Tangential Wind Stress Research Articles

Analysis of Seasonal Energy Characteristics of the Marmara Sea Upper Layer Dynamics

Purpose. The aim of the work is to assess the role of the basic forces in formation of the dynamic structures of the upper layer in the Marmara Sea in different hydrological seasons. Methods and Results. The numerical model developed in Marine Hydrophysical Institute, Russian Academy of Sciences, was used to calculate the hydrodynamic and energy characteristics of the sea circulation. The horizontal spatial resolution was 1.22 × 0.83 km, 18 horizons were used vertically, and the time step was 0.5 min. The characteristics of the waters inflowing through the Bosporus and Dardanelles straits corresponded to the available observational data. On the sea surface, the daily average fields of tangential wind stress, heat fluxes, precipitation and evaporation for 2008 were preset; these parameters were calculated using the regional atmospheric model MM5. Spatial distributions of the vortex structures and the corresponding energy flows in the upper sea layer for different seasons were considered. Having been analyzed, the average seasonal spatial distributions of the current velocity fields and the components of the kinetic energy budget showed that during the year, the vortices in the upper sea layer were formed mainly by two mechanisms. In the central part of the sea, generation of a large-scale anticyclone and its seasonal variability are basically conditioned by the wind forcing, whereas formation and evolution of coastal cyclonic eddies are caused by the buoyancy force. In the fields where the buoyancy, pressure and friction forcing takes place, the zones of local extremes are distinguished. Two of them are the areas of water inflow through the Bosphorus and Dardanelles straits. The strongest variability is observed in the Bosporus region that is certainly conditioned by the fact that the inflowing Black Sea waters have a decisive influence on seasonal nature of the Marmara Sea circulation. The extreme values in the other zones are the result of the coastline structure heterogeneities, that leads to formation of the coastal eddies, the energy source for which is the available potential energy. Conclusions. Analysis of the results of the performed numerical experiment makes it possible to conclude that in the upper layer of the Marmara Sea, formation and variability of the central anticyclone are conditioned by the wind forcing, while those of the coastal cyclones – by the buoyancy effect.

Analiz sezonnykh energeticheskikh kharakteristik dinamiki verkhnego sloya vod Mramornogo morya

Purpose. The aim of the work is to assess the role of the basic forces in formation of the dynamic structures of the upper layer in the Marmara Sea in different hydrological seasons. Methods and Results. The numerical model developed in Marine Hydrophysical Institute, Russian Academy of Sciences, was used to calculate the hydrodynamic and energy characteristics of the sea circulation. The horizontal spatial resolution was 1.22 × 0.83 km, 18 horizons were used vertically, and the time step was 0.5 min. The characteristics of the waters inflowing through the Bosporus and Dardanelles straits corresponded to the available observational data. On the sea surface, the daily average fields of tangential wind stress, heat fluxes, precipitation and evaporation for 2008 were preset; these parameters were calculated using the regional atmospheric model MM5. Spatial distributions of the vortex structures and the corresponding energy flows in the upper sea layer for different seasons were considered. Having been analyzed, the average seasonal spatial distributions of the current velocity fields and the components of the kinetic energy budget showed that during the year, the vortices in the upper sea layer were formed mainly by two mechanisms. In the central part of the sea, generation of a large-scale anticyclone and its seasonal variability are basically conditioned by the wind forcing, whereas formation and evolution of coastal cyclonic eddies are caused by the buoyancy force. In the fields where the buoyancy, pressure and friction forcing takes place, the zones of local extremes are distinguished. Two of them are the areas of water inflow through the Bosphorus and Dardanelles straits. The strongest variability is observed in the Bosporus region that is certainly conditioned by the fact that the inflowing Black Sea waters have a decisive influence on seasonal nature of the Marmara Sea circulation. The extreme values in the other zones are the result of the coastline structure heterogeneities, that leads to formation of the coastal eddies, the energy source for which is the available potential energy. Conclusions. Analysis of the results of the performed numerical experiment makes it possible to conclude that in the upper layer of the Marmara Sea, formation and variability of the central anticyclone are conditioned by the wind forcing, while those of the coastal cyclones – by the buoyancy effect

The impact of surface boundary conditions on the formation of the Black Sea haline stratification from the numerical simulations

Following a series of studies of the formation of the Black Sea haline stratification by various initial and boundary conditions, three numerical experiments were performed. For this purpose, the previously performed experiment with a spatially uniform salinity of 12 %º was chosen (experiment 1). New simulations were carried out with heat fluxes on the surface (experiment 2) obtained in the department of VAO FSBSI FRC MHI. In addition to heat fluxes, a constant cyclonic tangential wind stress curl (experiment 3) and a tangential wind stress curl with a seasonal variation (experiment 4) were also set. The results of numerical simulations show that in all experiments where heat fluxes were set on the surface, the upper mixed layer appears. The depth of the halocline does not change and is at about 20 m. Also in all experiments the relation between the southward and northward currents are close. Water exchange through the southward current is almost two times greater than that through the northward.

On the changes in the sea surface temperature in the benguela upwelling region. Part 2: the long-term tendencies

The paper examines the issue on the long-term trends in the sea surface temperature (SST) in the Benguela upwelling zone and their causes using the daily SST satellite data for 1985–2017’s and the daily near-surface wind for 1988–2017”s. It is shown that in the Benguela upwelling region, there is a significant intensification of driving winds in the last 20 yrs. This is accompanied by a decrease of the thermal upwelling index (taking into account the sign of the index or an increase of its absolute values) in the southern part of the Benguela upwelling, but practically does not influence this indicator in its northern part. The likely reason for this difference is the change in the wind field structure, as a result of which there are opposite trends in the magnitude of the vorticity of the tangential wind stress in different parts of the Benguela upwelling. In the southern part of the Benguela upwelling, both the Ekman’s upwelling and the vertical velocities due to the vorticity of the driving wind intensify, while in the northern part the corresponding trends have the opposite signs. This leads to a partial compensation of these two effects in the northern part of the Benguela upwelling. The reason for the change in the wind field structure is the displacement of the center of the Subtropical High to the south-east and the concomitant reversal of the near-surface wind vector in the coastal zone.

Energeticheskie perekhody v dvukhsloynoy vikhrerazreshayushchey modeli Chernogo morya

Purpose. Performed is the energy analysis of the numerical experiment results obtained from modeling of the large-scale circulation in the Black Sea within the framework of a two-layer eddy-resolving model under the tangential wind stress forcing. The paper is aimed at determining directions and magnitudes of the energy transitions accompanying formation of the large-scale flows and mesoscale eddies in the sea. Methods and Results. The analysis is carried out for the period of statistical equilibrium in which the average values of all the characteristics calculated in the model remain constant in time. According to the motion scales, the Reynolds averaging method permits to divide the energy characteristics (mechanical energy and its transitions) into those relating to the large-scale flows and – to the eddies. The large-scale currents are defined as average flows over a certain selected time interval, and the deviations from them are considered to be the vortices. The energy characteristics averaged over time and/or space, are analyzed. For the period of statistical equilibrium, calculated are the energy diagrams showing contribution of the large-scale currents and the vortices to the total mechanical energy, to the magnitudes and directions of energy transitions. The time-averaged fields both of the energy components and the forces involved in the energy balance were constructed for the same period. Conclusions. It is shown that baroclinic instability of a large-scale flow is the main cause of the Rim Current meandering, and the energy is transferred to the bottom layer due to baroclinic instability of the eddies. It has been revealed that a large portion of wind energy falls on the eastern part of the sea, whereas the energy losses take place in the western and northwestern regions of the basin. The basic part of energy dissipation takes place due to the friction forces’ work on the lower boundary of the up-per layer in the area where the layers’ interfaces intersect the bottom.

Wave boundary layers in rotating stratified fluid and near-inertial oscillations

Theory of wave boundary layers (WBLs) developed by Reznik (J Mar Res 71: 253–288, 2013, J Fluid Mech 747: 605–634, 2014, J Fluid Mech 833: 512–537, 2017) is extended to a rotating stratified fluid. In this case, the WBLs arise in the field of near-inertial oscillations (NIOs) driven by a tangential wind stress of finite duration. Near-surface Ekman layer is specified in the most general form; tangential stresses are zero at the lower boundary of Ekman layer and viscosity is neglected below the boundary. After the wind ceases, the Ekman pumping at the boundary becomes a linear superposition of inertial oscillations with coefficients dependent on the horizontal coordinates. The solution under the Ekman layer is obtained in the form of expansions in the vertical wave modes. We separate from the solution a part representing NIO and demonstrate development of a WBL near the Ekman layer boundary. With increasing time t, the WBL width decays inversely proportional to $$ \sqrt{t} $$ and gradients of fields in the WBL grow proportionally to $$ \sqrt{t} $$ ; the most part of NIO is concentrated in the WBL. Structure of the WBL depends strongly on its horizontal scale L determined by scale of the wind stress. The shorter the NIO is, the thinner and sharper the WBL is; the short-wave NIO with L smaller than the baroclinic Rossby scale LR does not penetrate deep into the ocean. On the contrary, for L ≥ LR, the WBL has a smoother vertical structure; a significant long-wave NIO signal is able to reach the oceanic bottom. An asymptotic theory of the WBL in rotating stratified fluid is suggested.

Diagnosis and forecast of the Caspian Sea level with the operational hydrodynamic model

The observational data on sea level at Tyulenii Island station were compared with the results of the Caspian Sea level simulation with the three-dimensional hydrodynamic model with different spatial horizontal resolution (3 and 1 nautical miles). The sea motion is induced by the air pressure gradients and tangential wind stress that are obtained from the COSMO model forecast and specified on the surface. The results of diagnostic calculations of the sea level for June-October 2014 and 60 forecasts for September 2014 demonstrated that the model with the 1-mile resolution meets the simulation accuracy requirements of the Hydrometcenter of Russia and can be used for the short-range forecasting of the Caspain Sea level.

An approach to modeling wind currents in Kerch Strait

The Kerch Strait plays an important role in the for� mation of the oceanographic regime of the Black Sea and the Sea of Azov, which are connected by this strait. In the conditions of the modern climate changes and anthropogenic influence on the marine ecosystems, the strait is a natural transport link for these interac� tions. In addition, the characteristics of the ecosystem in Kerch Strait are subjected to significant variations, which influence the character of its economical use. Systematic investigation of currents in the Kerch Strait connecting the Sea of Azov and the Black Sea is principally important for understanding the physical nature of circulation variability and its influence on the regional climate. The water level regime in Kerch Strait is formed under the influence of two main causes: fluctuations of the sea level in the Black Sea and the Sea of Azov and direct wind forcing with account for the coastline form and bottom topography. Wind forcing has a double effect: on the one hand, it causes sea level variations at the ends of the strait and in the strait that results in the gradient currents and, on the other hand, it influences the water surface directly and causes the velocity vari� ations and sometimes variations in the direction of the currents due to the tangential wind stress. Wind plays the main role in the formation of currents in Kerch Strait. The wind flows over the entire basin and over the strait. The current field is also influenced by the difference in the sea level at the ends of the strait caused by the onshore and offshore wind currents and differences in the freshwater balance in the basins of the Black Sea and the Sea of Azov. DESCRIPTION OF THE MATHEMATICAL MODEL The coastline of Kerch Strait is very complex. It is indented by bays and spits, and the existence of Tuzla Island makes the calculation domain multiply con� nected. It is also important that the bottom topogra� phy of the strait is very uneven. The depths of the nav� igation part of the strait are 8–10 m and increase in its southern part to 19 m. At the same time, the depth of the coastal region in Tamanskii and Dinskii bays ranges between 0.5 and 4 m. Such a distribution of depths allows us to state that the bottom topography is

Generation of waves in a bounded basin by a traveling front of atmospheric pressure and the field of wind stresses induced by this front

A plane problem of generation of barotropic seiches in a bounded rotating basin by traveling atmospheric fronts is studied within the framework of the linear theory of long waves. The front is characterized by disturbances of the baric field and the corresponding field of tangential wind stresses. We deduce the modified Ackerblom formulas for the wind stresses according to the given anomalies of atmospheric pressure in which the uniform transport of disturbances of the baric field is taken into account. We perform the numerical analyses of the dependences of the amplitudes of oscillations of fluid in the basin on the parameters of the atmospheric front and the choice of the formulas for the tangential wind stresses. The influence of the wind stresses leads to significant quantitative and qualitative changes in the oscillations of fluid in the basin as compared with the case of pure baric action.

On the transport of small river run off by an alongshore baroclinic sea current

A numerical simulation has been undertaken to study the process of the transport of small river runoff by alongshore baroclinic sea currents. The study is based on the implementation of the Princeton Ocean Model (POM) under the conditions of a circular stratified basin whose surface is exposed to a transient tangential wind stress to form an alongshore baroclinic current. A baroclinic current of the downwelling type (in the Northern Hemisphere directed to the left to a sea-viewing observer) was shown to provide the carrying out of the river discharge from estuary’s vicinity more effectively than that of the upwelling type (in the Northern Hemisphere directed to the right to a sea-viewing observer).

Physical aspects of extreme storm surges and falls on the Polish coast

Extreme sea levels – storm-generated surges and falls – on the Polish coast are usually the effects of three components: the volume of water in the southern Baltic (the initial level preceding a given extreme situation), the action of tangential wind stresses in the area (wind directions: whether shore- or seaward; wind velocities; and wind action duration), and the sea surface deformation produced by deep, mesoscale baric lows moving rapidly over the southern and central Baltic that generate the so-called baric wave. Among these factors, the baric wave is particularly important for, i.e. the water cushion underneath the baric depression, moving along the actual atmospheric pressure system over the sea surface.

Computation of Expected Total Water Levels along the East Coast of India

Extreme sea levels generated by strong tangential wind stress and normal atmospheric pressure gradients at the sea surface due to tropical cyclones were studied for the western sector of the Bay of Bengal (the east coast of India). The maritime states along the coast, hit by different intensities of cyclones, are West Bengal, Orissa, Andhra Pradesh, and Tamil Nadu. Based on precise historical data for the period 1891–2007, statistical projections have been made on the probable intensities of tropical cyclones for various return periods. With the help of numerical models developed at Indian Institute of Technology Delhi, maximum probable surges along the coast were simulated. The local tide effect was considered in the computations as rise of sea level would be greater if the cyclone crosses the coast during high tide. The total water elevation is computed by linearly adding the local tide and wave setup to the surge simulated along the coast. In the present study, an assessment of cyclone risk and vulnerability was evolved, which is an important component of the information used to create sustainable local-level development action plans for preparedness and mitigation.

Coastal flooding due to synoptic scale, meso-scale and remote forcings

Coastal flooding occurs due to storm surges generated by tropical and extra-tropical cyclones on the globe. The meteorological forcing fields for the generation of storm surges are the tangential surface wind stress on the ocean surface and the normal atmospheric pressure gradients associated with the weather systems. The large scale forcing from the cyclones is referred to as the synoptic scale and storm surge prediction from synoptic scale forcing is well developed and is reasonably satisfactory around the world. However, coastal flooding also occurs from weather systems, with forcing on a meso-scale and also from remote forcing. It is proposed here that the term “Storm surge” be used to only refer to coastal flooding from synoptic scale forcing and the terminology “Rissaga” be used for coastal flooding from meso-scale forcing. For flooding due to remote forcing, a new term “Kallakkadal” is proposed.

Investigation of the characteristics of surge phenomena in the Sea of Azov

We present the results of numerical simulation of currents and sea level for the Sea of Azov. In calculations, we use a three-dimensional nonlinear mathematical model taking into account the tangential wind stresses. We present the results of numerical analysis of the fields of currents and the amplitudes of oscillations of the sea level at the coastal stations as functions of the maximum velocity and the period of constant action of the west wind.

Simulation of surge phenomena in a bounded sea basin

By using the method of mathematical simulation, we study surge oscillations caused by the action of tangential wind stresses in a bounded basin whose geometric parameters correspond to Lake Donuzlav. The dependences of the amplitudes of surges on the magnitude and direction of the wind velocity are analyzed at several points of the coast.

Modeling drift currents in a shallow basin with allowance for variations in tangential stresses induced by wind waves

A coupling model for calculating wind-driven currents and waves in a shallow basin with allowance for current-wave interactions is introduced. The model is constructed on the basis of the three-dimensional σ-coordinate model of currents [3] and the SWAN (Simulating Waves Nearshore) spectral wave model [4]. The effect of waves on currents is taken into account in the coefficients of surface and bottom friction through roughness parameters. Results of combined modeling of stationary fields of currents and waves generated by spatially homogeneous wind are correlated with the corresponding results of separate modeling for a cylindrical basin of constant depth and the water area of Lake Donuzlav (the northwestern coast of the Crimea). The allowance for the effect of waves during calculation of tangential wind stresses in the model of currents is shown to be among major factors intensifying water circulation and forming spatial inhomogeneities of the vortex type. In addition, some cases of local decreases in tangential wind stresses are revealed; they appear when the lake is penetrated from the side of the open sea by relatively long waves, which significantly decrease the roughness of the water surface.

Wind-induced motion of water in shallow-water closed basins

The motion of water induced by tangential wind stresses in a circular basin of finite depth is studied by the finite-element method. The dependences of vertical displacements of the surface of the basin and the field of horizontal wave currents on the topography of the bottom and the direction of the wind are analyzed. The transformations of wind-induced disturbances observed after termination of the wind action are investigated.

Intrayear Variability of Geostrophic Zonal Currents in the Tropical Zone of the South Atlantic

On the basis of the results of calculation and analysis of the characteristics of space variability of the geostrophic component of large-scale zonal currents in the tropical zone of the South Atlantic, its intrayear oscillations, and the components of tangential wind stresses, we establish the regularities of seasonal variability of the system of gradient currents on the ocean surface depending on wind conditions. We describe the specific features of seasonal meridional fluctuations of the midstreams of the geostrophic component of zonal large-scale currents and the intrayear variations of their velocity.

Generation of internal waves in the sea by local perturbations of wind and density flows on the surface

On the basis of a nonlinear model taking into account viscosity, density diffusion, and currents, we perform the numerical analysis of propagation of perturbations formed in the abyssal part of the Black Sea as a result of the action of tangential wind stresses or density flows in the form of a strip on the sea surface. It is shown that, in the course of time, in the region of the bottom slope, these perturbations generate a train of waves in the sea, which causes oscillations of the fields of density and currents. There are some differences between the generation of internal waves by the flows of density and the wind. Unlike the wind action, in the presence of flows of density in the upper layer in the region of the strip, the initial period of generation is characterized by the formation of intense perturbations without train structure.

Generation of baroclinic wave signatures by moving atmospheric fronts

The paper addresses the plane linear problem on generation of an internal wave in a continuously stratified ocean by a moving atmospheric front. The front exhibits air pressure perturbations, as well as the field of tangential wind stress. In the frame of a model for the planetary atmospheric boundary layer, a relationship between the air pressure and wind fields has been derived, generalizing Ackerbloom's formulae for the case of a moving atmospheric anomaly. Using the Fourier transform, a relation has been obtained for the wave's signature in the wake of a moving atmospheric perturbation, and the respective analysis has been performed. Numerical estimates of the internal wave amplitudes have been acquired for the mean density stratification in the Kuril-Kamchatka region. Comparative analysis of the effectiveness of wave signature generation by the moving areas of surface pressures and tangential wind stresses has been carried out. It has been demonstrated that the latter field determines the effectiveness of baroclinic wave signature generation.