Features of Currents on the Black Sea Northwestern Shelf Based on the Numerical Simulation Results

Purpose. The work is aimed at studying the features of currents on the Black Sea northwestern shelf based of the reanalysis results, and at analyzing the reasons of these features. Methods and Results. To analyze the currents on the northwestern shelf, applied were the results of physical reanalysis of the Black Sea fields performed by the authors earlier, namely, the arrays of hydrodynamic fields on a regular grid with the 21-year duration (1992–2012). Surface currents on the northwestern shelf of the Black Sea are directed mainly to the southwest. Throughout the whole year (except for the summer months when the wind effect weakens), an intensive compensatory current directed to the south is formed along the western coast. The waters near the western coast are highly horizontally stratified that is caused by fresh water inflowing with the river runoffs. In winter seasons, the stratification is most pronounced, whereas in summer, the horizontal density gradient decreases. The horizontal density stratification leads to the following: starting from the depth ~ 20 m, the pressure gradient changes its sign and the along-coastal jet countercurrent directed to the north, occurs. Conclusions. The performed studies have shown that the water circulation on the Black Sea northwestern shelf is determined mainly by the following factors: the wind-induced water flows across the shelf boundary and strong horizontal water stratification near the western coast resulted from the river runoffs. As the currents on the sea surface are directed mainly to the southwest, the compensatory current directed to the south is formed near the western coast. Due to the strong horizontal stratification resulted from the river runoffs, a countercurrent directed to the north is formed in the subsurface layer near the western coast. In case the seawater flows to the shelf are extremely high, the countercurrent may be absent.

Features of Currents on the Black Sea Northwestern Shelf Based on the Numerical Simulation Results

Purpose. The work is aimed at studying the features of currents on the Black Sea northwestern shelf based of the reanalysis results, and at analyzing the reasons of these features. Methods and Results. To analyze the currents on the northwestern shelf, applied were the results of physical reanalysis of the Black Sea fields performed by the authors earlier, namely, the arrays of hydrodynamic fields on a regular grid with the 21-year duration (1992–2012). Surface currents on the northwestern shelf of the Black Sea are directed mainly to the southwest. Throughout the whole year (except for the summer months when the wind effect weakens), an intensive compensatory current directed to the south is formed along the western coast. The waters near the western coast are highly horizontally stratified that is caused by fresh water inflowing with the river runoffs. In winter seasons, the stratification is most pronounced, whereas in summer, the horizontal density gradient decreases. The horizontal density stratification leads to the following: starting from the depth ~ 20 m, the pressure gradient changes its sign and the along-coastal jet countercurrent directed to the north, occurs. Conclusions. The performed studies have shown that the water circulation on the Black Sea northwestern shelf is determined mainly by the following factors: the wind-induced water flows across the shelf boundary and strong horizontal water stratification near the western coast resulted from the river runoffs. As the currents on the sea surface are directed mainly to the southwest, the compensatory current directed to the south is formed near the western coast. Due to the strong horizontal stratification resulted from the river runoffs, a countercurrent directed to the north is formed in the subsurface layer near the western coast. In case the seawater flows to the shelf are extremely high, the countercurrent may be absent.

Intensification of Loop Current Frontal Eddies and their Interactions with the Loop Current and Surrounding Flow

Loop Current Frontal Eddies (LCFEs) are cold-core vortices located in the vicinity of the Loop Current (LC) and are known to intensify and play an essential role in the LC shedding. The amplification of the LCFEs also affects the local circulation. During the 2010 Deepwater Horizon oil spill, part of the oil was entrained around and inside an intensified LCFE. The goal of this research is to characterize the LCFE intensification and understand its effects on the LC and surrounding flow. Firstly, the LC-LCFE interaction was investigated using altimetry and a mooring array. The intensification of the observed LCFEs shows similar characteristics over time, independent of their location: a steep increase in kinetic energy, a corresponding decrease in SSH, and an increase in size. LCFE intensification is dependent on the distance from the LC front. As the LCFE grows, the flow at the interface with the LC becomes stronger and deeper, and the horizontal density gradient between the features increases. Further intensification of the LC front and the LCFEs is suggested to be driven by the advection (nonlinear) term and the pressure-gradient (linear) term in the momentum budget. Secondly, the ageostrophy of the LC meanders during LCFE intensification is assessed using HYCOM velocity and geostrophic velocity from altimetry. The results indicate that during strong meandering, especially before and during LC shedding and in the presence of frontal eddies, the centrifugal force becomes as important as the Coriolis and the pressure-gradient forces, i.e., the LC meanders are in gradient-wind balance. Finally, the ability of LCFEs to transport particles without exchange with the exterior (i.e., material coherence) is investigated. The results show that the frontal eddies can remain coherent for up to 20 days at the surface and up to 25 days at deeper layers. Particles inside the frontal eddies were tracked backward in time and showed that the material coherence of the eddies builds up from Gulf water and can drive cross-shelf exchange of particles, water properties, and nutrients.

Water exchange and renewal times of a micro-tidal fjord in isohaline coordinates

<p>In micro-tidal coastal systems, the hydrodynamics in fjords reduce to a competition between horizontal density gradient, friction and wind stress. Depending on the depth of the entrance sill, the importance of these factors for water exchange varies within the vertical layer structure of fjords. This study investigates these renewals of water bodies in an isohaline framework, using the example of the Gullmar Fjord on the west coast of Sweden, a transitional area between the brackish Baltic Sea and the northeastern region of the North Sea.</p><p>To estimate the influence of wind and baroclinic pumping on volume and salinity transport and their importance on the exchange time scales, a well-validated, realistic, and highly resolved 3D coastal ocean model (GETM) is used, calibrated with especially designed observations. Simulations were combined with passive numerical tracers and evaluated with the mathematical analysis framework of the Total Exhange Flow (TEF).</p><p>The results highlight the advantage of isohaline coordinates in the study of water mass transformations within the fjord, compared to geographic coordinates, and the high sensitivity of the exchange flow to sub-grid turbulence.</p>

Analysis of one-dimensional models for exchange flows under strong stratification

One-dimensional models of exchange flows driven by horizontal density gradients are well known for performing poorly in situations with weak turbulent mixing. The main issue with these models is that the horizontal density gradient is usually imposed as a constant, leading to non-physically high stratification known as runaway stratification. Here, we propose two new parametrizations of the horizontal density gradient leading to one-dimensional models able to tackle strongly stratified exchange flows at high and low Schmidt number values. The models are extensively tested against results from laminar two-dimensional simulations and are shown to outperform the models using the classical constant parametrization for the horizontal density gradients. Four different flow regimes are found by exploring the parameter space defined by the gravitational Reynolds number Reg, the Schmidt number Sc, and the aspect ratio of the channel Γ. For small values of RegΓ, when diffusion dominates, all models perform well. However, as RegΓ increases, two clearly distinct regimes emerge depending on the Sc value, with an equally clear distinction of the performance of the one-dimensional models.

A hydrodynamic model for Galveston Bay and the shelf in the northwestern Gulf of Mexico

We present a 3D unstructured-grid hydrodynamic model for the northwestern Gulf of Mexico that utilizes a high-resolution grid for the main estuarine systems along the Texas-Louisiana coast. This model, based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) with hybrid horizontal and vertical grids, is driven by the observed river discharge, reanalysis atmospheric forcing, and open boundary conditions from the global models. The model reproduces well the temporal and spatial variation of observed water level, salinity, temperature, and current velocity both in Galveston Bay and on the shelf. We apply the validated model to examine the remote influence from large rivers, specifically the Mississippi and Atchafalaya rivers, on the salinity regime along the Texas coast. Numerical experiments reveal that the Mississippi-Atchafalaya discharge could significantly decrease the salinity on the inner shelf along the Texas coast and its influence highly depends on the wind field and the resulting shelf current. Winter wind tends to constrain the Mississippi-Atchafalaya water against the shore, forming a narrow lower-salinity band all the way to the southwestern Texas coast. Under summer wind, the influence of the discharge on salinity is limited to the upper Texas coast while extended offshore. The decrease in salinity at the mouth of Galveston Bay due to the Mississippi-Atchafalaya discharge leads to a decrease in horizontal density gradient, a weakened estuarine circulation inside the bay, a decrease in the salt flux, and a smaller estuarine-ocean exchange. We highlight the flexibility of the model that simulates not only estuarine dynamics and shelf-wide transport but also the interaction between them.

Submesoscale Coherent Structures on the Continental Shelf

Discovery and analysis of submesoscale variability O(0.3–30) km on the continental shelf is made possible by a high-resolution (Δx = 75 m) Regional Oceanic Modeling System (ROMS) simulation of the Southern California Bight (SCB). This variability is manifest in ubiquitous yet ephemeral coherent structures: fronts, filaments, and vortices. Similar to their open-ocean counterparts, fronts and filaments on the shelf are identified by their strong vertical velocity, surface convergence, cyclonic vorticity, and horizontal density gradient. Life cycles of these features typically last 3–5 days, with the formation dominated by a horizontal advective tendency that increases density and velocity gradients (i.e., frontogenesis). The shape of the coastline and depth of the water column both influence the abundance and spatial orientation of shallow-water fronts and filaments. Closer to shore, fronts and filaments often align themselves parallel to isobaths, and headlands often act as sites of intense vorticity generation through bottom stress. A quasi-steady, approximate momentum balance among rotation, pressure gradient, and vertical mixing—known as turbulent thermal wind (TTW)—often is valid in the strong secondary circulations local to fronts and filaments. However, front and filament circulations subject to strong diurnal variation in surface heating and vertical mixing are inconsistent with steady-state TTW balance. The secondary circulations can induce ephemeral material trapping and substantial vertical heat fluxes on the shelf.

Downfront Winds over Buoyant Coastal Plumes

Downfront, or downwelling favorable, winds are commonly found over buoyant coastal plumes. It is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport, spatial extent, and stability of the plumes. In the present study, the interaction of the Ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets. The approach makes use of an idealized numerical model and scaling theory. It is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near-zero average potential vorticity q. For weak to moderate winds, the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratification. However, for very strong winds the wind stress overwhelms the geostrophic stress and leads to a reduction in q, which is balanced by the vertical mixing term. In the absence of winds, the geostrophic stress dominates mixing and the flow rapidly restratifies. Nonlinearity, extremes of relative vorticity and vertical velocity, and mixing are all enhanced by the presence of a coast. Scaling estimates developed for the eddy buoyancy flux, the surface potential vorticity flux, and the diapycnal mixing rate compare well with results diagnosed from a series of numerical model calculations.

Decomposition of Residual Circulation in Estuaries

The residual currents in estuaries are produced by a variety of physical mechanisms. To understand the contribution of each individual mechanism to the creation of residual circulation, it is necessary to separate the effect of one particular mechanism from the others. In this study, a method based on dynamics is developed to decompose the residual circulation into individual components corresponding to different forcing mechanisms. Specifically, residual flows are partitioned based on the separate contributions by river discharge, horizontal density gradient, internal tidal asymmetry, advection, semi–Stokes transport, and wind. The method includes the effects of the earth’s rotation and can be applied for general conditions. Under the precondition that the ratio between width and length of the estuary is small, the continuity equation can be simplified such that the method only requires the data at a cross-estuary section to decompose residual currents. This makes the method practicable for real estuaries. Results from a generic numerical model are used to illustrate the decomposition method and to demonstrate its validity for weakly stratified estuaries.

Residual Sediment Fluxes in Weakly-to-Periodically Stratified Estuaries and Tidal Inlets

In this idealized numerical modeling study, the composition of residual sediment fluxes in energetic (e.g., weakly or periodically stratified) tidal estuaries is investigated by means of one-dimensional water column models, with some focus on the sediment availability. Scaling of the underlying dynamic equations shows dependence of the results on the Simpson number (relative strength of horizontal density gradient) and the Rouse number (relative settling velocity) as well as impacts of the Unsteadiness number (relative tidal frequency). Here, the parameter space given by the Simpson and Rouse numbers is mainly investigated. A simple analytical model based on the assumption of stationarity shows that for small Simpson and Rouse numbers sediment flux is down estuary and vice versa for large Simpson and Rouse numbers. A fully dynamic water column model coupled to a second-moment turbulence closure model allows to decompose the sediment flux profiles into contributions from the transport flux (product of subtidal velocity and sediment concentration profiles) and the fluctuation flux profiles (tidal covariance between current velocity and sediment concentration). Three different types of bottom sediment pools are distinguished to vary the sediment availability, by defining a time scale for complete sediment erosion. For short erosion times scales, the transport sediment flux may dominate, but for larger erosion time scales the fluctuation sediment flux largely dominates the tidal sediment flux. When quarter-diurnal components are added to the tidal forcing, up-estuary sediment fluxes are strongly increased for stronger and shorter flood tides and vice versa. The theoretical results are compared to field observations in a tidally energetic inlet.