Japan/East Sea

Physical and geographical characteristics and hydrometeorological conditions.

The Japan/East Sea is located in the North-Western Pacific between the continental coast of Asia, the Japanese Islands and Sakhalin Island in the geographical coordinates of 34^o26’ - 51^o41’ N and 127^o20’ - 142^o15’ E. By the physical-geographical location, it is referred to the marginal seas, and it is separated from the adjacent basins by the shallow water barriers. In the north and north-east, the Japan/East Sea is connected with the Sea of Okhotsk by the Nevel’skoy Strait and Laperuz Strait (Soya), in the east - with the Pacific Ocean by the Tsugaru Strait, in the south - with the East-China Sea by the Korean (Tsushima) Strait. The shallowest of them is the Nevel’skoy Strait, its maximal depth is 10 m, and the deepest is the Tsugaru Strait - about 200 m. The hydrological mode of the basin is the most effected by subtropical waters flowing through the Korean Strait from the East China Sea. The width of this strait makes 185 km, and the largest depth of the threshold is 135 m. The second in the water exchange volume is the Tsugaru Strait, its width making 19 km. The Laperuz Strait, the third one in the water exchange volume, is 44 km wide and to 50 m deep. The area of the water surface makes 1062 thousand km², and the total volume of the seawater is 1631 thousand km³.

According to the bottom relief, the Japan/East Sea is subdivided to three parts: northern one - to the north of 44^o N, central one - between 40^o and 44^o N, and southern one - to the south of 40^o N. The bottom surface of the northern bathymetric step presenting itself a wide trough is gradually elevating to the north and merges with the surface of the Tartar Strait shoal. The basin of the central part with the sea depth maximal (to 3700 m) has an even floor and it is stretching from west to east, north-east. In the south, its margin is the underwater Rise of Yamato. The most complicated bottom topography is in the southern part of the sea. Here, the main geological feature is the underwater Rise of Yamato, formed by two ridges stretching to the east-north-eastern direction, and a closed basin situated between the ridges. Between the Yamato Rise and Honshu slope it is stretching the Honshu Basin with the depths of about 3000 m. In the south-western part of the sea there is a less deep Tsushima Basin. In the Korean Strait area the shoals of the Korean Peninsula and Honshu Island merge, forming a shallow water area 120-140 m deep.

Peculiar feature of the Japan/East Sea floor morphology is the poorly developed shelf stretching along the coast in a band of 15 - 70 km. The narrowest band of the shelf, which is 15-25 km wide, is observed along the southern coast of Primorye. The shelf is more developed in the Peter-the-Great Bay, in the northern part of the Tartar Strait, East-Korean Bay and in the Korean Strait area.

Total length of the sea coastal line is 7531 km. It is slightly cut (excepting the Peter-the-Great Bay), sometimes it is almost strait. Few islands are located mainly near the Japanese Islands and in the Peter-the-Great Bay.

The Japan/East Sea is located in two climatic zones: subtropical and moderate. In the bounds of these zones there are two sectors with different climatic and hydrological conditions: a severe cold northern one (in winter - partially covered with ice) and a mild, warm one adjacent to the Japanese and Korean coasts. The main factor forming the sea climate is monsoon circulation of the atmosphere.

The main atmospheric circulation systems over the Japan/East Sea are the Aleutian Low, Pacific High, and the Asian center of the atmosphere activity which is located above the continent. Their changes during a year precondition the monsoon character of climate in the Far East of Russia. The distribution of the atmospheric pressure above the Japan/East Sea predetermined by the main pressure formations possesses the following peculiarities: general decrease of pressure in direction from west to east, pressure increase from north to south, growth of excess of winter pressure value over the summer one in the direction from the north-east to the south-west, and also sharply expressed seasonal variability. In the annual changes of pressure for the largest part of the sea it is typical the pressure maximum occurrence in winter and the pressure minimum - in summer. In the north-eastern part of the sea - near the northern half of Honshu Island, Hokkaido Island and near the southern coast of Sakhalin Island there are two maximums of pressure: the first one - in February and the second one - in October, the minimum being in summer. The amplitudes of the annual pressure changes, as a rule, are decreasing from south to north. Along the continental coast the amplitude decreases from 15 mb in the south to 6 mb in the north, and along the coasts of Japan - from 12 to 6 mb, correspondingly. Absolute amplitude of pressure variations in Vladivostok makes 65 mb, on Hokkaido Island - 89 mb. To the south-east, in the central and southern parts of Japan, it increases to 100 mb. The main reason for the increase of the amplitudes of pressure variations in the south-eastern direction is deep cyclones and typhoons.

The above considered peculiarities of atmospheric pressure distribution precondition the general characteristics of the wind mode in the Japan/East Sea. Near the continental coast, during the cold period, strong winds of the north-western direction predominate, their velocity is 12-15 m/s. Repeatedness of these winds in the period from November to February makes 60-70%. In January and February, the repeatedness of predominant winds in some coastal places reaches 75-90%. From north to south, wind velocities gradually decrease from 8 m/s to 2.5 m/s. Along the eastern island coast, winds of the cold season are not so well expressed in direction, as near the continental coast. Here, wind velocities are less, but also generally decrease from north to south. Every year, at the end of summer, and at the beginning of autumn, in the Japan/East Sea the tropical cyclones (typhoons) appear, they are accompanied by the hurricane wind. During the cold season, the repeatedness of the storm winds induced by deep cyclones, is sharply increasing. During the warm period of a year, over the sea, the southern and south-eastern winds predominate. Their repeatedness makes 40-60%, and the velocities, as in winter, decrease from north to south, as a rule. As a whole, the wind velocity during the warm period of a year is considerably less, than in winter. During the transitional seasons (spring and autumn) the wind direction and velocity endure significant changes.

For the open areas of the north-western part of the sea, in winter, predominant winds are of the north-western and northern directions. South-westwards, the winds change their direction from the north-western to the western, and in the areas adjacent to the southern Sakhalin and Hokkaido - from the north-western to the northern and even to the north-eastern. During a warm season, such regular situation of the general structure of the wind field fails to be determined for the whole sea. Still, it is found that in the northern areas of the sea, the eastern and north-eastern winds predominate, and in the southern areas - the southern ones.

In the Japan/East Sea the air temperature is regularly changing both from north to south and from west to east. In the northern, the most severe climatic zone, the average annual temperature makes 2^o, and in the south, in the subtropical area - +15^o. In the seasonal changes of the air temperature the minimum occurs in winter (January - February), and the maximum - in August. In the north, the average monthly temperature in January is about -19^o, and the absolute minimum equals -32^o. In the south, the average monthly temperature in January makes 5^o, and the absolute minimum - 10^o. In August, in the north, the average temperature is 15^o, and the absolute maximum - +24^o; in the south, correspondingly, 25^o and 39^o. Temperature changes from west to east are of less amplitude. The western coast during the whole year is colder than the eastern one, the temperature differences increasing from south to north. In winter they are larger than in summer, and in average they make 2^o, but in some latitudes they can reach 4-5^o. A number of cold days (the average temperature being below 0^o) is sharply decreasing from north to south.

As a whole, the sea possesses a negative (about 50w/m²) annual radiation heat balance on the surface which is compensated at the expense of the constant heat inflow with the waters supplied through the Korean Strait. Seawater balance is mainly determined by its water exchange with the adjacent basins through three straits: Korean (inflow), Tsugaru and Laperuz (flow). As compared with the water exchange through the straits, the input to the water balance by precipitation, evaporation and continental runoff is negligibly small. Continental runoff, due to its insignificance impose its effect just on the coastal areas [Demenitskaya et al, 1974; Dobrovol'skii and Zalogin, 1982; Stepanov, 1961].

All values of the air and water temperature are given by Celsius (° C), and salinity - per mille (1 g/kg = 1 ‰ ).

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Hydrological characteristics

Main factors predetermining the hydrological mode of the Japan/East Sea are the interaction of its surface waters with the atmosphere on the background of changing climatic conditions and the water exchange through the straits with the adjacent water basins. The first of these factors is the major one for the northern and north-western parts of the sea. Here, under the influence of the north-western monsoon winds bringing the cold air masses from the continental areas in winter, as a result of heat exchange with the atmosphere, the surface waters are considerably cooling. With this, in the shallow water areas of the continental coast, Peter-the-Great Bay and Tartar Strait, it is formed the ice cover, and in the adjacent open sea areas the convection processes are developed. The convection involves considerable water layers (till the depths of 400-600 m), and in some extremely cold years it reaches the near-bottom layers of the deep water basin while ventilating the cold, relatively homogeneous deep-water mass making 80% of the total sea water volume. During the whole year, the northern and north-western parts of the sea stay colder than the southern and south-western ones.

Water exchange through the straits impose predominant influence on the hydrological mode of the southern and eastern parts of the sea. Subtropical waters of the Kuroshio branch which are in-flowing through the Korean Strait, during the whole year are heating the southern sea areas and the waters adjacent to the Japanese Islands till the Laperuz Strait, as a result, the eastern sea waters are always warmer than those of the western part.

The given paragraph briefly presents the general knowledge on the spatial distribution and variability of the seawater temperature and salinity, water masses, currents, tides, and ice conditions of the Japan/East Sea based mainly on the following research papers, monographs, atlases and reference books: [Bogdanov et al, 1991; Chu et al, 2001; Demenitskaya et al, 1974; Dobrovol'skii and Zalogin, 1982; Preller and Hogan, 1998; Stepanov, 1961; Yakunin, 1995; Yurasov and Yarichin, 1991], as well as on graphic materials of the Atlas.

All values of the air and water temperature are given by Celsius (° C), and salinity - per mille (1 g/kg = 1 ‰ ).

Horizontal distribution of temperature

On the maps of horizontal distribution of the water temperature on the surface, the northern and southern parts of the sea are distinctly separated by a thermal front, which location is almost the same during all seasons. This front separates the warm and salty waters of the southern sea sector from the colder and freshened waters of the northern sea part. Horizontal gradient of temperature on the surface transverse the front during a year varies from maximal values (16^o/100 km) in February to the minimal ones (8^o/100 km) in August. In November-December, northward of the main front parallel to the Russian coast it is formed a secondary front with the gradient of 4^o/100 km. The temperature difference in the bounds of the whole sea water area during all seasons stay almost constant and equals 13-15^o. The warmest month is August, when the temperature in the north equals 13-14^o, and in the south, in the Korean Strait it reaches 27^o. The lowest temperature (0...-1.5^o) is typical for February, when in the northern shallow-water areas it is formed ice, and in the Korean Strait the temperature decreases to 12-14^o. Seasonal changes of the water temperature on the surface as a whole, increase from the south-east to the north-west from the minimal values (12-14^o) near the Korean Strait to the maximal ones (18-21^o) in the central part of the sea and near the Peter-the-Great Bay. Relatively to the average annual values, the negative temperature anomalies take place in the period from December to May (during the winter monsoon activity), and the positive ones - from June to November (summer monsoon). The strongest cooling (negative anomalies are to -9^o) takes place in February in the area of 40-42^o N, 135-137^o E, and the greatest warming (positive anomalies exceed 11^o) is observed in August near the Peter-the-Great Bay.

With the depth, the range of spatial temperature changes and its seasonal variations on different levels is getting considerably narrower. It is the horizon of 50 m where seasonal temperature variations do not exceed 4-10^o. Maximal amplitudes of temperature variations at this depth are observed in the south-western part of the sea. On the level of 200 m the average monthly water temperature values during all seasons increase from 0-1^o in the north of the sea to 4-7^o in the south. The main front location here does not change with regard to the surface one, but it occurs its meandering in the area between 131^o and 138^o E. In the central part of the basin to the north of the main front the temperature on this horizon is 1-2^o, and to the south - it increases sharply till 4-5^o. At the depth of 500 m, the temperature in the whole sea varies non-considerably. It makes 0.3-0.9^o and practically does not endure any seasonal variations. A zone of frontal division does not exhibit itself, though in the area adjacent to the coast of Japan and Korea it is observed some temperature increase conditioned by the heat transfer to the deep layers by the vortex formations being actively formed in this part of the sea.

Out of the regional peculiarities of the horizontal temperature distribution it is necessary to distinguish the upwelling zones, eddy formations and coastal fronts.

The upwelling near the southern coast of Primorye is intensively developed at the end of October - beginning of November, but the particular cases of its short-period manifestation can be identified in September - beginning of October. Diameter of a cold water spot in the upwelling zone is 300 km, and the temperature difference between its center and the adjacent waters can reach 9^o. The upwelling origin is conditioned not only by the deep-water circulation increase, but also, mainly, by the monsoon shift of the wind which is confined just to this time interval. Strong north-western winds blowing from the continent create favorable conditions for the upwelling development in this area. At the end of November, under the influence of cooling, it occurs the destruction of stratification in the upwelling zone and the temperature distribution on the surface becomes more homogeneous.

In the coastal area of the north-western part of the Japan/East Sea (in the area of Primorye Current) the front is formed at the beginning of summer on the background of the general temperature increase of the surface layer. The main front is parallel to the coastline. Apart it, there are secondary fronts oriented perpendicular to the coast. In September-October the main front is just in the northern part of the sea, and southward there are isolated cold water spots bounded by fronts. Probably, the cold water cells appearing near the coast are conditioned by the rapid cooling of the surface layer in the shallow water areas. These waters, after the terminal destruction of the thermocline are propagating towards the open sea as the continuous intrusions.

The most active eddies are formed on both sides of the front and, embracing the considerable water thickness, they bring the anomalies to the field of the horizontal temperature distribution.

The absence of the water exchange of the Japan/East Sea with the adjacent basins at the depths of more than 200 m, as well as the active ventilation of the deep-water layers, due to the autumn-winter convection in the northern and north-western areas lead to the distinct separation of the water thickness into two layers: a subsurface activity layer, characterized by seasonal variability, and a deep-water layer, where both seasonal and spatial variability is almost not traced. According to the available estimates, a margin between these layers is located at the depths of 300-500 m. Extreme depths (400-500 m) are confined to the southern sea part. This is related to the observed descending water changes in a center of a vast anticyclone meander of the East-Korean Current, as well as to variations of the frontal zone location on its northern and eastern margins. Till the depth of 400 m, seasonal temperature variations are traced near the coast of Japan, which is due to the waters sinking in anti-cyclone vortices formed at the Tsushima Current interaction with the continental slope. Great depths of seasonal temperature variations (to 400-500 m) are found in the Tartar Strait. It is mainly related to the convection processes and considerable variability of the surface waters parameters, as well as to the annual variability of intensity and spatial location of the Tsushima Current branch waters. Near the Southern Primorye coast, seasonal variations of the water temperature are exhibited just in the upper three hundred meter layer. Below this margin, seasonal temperature variations are almost not traced. As the vertical sections of the temperature field show, the activity layer characteristics undergo considerable changes not only with the seasonal changes, but also from area to area. Waters of the deep layer occupying about 80% of the sea volume are slightly stratified and possess the temperature of 0.2-0.7^o.

Thermal structure of the activity layer waters is composed of the following elements (layers): upper quasi-homogeneous layer (UQL), seasonal layer of the temperature jump, and the main thermocline. Characteristics of these layers in different seasons in the sea water area possess regional distinctions. Near Primorye coast, in summer the lower boundary of the UQL is at the depth of 5-10 m, and in the southern sea areas it is deepening to 20-25 m. In February, the lower boundary of the UQL in the southern sector is at the depths of 50-150 m. Seasonal thermocline is intensifying from spring to summer. In August, the vertical gradient reaches the maximum - 0.36^o/m. In October, the seasonal thermocline is destroying itself and merges with the main one located during the whole year at the depths of 90-130 m. In the central sea areas the observed regularities are preserved on the background of the general decrease of contrasts. In the northern and north-western sea areas the main thermocline is weak and sometimes it is even absent. Here, seasonal thermocline starts to form with the beginning of spring water heating and occurs till the winter period, when it is completely destroyed by convection in the bounds of all water thickness of the activity layer.

Large-scale peculiarities of salinity distribution on the surface are determined by the sea water exchange with the adjacent sea basins, balance of precipitation and evaporation, ice formation and ice melting, as well as by the continental runoff in coastal areas.

In winter, on the largest area of the sea surface, water salinity exceeds 34‰ which is mainly conditioned by high-salinity waters (34.6‰) supplied from the East-China Sea. Less saline waters are concentrated in the coastal areas of the Asian continent and islands where their salinity decreases to 33.5 - 33.8‰. In the coastal areas of the southern sea area the salinity minimum on the surface is observed during the second half of summer and at the beginning of autumn, which is related to the cloud-burst precipitation in the second half of summer and the freshening of waters brought from the Eastern Kamchatka Sea. In the northern part of the sea, apart from the summer-autumn decrease it is formed the second salinity minimum in spring, during ice melting in the Tartar Strait and Peter-the-Great Bay. The highest values of salinity in the southern part of the sea occur in spring-summer, when it is enhanced the inflow of saline Pacific waters from the East China Sea. It is typical a gradual delay of salinity maximums from south to north. While in the Korean Strait the maximum occurs in March-April, near the northern coast of Honshu Island it is observed in June, and near the Laperuz Strait - in August. Along the continental coast, the salinity maximum takes place in August. The most saline waters are located near the Korean Strait. In spring, these peculiarities are generally preserved, but the area of the low salinity in the coastal areas, due to the ice melting and the increase of the continental discharge, as well as the precipitation is increased. Closer to the summer period, after the surface waters of the East China Sea (which are freshened due to the abound precipitation) are supplied to the sea through the Korean Strait, general salinity background in the sea water area is decreasing to the values less than 34‰. In August, the range of salinity variability in the whole sea makes 32.9-33.9‰. At that time, in the north of the Tartar Strait, salinity decreases to 31.5‰, and in some particular areas of the coastal zone - to 25-30‰. In autumn, with the increase of the northern winds, it occurs the water surge and mixing of the upper layer waters and it is observed some increase of salinity. Minimal seasonal changes of salinity on the surface (0.5-1.0‰) are found in the central part of the sea, and the maximal ones (2-15‰) - in the coastal areas of the north-western part and in the Korean Strait. At great depths, along with the general increase of salinity values, it takes place sharp decrease of its variability range, both in space and in time. By the average long-term data, even at the depth of 50 m, seasonal salinity variations in the central part of the sea do not exceed 0.2-0.4‰, and in the north and south of the water area - 1-3‰. On a level of 100 m the horizontal salinity changes are over the range of 0.5‰, and on a level of 200 m, during all seasons of a year, they do not exceed 0.1‰, i.e. the value which is typical for deep waters. Somewhat larger values are observed just in the south-western part of the sea. We should note, that the horizontal salinity distribution at the depths exceeding 150-250 m, is very similar: minimal salinity values are confined to the northern and north-western parts of the sea, and the maximal - to the southern and south-eastern ones. Along with this, a haline front weakly expressed at these depths, completely follows the outlines of the thermal one.

Vertical structure of salinity field in different parts of the Japan/East Sea is characterized by considerable diversity. In the north-western part of the sea it is observed the monotonous increase of salinity with the depth during all year seasons, excepting the winter season, when it is almost constant in all water thickness. In the southern and south-eastern parts of the sea during the warm period of a year, below the freshened surface waters, it is distinctly distinguished an intermediate layer of increased salinity, which is formed by high salinity waters (34.3-34.5‰) inflowing through the Korean Strait. Its core is located at the depths of 60-100 m in the north and somewhat deeper - in the south of the sea. Northward, the salinity in the core of this layer is decreasing and on the periphery it reaches the value of 34.1‰. In winter, this layer is not exhibited. At that time, vertical salinity changes on the largest part of the water area do not exceed 0.6-0.7‰. In the bounded area located to the east of the Korean Peninsula, at the depths of 100-400 m it is distinguished an intermediate layer of decreased salinity formed in winter at the expense of the surface waters sinking in a zone of frontal division. Salinity in the core of this layer equals 34.00-34.06‰. Seasonal variations of the vertical structure of salinity field are well seen just in the upper 1-250 m layer. Maximal depth of seasonal salinity variations (200-250 m) is found in a zone of the Tsushima Current. This is related to the peculiarities of the annual salinity changes in the sub-surface Pacific waters flowing to the sea through the Korean Strait. In the northern part of the Tartar Strait, near the coast of Primorye, near Korea Peninsula, and in the area to the south and south-west of Peter-the-Great Bay, seasonal variations of salinity are exhibited just in the upper 100-150 m layer. Here, the influence of Tsushima Current waters is weak, and the annual salinity changes of the surface water layer related to the processes of ice melting and river discharge are limited by the water areas of the bays and inlets. This area with the minimal depth of seasonal salinity variations is alternating with the higher values areas originated with the high-salinity waters of the Tsushima Current branches penetrating to the north-western coast of the sea. General notion on the vertical structure of the salinity field is given by the spatial sections of this parameter distribution and the table values given in the Atlas.

In accordance with the considered peculiarities of spatial-temporal variability of temperature and salinity, the Japan/East Sea water thickness is composed by different water masses. We suggest the classification based on the extreme elements of the vertical salinity distribution.

By the vertical, water masses in the open part of the Sea of Japan are subdivided into the surface one, intermediate one, and deep one. Surface water mass (its varieties are: SSA - Surface Subarctic, SFZ - Surface Front Zone, SST - Surface Subtropical) is located in the bounds of the upper mixed layer and it is limited beneath by a seasonal thermocline. In the southern warm sector surface water mass is formed due to the mixing of waters supplied from the East China Sea and the coastal waters of Japanese Islands, and in the cold northern sector (SSA) - by mixing of the coastal waters (freshened by the continental runoff) with the waters of the open areas of the adjacent sea area. As shown above, during a year, the temperature and salinity of the surface waters vary in a wide range, and their thickness varies from 0 to 120 m.

In the lower intermediate layer of waters, on the most part of the sea area, during the warm season high salinity (HS) water mass observed (its varieties: HSST - Subtropical, HSSTT - Transformed), its core is located at the depths of 60-100 m, and the lower boundary - at the depth of 120-200 m. Salinity in its core is ~ 34.1-34.8‰. In the local area to the east of the Korean Peninsula coast, the low salinity water mass is observed at depths of 200-400 (34.0-34.06‰).

Deep water mass is usually called the Japan Sea proper water. It occupies all lower layer (deeper than 400 m) and is characterized by homogeneous values of temperature (0.2-0.7^o) and salinity (34.07-34.10‰). High content of dissolved oxygen in its core suggest active renewal of the deep layers by the surface waters.

In the coastal areas of the north-western part of the sea, due to the considerable freshening by the continental discharge, tidal mixing, wind upwelling and winter convection, it is formed a specific coastal water structure presented vertically by the coastal surface (CS) waters. Its salinity is less than for the waters of the adjacent areas of the open sea and possessing more considerable temperature variations, and also the sub-surface water (CSS) of higher salinity and low temperature, which is formed in the course of winter convection. In some areas (Tartar Strait, Peter-the-Great Strait) in the course of intensive ice formation in winter it is formed bottom shelf water mass (BS) of high salinity (up to 34.7‰) and very cold (to -1.9^o). Propagating near the bottom it can reach the shelf edge and flow down the continental slope while taking part in the deep layers ventilation.

On the shelf area, where the water freshening by the continental runoff is small, it occurs weakening or even destruction of the water stratification by the tidal mixing. As a result, it is formed a poorly-stratified shelf structure, composed by the relatively cold freshened surface shelf water mass (SS) and the relatively warm and freshened shelf modification of deep waters (DS). At definite orientation of predominant winds this structure is distorted by the upwelling. In winter it is destroyed by a more powerful mechanism - convection. The waters formed in zones of tidal mixing are involved into the circulation occurring in the north-western part of the sea and are propagating beyond the bounds of their origin area, and they are usually referred to as the "waters of Primorye Current".

Characteristics of the water structures and water masses in the north-western part of the Japan/East Sea (numerator - February, denominator - August) [Zuenko and Yurasov, 1995]

Note: in February, the surface and deep water masses of Subarctic structure do not differ in their thermohaline characteristics.

Major elements of the water circulation scheme given in the Atlas are the warm currents of the southern and eastern sector and the cold currents of the north-western sector of the sea. Warm currents are initiated by the sub-tropical waters in-flowing through the Korean Strait, they are represented by two flows: the Tsushima Current consisting of two branches (a calm - marine one, and a more turbulent - moving near the very coast of Honshu Island), and the East-Korean Current propagating in a single flow along the Korean Peninsula coast. On latitude 38-39^o N the East-Korean Current is dividing into two branches, one of them, while bending around the Yamato Rise in the north, moves in the direction of Tsugaru Strait, another one - while deviating to the south-east, by a part of its waters closes the anti-cyclone circulation near the southern coast of Korea, and by another part - merges with the marine branch of Tsushima Current. All branches of Tsushima and East-Korean Currents are jointed into one flow near Tsugaru Strait, through which it occurs the removal of the majority (70%) of supplied warm sub-tropical waters. The remaining part of these waters propagates farther to the north towards the Tartar Strait. While reaching the Laperuz Strait, the main mass of this flux is removed from the sea and just its insignificant part, while propagating in the bounds of the Tartar Strait, gives the origin to the cold current propagating to the south along the continental coast of Primorye. On 45-46^o N, by a divergence zone, this current is divided into two parts: the northern one - Limannoye Current (Shrenk Current) and southern one - Primorskoye Current, which to the south of the Peter-the-Great bay is dividing into two branches, one of them gives the origin to the cold North-Korean Current, and another one turns to the south and, touching the northern flow of the East-Korean Current, forms a large scale cyclone vortex, which center is 42^o N and 138^o E above the Japan Sea Basin. The cold North-Korean Current reaches 37^o N and then it is merged with the strong flow of the warm East-Korean Current forming a frontal zone together with the southern branch of Primorskoye Current. The least expressed element of the general scheme of circulation is the West-Sakhalin current flowing in the southern direction from 48^o N along the southern coast of Sakhalin Island and transporting some part of the waters of Tsushima Current separated from it in the water area of the Tartar Strait.

During a year, singled out peculiarities of the water circulation are practically preserved, but the power of the main currents changes. In winter, due to the decrease of the waters supply, the velocity of both branches of Tsushima Current does not exceed 25 cm/s, the coastal branch being of larger intensity. Total width of the current of about 200 km is also preserved in summer, but the velocities increase up to 45 cm/s. The East-Korean Current also intensifies in summer, when its velocities reach 20 cm/s, and the width - 100 km, and extinguishes in winter to 15 cm/s, its width reducing to 50 km. Velocities of cold currents during a year do not exceed 10 cm/s, and their width is limited to 50-70 km (the maximum being in summer). In transitional seasons (spring, autumn) the current characteristics possess the values average between the summer and winter ones. Current velocities in a layer of 0-25 are almost constant, and with the subsequent increase of the depth, at 100 m they decrease till half of the surface value. The Atlas presents the schemes of the water circulation on the Japan/East Sea surface during different seasons, they are obtained by calculation methods.

Tidal phenomena in the Sea of Japan are formed mainly by the semi-diurnal tidal wave M₂, which is almost purely standing, with two amphidromic systems located near the margins of the Korean and Tartar Straits. Synchronous variations of the sea level and tidal currents in the Tartar Strait and Korean Strait occur according to the law of two-nodal seiche which swelling occupies all central deep water part of the sea, and the nod lines are located near the margins of the given straits.

In its turn, the relationship of the sea with the adjacent basins through three major straits provides the formation of the induced strait there, which influence, due to the morphological peculiarities (shallowness of straits, as compared to the sea depth), is observed in the straits and areas directly adjoining them. In the sea, semi-diurnal, diurnal and mixed tides occur. The largest level variations are observed in the terminal southern and northern sea areas. By the southern exit of the Korean Strait the tide value reaches 3 m. In its movement to the north, it is rapidly decreasing, and near Pusan it does not exceed 1.5 m so far. In the middle part of the sea, the tides are not large. Along the eastern coast of Korea and Russian Primorye, till the exit of the Tartar Strait they do not exceed 0.5 m. The tides near the western coast of Honshu Island, Hokkaido Island and south-western Sakhalin are of the same range. In the Tartar Strait the tidal range is ~ 2.3-2.8 m. The increase of the tide range in the northern part of the Tartar Straits is due to its funnel shape.

In the open sea areas, generally the semi-diurnal tidal currents occur, their velocities making 10-25 cm/s. More complicated are tidal currents in the straits, where they possess quite considerable velocities. For instance, in the Tsugaru Strait the velocities of tidal currents reach 100-200 cm/s, in the Laperuz Strait - 50-100 cm/s, in the Korean Strait - 40-60 cm/s.

According to the ice conditions, the Japan/East Sea can be divided into three areas: the Tartar Strait, the area along the coast of Primorye from Povorotny Cape to Belkin Cape and Peter-the-Great Bay. In winter, the ice is constantly observed just in the Tartar Strait and Peter-the-Great Bay, in the rest water area, excluding the closed bays and inlets in the north-western sea area, it is formed from time to time. The coldest area is the Tartar Strait, in winter, there it is formed and localized more than 90% of the total ice observed in the sea. By the long-term observation data, the duration of the ice period in the Peter-the-Great Bay makes 120 days, and in the Tartar Strait - from 40-80 days in the southern part of the strait, to 140-170 days - in its northern part.

The first ice appear in the top parts of the bays and gulfs protected from the wind and wave, and possessing the freshened surface layer. During moderate winters, in the Peter-the-Great Bay the first ice is formed in the second decade of November, and in the Tartar Strait, in the upper part of Sovetskaya Gavan’ Bay, Chekhachev Bay and Nevel’skoy Strait the initial ice forms are observed as early as the beginning of November. Early ice-formation in the Peter-the-Great Bay (the Amur Bay) starts at the beginning of November, in the Tartar Strait - in the second half of October. Later - at the end of November. At the beginning of December, the ice cover development along the coast of Sakhalin Island takes place faster than near the continental coast. Correspondingly, in the eastern part of the Tartar Strait at that time the ice area is larger than in the western one. Till the end of December the ice area in the eastern and western areas is equalized, and after reaching the Syurkum Cape parallel the edge direction is changed: its shift along Sakhalin coast is getting slower, and along the continental coast - it is getting more active.

In the Sea of Japan the ice cover reaches its maximal development in the middle of February. In average, the ice covers 52% of the Tartar Strait area and 56% of the Peter-the-Great Bay.

Ice melting starts in the first half of March. In the middle of March, the open areas of the Peter-the-Great Bay and all Primorye coast till Zolotoy Cape are cleared of ice. The ice cover margin in the Tartar Strait moves back to the north-west, and in the eastern part of the strait at that time, the sea is cleared of ice. Early sea clearance occurs in the second decade of April, later - at the end of May-beginning of June.

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Hydrological conditions of the Peter-the-Great Bay and the coastal zone of Primorski Krai

Physical characteristics and hydrometeorological conditions

Peter-the-Great Bay is the largest one in the Japan/East Sea. It is located in the north-western sea area between the parallels of 42^o 17’ and 43^o 20’ N and the meridians of 130^o 41’ and 133^o 02’ E. Waters of the Peter-the-Great bay on the sea-side are bounded by a line joining the Tyumen’-Ula River mouth with the Povorotny Cape.

By the Murav’ev-Amursky Peninsula and a group of islands located to the south-west from it, Peter-the-Great Bay is divided into two large bays: the Amur Bay and Ussuri Bay. The Amur Bay presents itself a north-western part of the Peter-the-Great Bay. In the west, it is bounded by the continental coast, in the east - mountainous Peninsula of Murav’ev-Amursky and the Islands of Russkiy, Popova, Reineke, Rikorda. Southern boundary of the Amur Bay is a line joining the Bryus Cape with the islands of Tsivol’ko and Zheltukhin. Ussuri bay occupies the north-eastern part of the Peter-the Great Bay. In the north-west it is bounded by the Murav’ev-Amursky Peninsula, Russkiy Island, and the islands located to the south-west from the latter. Southern boundary of the Bay is a line joining southern termination of Zheltukhin Island and Askol’d Island.

Peter-the-Great Bay area makes about 9000 square km. In its vast water area there are many islands different in area, they are located mainly in the western part of the bay as two groups. The northern group of islands is located to the south-west from Murav’ev-Amursky Peninsula and is separated from it by the Strait of Bosfor-Vostochny. This group is composed by four large and a lot of small islands. The largest in this group is Russkiy Island. Southern group - the Rimsky-Korsakov Islands - involves eight islands and many small isles and rocks. The largest is Bol’shoy Pelis Island. In the eastern part of the Bay there two more large islands: Putyatin Island located in the Strelok Bay and Askol’d Island located to the south-west of Putyatin Island.

The most significant Strait is Bosfor-Vostochny, separating Russkiy Island from the Murav’ev-Amursky Peninsula. The straits between the Islands of Rimsky-Korsakov are deep and broad; between the islands adjacent to the Murav’ev-Amursky Peninsula the straits are more narrow.

The coast line of the Peter-the-Great Bay is very curvy and forms many secondary bays and inlets. The largest among them are the Bays of Pos’et, Amur, Ussuri, Strelok, Vostok, and America. The capes protruding far to the sea form rocky, generally steep coast framed by stones. The largest peninsulas are Gamov, Bryus, Murav’ev-Amursky.

Bottom topography of the Peter-the-Great Bay is characterized by the developed shallow-water and steep continental slope cut by the subsurface canyons. The continental slope lies 18 and 26 miles to the south from Islands of Askol’d and Rikord almost parallel to the line joining the Tyumen’-Ula River mouth and Povorotny Cape. The Peter-the-Great bottom is quite even and it is gradually elevating from south to north. In the eastern part, the depths reach 100 m and more, and in the western one they do not exceed 100 m. Off the bay exit, the depth is sharply increasing. On the continental slope, in a band of 3 to 10 miles wide the depths vary from 200-2000 m. Secondary bays - Amur, Ussuri, America are shallow water. In the Amur Bay the bottom relief is quite even. From the bay top coast the vast shoals are stretching. From the north-western coast of Russkiy Island to the opposite coast of the bay it is stretching the subsurface threshold, its depths making 13-15 m. In the Amur Bay the smallest depths are in its northern part, as well, there are a lot of rocky banks. In Uglovoy Bay, in the fairways leading to the bay, the depth makes 2.4-3.6 m; to the north of Razdel’ny Cape (eastern termination of De-Friz Peninsula) the depths are sharply decreasing to 2 m and less. By the exit of the Ussuri Bay the depths make 60-70 m, further on they are decreasing to 35 m in the middle part of the bay and to 2-10 m - in the top. In the America Bay the depths reach 23-42 m by the exit, in the middle part - 20-70 m, and the top of the bay is occupied by the shallow water with the depths less than 10 m.

Meteorological mode of the Peter-the-Great Bay is conditioned by the atmosphere monsoon circulation, geographical location of the region, influence of the cold Primorye Current. From October-November to March, due to the effect of the stationary pressure systems centers of actions (Asian High and the Aleutian Low) the cold continental air is transported from the continent to the sea (winter monsoon). As a result, in the Peter-the-Great Bay it is established the frosty low-cloud weather with the small precipitation amount and predominant winds of the northern and north-western directions. In spring, the wind mode is not stable, the air temperature is comparatively low and the long periods of dry weather are possible. Summer monsoon occurs from May-June to August-September. With this, it takes place the sea air transport to the continent and it is observed warm weather with the relatively large amount of precipitation and fog. The autumn season in the Peter-the-Great Bay is the best time period of a year - as a rule, it is warm, dry, with predominant clear sunny weather. In certain years, warm weather lasts to the end of November. Monsoon character of the weather is destroyed by intensive cyclone activity. Cyclones are accompanied by the increase of cloudiness till the complete one, strong cloud-burst precipitation, worsening of visibility and strong storm activity.

Average annual air temperature is about 6^o. The coldest month is January, when the average monthly air temperature in the northern part of the Amur and Ussuri Bay makes -16^o...-17^o. In the top of the Amur and Ussuri Bays the air temperature may decrease to -37^o. The warmest month is August when the average monthly temperature increases up to +21^o.

During winter monsoons, from October-November to March, the winds of the northern and north-western direction predominate. In spring, at the shift of the winter monsoon to the summer one, the winds are not stable. In summer, the south-eastern winds predominate in the Bay. Calm weather is more often in summer. Average annual wind velocity varies from 1 m/s (in the northern part of the Amur Bay) to 8 m/s (Askol’d Island). In certain days, the wind velocity can reach 40 m/sec. In summer, the wind velocity is lower. In the tops of the Amur Bay and Ussuri Bay the average monthly wind velocity is 1 m/s, in the bays and inlets - 3-5 m/s. Storms are generally related to the cyclone activity and they are mainly observed during the cold period of a year. The largest number of days characterized by the storm wind is observed in December-January and makes 9-16 days a month. In the tops of the Amur Bay and Ussuri Bay the storm winds are observed not every year.

To the Peter-the-Great Bay the typhoons come, which originate in the tropical latitudes, in the Philippine Islands area. To the Japan/East Sea and Primorski Krai, mainly in August-September, about 16% of all originated tropical cyclones come. Their paths are quite various, no one repeats the trajectory of another one. If the typhoon does not enter the Peter-the-Great Bay but is observed just in the southern part of the Japan/East Sea, still it effects the weather in this area: it rains heavily and the storm wind starts to blow.

Horizontal distribution of temperature

Water temperature on the surface is characterized by significant seasonal variability due to the interaction of the surface layer with the atmosphere. In spring, water temperature in the surface layer in the bay water area ranges from 4 to 14^o. In the tops of the Amur and Ussuri Bays it reaches correspondingly 13-14^o and 12^o. As a whole, the Amur Bay is characterized by higher temperatures than the Ussuri Bay. In summer, the bay waters are well warmed. At that time, in the tops of the Amur Bay and Ussuri Bay it reaches 24-26^o, in the America/Nakhodka Bay - 18^o, and in the open part of the bay - 17^o. In autumn, the temperature decreases to 10-14^o in the secondary bays, and to 8-9^o - in the open part. In winter, the whole water mass is cooled, its temperature varies from 0 to -1.9^o. Negative temperature values occur in all shallow water area, as well as in the secondary bays. 0^o isotherm location approximately coincides with the 50 m isobath. At that time, the waters in the open part of the bay are warmer than the coastal ones and are characterized by positive temperature values. With the depth growth the range of temperature variation decreases and at the depth of 50 m it does not exceed 3^o, and at the depths of more than 70 m the seasonal variations are almost not exhibited.

During the warm period of a year (April-November) it is observed monotonous temperature decrease with the depth. At that time, in the subsurface levels it is formed a layer of seasonal thermocline - in all places, except the shallow water area, where all water thickness is well heated and mixed. In autumn, with the start of the winter monsoon activity and cooling it occurs the upwelling of the deep cold waters on the shallow water areas and at the depth of 40 m it is formed the second layer of the temperature jump. In December, both layers of the thermocline are destroyed under the influence of convection, and during all winter season (from December to March) the temperature is constant in the bounds of all water thickness in the bay.

Orographic conditions in the bay and the continental slope influence create a specific mode of salinity distribution and variability. In some coastal areas of the bay the water is freshened till somewhat saline one, and in the open areas - its salinity is close to that of the adjacent sea area. Annual salinity changes is characterized by the minimum in summer and the maximum in winter. In spring, on the surface, the minimal values of salinity are confined to the top of the Amur Bay, where they make 28‰. In the top of the Ussuri Bay, the salinity is 32.5‰, in the rest water area it is growing to 33-34‰. In summer, the surface layer is subjected to the greatest freshening. In the top of the Amur Bay the salinity makes 20‰, and as a whole, in the surface waters and secondary bays it does not exceed 32.5‰ and increases to 33.5‰ in the open areas. In autumn, the horizontal salinity distribution is similar to that of the spring. In winter, in all water area of the bay the salinity is close to 34‰. At the depth exceeding 50 m the salinity varies in the bounds of the bay water area in the range of 33.5-34.0‰.

With the depth growth, the salinity, as a rule, increases (spring-autumn) or stays constant (winter). In the near-bottom layer of the bay, due to the process ice formation, in winter months salinity increases and waters of high density are formed, their temperature being less than -1.5 ^o, and salinity making 34.2-34.7‰. During the years of extreme ice conditions the high density waters, while propagating near the bottom, reach the shelf edge, then slide along the slope and ventilate the deep-water layers of the sea.

In winter season, in the Peter-the-Great Bay the water by its characteristics through all water thickness corresponds to the deep water mass of the Japan/East Sea (the temperature is less than 1^o, salinity - about 34‰). In the near-bottom layer of 20 m at that period it is distinguished water mass of increased density with low temperature (to -1.9^o) and high salinity (to 34.8‰) which as early as in the middle of March vanishes while mixing with the adjacent waters.

In summer, due to the increase of heat supply and the continental runoff, the layering of water thickness takes place. In the coastal areas, especially in zones of the fresh water supply from the river mouths, it is distinguished the estuary water mass with low salinity (in average, 25‰), high temperature (in average, 20^o) in summer, and the propagation depth of up to 5-7 m. Water masses in the open bay areas are separated by a seasonal thermocline into: surface coastal mass (it is propagating from the surface up to the depth of 40 m, its indices in summer are: temperature - 17-22^o, salinity - 30-33‰); subsurface water mass - till the depth of 70 m, with the temperature of 2-16^o and salinity making 33.5-34.0‰; and the deep water shelf mass - below the horizon of 70 m till the bottom with the temperature of 1-2^o and salinity of about 34‰.

Water circulation in the Peter-the-Great Bay is formed under the influence of the constant currents of the Japan/East Sea, tidal, wind, and runoff currents. In the open bay it is distinctly traced the Primorye current, which is propagating to the south-west at the velocity of 10-15 cm/s. In the south-western bay area it turns to the south and gives the origin to the North-Korean Current the most expressed in the sub-surface levels. In the Amur Bay and Ussuri Bay the Primorye Current influence is distinctly manifested just at the wind absence, when in the Ussuri Bay it is formed the anticyclonic circulation, and in the Amur Bay - cyclonic circulation. The wind, tidal phenomena and the runoff of the Razdol’naya River (in the Amur Bay) induce considerable effect on the current field. Scheme of major component total currents of the Amur Bay and Ussuri Bay [Savel'eva, 1989] given in the Atlas, show that the largest input is made by the wind currents, in winter they enhance the anticyclonic circulation in the Ussuri Bay, and in summer they change it to the cyclonic one. During the passage of cyclones, the velocities of the total currents on the surface can reach 50 cm/s.

Semi-diurnal tidal wave enters the Peter-the-Great Bay from the south-west and propagates to the secondary Bays of Pos’et, Ussuri, and America. It moves round the bay for the time period of less than an hour. Time of full water occurrence of the semi-diurnal tide is slower in the closed bays and secondary bays separated by the islands and peninsulas. Maximal possible tide value (during day and night) in the bay makes 40-50 cm. The most largest tides range are in the Amur Bay, in its north-western area, where the maximal tidal range somewhat exceed 50 cm, and the least developed - in the Ussuri Bay and in the Strait between Putyatin Island and the continent (tidal range is up to 39 cm). Tidal currents in the bay are insignificant and their maximal velocities do not exceed 10 cm/s.

Ice in the area practically does not prevent the regular navigation during the whole year. In the bay, the ice is found in winter season as the fast ice and drifting ice. Ice formation starts in the middle of November in the Amur Bay inlets. At the end of December, the majority of the Amur Bay inlets and some part of the Ussuri Bay are completely covered with ice. In the open sea area it is observed the drifting ice. Maximal development of the ice cover is at the end of January - middle of February. From the end of February the ice conditions are getting easier, and in the first half of April, generally, it takes place full clearance of the bay water area from the ice. In severe winters, especially in the first decade of February the ice reaches great compaction, that excludes the possibility of the vessel navigation without using the ice-breakers.

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Hydrochemical characteristics

In the given Atlas the hydrochemical characteristics are presented as the maps of distribution on different levels of the average for-many-years values of the content of the dissolved oxygen (ml/l), phosphates (µM), nitrates (µM), silicates (µM) and chlorophyll (µg/l) for winter, spring, summer and autumn without additional description. In the source of the used data (WOA’98) the temporal bounds of the hydrological seasons are determined as follows. Winter: January-March. Spring: April-June. Summer: July-September. Autumn: October-December [Antonov et al, 1999].

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Acoustic Characteristics

Main changes of sound speed values, both seasonal and spatial, occur in a layer of 0-500 m. The difference in sound speed values during one and the same season on the sea surface reaches 40-50 m/s, and at the depth of 500 m - 5 m/s. Maximal values are found in the southern and south-eastern parts of the sea, and the minimal ones - in the northern and north-western. The range of seasonal changes of sound speed in both zones is approximately similar and reaches 35-45 m/s. Frontal zone is stretching from the south-west to the north-east across the central part of the sea. Here, in a layer of 0-200 m the maximal horizontal gradients of sound speed values are observed at any time (from 0.2 s^-1 in summer to 0.5 c^-1 in winter). With this, the maximal changes of sound speed values by horizontal are observed in summer at the depth of 100 m.

By the vertical distribution of sound speed in the southern and south-eastern sea part it may be distinguished:

The axis of the underwater sound channel (USC) is located at the depths of 300-500 m, and near the coast of Japan at 40^o N it lowers to 600 m. Sound channel is propagating from the surface to the bottom. In the northern and north-western sea part the homogeneous layer, but with the minimal values of sound speed (less than 1455 m/s) is formed in winter and is related to the winter convection. A layer thickness can reach 600 m; with this, it is formed the surface sound channel. During the rest period of time, the changes of sound speed with the depth are characterized by the negative gradients increasing from spring to autumn up to 0.5-0.8 c^-1 in a layer of 0-100 m, with the minimal gradients in a layer of up to 500 m thick and further on - sound speed increase at the constant gradient value. The USC axis with the minimal values of sound speed of 1455-1460 m/s in this part of the sea in winter comes out to the surface, and from spring to autumn it is gradually sinking to the depth of 200-300 m. With the movement to the south, in the area of the USC the axis is sharply deepening to 300 m. In the central part of the sea, the width of the sound channel in winter does not exceed 1000-1200 m, in spring it increases up to 1500 m, in summer and at the beginning of autumn it is conditioned just by the depth of the place.

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