On the cyclonic circulation mechanism in the Black Sea

This paper discusses the results of numerical experiments on the generation of a large-scale circulation in the sea carried out in the framework of a multilayer quasi-isopycnic model. The role played by wind vorticity, non-uniformity of the heat flux through the sea surface, and the fluxes of mass, salt, and heat across the liquid lateral boundaries is studied separately. It is shown that wind with cyclonic vorticity induces cyclonic vorticity in the upper layers of the sea and anticyclonic vorticity in the lower layer. A uniform wind having even a non-uniform heat flux does not give rise to a cyclonic circulation in the sea. Within the period calculated (410 years), the heat, mass, and salt fluxes through the lateral boundaries also do not result in a cyclonic circulation; this is apparently connected with the fact that the solution has not reached steady-state conditions in this period.Translated by Mikhail M. Trufanov.     

Three-dimensional properties of mesoscale cyclonic warm-core and anticyclonic cold-core eddies in the South China Sea

In general, a mesoscale cyclonic (anticyclonic) eddy has a colder (warmer) core, and it is considered as a cold (warm) eddy. However, recently research found that there are a number of “abnormal” mesoscale cyclonic (anticyclonic) eddies associated with warm (cold) cores in the South China Sea (SCS). These “abnormal” eddies pose a challenge to previous works on eddy detection, characteristic analysis, eddy-induced heat and salt transports, and even on mesoscale eddy dynamics. Based on a 9-year (2000–2008) numerical modelling data, the cyclonic warm-core eddies (CWEs) and anticyclonic cold-core eddies (ACEs) in the SCS are analyzed. This study found that the highest incidence area of the “abnormal” eddies is the northwest of Luzon Strait. In terms of the eddy snapshot counting method, 8 620 CWEs and 9 879 ACEs are detected, accounting for 14.6% and 15.8% of the total eddy number, respectively. The size of the “abnormal” eddies is usually smaller than that of the “normal” eddies, with the radius only around 50 km. In the generation time aspect, they usually appear within the 0.1–0.3 interval in the normalized eddy lifespan. The survival time of CWEs (ACEs) occupies 16.3% (17.1%) of the total eddy lifespan. Based on two case studies, the intrusion of Kuroshio warm water is considered as a key mechanism for the generation of these “abnormal” eddies near the northeastern SCS.     

Dynamics of the mesoscale cyclonic eddy in the POLYMODE area

The evolution of the mesoscale cyclonic eddy is analysed by means of a dynamic baroclinic quasi-geostrophic model. The initial field and boundary conditions defined in the course of observations made in the POLYMODE area during 24 July–23 August 1978 are used. The estimation model eddy field obtained is compared with the observation results.Translated by Mikhail M. Trufanov.     

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.     

Eddies in the southwestern East/Japan Sea

Closed loop mesoscale eddies were identified and tracked in the Ulleung Basin of the southwestern Japan/East Sea (JES) using the winding-angle (WA) methodology, for mapping the absolute geostrophic currents into surface streamlines of flow. The geostrophic velocity used here was the sum of the Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO), time variable velocity and the 1992–2007 mean geostrophic velocity. Local sampling bias was removed using the drifter observations. This WA methodology of deriving the Lagrangian path lines that drifters followed over a 7-day period was validated by individual drifter tracks and it demonstrated closed looping eddy motions. The WA method demonstrated that less than 6% of the closed streamlines appeared when drifters did not show a closed loop in their vicinity, compared to 30% of the excess detection rate by the Okubo–Weiss method of locating closed loop structures. Three groups of eddies were identified: (1) Coastal Cold and Warm Eddies, which appeared in the area between the coast of southern Korea and the East Korean Warm Current (EKWC), when a southward coastal current was present, (2) Frontal Cold and Warm Eddies, which were formed in the region of the seaward extension of the meandering EKWC, north of Ulleung Island and (3) Ulleung Warm Eddies (UWE) and Dok Cold Eddies (DCE), which appeared during meanders of the EKWC, in the Ulleung Basin. No seasonal concentration for eddy generation and eddy population was found. The average radius of eddies was about 38–60 km. These were born, moved in an erratic pattern and then died in the vicinity where the EKWC separated from the coast and formed a large meander. The time-mean large meander formed meridionally concentrated bands of positive and negative relative vorticity. The cyclonic (cold) eddies tend to reside within the band of positive time-mean relative vorticity, and the anticyclonic (warm) eddies reside within the bands of negative relative vorticity. Six UWE and four warm eddies, in the Yamato Basin (about 10% of warm eddies), were sustained longer than a year. Because the large meander of the EKWC appeared to be controlled by topography, and the JES is a nearly enclosed basin with rapid flow-out to the east through the narrow Tsugaru Strait, there was little eddy energy propagation to the west. The warm eddies in the southwestern part of the JES appeared to be interacting very locally with the mean flow.     

Seasonal fields of tangenital wind stress over the Black Sea

Tangential wind stress fields with a 40×60 spatial step are calculated from the data on atmospheric pressure distribution over the Black Sea over a decade. Their space-time variability is studied. It is shown that maximum values of the tangential wind stress in all seasons are located in the areas southwest of the Crimea and south of the Taman Peninsula. The tangenital wind stress retrieved from wind measurements is compared with the one determined from the atmospheric pressure data.Translated by V. Puchkin.     

On the large-scale and mesoscale variability of thermohaline characteristics in the north-eastern part of the Black Sea

The data collected during an 18-day station and nine hydrologic surveys have been analysed. Mesoscale and large-scale temperature and salinity oscillations were revealed. Mesoscale oscillations in the sea surface layer are induced by the diurnal course of solar radiation, and in the seasonal thermocline layer by internal waves with a predominant 6–10 h periodicity. Large-scale fluctuations are related to the passage of clockwise (cold) and anticlockwise (warm) meanders and vortices. It has been determined that the contribution of large-scale temperature and salinity oscillations to the total variability is 1·5 to 3-fold larger than that of the mesoscale ones.Translated by Vladimir A. Puchkin.     

Numerical simulation of a quasi-tropical cyclone over the Black Sea

The paper describes results of numerical experiments on the simulation of a mesoscale quasi-tropical cyclone, a rare event for the Black Sea, with the MM5 regional atmospheric circulation model. General characteristics of the cyclone and its evolution and physical formation mechanisms are discussed. The balances of the momentum components have been estimated, and sensitivity experiments have been performed. It is shown that, according to its main physical properties and energy supply mechanisms, the cyclone can be related to quasi-tropical cyclones.     

吕宋海峡两侧中尺度涡统计

利用1993-2000年间的T/P卫星高度计轨道资料的时间序列和MODAS同化产品中的卫星高度计最优插值资料对南海东北部海区中尺度涡旋进行动态追踪。按照给定的标准从2种资料中提取了涡旋信息并对其特征量进行统计分析。结果表明,南海东北部海区中尺度涡旋十分活跃,平均每年6个,其中暖涡4个,尺度一般为200~250 km,平均地转流速为44 cm/s;冷涡每年平均2个,尺度一般为150~200 km,平均地转流速为-37 cm/s。吕宋海峡两侧涡旋的比较分析表明,南海东北部海区仍属于西北太平洋副热带海区的涡旋带,冷、暖涡旋处于不断的形成—西移—消散过程中。南海东北部中尺度冷涡大多是南海内部产生的,而暖涡与吕宋海峡外侧暖涡有一定的联系又具有相对的独立性。分析认为西北太平洋的西行暖涡在到达吕宋海峡时,受到黑潮东翼东向下倾的等密度面的抑制和岛链的阻碍,涡旋停滞于吕宋海峡外侧并逐渐消弱,被阻挡于吕宋海峡东侧涡旋释放的能量,形成一支横穿吕宋海峡(同时横穿过黑潮)的高速急流,把能量传递给吕宋海峡西侧的涡旋,使其得到强化,这是吕宋海峡两侧涡旋联系的一种重要机制。     

Numerical modeling of quasi-two-dimensional vortices in the atmosphere over the Black Sea

We consider quasi-two-dimensional rapidly dissipating mesoscale atmospheric vortices generated over the Black Sea near the Crimean and Caucasian coasts. Based on the results of numerical modeling for a characteristic example of the Crimean eddy, we determine its structure and parameters and estimate the rate of decay of kinetic energy and enstrophy. In addition to the large-scale secondary circulation in the vortex, we also consider a small-scale secondary circulation induced by Raleigh-Bénard convection.     

Mesoscale Eddies in the Bering Sea from Satellite Altimetry Data

Izvestiya, Atmospheric and Oceanic Physics - The dynamics of mesoscale eddies in the Bering Sea is studied by the method for automated eddy identification on the basis of altimetry-derived...     

On numerical modelling of currents over the north-west shelf of the Black Sea

Water circulation in the north-west part of the Black Sea during the summer period is calculated by means of a complete non-linear thermohydrodynamic model. We have also studied the contribution of wind forcing, the thermohaline regime, the coastline configuration, and the bottom topography to the generation and transformation of a flow field. It has been shown that the thermohaline regime and wind forcing account for 30 and 70% of the total circulation, respectively. Variation in the nature of circulation at weak winds is due to thermohaline factors. Vertical motion plays a crucial role in theT, S- andH ₂ S-field formation.Translated by V. Puchkin.     

Selective depressions of surface silicic acid within cyclonic mesoscale eddies in the oligotrophic western North Pacific

To reveal spatial dynamics of silicic acid [Si(OH)₄] in the poorly sampled oligotrophic western North Pacific, we investigated the surface distribution of Si(OH)₄ and associated biogeochemical parameters by using an underway survey system with a highly sensitive nutrient analyzer along the 138°E transect (between 30 and 34°N) and the 155°E transect (between 10 and 35°N) during the summers of 2007 and 2008. Surface Si(OH)₄ concentrations ranged from the detection limit (11 nmol L⁻¹) to 2462 nmol L⁻¹. High Si(OH)₄ concentrations (>1000 nmol L⁻¹) and dynamic fluctuations were generally observed north of 23°N, while consistently stable low concentrations of 415–751 nmol L⁻¹ were observed south of 23°N. Surface nitrate+nitrite (N+N) and phosphate (PO₄³⁻) were typically depleted to <20 nmol L⁻¹, except for PO₄³⁻ in the area south of 16°N. The majority of the study area was characterized by high-Si(OH)₄ and low-N+N and PO₄³⁻. However, submesoscale/mesoscale depressions of Si(OH)₄ were locally observed in the cyclonic eddy fields north of 23°N. Among a total of six Si(OH)₄ depressions within the eddies, a complete Si(OH)₄ depletion (<11 nmol L⁻¹) was observed on the cyclonic side near the Kuroshio axis (33.1°N, 138°E). This depletion was closely coupled with a diatom bloom, suggesting that Si(OH)₄ was exhausted by diatoms. All of the Si(OH)₄ depressions were selective and not accompanied by local depressions of N+N and PO₄³⁻. This unique phenomenon might be driven by biogeochemical processes such as selective Si export (Si pump), anomalous Si uptake associated with diatom physiology, and/or Si uptake supported by N₂ fixation.     

Statistical characteristics and the variability of currents over the western Black Sea shelf

Statistical characteristics and the parameters of the temporal variability of currents are analysed using data on current measurements conducted by five moored autonomous buoy stations over Bulgaria's shelf. Estimates of the specific contribution to the variability of synoptic (60%) and mesoscale (40%) oscillations are derived. The qualitative spectrum of the currents is defined, constituted by synoptic oscillations, short-period synoptic oscillations, inertial oscillations, and internal waves. Estimates of periods and oscillation amplitudes are also derived. The kinetic energy of the currents is calculated and the specific contribution of the energy of different-scale oscillations and their interrelations are determined.Translated by V. Puchkin.     

Three-dimensional structure of an observed cyclonic mesoscale eddy in the Northwest Pacific and its assimilation experiment

Mesoscale eddies play an important role in modulating the ocean circulation. Many previous studies on the threedimensional structure of mesoscale eddies were mainly based on composite analysis, and there are few targeted observations for individual eddies. A cyclonic eddy surveyed during an oceanographic cruise in the Northwest Pacific Ocean is investigated in this study. The three-dimensional structure of this cyclonic eddy is revealed by observations and simulated by the four-dimensional variational data assimilation(4 DVAR) system combined with the Regional Ocean Modeling System. The observation and assimilation results together present the characteristics of the cyclonic eddy. The cold eddy has an obvious dual-core structure of temperature anomaly.One core is at 50–150 m and another is at 300–550 m, which both have the average temperature anomaly of approximately-3.5°C. The salinity anomaly core is between 250 m and 500 m, which is approximately-0.3. The horizontal velocity structure is axis-asymmetric and it is enhanced on the eastern side of the cold eddy. In the assimilation experiment, sea level anomaly, sea surface temperature, and in situ measurements are assimilated into the system, and the results of assimilation are close to the observations. Based on the high-resolution assimilation output results, the study also diagnoses the vertical velocity in the mesoscale eddy, which reaches the maximum of approximately 10 m/d. The larger vertical velocity is found to be distributed in the range of 0.5 to 1 time of the normalized radius of the eddy. The validation of the simulation result shows that the 4 DVAR method is effective to reconstruct the three-dimensional structure of mesoscale eddy and the research is an application to study the mesoscale eddy in the Northwest Pacific by combining observation and assimilation methods.     

A quasitropical cyclone over the Black Sea: Observations and numerical simulation

A long-living mesoscale cyclone observed over the Black Sea in September 2005 was characterized by a high wind velocity (25 m/sec) and induced an abnormal lowering of the sea-surface temperature (by more than 10°C). We performed the numerical simulation of this cyclone by using the MM5 model and the data of global operative analysis. The evolution of the cyclone and its structure similar to the structure of a tropical cyclone are reproduced. The primary azimuthal and secondary radial circulations, the clear-sky eye with warm core at medium levels, and the eyewall with strong air updraft and intense convection and precipitation are well visible. The fluxes of sensible and latent heat on the sea surface played the key role in the initiation and development of this cyclone. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 3, pp. 41–55, May–June, 2008.     

11月初北黄海一次海龙卷风暴的中尺度特征分析

利用多普勒天气雷达资料及反演风场和常规观测资料,对2014年11月2日发生在北黄海（山东半岛北部海上）一次罕见海龙卷风暴的中尺度特征进行了分析。结果表明：冷空气、暖湿海面热力边界、山东半岛北部近海岸西北风与偏西风的辐合线是海龙卷风暴发生的天气背景。海龙卷风暴发生时雷达回波PPI最大分贝反射率因子为60 dBZ,高度为2.0 km,最高风暴顶为4.5 km,最大垂直累积液态水含量VIL为21 kg·m-2。利用雷达反演风场进行中尺度特征分析,结果表明：在海龙卷风暴发生发展过程中,低层风辐合对应4.0 km高度上是风辐散,海上有较强的偏南暖湿气流输送到雷暴区。中尺度动力特征：最大正涡度和散度辐合在1.0 km以下,低层正涡度和散度辐合、高层散度辐散是雷暴发生初期动力特征;低层没有正涡度和散度辐合、高层为正涡度和散度辐合是雷暴开始发展的动力特征;低层和高层为大的正涡度和散度辐合是雷暴成熟阶段的动力特征。高空冷空气叠加上低空强的偏南气流,造成局地涡度加大和低层辐合加强,使低层暖湿气流倾斜上升。海龙卷与辐合区的冷空气和暖湿气流有关。     

Comparing satellite and meteorological data on wind velocity over the Black Sea

Wind-velocity data obtained from in situ measurements at the Golitsyno-4 marine stationary platform have been compared with QuikSCAT scatterometer data; NCEP, MERRA, and ERA-Interim global reanalyses and MM5 regional atmospheric reanalysis. In order to adjust wind velocity measured at a height of 37 m above the sea surface to a standard height of 10 m with stratification taken into account, the Monin–Obukhov theory and regional atmospheric reanalysis data are used. Data obtained with the QuikSCAT scatterometer most adequately describe the real variability of wind over the Black Sea. Errors in reanalysis data are not high either: the regression coefficient varies from 0.98 to 1.06, the rms deviation of the velocity amplitude varies from 1.90 to 2.24 m/s, and the rms deviation of the direction angle varies from 26° to 36°. Errors in determining the velocity and direction of wind depend on its amplitude: under weak winds (<3 m/s), the velocity of wind is overestimated and errors significantly increase in determining its direction; under strong winds (>12 m/s), its velocity is underestimated. The influence of these errors on both spatial and temporal estimates of the characteristics of wind over the Black Sea is briefly considered.     

Submesoscale Eddies in the White Sea Based on Satellite Radar Measurements

Izvestiya, Atmospheric and Oceanic Physics - This paper presents the results of the analysis of the archive of radar images of Envisat Advanced Synthetic Aperture Radar (ASAR) and Radarsat-1,2 for...