Vertical Salt Flux Research Articles

Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea

Abstract. Turbulent diapycnal mixing is important for the estuarine circulation between basins of the Baltic Sea as well as for its local ecosystems, in particular with regard to eutrophication and anoxic conditions. While the interior of the basins is overall relatively calm, stratified flow over steep bathymetric features is known as a source of strong turbulent mixing. Yet, current in situ observations often cannot capture the spatio-temporal development of dynamic and intermittent turbulent mixing related to overflows over rough bathymetry. We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the sparsely sampled Southern Quark located in the Åland Sea, connecting the northern Baltic Proper with the Bothnian Sea. Our data were acquired during two 1-week cruises on R/V Electra in February–March 2019 and 2020. We collected high-resolution broadband acoustic observations of turbulent mixing together with in situ microstructure profiler measurements, and current velocities from acoustic Doppler current profilers. We found that a temporally reversing non-tidal stratified flow over the steep bathymetric sill created a dynamic and extremely energetic environment. The observed flow reversed during both cruises on timescales of a few days. Saltier, warmer, and less oxygenated deep water south of the sill was partly blocked, the reversing flow was at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill, and high spatial variability occurred in the surface layer on small scales. Dissipation rates of turbulent kinetic energy, vertical turbulent diffusivities, and vertical salt flux rates were increased by 3–4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow with average dissipation rates near the sill between 10−7 and 10−6 W kg−1, average vertical diffusivities of 0.001 m2 s−1 in the halocline and up to 0.1 m2 s−1 below the halocline, and average vertical salt flux rates around 0.01 g m−2 s−1 in the halocline and between 0.1 and 1 g m−2 s−1 below the halocline. We suggest, based on acoustic observations and in situ measurements, that the underlying mechanism for the highly increased mixing across the halocline is a combination of shear and topographic lee waves breaking at the halocline interface. We anticipate that the resulting deep- and surface-water modification in the Southern Quark directly impacts exchange processes between the Bothnian Sea and the northern Baltic Proper and that the observed mixing is likely important for oxygen and nutrient conditions in the Bothnian Sea.

Generation of a Vertical Fine Structure by Internal Waves on the Sea Shelf

Free internal waves are considered in the Boussinesq approximation with account of horizontal turbulent viscosity and diffusion in a two-dimensional flow in the sea of constant depth. In the linear approximation the boundary-value problem for the amplitude of vertical velocity has complex coefficients and is solved numerically using the third-order implicit Adams method. The vertical component of the Stokes drift velocity and the vertical wave fluxes of mass and salt are found in the second order in the wave amplitude. These fluxes lead to generation of a vertical fine structure that has an irreversible nature.

Otsenka intensivnosti vertikal'nogo turbulentnogo obmena v sloye osnovnogo piknoklina na prikerchenskom uchastke shel'fa Chernogo morya

The paper investigates the seasonal variability of the vertical turbulent exchange coefficient in the upper stratified layer of the Black Sea. The expedition data used in this work containing information on the microstructure of physical fields were obtained in different hydrological seasons covering the northeastern part of the Black Sea in the Prikerchensky area of the shelf slope. The data were collected during cruises of r/v “Professor Vodyanitsky” in 2016–2019 using “Sigma-1” sounding complex. Based on the semi-empirical methods of assessment of vertical turbulent exchange in the deep-water area of the Black Sea, the dependence of the vertical turbulent diffusion coefficient K on the buoyancy frequency N in the studied layer was established from the flow fluctuation characteristics, with the corresponding graphs and their approximating power-law dependences K  A  N  plotting. In addition, the vertical distribution of the K coefficient with depth was analyzed. Comparative analysis of the obtained dependences with the results of the 1.5D model was carried out. The analysis of the measurement data showed that the results obtained in this work do not contradict the original model. The results can also be used to assess the vertical fluxes of heat, salt and other dissolved chemical and biological substances depending on stratification in the studied part of the Black Sea for different seasons.

Vertical redistribution of salt and layered changes in global ocean salinity

Salinity is an essential proxy for estimating the global net freshwater input into the ocean. Due to the limited spatial and temporal coverage of the existing salinity measurements, previous studies of global salinity changes focused mostly on the surface and upper oceans. Here, we examine global ocean salinity changes and ocean vertical salt fluxes over the full depth in a dynamically consistent and data-constrained ocean state estimate. The changes of the horizontally averaged salinity display a vertically layered structure, consistent with the profiles of the ocean vertical salt fluxes. For salinity changes in the relatively well-observed upper ocean, the contribution of vertical exchange of salt can be on the same order of the net surface freshwater input. The vertical redistribution of salt thus should be considered in inferring changes in global ocean salinity and the hydrological cycle from the surface and upper ocean measurements.

Stratification Has Strengthened in the Baltic Sea – An Analysis of 35 Years of Observational Data

Stratification of the water column, consisting of the three layers (upper, intermediate, and deep layer), is an important factor for the functioning of the brackish Baltic Sea. In the present work, changes in the vertical structure of temperature and salinity, as well as heat content, salt mass and stratification conditions were estimated on the basis of in-situ and remote sensing data in 1982-2016. The seasonal thermocline and the halocline have strengthened in most of the sea by a rate of 0.33 – 0.39 kg m-3 and 0.70 – 0.88 kg m-3, respectively, during 35 years. The upper layer has warmed by 0.03 – 0.06 °C year-1 and sub-halocline deep layer 0.04 – 0.06 °C year-1 in most of the sea. The total warming trend in the whole Baltic has been 1.07 °C for 35 years, being approximately twice higher compared to the upper 100 m in the Atlantic Ocean. Average upper layer warming of the sea from May to September has been 0.07 – 0.08 °C year-1 while in winter, trends were mostly statistically not significant. More rapid warming during summers has occurred in shallower, closed-end areas of gulfs. Possible reasons for high warming there might be shallow depths and limited water exchange, stronger stratification, and/or higher turbidity. Sea surface temperature trends estimated by in-situ and satellite data agree well. Trends of freshening (-0.005 to -0.014 g kg-1 year-1) of the upper layer and increasing salinity (0.02 to 0.04 g kg-1 year-1) in the sub-halocline deep layer were detected. Increased salinity in the deep layer is likely caused by the increased lateral import of saltier water from the North Sea. Changes in the upper layer salinity might not be related to the accumulated river runoff only, but decadal changes of vertical salt flux might also contribute. The vertically distinct changes cancel each other and no significant trend in the mean salinity of the Baltic Sea was detected. No remarkable changes have occurred in the cold intermediate layer. In conclusion, different dominating processes have caused distinct long-term trends in the three layers of the Baltic Sea.

Measurements of buoyancy flux in a stratified turbulent flow

We present new experimental data on the controls on the buoyancy flux in a stratified turbulent flow. The inner cylinder of an annulus of fluid with vertical axis is rotated to produce a turbulent flow field with Reynolds numbers of up to $10^{5}$, while a flux of saline fluid is supplied to the base of the tank, and an equal flux of fresh fluid is supplied to the top of the tank. In addition, fluid is vented from the base and the top of the tank with the same volume fluxes as the supply. The steady-state vertical flux of salt is explored. When the salt flux supplied to the base of the tank is very small, the tank becomes nearly well-mixed, and the vertical salt flux is approximately equal to one-half the source flux. As the source salt flux increases, a weak stable salinity gradient develops across the tank, and the vertical salt flux increases. As the source flux continues to increase, eventually the vertical salt flux reaches a maximum, and further increases in the source salt flux can lead to an increase in the vertical salinity gradient but not the vertical flux. We interpret the transition in the vertical buoyancy flux as representing a change from a source-limited regime, where the buoyancy flux and buoyancy frequency, $N$, are related, to a mixing-limited regime, in which the buoyancy flux is independent of $N$. In the mixing-limited regime, the effective eddy diffusivity is proportional to $u_{rms}^{3}/LN^{2}$ while in the source-limited regime, the eddy diffusivity is approximately proportional to $u_{rms}^{2}/N$, where $u_{rms}$ and $L$ are the characteristic turbulence speed and length scale. This transition may have implications for the balance between upwelling and diapycnal mixing in the ocean, if the intensity of the turbulence varies in space or the flux of deep water varies in time.

Vertical turbulent exchange features in the layer of seasonal pycnocline in the northwestern part of the Black Sea

On the basis of experimental physical field’s microstructure data in the upper stratified layers of the Black Sea, the annual variability of the vertical turbulent exchange coefficient in the seasonal thermocline is investigated. The data were collected during eight expedition periods in 2007 – 2017, and they cover the north-western part of the Black Sea. The studies were conducted in different hydrological seasons from April to November. The measurements were carried out by the probing complex “Sigma-1”, the evaluation of turbulence energy dissipation was made by velocity pulsation gradients. The desired coefficient of vertical turbulent diffusion was calculated on the basis of the obtained experimental estimates of energy dissipation by known Osborn’s relationship. The analysis of these measurements makes it possible to calculate and use the vertical turbulent diffusion coefficient dependence on stratification to evaluate the vertical fluxes of heat, salt and other dissolved chemical and biological substances in the studied Black Sea region.

A three-dimensional numerical study of river plume mixing processes in Otsuchi Bay, Japan

The three-dimensional numerical model SUNTANS is applied to investigate river plume mixing in Otsuchi Bay, an estuary located along the Sanriku Coast of Iwate, Japan. Results from numerical simulations with different idealized forcing scenarios (barotropic tide, baroclinic tide, and diurnal wind) are compared with field observations to diagnose dominant mixing mechanisms. Under the influence of combined barotropic, baroclinic and wind forcing, the model reproduces observed salinity profiles well and achieves a skill score of 0.94. In addition, the model forced by baroclinic internal tides reproduces observed cold-water intrusions in the bay, and barotropic tidal forcing reproduces observed salt wedge dynamics near the river mouths. Near these river mouths, vertically sheared flows are generated due to the interaction of river discharge and tidal elevations. River plume mixing is quantified using vertical salt flux and reveals that mixing near the vicinity of the river mouth, is primarily generated by the barotropic tidal forcing. A 10 ms−1 strong diurnal breeze compared to a 5 ms−1 weak breeze generates higher mixing in the bay. In contrast to the barotropic forcing, internal tidal (baroclinic) effects are the dominant mixing mechanisms away from the river mouths, particularly in the middle of the bay, where a narrow channel strengthens the flow speed. The mixing structure is horizontally asymmetric, with the middle and northern parts exhibiting stronger mixing than the southern part of the bay. This study identifies several mixing hot-spots within the bay and is of great importance for the coastal aquaculture system.

Vertical heat and salt fluxes induced by inertia-gravity internal waves on sea shelf

Free inertia-gravity internal waves are considered in a two-dimensional vertically nonuniform flow in the Boussinesq approximation. The equation for vertical velocity amplitude includes complex factors caused by the gradient of the flow velocity component transverse to the wave-propagation direction; therefore, the eigenfunction and wave frequency are complex. It is shown that the decrement of damping (imaginary correction to the frequency) of 15-min internal waves is two orders of magnitude smaller than the wave frequency; i.e., the waves weakly damp. Vertical wave fluxes of heat and salt are nonzero due to the phase shift between fluctuations of the vertical velocity and temperature (salinity) different from π 2. The vertical component of the Stokes drift speed is also nonzero and contributed into the vertical transport.

Topographic enhancement of vertical turbulent mixing in the Southern Ocean

It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.

The influences of winds on the salt balance of the Tampa Bay during the fall of 2001

A three-dimensional, high resolution, Finite Volume Coastal Ocean Model (FVCOM) is used to diagnose the influences of winds during the fall of 2001 on the salt balance of the Tampa Bay. To distinguish the wind effecting, two experiments are designed: one is driven by tides and rivers; the other is driven by tides, rivers and winds. First, the salinity change induced by wind is provided. Wind forcing can significantly increase the salinity inside the Tampa Bay, along with decreases of horizontal and vertical salt gradients. Subsequently, the salt balance principles are detailed addressed. The primary salt balance is between the total (horizontal plus vertical) advective salt flux divergence and vertical diffusive salt flux divergence except at the channel bottom where horizontal diffusive salt flux divergence comes into play. At last, the salt balance variation induced by winds is further addressed. Wind forcing does not change the relative importance of salt balance terms. The variations of the salt balance terms influenced by winds are highly dependent on the specific locations. Besides, the variations of the total advective salt flux divergence and the vertical diffusive salt flux divergence are nearly couterbalance, and both of two terms are much greater than that of the horizontal diffusive salt flux divergence.

An Assessment of the Seasonal Salinity Budget for the Upper Bay of Bengal

AbstractDuring each summer monsoon, the Bay of Bengal is inundated by a large amount of rain and river discharge. The effects of this freshening are gradually reversed over the course of the year, with near-surface salinities typically returning to their presummer monsoon levels before the start of the next rainy season. While the forcing responsible for the summertime freshening is clear, the processes that act to restore the bay’s salinity are not well understood. To examine these processes, the authors construct a basin-integrated, near-surface, seasonal salinity budget using data-assimilated output from the Hybrid Coordinate Ocean Model (HYCOM). From this salinity budget, it is deduced that vertical salt fluxes are primarily responsible for counterbalancing the near-surface freshening caused by the summertime freshwater fluxes. These vertical salt fluxes are largest during the months that immediately follow the summer monsoon, when the near-surface halocline is strongest. These results must be tempered with the knowledge that HYCOM misrepresents some key features of the bay’s salinity field. In particular, the model tends to overestimate salinity along the East Indian Coastal Current during its equatorward phase. Notwithstanding these biases, these results still suggest that vertical processes have a prominent role in the bay’s near-surface salinity budget.

Seasonal hydrography and surface outflow in a fjord with a deep sill: the Reloncaví fjord, Chile

Abstract. Seasonal data on temperature, salinity, dissolved oxygen (DO) and chlorophyll, combined with meteorological and river discharge time series, were used to describe the oceanographic conditions of the Reloncaví fjord (41°35′ S, 72°20′ W). The winds in the fjord valley mainly blow down-fjord during the winter, reinforcing the upper layer outflow, whereas the winds blow predominantly up-fjord during the spring and summer, contrary to the upper layer outflow. The fjord, with a deep sill at the mouth, was well stratified year-round and featured a thin surface layer of brackish water with mean salinities between 10.4 ± 1.4 (spring) and 13.2 ± 2.5 (autumn). The depth of the upper layer changed slightly among the different studied seasons but remained at 4.5 m near the mouth. This upper layer presented a mean outflow (Q1) of 3185 ± 223 m3 s−1, which implies a flushing time of 3 days for this layer. The vertical salt flux was ∼ 37 tons of salt per second, similar to the horizontal salt flux observed in the upper layer. These estimates will contribute to better management of the aquaculture in this region.

Adrift Upon a Salinity-Stratified Sea: A View of Upper-Ocean Processes in the Bay of Bengal During the Southwest Monsoon

The structure and variability of upper-ocean properties in the Bay of Bengal (BoB) modulate air-sea interactions, which profoundly influence the pattern and intensity of monsoonal precipitation across the Indian subcontinent. In turn, the bay receives a massive amount of freshwater through river input at its boundaries and from heavy local rainfall, leading to a salinity-stratified surface ocean and shallow mixed layers. Small-scale oceanographic processes that drive variability in near-surface BoB waters complicate the tight coupling between ocean and atmosphere implicit in this seasonal feedback. Unraveling these ocean dynamics and their impact on air-sea interactions is critical to improving the forecasting of intraseasonal variability in the southwest monsoon. To that end, we deployed a wave-powered, rapidly profiling system capable of measuring the structure and variability of the upper 100 m of the BoB. The evolution of upper-ocean structure along the trajectory of the instrument's roughly two-week drift, along with direct estimates of vertical fluxes of salt and heat, permit assessment of the contributions of various phenomena to temporal and spatial variability in the surface mixed layer depth. Further, these observations suggest that the particular ``barrier-layer'' stratification found in the BoB may decrease the influence of the wind on mixing processes in the interior, thus isolating the upper ocean from the interior below, and tightening its coupling to the atmosphere above.

Double-Diffusive Convection in a Stochastic Shear

AbstractAn attempt is made to quantify the impact of stochastic wave–induced shears on salt fingers associated with internal waves in the ocean. The wave environment is represented by the superposition of Fourier components conforming to the Garrett–Munk (GM) spectrum with random initial phase distribution. The resulting time series of vertical shear are incorporated into a finger-resolving numerical model, and the latter is used to evaluate the equilibrium diapycnal fluxes of heat and salt. The proposed procedure makes it possible to simulate salt fingers in shears that are representative of typical oceanic conditions. This study finds that the shear-induced modification of salt fingers is largely caused by near-inertial motions. These relatively slow waves act to align salt fingers in the direction of shear, thereby rendering the double-diffusive dynamics effectively two-dimensional. Internal waves reduce the equilibrium vertical fluxes of heat and salt by a factor of 2 relative to those in the unsheared three-dimensional environment, bringing them close to the values suggested by corresponding two-dimensional simulations.

A modeling study of the processes of surface salinity seasonal cycle in the Bay of Bengal

AbstractIn response to the Indian Monsoon freshwater forcing, the Bay of Bengal exhibits a very strong seasonal cycle in sea surface salinity (SSS), especially near the mouths of the Ganges‐Brahmaputra and along the east coast of India. In this paper, we use an eddy‐permitting (∼25 km resolution) regional ocean general circulation model simulation to quantify the processes responsible for this SSS seasonal cycle. Despite the absence of relaxation toward observations, the model reproduces the main features of the observed SSS seasonal cycle, with freshest water in the northeastern Bay, particularly during and after the monsoon. The model also displays an intense and shallow freshening signal in a narrow (∼100 km wide) strip that hugs the east coast of India, from September to January, in good agreement with high‐resolution measurements along two ships of opportunity lines. The mixed layer salt budget confirms that the strong freshening in the northern Bay during the monsoon results from the Ganges‐Brahmaputra river discharge and from precipitation over the ocean. From September onward, the East India Coastal Current transports this freshwater southward along the east coast of India, reaching the southern tip of India in November. The surface freshening results in an enhanced vertical salinity gradient that increases salinity of the surface layer by vertical processes. Our results reveal that the erosion of the freshwater tongue along the east coast of India is not driven by northward horizontal advection, but by vertical processes that eventually overcome the freshening by southward advection and restore SSS to its premonsoon values. The salinity‐stratified barrier layer hence only acts as a “barrier” for vertical heat fluxes, but is associated with intense vertical salt fluxes in the Bay of Bengal.

Non-invasive turbulent mixing across a density interface in a turbulent Taylor–Couette flow

In this paper, we present new experimental measurements of the turbulent transport of salt across an interface between two layers of fluid of equal depth but different salinities. The fluid is confined to a cylindrical annulus with a vertical axis. The outer cylinder is stationary and the inner cylinder rotates to produce a turbulent flow field consisting of an approximately irrotational mean azimuthal flow, with narrow boundary layers on the inner and outer cylinders. We focus on the limit of high-Richardson-number flow, defined as Ri = gΔρH/(ρ0u2rms), where ρ0 is a reference density, Δρ is the time-dependent difference of the layers' mean densities, urms is the root mean square of the turbulent velocity fluctuations and H is the layer depth. The mean flow has Reynolds number of the order of 104−105, and the turbulent fluctuations in the azimuthal and radial directions have root-mean-square speed of order 10% of the mean azimuthal flow. Measurements based on our experimental system show that when the Richardson number is in the range 7 < Ri < 200, the interface between the two layers remains sharp, each layer remains well mixed, and the vertical flux of salt between the layers, Fs ~(1.15 ± 0.15)Ri−1𝒜(H/ΔR)urmsΔS, where ΔS is the spatially-averaged time-dependent salinity difference between the layers and in general 𝒜(H/ΔR) is a dimensionless function of the tank aspect ratio, here taken to be unity, with ΔR being the gap width of the annulus. The salt transport appears to be caused by turbulent eddies scouring and sharpening the interface and implies a constant rate of conversion of the turbulent kinetic energy to potential energy, independent of the density contrast between the layers. For smaller values of Ri, the flow regime changes qualitatively, with eddies penetrating the interface, causing fluid in the two layers to co-mingle and rapidly homogenize.

A HYCOM modeling study of the Persian Gulf: 1. Model configurations and surface circulation

The circulation and water mass transformation processes in the Persian Gulf and the water exchange with the Indian Ocean through the Strait of Hormuz are studied using the Hybrid Coordinate Ocean Model (HYCOM). Model results show that the Indian Ocean Surface Water (IOSW) propagates in two branches into the gulf, one along the Iranian coast toward the northern gulf and the other one onto the southern banks driven by the Ekman drift due to the prevailing northwesterly winds. These two branches of inflow form two cyclonic gyres in the northern and in the southern gulf. A salinity front separates the fresher intruding IOSW from the saltier waters in the gulf. Eddies with size of about 100 km are fully developed along the salinity front in summer. The intrusion of the IOSW in the model extends much farther into the gulf in summer than in winter. By analyzing the salt balance in the basin and conducting sensitivity experiments, we show that it is the balance between the advection of IOSW and vertical salt flux induced by mixing that mainly controls the seasonal variation of the surface salinity. Surface wind stress plays a secondary role in modulating the seasonal intrusion of the IOSW. High‐frequency atmospheric forcing produces more realistic surface temperatures than obtained from climatological forcing, as a result of increased heat loss in winter. However, the high‐frequency forcing does not change significantly the general features of the circulation.

Evolution of salt finger convection with steady wind shear

In oceanographics situations where salt fingers may be an important mechanism for the transport of heat and salt in the vertical direction, velocity shears may also be present. Salt finger convection is analogous to Bénard convection in that the kinetic energy of the motions is obtained from the potential energy stored in the unstable distribution of a stratifying component. On the basis of the thermal analogy it is of interest to discover whether salt fingers are converted into two-dimensional sheets by wind shear, and how the vertical fluxes of heat and salt are changed by wind shear. Salt finger convection under the effect of steady wind shear is theoretically examined in this paper. The evolution of instability developing in the presence of a vertical density gradient disturbance and the horizontal Couette flow is considered near the onset of salt fingers under a moderate rate of shear. We use velocity as the basic variable and solve the pressure Poisson equation in terms of the associated Green function. Growth competition between the longitudinal rolls (LR) and the transverse rolls (TR), whose axes are, respectively, in the direction parallel to and perpendicular to the Couette flow, is investigated by the weakly nonlinear analysis of coupled-mode equations. The results show that the TR mode is stable under a small wind shear and the LR mode is stable for a higher wind shear.

Ebb-Tide Dynamics and Spreading of a Large River Plume*

Abstract Momentum balances in the near-field region of a large, tidally pulsed river plume are examined. The authors concentrate on a single ebb tide of the Columbia River plume, using the Regional Ocean Modeling System (ROMS) configured to hindcast flow conditions on the Washington and Oregon shelves and in the Columbia River estuary. During ebb, plume-interior streamwise balances are largely between advection, pressure gradient, and frictional forces. Stream-normal balances in this region reduce to centrifugal, cross-stream pressure gradient, and Coriolis terms (i.e., the “gradient wind” balance commonly assumed in river plume bulge investigations). Temporal derivatives are most important at the plume front and as the ebb progresses. Winds were light and contributed little to the force balance. Midebb stress and vertical salt flux were largest at a midplume depth, where stratification and vertical shear were also high, consistent with shear-induced mixing. Internal stress slows the spreading plume considerably. A kinematic description of the spreading process relates lateral spreading to the momentum dynamics and illustrates that plume spreading is largely a competition between the cross-stream pressure gradient and Coriolis forces. However, the very near-field dome of buoyant water is instrumental in setting initial flow pathways.