Vertical exchange in the ocean

We analyze the expressions for vertical turbulent flows and the coefficients of turbulent exchange in the ocean and their dependences on the stratification of density. It is shown that, in the case of high density gradients, turbulent mixing is localized in separate bounded relatively small zones (turbulized spots or layers). Formulas for typical vertical sizes of turbulent spots and local coefficients of vertical turbulent exchange depending on the Väisälä-Brunt frequency are presented. We consider a model of vertical turbulent exchange for the case of spot-like intermittent turbulence (a similar model was independently developed at the Marine Hydrophysical Institute of the Ukrainian Academy of Sciences with certain differences in the statement the problem and methodology). We emphasize the priority of A. G. Kolesnikov in the investigations of vertical turbulent exchange in the ocean by the direct method, in the development and construction of quick-response measuring devices with detectors of pulsations of the components of velocity and temperature, and in the first determination of the characteristics of turbulence in lakes, seas, and oceans performed by using this equipment.     

夏季长江口及其邻近海域湍流特征分析

利用2019年7月在长江口科学考察实验研究夏季航段(NORC2019-03-02)中获得的MSS90L湍流剖面仪的直接观测数据,本文计算并分析了该断面的湍动能耗散率ε和垂向湍扩散系数K_Z的分布情况。湍动能耗散率的大小为1.72×10?10～2.95×10?5 W/kg;垂向湍扩散系数的大小为3.24×10?7～4.55×10?2 m2/s。湍动能耗散率和垂向湍扩散系数的分布相似,均为上层最强,底层次之,中层最弱。上层由于风应力的作用,使得湍动能耗散率和垂向湍扩散系数较大;温跃层处层化较强,抑制了湍动能的耗散和垂向上的湍混合。盐度锋面的次级环流会促使低盐水团脱离,锋面引起的垂向环流会加强海洋的湍混合。低盐水团与外界的能量交换较少,湍动能耗散率较弱。长江口海区存在明显的上升流和下降流,它们是由锋面的次级环流产生的;上升流和下降流的存在促进湍动能的耗散与湍混合。     

Experimental estimation of the coefficient of vertical turbulent exchange in a stratified layer of the black sea near the continental slope

On the basis of the data of field measurements, we present the results of numerical analysis of the intensity of vertical turbulent exchange in stratified layers of the Black Sea in the region of the shelf–continental-slope boundary depending on the local stratification. The experiments were carried out within the framework of the GEF/BSERP and Black Sea-2004 international projects. The data were obtained by using a probing version of the Sigma-1 measuring complex. In processing the data of measurements, we apply a procedure of evaluation of the coefficient of vertical turbulent diffusion depending on the external conditions based on the analysis of the spectra of the gradient of temperature fluctuations. For the two studied regions of the shelf, the coefficients of turbulent exchange turn out to be much higher (by about an order of magnitude) than for the open sea under similar conditions. This can be explained by the specific features of the bottom topography affecting the dynamics of quasiinertial waves playing to role of the main causes of small-scale mixing and vertical diffusion. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 14–24, November–December, 2008.     

Numerical analysis of the mesoscale features of circulation in the Black Sea coastal zone

The results of simulating the hydrophysical fields of the Black Sea with a resolution of 1.64 × 1.64 km for January–September 2006 with the use of real atmospheric forcing are analyzed. Both vertical turbulent momentum exchange and vertical turbulent heat and salt diffusions are parameterized using the Mellor-Yamada level 2.5 scheme. The results of this numerical experiment are compared with similar data obtained with a horizontal resolution of 5 km. The features of the meso- and submesoscale dynamics of waters in individual sea regions are given. Possible physical mechanisms of forming meso- and submesoscale vortices are studied on the basis of energy analysis. It is shown that, in the absence of significant wind forcing, the main contribution to kinetic energy is made by the buoyancy force and wind-field inhomogeneities result in significant variations in both total vertical viscosity and total vertical diffusion.     

Staircase and inversion structures of thermocline in the north-west Tropical Atlantic

The peculiarities of the vertical fine thermohaline structure of waters in the north-west Tropical Atlantic are considered on the data of STD surveys recorded in winter-spring 1984. The variability of the characteristics of staircase and inversion elements of stratification with depth is analysed over the horizontal as well as related to the mesoscale and large-scale dynamics of waters. The coefficients of horizontal turbulent exchange are estimated within the framework of Joyce's hypothesis on quasi-compensation of vertical and turbulent horizontal transport. The effects of double diffusion are considered to dominate in vertical transport.UDK 551.465.15Translated by Mikhail M. Trufanov.     

Influence of the dynamics of currents on the hydrophysical structure of the waters and the vertical exchange in the active layer of the Black Sea

An analysis of the data of measurements of the fine structure and microstructure fluctuations of hydrophysical fields in the upper 200-m layer of the Black Sea carried out using CTD profilers and a Baklan free falling microstructure and turbulence profiler revealed the existence of a positive correlation between the intensity of the fine structure and microstructure fluctuations and the dynamics of the currents. On the other hand, the level of the fine structure and microstructure fluctuations reflects the rate of the vertical turbulent exchange. It was shown that, in the case of the absence of the Black Sea Rim Current (BSRC) jet or clearly manifested mesoscale eddy structures, the vertical turbulent exchange in the pycnocline is weak, while, in the opposite case, it is stronger. The results obtained support the supposition that the interbasin dynamics play an important role in the maintenance of the rate of small-scale mixing in the pycnocline and halocline and provide the vertical transport of dissolved oxygen from the cold intermediate layer into the deeper layers of the sea.     

Transport of Sediments Entrapped by Topographic Waves

In the Boussinesq approximation, for topographic waves entrapped by a sloping bottom, we determine mean currents induced by a wave due to nonlinearity with regard for turbulent viscosity and diffusion. We determine the thickness of the bottom boundary layer, the vertical turbulent exchange coefficients, and turbulent stresses on the upper boundary of the boundary layer depending on the parameters of the wave. In the diffusion approximation, we find the vertical distribution of the concentration of sediments suspended by the wave and the flow rates of sediments along and perpendicular to the isobaths. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 13–24, September–October, 2005.     

Vertical turbulent exchange in the Black Sea pycnocline and its relation to water dynamics

Estimates of vertical turbulent diffusion coefficient (K _t ) in the Black Sea pycnohalocline have been obtained from data of simultaneous observations of seawater temperature, salinity, density, and horizontal current velocity, obtained in the northeastern part of the Black Sea during 2013–2014 with a moored Aqualog profiler. A Munk and Andersson (1948) type parameterization, adapted for the Black Sea environment, is proposed for calculating K _t . Strong short-period (several days) variability of turbulent exchange is revealed, induced by vertical shear variations of the current velocity.     

Transport properties of entrained topographic waves during their interaction with turbulence

In the Boussinesq approximation, topographic waves entrained by a sloping bottom are considered with allowance for turbulent viscosity and diffusion. The mean flows induced by a wave through nonlinearity are determined. The turbulent exchange coefficients are expressed in terms of the density of turbulent energy by using the relations of the semiempirical theory of turbulence. The equation for boundary-layer wave solutions and the equation of turbulent energy balance are solved jointly, which makes it possible to determine the vertical distribution of the density of turbulent energy in the area of the given wave. In the diffusion approximation, the vertical distribution of the concentration of wave-suspended sediments is obtained for the case when the tangential bottom stress exceeds the critical values corresponding to the start of sediment motion.     

Turbulence in the ocean: Development of investigations, evolution of ideas, and prospects

We present an analytic survey of a series of works devoted to the direct methods for the investigation of turbulence in natural basins originated under the guidance of A. G. Kolesnikov in the 50s. We describe the principal results of the early stage which formed the basis for future investigations. The main scientific directions of the latest period are also considered from the viewpoint of the development of Kolesnikov's ideas: investigation of the processes of generation and energy supply of small-scale turbulence, relationship between turbulence and the fine vertical structure and internal waves, and the mechanisms of vertical turbulent exchange in the ocean. A model of vertical exchange under the conditions of intermittent “source” turbulence is distinguished. We also discuss the directions of development and prospects of the investigation of turbulence. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Alteration of estuarine circulation pattern due to channel modification in the North Passage of the Changjiang River Estuary

The exchange flow structure was examined in the North Passage of Changjiang River Estuary, where a deep waterway project (DWP) was carried out to improve the navigability. Before the construction of the DWP, the friction effect played a significant role in shaping the transverse structure of the exchange flow. The turbulent eddy viscosity generated near the seabed can be transferred to the upper water column, which facilitated vertical momentum exchange. As a result, the landward inflow extended to –2 m below the water surface and the seaward outflow was concentrated on the shallow shoal on the southern side of the cross section. After the construction of the DWP, the turbulent mixing was suppressed as a result of density stratification. The friction felt by the water was constrained in the lower half of the water column and the vertical momentum exchange was reduced. Meanwhile, the channel became dynamically narrowed with a Kelvin number of 0.52. Therefore, the Coriolis played a minor role in shaping the transverse structure of the exchange flow. As a consequence, the exchange flow featured a vertically-sheared pattern, with outflow at the surface and inflow underneath. Additionally, the gravitational circulation was enhanced due to increase in along-channel density gradient and stratification. The exchange flow components associated with the lateral processes (residual currents induced by eddy viscosity-shear covariance and lateral advective acceleration) were reduced, which suggests that lateral processes played a minor role in modifying the along-channel dynamics when the estuary becomes dynamically-narrowed.     

About the effect of chemical-biological processes on the diurnal variance of hydrochemical parameters (numerical model)

Data on the diurnal variability of phosphate and oxygen concentrations are analysed using a numerical model, which considers chemical-biological processes and vertical turbulent exchange. We consider how the variation of the photosynthetic rate due to the diurnal course of illumination may affect the variability of these elements. Some modelling experiments were conducted to show the feasibility of assessing primary production from data on the vertical distribution of biogenic elements.Translated by V. Puchkin.     

The influence of the spatial heterogeneity of vegetation cover and surface topography on vertical CO<Subscript>2</Subscript> fluxes within the atmospheric surface layer

The influence of the spatial heterogeneity of vegetation cover and topography on CO₂ fluxes in the atmospheric surface layer is estimated using a two-dimensional (2D) hydrodynamic model of turbulent exchange. A ~4.5-km-long profile that crossed a hilly area with a mosaic vegetation cover in Tula region was selected for model experiments. During the first experiment, a wind field and vertical fluxes were calculated by the 2D model for the entire selected profile with respect to the horizontal heterogeneity of the vegetation cover and surface topography. In the second experiment, the profile was considered an assemblage of elementary independent homogeneous segments; for each of them, vertical fluxes were calculated by the 2D model with the assumption of ‘zero’ horizontal advection, i.e., the required functions are independent of the horizontal coordinates. The influences of any boundary effects that appear at the interface between the different vegetation communities and at topographical irregularities on the turbulent regime are ignored in this case. For the profile selected, ignoring the horizontal advection, disturbances in the wind field that appeared at surface topography irregularities, and boundaries between different vegetation communities can lead to a 26% underestimation of the total CО₂ absorption by the ground surface on a clear sunny day under summer weather conditions.     

Experimental determination of small-scale turbulence coefficients in the near-surface layer of the sea

The results of numerous experiments on the diffusion of fluorescent dyes are presented. To determine the turbulent diffusion coefficients, the technique involving under water photography of a jet with the subsequent processing of negatives by a microphotometer was used. The vertical coefficients were found to be slightly larger (by about 20%) than the horizontal ones, with the scale of the phenomenon being 0.3–1 m. Empiric relationships have been derived for the turbulent diffusion vertical coefficient on the parameters of surface waves.Translated by V. Puchkin.     

Estimates of the characteristics of the horizontal turbulent exchange in the northwestern part of the Pacific Ocean

Estimates of the characteristics of the horizontal turbulent exchange (coefficients of horizontal exchange, scales of deviations of the velocity of geostrophic currents from the background flows, and horizontal scales of perturbations) in the upper ocean layer were obtained on the basis of the data of expeditions held in 1978 and 1980 in the northwestern part of the Pacific Ocean. It is shown that the characteristics of the horizontal turbulent exchange strongly depend on the structure of the background flows, and one of the causes of their variability is related to tropical cyclones. A tendency to a decrease (to different degrees) of the turbulent coefficients in the wake of a tropical cyclone is noted.     

夏季黄东海硝酸盐垂向扩散通量的分析

湍流扩散过程导致的硝酸盐垂向输运对海水表层的浮游植物生长和初级生产力的大小有着重要影响。本文基于2018年夏季黄、东海水文环境、硝酸盐浓度和湍动能耗散率的同步、原位数据，分析了海域温度、盐度和硝酸盐的空间分布特征，结果表明营养盐含量丰富的黄海冷水团、长江冲淡水、东海北部底层混合水与黑潮次表层水是影响研究海域硝酸盐分布的主要水团。利用垂向湍扩散硝酸盐通量公式，计算了三个选定断面上的硝酸盐垂向扩散通量，其高值区与湍流扩散系数的高值区的位置基本一致。针对存在明显硝酸盐跃层的站位，计算得到跨硝酸盐跃层的垂向通量F_ND的范围在-9.78—36.60mmol/（m²·d）之间，在黄海冷水团区，夏季温跃层限制了该区营养盐向近表层的湍流垂向扩散；东海北部底层混合水区，湍流垂向扩散向上层补充了大量硝酸盐，促进了跃层之上浮游植物的生长；黑潮次表层水影响海区，夏季中层水体混合较弱，跨跃层的垂向通量也普遍偏低。开展硝酸盐垂向扩散通量的计算与分析，对进一步明确营养盐的输运机制有着重要研究意义。     

Observed characteristics of flow,water mass,and turbulent mixing in the Preparis Channel

Preparis Channel is the very important exchange path of energy and materials between the northern Bay of Bengal and Andaman Sea(AS). A set of hydrographic measurements, a microstructure profiler, and a deep mooring were used to determine the characteristics of water masses, turbulent mixing, and flows in the Preparis Channel. The unprecedented short-term mooring data reveal that a deep current in the deep narrow passage(below 400 m) of the Preparis Channel flows toward the Bay of Bengal(BoB) wit...     

2014年大西洋水在楚科奇边陲区域的上边界混合

This study presents an analysis of the CTD data and the turbulent microstructure data collected in 2014, the turbulent mixing environment above the Atlantic Water(AW) around the Chukchi Borderland region is studied.Surface wind becomes more efficient in driving the upper ocean movement along with the rapid decline of sea ice,thus results in a more restless interior of the Arctic Ocean. The turbulent dissipation rate is in the range of4.60×10~(–10)~(–3.31×10~(–9) W/kg with a mean value of 1.33×10~(–9) W/kg, while the diapycnal diffusivity is in the range of1.45×10~(–6)–1.46×10~(–5)m~2/s with a mean value of 4.84×10~(–6) m~2/s in 200–300 m(above the AW). After investigating on the traditional factors(i.e., wind, topography and tides) that may contribute to the turbulent dissipation rate, the results show that the tidal kinetic energy plays a dominating role in the vertical mixing above the AW. Besides, the swing of the Beaufort Gyre(BG) has an impact on the vertical shear of the geostrophic current and may contribute to the regional difference of turbulent mixing. The parameterized method for the double-diffusive convection flux above the AW is validated by the direct turbulent microstructure results.     

Turbulent mixing in a stratified estuarine tidal channel: Hikapu Reach,Pelorus Sound,New Zealand

Channel constrictions within an estuary can influence overall estuary-sea exchange of salt or suspended/dissolved material. The exchange is modulated by turbulent mixing through its effect on density stratification. Here we quantify turbulent mixing in Hikapu Reach, an estuarine channel in the Marlborough Sounds, New Zealand. The focus is on a period of relatively low freshwater input but where density stratification still persists throughout the tidal cycle, although the strength of stratification and its vertical structure vary substantially. The density stratification increases through the ebb tide, and decreases through the flood tide. During the spring tides observed here, ebb tidal flow speeds reached 0.7?m?s^?1 and the buoyancy frequency squared was in the range 10^?5 to 10^?3?s^?2. Turbulence parameters were estimated using both shear microstructure and velocimeter-derived inertial dissipation which compared favourably. The rate of dissipation of turbulent kinetic energy reached 1?×?10^?6?m²?s^?3 late in the ebb tide, and estimates of the gradient Richardson number (the ratio of stability to shear) fell as low as 0.1 (i.e. unstable) although the results show that bottom-boundary driven turbulence can dominate for periods. The implication, based on scaling, is that the mixing within the channel does not homogenise the water column within a tidal cycle. Scaling, developed to characterise the tidal advection relative to the channel length, shows how riverine-driven buoyancy fluxes can pass through the tidal channel section and the stratification can remain partially intact.