant stratification maximum, which formed when the summer mixed layer shoaled. Seasonally, the stratification maximum was never in a steady state. It was largest in October (4.8 × 10−3 rad2/sec2) and decreased during all winter months (to 2.3 × 10−3rad2/sec2 in June), indicating that surface forcing and interior vertical mixing were never in equilibrium during the year. Interannually, the period from 2011-2018 had a higher stratification maximum than then the period from 2005-2010 regardless of the season. The remnant stratification maximum was consistently weaker than the winter stratification maximum from which it formed. The initial evolution of the remnant stratification maximum is used to estimate an effective vertical diffusivity of order 10−6m2/s. No significant geographic variability was found, in part due to high temporal and small scale variability of the stratification maximum layer. Implications for heat transport through to the sea ice cover are discussed.

scholarly journals Application of Multi-Source Data Fusion Method in Updating Topography and Estimating Sedimentation of the Reservoir

Water ◽  
2020 ◽  
Vol 12 (11) ◽  
pp. 3057
Author(s):  
Yu Liu ◽  
Shiguo Xu ◽  
Tongxin Zhu ◽  
Tianxiang Wang
Keyword(s):  
Data Fusion ◽  
Northeast China ◽  
Bottom Topography ◽  
Measurement Data ◽  
Average Error ◽  
Source Data ◽  
Sedimentation Volume ◽  
Simulation Results ◽  
The Difference ◽  
Total Sedimentation

The underwater terrain of a reservoir can experience significant changes due to the effects of erosion and siltation during decades of operation. Therefore, existing topographic data no longer reflect current reservoir terrains and need to be updated. In this paper, we propose a fast and economical method for updating the topography of a reservoir. According to multi-source data fusion, we effectively integrated sonar sounding data, cartographic data, and manual measurement data to update and reconstruct the bottom topography of a reservoir in Northeast China. By comparing the updated topography with the measured elevation, the average error of the simulation results is only 0.56%, which shows that the updated topography can accurately reflect the actual topography of the reservoir. Furthermore, by using the surface volume tool in ArcGIS, we developed the original and updated the elevation and volume curves of the reservoir. Finally, the amount of silting and its distribution in the reservoir were obtained by calculating the difference between the original and updated elevation and volume curves. The results show that the total sedimentation volume in the researching reservoir is about 4.3 million m3, which is mainly concentrated in the areas with an elevation below 50 m and above 60 m.

scholarly journals Spring Circulation Associated with the Thermal Bar in Large Temperate Lakes

1995 ◽  
Vol 26 (4-5) ◽  
pp. 331-358 ◽  
Author(s):  
Joakim Malm
Keyword(s):  
Numerical Model ◽  
Water Exchange ◽  
Bottom Topography ◽  
Practical Importance ◽  
Circulation Pattern ◽  
Maximum Density ◽  
Thermal Bar ◽  
Temperature Of Maximum Density ◽  
Induced Currents ◽  
Bottom Depth

The overall circulation pattern in spring is rather specific as density-induced currents may be of significance. The density-driven circulation perpendicular to the shore can be described as consisting of two circulation cells, with a zone of convergence, referred to as thermal bar, in between. The thermal bar, which coincides with the 4°C isotherm (the temperature of maximum density), inhibits horizontal water exchange, implying its practical importance. In this paper, a hydrodynamic numerical model is used to study the relative influence of wind- and density-driven currents in a large temperate lake during spring. The study shows that the general density-driven circulation is strongly dependent on the bottom topography, with a more pronounced circulation and considerable descending motions in the thermal bar zone in lakes with steep sloping bottoms. In shallow lakes, the wind-driven circulation dominates, and the effect of density-induced currents is marginal, except at locations with a drastic change in bottom depth.

scholarly journals Small-Scale Wind Fluctuations in the Tropical Tropopause Layer from Aircraft Measurements: Occurrence, Nature, and Impact on Vertical Mixing

2017 ◽  
Vol 74 (11) ◽  
pp. 3847-3869 ◽  
Author(s):  
Aurélien Podglajen ◽  
T. Paul Bui ◽  
Jonathan M. Dean-Day ◽  
Leonhard Pfister ◽  
Eric J. Jensen ◽  
...  
Keyword(s):  
Deep Convection ◽  
Vertical Mixing ◽  
Turbulence Theory ◽  
Small Scale ◽  
Tracer Transport ◽  
Tropical Tropopause Layer ◽  
Airborne Measurements ◽  
Tropical Tropopause ◽  
Vertical Scale ◽  
The Impact

Abstract The contribution of turbulent mixing to heat and tracer transport in the tropical tropopause layer (TTL) is poorly constrained, partly owing to a lack of direct observations. Here, the authors use high-resolution (20 Hz) airborne measurements to study the occurrence and properties of small-scale (&lt;100 m) wind fluctuations in the TTL (14–19 km) over the tropical Pacific. The fluctuations are highly intermittent and appear localized within shallow (100 m) patches. Furthermore, active turbulent events are more frequent at low altitude, near deep convection, and within layers of low gradient Richardson number. A case study emphasizes the link between the turbulent events and the occurrence of inertio-gravity waves having small horizontal or vertical scale. To evaluate the impact of the observed fluctuations on tracer mixing, their characteristics are examined. During active events, they are in broad agreement with inertial-range turbulence theory: the motions are close to 3D isotropic and the spectra follow a −5/3 power-law scaling. The diffusivity induced by turbulent bursts is estimated to be on the order of 10−1 m2 s−1 and increases from the top to the bottom of the TTL (from ~2 × 10−2 to ~3 × 10−1 m2 s−1). Given the uncertainties involved in the estimate, this is in reasonable agreement (about a factor of 3–4 lower) with the parameterized turbulent diffusivity in ERA-Interim, but it disagrees with other observational estimates from radar and radiosondes. The magnitude of the consequent vertical transport depends on the altitude and the tracer; for the species considered, it is generally smaller than that induced by the mean tropical upwelling.

scholarly journals Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake

2021 ◽  
Vol 83 (2) ◽  
Author(s):  
Bieito Fernández Castro ◽  
Oscar Sepúlveda Steiner ◽  
Deborah Knapp ◽  
Thomas Posch ◽  
Damien Bouffard ◽  
...  
Keyword(s):  
Thin Layer ◽  
Vertical Mixing ◽  
Stratified Medium ◽  
Vertical Position ◽  
Surface Mixed Layer ◽  
Convective Mixing ◽  
Vertical Diffusivity ◽  
Ozmidov Scale ◽  
Thorpe Scale ◽  
Lake Zürich

AbstractHarmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline ($$N \gtrsim 0.05$$ N ≳ 0.05  s$$^{-1}$$ - 1 ) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates ($$\varepsilon \approx 10^{-8}$$ ε ≈ 10 - 8  W kg$$^{-1}$$ - 1 ). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale ($$L_O \approx 1$$ L O ≈ 1  cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.

scholarly journals Research on Numerical Modelling in Lake Dynamics

2019 ◽  
Vol 8 (9S2) ◽  
pp. 496-498
Keyword(s):  
Numerical Simulation ◽  
Coriolis Force ◽  
Lake Water ◽  
Hydrodynamic Model ◽  
Bottom Topography ◽  
Vertical Mixing ◽  
Boussinesq Approximation ◽  
Strong Wind ◽  
Large Lakes ◽  
Lake Dynamics

Lakes are natural water bodies, where flow from single or various rivers is impounded by a natural impediment. Lake water environmental problem has severe effect on human health and the socio-economic sustainable development. So, it’s very important to find the more effective way of controlling the water pollution. Dynamics of lakes is the vast topic, which includes important concepts such as circulation of lakes, pollutant transport and interaction between lakes and hydrology. The hydrological dynamics of lake has been influenced by land cover modification, climate change, and increase in population and development activities within the catchment. Due to less velocity, lake impounds water for some time, and a significant characteristic of a lake is its retention time. Wind is the prevalent force in driving the circulation and in developing turbulent mixing in the lakes. Vertical mixing is caused by this turbulence. During circulation, summer and winter has different wind patterns. Strong wind would cause storm surge, which results in increased, mixing and transport in the surface water systems. Air temperature influence surface waters through heat flux and evaporation exchange between the air and the water. The Coriolis force is certain in large lakes due to earth rotation. Precipitation, tributaries inflow, runoffs, etc are lake water inputs. During numerical simulation of lakes, generally Boussinesq approximation and hydrostatic approximation are considered due to actual density distribution variations in water depth concepts respectively. It is essential to calibrate and verify the model before predictive applications anywhere. A simple numerical hydrodynamic model of a lake includes wind stress, bottom friction, Coriolis force, inflow, outflow, and the bottom topography of the lake. The hydrodynamic model has to be tested for stability, convergence, and sensitivity to parameters such as wind shear, wind direction, and vertical eddy viscosity effects. In this paper, the numerical simulation of lake dynamics has been discussed in detail

A discussion on ocean currents and their dynamics - Time-varying currents

1971 ◽  
Vol 270 (1206) ◽  
pp. 371-390 ◽  
Keyword(s):  
Bottom Topography ◽  
Vertical Mixing ◽  
Southwest Monsoon ◽  
Dynamical Theory ◽  
Ocean Current ◽  
Nonlinear Interactions ◽  
Time Varying ◽  
Output Data ◽  
The Indian Ocean ◽  
Heat And Momentum Transfer

It would be desirable to have a dynamical theory of how ocean current patterns vary with time in response to variation in the patterns of mass, heat and momentum transfer at the surface, but severe difficulties, particularly the uncertain effects of vertical mixing, nonlinear interactions and bottom topography, oppose the development of such a theory, while its evaluation through comparison with observation is impeded by the insufficiency of both input and output data. The characteristic wavenumber components associated with different parts of the input frequency spectrum at different latitudes can, however, be expected to play a particularly important role in determining response. Several of the means by which this may happen are discussed, with some particular reference to the dynamic response of the Indian Ocean to onset of the Southwest Monsoon.

scholarly journals The Effect of Localized Mixing on the Ocean Circulation and Time-Dependent Climate Change

2006 ◽  
Vol 36 (1) ◽  
pp. 140-160 ◽  
Author(s):  
Oleg A. Saenko
Keyword(s):  
Ocean Circulation ◽  
Climate Models ◽  
Vertical Mixing ◽  
Mean Diffusivity ◽  
Deep Ocean ◽  
Heat Diffusion ◽  
Western Boundary ◽  
Diffusivity Coefficient ◽  
Mean Values ◽  
Vertical Diffusivity

Abstract Observations indicate that intense mixing in the ocean is localized above complex topography and near the boundaries. Model experiments presented here illustrate that accounting for this fact can be important. In particular, it is found that in the case of localized mixing, the rate of overturning circulation is proportional to the net rate of generation of potential energy by the vertical mixing, linked to the net downward heat diffusion, rather than to the value of the mean vertical diffusivity coefficient. Furthermore, it is shown that two climate models, having the same vertical profile of diffusivity but differing in their distribution (horizontally uniform versus topography/boundary intensified) can simulate significantly different meridional oceanic circulations, vertical heat transfers, and responses of simulated climate to atmospheric CO2 increase. This is found for relatively large [O(1.0 cm2 s−1)] horizontal-mean values of vertical diffusivity in the pycnocline. However, in cases of relatively small [O(0.1 cm2 s−1)] mean diffusivity in the pycnocline, the simulated integral quantities such as meridional mass and heat transports do not depend much on the details of the mixing distribution. Even so, it is found that the deep western boundary currents are more localized near the boundaries in the case of topography/boundary-intensified mixing; also, the stratification in the deep ocean is set through the localized regions of intense vertical mixing. In addition, it is shown that reconciling the observed basin-mean values of diffusivity in the abyssal ocean of O(10 cm2 s−1) with realistic stratification can be problematic, unless the regions of enhanced vertical mixing are localized.

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