The Okhotsk Sea

Physical and geographical characteristics and hydrometeorological conditions

The Okhotsk Sea is located in the North-Western Pacific near the Asian coast, it is separated from the ocean by a chain of the Kuril Islands and Kamchatka Peninsula. In the south and west it is bounded by the coast of Hokkaido Island, eastern coast of Sakhalin Island and the coast of the Asian continent. The sea is strongly elongated from the south-west to the north-east in the bounds of a spherical trapezium with the coordinates 43^o 43’ - 62^o 42’ N and 135^o 10’ - 164^o 45’ E. The largest extension of the water area in this direction is 2463 km, and the width reaches 1500 km. The area of the sea water surface by some estimates makes 1603 thousand km², the coast line length is 10460 km, and the total volume of the sea water makes 1316 thousand km². By the geographical situation it is related to the marginal seas of the mixed continent-marginal type. The Sea of Okhotsk is linked with the Pacific Ocean by the numerous straits of the Kuril Island Ridge, and with the Sea of Japan - through the Laperuze Strait and through the Amur drowned river - by the Nevel’skoy Strait and the Tartar Strait. Average value of the sea depth makes 821 m, and the largest one - 3374 m (in the Kuril Basin). Some sources propose different values of the maximal depth - 3475 and even 3521 m.

Major morphological zones of the bottom relief are: the shelf (continental and island shoal of Sakhalin Island), continental slope, on which the particular under-sea rises, troughs and islands, and a deep-water depression are distinguished. Shelf zone (0-200 m) has the width of 180-250 km and it occupies about 20% of the sea area. The continental slope wide and gently-sloping in the central part of the basin (200-200 m) occupies about 65%, and the deepest basin (more than 2500 m) located in the southern part of the sea makes 8% of the sea area. In the continental slope area there are several rises and troughs where the depths sharply change (USSR Academy of Sciences Rise, Institute of Oceanology Rise, Deryugin Basin, TINRO Basin). The floor of the deep-water basin presents itself a plane abyssal valley, and the Kuril Archipelago is a natural threshold separating the sea basin from the ocean.

The straits linking the Sea of Okhotsk with the adjacent areas of the Sea of Japan and the Pacific Ocean provide the possibility of the water-exchange between the basins, which, in its turn, imposes significant influence on the distribution of hydrological characteristics. The Straits of Nevel’skoy and Laperuz are relatively narrow and shallow-water, which is the reason of the comparatively weak water exchange with the Sea of Japan. The Kuril Archipelago stretching is about 1200 km. Straits of the Kuril Island Ridge are more deep-water and their total width is 500 km. The deepest straits are Bussol Strait (2318 m) and Kruzenshtern Strait (1920 m).

The Sea of Okhotsk is located in the monsoon climate zone of the moderate latitudes, but for the northern part of the sea, which is deeply intruding into the Asian continent, it is typical some peculiar features of the Arctic Sea climate. Monsoon climate conditioned by the changes of the location and by the character of interaction of the pressure formations, as well as the sea location on the margin of the Asian continent and the Pacific Ocean, are major factors forming the climate and hydrological mode of the sea. Major pressure formations which condition the atmosphere circulation and the air masses transport, are the Aleutian Low, North Pacific High, Siberian High (in winter), as well as the Far Eastern Low and the Okhotsk High (in summer). Monsoon character of circulation and the wind mode is often violated by the deep cyclones passing from the south-west to the north-east. Here, especially in the northern part of the sea, the winter is long and severe, with the frequent storm winds and snow-storms. Summer is cool with the large quantity of precipitation and dense fog. Spring and autumn are short, cold and cloudy. As a whole, the Sea of Okhotsk is the coldest one out of the Far Eastern Seas of Russia. Here, the cold period of a year lasts from 120-130 days in the south to 210-220 days in the north of the sea. The cooling factor influence is stronger than that of the warming factors, and the resulting heat exchange on the surface is negative. As a whole, by its climate conditions, the Sea of Okhotsk is the coldest one of the Far Eastern Seas.

From May to September over the sea water area the weak winds of the southern quarter predominate (2-5 m/s). Cases of the short-period sharp increase of the wind (up to 20 m/s and more) are related to the particular cyclones and typhoons appearing over the sea most frequently in August-September. Usually, there are 1-2, rare 3-4 typhoons a year. During the cold period of a year over the sea strong winds of the northern quarter with the most probable values of velocity being 5-10 m/s (in particular months - 10-15 m/s) predominate. Repeatedness of the storm winds with the velocity of more than 15 m/s in average during a year makes about 10%. Probability characteristics of the velocity and wind direction differ distinctly for the particular sea areas. Maximal wind velocities reach 25-30 m/s in the north-eastern and western parts of the sea, 30-35 m/s - in the central and eastern parts, and more than 40 m/s - in the south. Autumn-winter storm winds, as compared to the summer ones, are characterized by the stronger force and duration. The most non-convenient are southern and south-eastern areas of the sea. Considerable horizontal sea length, frequent and strong winds over the water area provide the development of the strong wind waves and swell (wave height is 4-6 up to 10-11 m), and all integrity of hydrometeorological conditions makes the preconditions for the dangerous icing of the vessels and installations located in the sea.

The average annual values of the air temperature over the sea of Okhotsk gradually decrease from south to north from 4-5^o to -4...-5^o. The range of the average monthly temperature variations in this direction, vise versa increases from 15-18^o to 30-36^o. The coldest month is January, the warmest one - August. Minimal factual values of the air temperature observed on the coastal stations are -36...-51^o in the north and -12...-16^o in the southern sea areas. Maximal values (31-36^o) occur in the south-western sea area. During the cold period, at the change of the synoptic situations, sharp variations of the air temperature occur in the bounds of the whole territory, their range can exceed 20^o [Demenitskaya et al, 1974; Dobrovol'skii and Zalogin, 1982; Grishin, 1959; Polyakova, 1999: Terziev et al, 1998].

The Sea of Okhotsk, along with the Bering Sea, is a highly productive marine ecosystem and it is very significant for the fisheries of Russia.

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Hydrological characteristics

Hydrological mode of the sea is conditioned by peculiarities of its geographical situation, considerable meridional extension, severe climate conditions, character of the vertical and horizontal circulation, water exchange with the Pacific Ocean and the Sea of Japan, as well as the bottom topography. Near the coast, significant importance is additionally acquired by the continental discharge, tidal phenomena, the coast line configuration. Complex of these factors creates quite a complicated scheme of hydrological characteristics distribution on the surface and intermediate depths. 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 Bering Sea based mainly on the following research papers, monographs, atlases and reference books: [Belkin, 2001; Bogdanov et al, 1991; Demenitskaya et al, 1974; Dobrovol'skii and Zalogin, 1982; Kowalik and Polyakov, 1998; Preller and Hogan, 1998; Tally and Nagata, 1995; Terziev et al, 1998; Yakunin, 1995], as well as on graphic materials of the Atlas.

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

Observed characteristics of the field of the water temperature horizontal distribution on the surface and on deep levels of the Okhotsk Sea are formed and are constantly changing under the influence of the physical processes of different scale and intensity on the surface and in the sea water column. Variations of these parameters, as in the other Far Eastern Seas are most distinctly exhibited in the active surface layer of the sea, where it is distinctly traced their short-period and daily variability, seasonal annual and inter-annual climate motion, non-periodic variations of various nature. Physics of these processes and regional peculiarities of the thermal mode in this water area are comparatively well studied. Assimilation of the long-period hydrological observation data allows to construct the generalized seasonal schemes of spatial temperature distribution on different levels.

Sea surface temperature (SST), except particular summer months, when there is more diverse picture, generally decreases from south to north. In the south the average annual temperature values make 5-7^o, in the north - about 2-3^o. Annual variations of the SST are quite considerable on the whole area. It is quickly extinguish with the depth. Variations of the SST are 10-19^o. Maximal average values of the annual variation amplitudes are observed in the southernmost part of the sea and somewhat less - in its all western part. Minimal ones - near the central and northern part of the near-Kuril area. During the period from May to November the average monthly water temperature values are positive in all places. At the expense of the non-regular heating and mixing of the surface layer, as well as at the expense of the advection processes, at that time the horizontal distribution of temperature is the most non-homogeneous. While in May the average temperature values on the surface vary from 0 up to 5^o, in August, the warmest month, these values increase up to 8-18^o. The warmest waters are located in the southernmost part of the sea near the Laperuze Strait and Hokkaido Island. Note that the time of the temperature maximum on the surface in some areas can vary for 1-2 months and it is somewhat late on the sub-surface horizons. In October, the water temperature on the surface about two times decreases, and in November its spatial distribution passes to the winter type. In February-March, when a considerable part of the sea is covered by ice, the lateral gradients of the temperature are smoothed and temperature reaches -1.0...-1.8^o. In the south-eastern part of the sea and to the north-west from the Kuril Islands the SST almost never falls till the negative values.

Seasonal changes of temperature are observed in the upper activity layer (up to 100-250 m) with the well developed seasonal thermocline. Annual temperature values on the depth 50 m do not exceed 3-4^o_, and at the depth of 75-100 m - 2.0-2.5^o. At 50 m the temperature maximum occurs in October-November. At that time, the water temperature makes 6-8^o in the south, and 0-2^o in the north-western part of the sea. In December, at this depth the negative values of temperature appear. On 100 m level the temperature negative values in the north-western part of the sea are preserved during the whole year, and at 2000 m depth in the averaged fields they almost do not manifest themselves. Here, the temperature in the whole sea basin varies from 0.5^o up to 1.5-2.0^o. On the below-lying levels of 200-1000 m the average long-term values of temperature in all places somewhat increase (up to 2.3-2.4^o on 1000 m depth). Below 1000-1200 m, the temperature values on different levels are somewhat lower (1.95-2.00^o at 2000 m depth).

As in any other sea, the above data exhibit the background characteristics of the large-scale distribution and variability of the water temperature which can vary from year to year (interannual variability) and be detailed in the process of the new data accumulation. To solve many practical tasks, along with the general, background characteristics of marine environment, more detailed data on the factual distribution of its parameters in particular areas are needed; such data should be approximated to the real time scale. Results of studies show that a significant role in the formation of the smaller meso-scale non-homogeneities of the temperature field on the surface horizons is plaid by the frontal zones, eddy formations, some circulation cells and the water upwelling zones which occur in the coastal zone, on the shelf, in the deep-water basin and are an object of special investigation. The Atlas provides a generalized scheme of the surface thermal fronts in the Okhotsk Sea which is based on the satellite observation data for the warm period of a year.

By the character of the temperature vertical distribution, the Sea of Okhotsk waters stratification is referred to the Subarctic type in which for the largest part of a year the cold intermediate layer (CIL) (sub-surface one) and the warm deep-water layer (WDL) are well expressed. More detailed consideration gives the opportunity to distinguish three major varieties of this structure, i.e. that of the Sea of Okhotsk, the Pacific one, and the Kuril one possessing qualitative differences in the water mass characteristics. The greatest variability from area to area, and especially in the annual motion, is typical for the water mass structure of the upper layer of 100-150 m thick (in the south-east - 200-250 m). In different months the SST changes from -1.8 up to +18^o. During a warm period, as a result of heating and vertical mixing, in its upper part it is formed a thin surface quasi-homogeneous layer (QHL) and seasonal thermocline (ST). The QHL thickness is 10-20 m, and that of the ST - 15-25 m (in places - even more). Vertical gradients in the thermocline reach the values of 5-10^o/m. At that time, in the layer of 40-120 m it is distinctly distinguished the CIL core, which lower boundary occurs at the depths of 100-250 m (thermal mode of this layer has been considered above). The advection processes lead to the CIL splitting and to the formation of separate «cold core» in its structure. Below this layer, during the whole year the temperature is monotonously increasing with the depth, reaching the local maximum (2.2-2.4^o) in the WDL core at the depths of 800-1200 m. Note, that in particular years the negative temperature values can be measured at the depths of up to 500 m. In the deep-water layer below the WDL core, the temperature is gradually decreasing with the depth up to 1.7-1.9^o near the floor. General notion on peculiar features of spatial distribution of the distinguished elements of stratification and their temporal dynamics is provided by the vertical zonal and meridional sections of the temperature field, presented by the Atlas.

By the data of daily and longer series of continuous observations in the warm period of a year, the profiles of the vertical temperature distribution on the surface and in a layer of the temperature abrupt change undergo significant variations in time. For instance, the value of diurnal variations of the water temperature on some horizons of the periphery sea areas can reach 8-12^o.

Large-scale characteristics of a salinity field are predetermined by peculiarities of the fresh water flux in the Okhotsk Sea (precipitation - evaporation, ice formation and melting), continental fresh water run-off in the coastal areas, as well as by the water exchange through the straits and the transport from the adjacent areas. At the expense of the integral influence of these processes the schemes of spatial distribution of salinity are significantly non-homogeneous and considerably change from season to season. In the course of a year, the sea-surface salinity in the coastal areas of north-western part of the sea ranges from 20-25 up to 30-33‰. In summer and at the beginning of autumn here the water salinity is less than in winter. In winter it increases at the expense of ice formation processes and the coastal run-off decrease. Salinity maximum in these areas is observed from December to March. In the open sea and in its south-western part the range of these changes is considerably lower (31.0-33.5‰.). Important role in the salinity field formation in this area is played by the water exchange through the Laperuz Strait and the Kuril straits. Here, the periods of appearance of both maximum and minimum of salinity are different for different areas. As a result, salinity distribution on the Okhotsk Sea surface in some months is characterized by considerable alternation. In February, in the areas free of the ice cover, the average long term monthly values of salinity on the surface vary from 32.6-33.3‰. In May, salinity in the coastal zone and near Sakhalin Island decreases to 30-32‰. At that time, in the open sea it makes 32.5-33.0‰, and near the Kuril Islands and Hokkaido Island - 33.0-33.5‰. In August-September the maximal freshening of all surface layer takes place. Near the northern termination of Sakhalin Island, in the continental bays and inlets of the coastal band, in summer salinity decreases to 20-30‰, and in the open sea - to 32.0-32.5‰. In November-December, in all water area of the sea, the salinity increases again. During the warm period, even in the maps of distribution of the averaged values of salinity by the month in the particular areas of the coastal zone (Sakhalin Island, Kamchatka Peninsula, Tauiskaya Guba, etc.) the zones of maximal horizontal gradients, i.e. salinity fronts, are distinctly expressed.

With the depth, the salinity, both in the surface layer, and in the lower layers is constantly increasing in all area and during all seasons. The range of its spatial and temporal changes is sharply narrowing, and the areas of maximal and minimal values are shifting. For instance, on a level of 50 m, the average values of salinity in all water area vary from 32.0 to 33.5‰ and seasonal variations do not exceed 0.5-1.5‰. On 100 m level the value of annual salinity variations decrease to 0.5-1.0‰, and horizontal gradients of salinity field are weak. On 200 m level the background values of spatial salinity changes do not exceed 0.2-0.3‰, and those of temporal ones - 0.10-0.15‰. On levels of 500 m and 1000 m the salinity values somewhat increase in direction from the south-east to the north-west (from 33.58 to 34.85‰, and from 34.18 to 34.42‰, correspondingly), that is related to the peculiar features of the Pacific water distribution and the vertical circulation. In the lower layers, the salinity as a whole is somewhat growing with the depth, and the range of spatial salinity changes is narrowing from 34.37 to 34.54‰ (1500 m level) to 34.38-34.52‰ (2000 m).

As in the case of the temperature field, the above data exhibit just the large scale, background characteristics of the horizontal salinity distribution in the Okhotsk Sea. The available data of hydrological survey allow, if necessary, to precise some details of this matter and to trace its dynamics retrospectively.

Salinity profiles are almost identical for all seasons, and as a whole, are characterized by the monotonous salinity increase from the surface to the bottom. As in the temperature field, the seasonal changes are exhibited mainly in the upper 50-100 m layer (in places - to150-200 m). During the warm period, the surface layer is freshened, the vertical gradients of salinity are increasing, and here it is formed a seasonal halocline. Below it, till the depth of 600-800 m (in the central part of the basin) and 800-1000 m (in the south of the sea) the major halocline is located, in which thickness there is a gradual decrease of the vertical gradients. With the beginning of the winter convection mixing accompanied by the ice formation, the vertical gradients of salinity in the upper layer are rapidly decreasing till the inverse values (the change of the gradient sign). General notion on the vertical structure of the salinity field is given by zonal and meridional sections. With regard to the local hydrological conditions, in some bays and straits, both the absolute values of salinity and its stratification can significantly differ from the similar characteristics in the open sea.

In the central Okhotsk Sea, Kuril Basin and in the periphery areas there are several water masses and their modifications with their proper hydrological characteristics, sources of formation and the distribution area. These water masses form the main components (particular layers and extremums) of the vertical structure of the water thickness. The main water mass of the sea is of the Pacific origin. For the Sea of Okhotsk basin it is typical the western variety of the Subarctic structure of waters, which main feature is a cold intermediate layer (in winter - sub-surface one) and an under-laying layer with the temperature maximum, which different water masses. By its origin, location and characteristics, here, four main water masses are distinguished: surface, cold intermediate (subsurface), deep water Pacific and the near-bottom ones. In the periphery sea areas, in the shelf zone, we distinguish different local, seasonal varieties and modifications of water masses (a list and parameters are presented in table). Their origin is determined by the difference in geographical location and by the hydrological process peculiarities on the shelf, in the near-estuary zones, near the straits, etc. Surface water mass occurs during the warm period, it is characterized by the temperature values maximal for the whole water thickness (to 18-19^o in the south of the sea) and the salinity values minimal for all seasons (less than 20‰ in the near-estuary areas). Its core is located on the surface and it is characterized by the maximal range of parameter variability during the annual motion. Cold intermediate (subsurface) water mass is formed during sea surface cooling and the autumn-winter convection. Its upper margin is located under the surface water mass on the depths of 25-50 m (in the south - 75-175 m) and in winter it is getting narrower towards the surface, and the cold core is at the depth of 40-120 m (in the south - 150-200 m). The lower margin is getting deeper from the north-west to the south-east from 200-250 m to 500-600 m. In winter, the water temperature in a layer occupied by the upper part of this water mass is decreasing till the negative values of -1.5...-1.8^o (in the south-western part - +0.5-1.0^o) which are preserved in summer either. Salinity in the core 32.5-33.4‰. The warm core of the deep Pacific water mass is situated between 500 and 1200 m (near-Kuril area). Water temperature in the core is 1.3-2.5^o, and salinity is 33.6-34.4‰. In a layer of the near-bottom water mass the temperature is gradually decreasing with the depth till 1.7-1.9^o near the floor, where salinity is 34.6-34.7‰. Water masses differ not only in the thermohaline parameters but also in hydrochemical and biological values. Table presents water mass characteristics of the Okhotsk Sea near-coastal areas.

Typical characteristics of the water masses on the Okhotsk Sea shelf

(numerator - February, denominator - August) [Zuenko, Yurasov, 1997].

Note: levels of occurrence of the surface and sub-surface Subarctic water masses in winter are not determined, as they do not differ in their thermohaline characteristics.

General scheme of circulation and summarized currents in particular areas of the Okhotsk Sea are formed as a result of composition of different types of the water motion with different spatial-temporal scales: relatively constant non-periodic currents, variations of seasonal and synoptical scales, tidal, inertial, and surge phenomena. Their observed characteristics can differ significantly in this or that point of spatial coordinates, and the generalized ones are in dependence on the time scale, accepted for the averaging. Available schemes of the sea water circulation are based either on the uncoordinated data of in situ observations, or they are obtained by the calculation methods and are mainly referred to the warm period of a year, when the sea surface is free of the ice cover.

The main peculiarity of the Okhotsk Sea circulation is the general cyclonic motion of waters (counter-clockwise) along the margins of the whole basin. On the background of the general circulation in the various areas there are the local anti-cyclonic and cyclonic circulation, and the eddy formations of a smaller scale. To the areas with the stable anti-cyclonic circulation we refer the vortices located above the TINRO Basin, to the west of the southern tip of Kamchatka, and in the area of the Kuril Basin. Relatively stable links of the total water gyre in the Sea of Okhotsk during the warm period are called the independent currents with the corresponding geographical referencing: Kamchatka (West-Kamchatka Current) and Compensatory, Yamskoye, Northern Okhotsk Current, East-Sakhalin Current, and Soya Current.

By the observation data and diagnostic calculations a general scheme of the circulation in the upper sea layer endues considerable changes from season to season. In autumn, the current velocities are somewhat growing. In winter, in the areas free of ice the currents of the southern, south-western direction are generally observed. Velocities of non-periodic currents in the surface layer reach considerable values in the southern part and on the periphery of the sea - in the coastal band, bays, straits and narrow areas. With the common synoptic situations above the Kuril Basin and near the western coast of Kamchatka they reach 10-20 cm/s, in the Shelikhov Bay - 20-30 cm/s, in Sakhalin Bay - 30-45 cm/s, in the Kuril straits area - 15-40 cm/s, in the Soya Current near the coast of Hokkaido Island - 50-90 cm/s, in the Kamchatka Current - 10-15 cm/s. In the central part of the basin the current velocities are smaller - about 2-10 cm/s. The influence of the atmospheric circulation on the currents in the subsurface and deep water layers is decreasing. On a horizon of 100 m the velocities of constant currents decrease to 5-10 cm/s in the central part and in the north of the sea, and to 15-20 cm/s in the south. In the lower layers the current velocities continue to decrease with the depth and on the horizon of 100 m, as a rule, they do not exceed 10 cm/s. But in the deep-water Straits of Bussol and Kruzenshtern, in a layer of 1000-2000 m the velocities of non-periodic currents may exceed 30-45 cm/s.

On the background of the general water circulation on the sea surface somewhat smaller elements are observed - quasi-stationary eddy formations and meanders of currents. For instance, in the Kuril Basin area annually there are 3-4 anticyclonic vortices of 100-150 km in diameter, they form the local peculiarities of the water motion.

In the Okhotsk Sea, well exhibited are periodic tidal currents which possess the rotational character in the open areas, and in the coastal areas - the reverse one. Far from the coast, the velocities of these currents are small - 5-10 cm/s, and near the coast, under-water shoals, in the bays and straits they reach extremely high values. For instance, near the Amurskiy Liman - up to 234 cm/s, in the Shantary area - 433 cm/s, in the northern and north-eastern coast - 300 cm/s, in the some Kuril straits - 360 cm/s and more, in the Laperuz Strait - 360 cm/s, in the bays of the eastern coast of Sakhalin - 260 cm/s.

Tidal phenomena in the Okhotsk Sea are related to the tidal wave propagation from the Pacific Ocean through the straits of the Kuril Islands. They induce considerable variations of the sea level, velocities and directions of currents. By the character of the sea level variability here, to different extent, all types of tides are exhibited: semi-diurnal, non-regular semi-diurnal, non-regular diurnal and diurnal. On the largest part of the water area the diurnal, non-regular diurnal and non-regular semi-diurnal tides are observed. Values of maximally possible tidal range of the level surface vary from several dozens of centimeters (northern and central coast of Sakhalin Island) up to 9.7 m in the Udskaya Guba, 10.1 m in the Tugur Bay and 13.9 m in the Penzhinskaya Guba. In other places they vary from 0.8 to 4.0 m, gradually increasing from south to north up to 5-7 m near the Shantary Islands and near the exit of the Penzhinskiy Bay.

A long winter leads to the strong cooling of the sea surface accompanied by the intensive ice-formation almost in all sea areas. The Sea of Okhotsk ice is of exclusively local origin. Here, both the fast ice and the drifting ice occur, they present themselves the most widespread form of the sea ice. As a whole, by the severity of the ice conditions the Sea of Okhotsk is comparable to the Arctic Seas. Average ice period in the north-western area of the sea makes 260 days, in the northern areas and near the coast of Sakhalin Island - 190-200 m, in the south - 110-120 days a year. During the most severe winters the ice cover occupies up to 99% of the total sea water area, and in warm winters - 65%. Maximal duration of the ice period reaches 290 days. Ice formation usually begins in November in the north-western part of the sea, and in the places of considerable water freshening - in October. The ice cover is gradually propagating to the south along the western and eastern coasts and appears in the open part of the sea. In December, in the bays and inlets it is formed motionless solid coastal fast ice. In January and February, the ice fields occupy the whole north-western and middle part of the sea. Drifting ice acquires strong convergence and under the influence of currents and winds it is subjected to the strong compression and hummocking. In the open part of the sea it is never observed motionless solid ice. The farthest ice motions to the south and south -east occurs in February and March. At that time it is found in all places. The eastern and western halves of the central part of the Okhotsk Sea differ sharply both in the continuation of the ice period and by the character of the ice conditions. During the long period from April to June it takes place the destruction and melting of the ice cover. In the north-western part of the sea the ice is preserved till July. The southern coast of Kamchatka, central and northern Kuril Islands are characterized by small ice amount and by considerably less period of ice existence. But during severe winters the drifting ice can be pressed to these islands and can block particular straits. In the coastal and shallow-water areas, in December-January the ice thickness (without considering the hummocks formation) reaches 40-50 cm, in the Shelikhov Bay and Kamchatka coast - 30-40 cm, in the open sea (in the moderately severe winters) - 40-70 cm. Maximal values of the ice thickness (90-160 cm) are observed in severe winters in the Sakhalin Bay and in the sea area to the north-east of Elizabeth Cape (Northern Sakhalin). Hummock height in the open sea doesn’t exceed 1 m, in particular bays - 1.5-3.0 m. Statistical characteristics of the ice cover distribution in the water area and variability of different parameters of the ice cover are comparatively well studied on the basis of the long-period observations and are described in detail in special literature.

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Hydrological conditions of the Kuril Island zone and the adjacent areas

The Kuril Islands zone is the intensive fishery and shipping area. This region is characterized by high rate of the spatial-temporal variability of the water structure and dynamic parameters. Intricate nature of hydrological regime is caused by the presence of frontal zone created by the distinctly modified waters (Pacific ocean – Okhotsk Sea) transferred by streams of the near-Kuril currents of the Okhotsk Sea and Pacific Ocean. The exchange processes that are intricate by multiform factors (first of all distinct tidal fronts and connected with them significant tidal currents above complicated of bottom relief) are forming specific water structure in the straits [Bogdanov and Moroz, 2000].

Tidal events in the Kuril Island region

Tides are the dominant factor that determines dynamics of water in the straits, vertical and horizontal water structure alterations. The tides in the Kuril Islands and in the Okhotsk Sea are formed generally by tidal waves that are come from the Pacific Ocean. The Okhotsk Sea tides conditioned by immediate influence of tide-generating forces is disdain small. Tidal waves in the north western part of the Pacific Ocean have mainly onward nature moving to the South-West along the Kuril Islands chain. Propagation speed of tidal waves in the ocean approaching to the Kuril Islands gains 1-2 m/s. Amplitude of sea level tidal fluctuations in the island zone does not exceed 1m. Speed of tidal currents is about 10-15 cm/s. In straits the amplitude of sea-level variations increases to 1.7-2.5 m. Speeds of tidal currents increase to 2.5 m/s. Owing to multiple reflection of tidal waves from the Okhotsk Sea coast the complex progressive-standing waves occur in the straits themselves. Tidal currents in the straits possess clear reverse character that is confirmed by measurements on the diurnal stations in the straits. Tidal ellipses, as a rule, are similar on their shape of directed line oriented along the straits.

Fig 1. Tidal maps of main constituents: M₂, S₂, K₁ and O₁.

Influence of atmospheric circulation on the thermodynamic parameters of water

The peculiarity of atmospheric processes in the Kuril Islands area as well as in the all Far Eastern Seas is the monsoon nature of atmospheric circulation. It is dominated by southern-eastern winds during summer monsoon period and opposite direction winds in winter. Intensity of monsoon development is identified by progress of large-scale atmospheric processes associated with the state of principal atmospheric centers action that regulates atmospheric circulation over the seas of the Far Eastern region namely Aleutian Low and Pacific High [Polyakova, 1999].

Fig.2. Prevailing types of atmospheric circulation: NW – North-western, CO – cyclones above the ocean, OA – Okhotsk-Aleutian [Polyakova A.M., 1999].

Water mass structure and dynamic peculiarities in the Kuril region

Water mass structure of the Kuril region is linked with Kuril Current being western boundary current of Western Subarctic Gyre in the Pacific Ocean The current is traced in the Subarctic water mass structure (Western modification) having the following features [Bogdanov and Moroz, 2000]:

1. Surface water mass (0-60 m); in spring T=2-3°C, S=33.0‰; in summer T=8°C, S=33.0‰.

2. Cold intermediate layer (60-200 m); T_min=0.3°C, S=33.3‰ with the core at the depth of 75-125 m.

3. Warm intermediate layer (200-800 m); T_max=3.5°C, S=34.1‰ with the core at the depth of 300-500 m.

4. Deep (800-3000 m); T=1.7°C, S=34.7‰.

5. Bottom (>3000 m); T=1.7°C, S=34.7‰.

Pacific water of northern Kuril straits is discriminated considerably from the waters of the Southern Straits. Water of Kuril current, that is being formed by very cold and freshened waters of East-Kamchatka Current and Pacific waters in the Kuril straits, mixes with the Okhotsk Sea transformed water. Then the Oyashio Current (to south of Bussol Strait) water is being formed by mix of the Okhotsk Sea water that is transformed in straits and by Kuril Current (to north of Bussol Strait) water.

The Okhotsk Sea water structure, determined as its variant of Subarctic water structure, consist of the following water masses [Bogdanov and Moroz, 2000]:

1. Surface water mass (0-40 m); in spring T=2.5°C, S=32.5‰; in summer T=10-13°C, S=32.8‰.

2. Cold intermediate layer (40-150 m); T_min= -1.3°C, S=32.9‰ with the core at the depth of 100 m. (It is formed in the Okhotsk Sea in winter).

Along the Kuril Islands in the Okhotsk Sea it is observed the abrupt “end” of cold intermediate layer core with minimal temperature lower than +1°Cat the distance of 40-60 miles from the islands coast. The abrupt “end” of cold intermediate layer evidences existence of clear front between intermediate water of the Okhotsk Sea and the mixed water by tides in the straits. The front restricts spread of colder surface water along the Kuril Islands area. That is cold intermediate layer in the Okhotsk Sea is not connected with the same in Oyashio current and is identified by winter temperature conditions in the region.

3. Okhotsk Sea intermediate water mass (150-600 m) forming as result of tidal mixing of upper layer in Pacific and Okhotsk water of the Kuril straits zone (T=1.5°C, S=33.7‰).

4. Deep water mass (600-1300 m) appearing in the Okhotsk Sea as a warm intermediate layer with T=2.3°C, S‰=34.3‰ at the depth of 750-1000 m.

5. Deep water mass of Okhotsk Sea (more than 1300m) with parameters T=1,85°C, S=34.7‰.

In the southern Okhotsk Sea the surface water mass has three modifications. First modification is low salinity water (S<32.5‰). It is formed generally during melting of ice and situated till 30 m depth in the period since April till October. The second is in the East Sakhalin current and is observed in 0-50 m layer being characterized by low temperature (<7°C) and low salinity (<32.0‰). The third modification is in warm and salt water of Soya Current being the extension of Tsusima Current branch that cover area along the Okhotsk Sea coast of the Hokkaido Island (in the layer of 0-70 m) from the Laperouz Strait till the South Kuril Islands. Since March till May there is taking place “a precursor” of Soya Current (T=4-6°C, S=33.8-34.2‰) but since June to November in the full sense Soya Current with high temperature (till 14-17°C) and more salinity (till 34.5‰).

The Kuril straits

The Kuril archipelago of approximately 1200km long consists of 28 relatively large islands and many small ones. Total width of the Kuril straits is some 500 km. About 43.3% are taken by the Bussol Strait (depth of sill is 2318 m), 24.4% - by Kruzenshtern Strait (depth of sill is 1920 m), 9.2% - by Friz Strait and 8.1% - by Chetvertiy Kuril Strait. But the depth of even the deepest Kuril Strait is significantly lower than maximal depth of adjacent to the Kuril Island areas of the Okhotsk Sea (some 3000 m) and of the Pacific Ocean (more than 3000 m). Therefore the Kuril archipelago presents by itself a natural sill that divides sea basin from the ocean. Along with it Kuril straits are just that zone where water exchange between mentioned basins occurs. This zone has its own peculiarities of hydrological regimen that are distinct from the regimens peculiarities of topography and relief of bottom in this zone induce corrective influence to formation of water structure and dynamics.

On the base of summary of multi-year observations there was disclosed that in the straits zone more complicated, than believed earlier, water hydrological structure is observed.

First of all the transformation of water in the straits is exposed differently. Transformed structure of water possessing intrinsic signs of the Kuril type of Subarctic water structure is observed mainly on the Islands shelf where tidal mixing is more evident. This structure is characterize by negative anomalies of temperature and positive ones of salinity on surface during warm half year and by more mighty cold intermediate layer and smoother extremes of intermediate water masses including positive anomaly of minimal temperature. In the shallow water zone the tidal transformation results in formation of vertically homogenous water structure. In deep water areas of straits highly stratified water is observed.

Secondly, the problem is that of the characteristic presence of variously scaled inhomogeneous that is being formed at eddy-formation and front-genesis during the process of currents touching that takes place on the background of tidal mixing. Along with it alteration of location of intermediate layers boundaries and extremes occurs in thermohaline field structure. In the eddies and also in the currents stream that carry and preserve own parameters it is observed location of homogeneous cores of minimal temperature in cold intermediate layer.

Thirdly the structure of water in straits zones is being corrected by variability of water exchange in the straits. In every of the main Kuril straits in various years depending on development any link of currents system in the region there is possible both a dominant outflow the Okhotsk Sea water, a dominant inflow the Pacific Ocean water or bilateral water circulation.

Chetvertiy Kuril Strait

Chetvertiy Kuril Strait is one of the main north Kuril straits. Cross section of this strait is 17.38 km² and depth is some 600 m. Topographical peculiarity of the strait is its openness into the Okhotsk Sea side and presence of sill with some 400 m depth from the Pacific Ocean side.

Thermohaline structure in the Chetvertiy Kuril Strait [Bogdanov and Moroz, 2000]

For the bottom complex relief the volume of water mass in the strait is different. In the shallow water vertical mixing results in homogenization of water. In these cases only surface water mass is placed. In the main part of the strait with depth of 500-600 m only a surface and cold intermediate water masses were observed. The sill of some 400 m depth exists from the Pacific Ocean side. Water exchange between the ocean and sea occur till the sill depth. This means that the Pacific Ocean and Okhotsk Sea water masses spreading on great depth have no contact in the strait zone.

Fig. 3. Distribution of hydrological parameters in Chetvertiy Kuril Strait in summer: (a)-temperature on vertical section, (b, c)-temperature and salinity on surface, (d, e) -schemes of geostrophic circulation at 0 and 200 m.

Kruzenshtern Strait

Kruzenshtern Strait is one of the widest and deepest strait of the Kuril Islands. Cross section square of the strait is 40.84 km². The strait sill of 200-400 m is situated on its oceanic side. There is the trench of 1200 m till 1990 m through which the water exchange between deep water of the ocean and sea occurs in the strait. North-eastern part of the strait is shallow with less than 200 m depth. The system of islands and straits that form Kruzenshtern strait includes group of small islands and rocks restricted from south by Simushir Island and from north by Shiashkotan Island.

Thermohaline structure in the Kruzenshtern Strait [Bogdanov and Moroz, 2000]

Fig.4. Spatial distribution of hydrological parameters in Kruzenshtern Strait zone in summer: (a)-temperature on vertical cross section, (b, c)-temperature and salinity on surface, (d, e) -schemes of geostrophic circulation on 0 and 200 m.

Bussol Strait

Bussol is the widest and deepest strait of the Kuril Islands. It located in their middle part between Simushir and Urup Islands. Dew to its great depth, its section is equal to almost area half (43.3%) from all cross sections of these Islands Straits and amounts to 83.83 km². Topography of the strait is distinguished by sharp alterations of depth. In the strait central part there is a sill till 515 m depth where it is divided by two trenches, namely by Western one with 1334 m depth and by Eastern trench of 2340 m depth. Presence of great depth in the strait creates conditions to preserve vertical stratification of waters and penetration Pacific Ocean water into the sea great depth.

Thermohaline water structure in the Bussol Strait [Bogdanov and Moroz, 2000]

Fig.5. Spatial distribution of hydrological parameters in Bussol Strait zone in summer: (a)-temperature on vertical cross section, (b, c)-temperature and salinity on surface, (d, e)-schemes of geostrophic circulation on 0 and 200 m.

Friz Strait

Friz Strait is one of the main straits in southern part of the Kuril Islands. The strait lies between Urup and Iturup Islands. Cross section - amounts to 17.85 km². The strait depth is some 600 m. On the Pacific side there is a sill with 500 m depth approximately.

Thermohaline water structure in the Friz Strait [Bogdanov and Moroz, 2000]

In the deepest part of the strait where depth equals some 500 m only two water masses that are surface and cold intermediate were observed. On the deeper stations the observed warm intermediate water mass is near-bottom (due to small strait depth of some 600 m). Warm intermediate layer in the strait zone has smoothed parameters more closed to the indices of warm intermediate layer of the Okhotsk Sea. Because of small depth in the strait the deep Okhotsk Sea and Pacific water masses have no contact in the strait zone.

Considerable range of thermohaline parameters in the surface water mass is related with peculiarities of water circulation in this region especially with seasonal and interannual variations of Soya current intensity. The current origins in southern part of the Okhotsk Sea in spring intensifies and maximally is spread in summer weakening in autumn. Meantime border of the current spread depends on its mighty and alters from one year to the other. Totally Friz Strait does not belong to the discharging nor to the supplying one, although in several years it is just of such type that is considerably determined by influence of atmospheric circulation.

Fig. 6. Spatial distribution of hydrological parameters in Friz Strait zone in summer: (a)-temperature on vertical cross section, (b, c)-temperature and salinity on surface, (d, e)-schemes of geostrophic circulation on 0 and 200 m.

Ekaterina Strait

The Ekaterina Strait is situated between Iturup and Kunashir Islands. The strait narrowest width is equal to 22 km, depth of sill is 205 m, area of cross section is some 5 km². From North, that from side of the Okhotsk Sea, the trench of more than 500m depth approaches there, which extension with depth of more than 300 m runs through the central deep part of the strait. From oceanic side the depth is no more than 200-250 m.

In Ekaterina Strait the water structure essentially differs from the main Kuril straits structure. The Okhotsk Sea coastal water near Kunashir Island in surface mass is formed by warmer water of Soya current and surface waters of the Okhotsk Sea. The first one lies adjacent to northern coast of Kunashir Island embracing usually layer from surface till 50-100 m depth. The second ones usually are spread to the open part of Sea from northern boundary of Soya Current and in case of poor formation of the current they approach from North to Ekaterina Strait. Their distribution along the profile rarely exceeds 50-75 m depth from surface. Both mentioned surface water masses are “propped up” by the Okhotsk Sea intermediate water that is compiling intermediate cold layer during warm half year. The water structure in oceanic side of Ekaterina Strait depends on Oyashio Current.

Thermohaline indices and vertical boundaries of water masses in Ekaterina Strait [Bogdanov and Moroz, 2000]

For horizontal distribution of water temperature in the strait there is intrinsic non-steady patches-like structure which possibly is a result of interaction of non-periodical currents and tides flowing on the background of changeable bottom relief.

Seasonal variability of water circulation in the Kuril straits

Geostrophic circulation for Kuril region has two schemes of currents in straits. Along with it a picture of water circulation in special strait is not steady one. Jointly with tidal events it is influenced essentially by dynamics of adjacent water of the ocean and sea areas that results in alteration of water exchange character through the strait (is flow out or contrary to it). To estimate variability of water exchange the number of accountings was fulfilled using Vasiliev' mathematical model [Bogdanov, Moroz, 2000] in the strait zone that includes in itself the most active according to dynamics the Kuril Islands region (Friz Strait, Bussol Strait with adjacent areas).

Accordingly results of model accounting in Friz Strait the dominant water flowing out of the Okhotsk Sea in winter and in spring at NW type of atmospheric circulation and also in winter and in autumn at CO type of atmospheric circulation. Bilateral currents scheme occur at NW type of atmospheric circulation in summer and autumn. Dominant penetration of Pacific Ocean water is observed at OA type in summer. In Bussol Strait the prevailing flowing out of the Okhotsk Sea was registered at NW type in summer. Bilateral scheme of water circulation in the strait is clearly enough traced and is formed at the same type of atmospheric circulation in winter and in spring. There was traced seasonal variability of intensification of water integral transfer in the straits. From cold period of half-year to warm one its figures are increasing tenfold.

Integral water transfer in Kuril straits [Bogdanov and Moroz, 2000] (in "Sverdrup", 1 Sv = 10⁶ m³/c )

(positive figures – influx of Pacific Ocean, negative ones – outflow from the Okhotsk Sea)

Hydrological zoning

From the result of T-S analysis and systematization of the observation data, following water thermohaline structure types taking place in given region, were distinguished [Bogdanov and Moroz, 2000]:

 1. Pacific Ocean type (subarctic structure – Pacific Ocean water that is transferred by Kuril and Oyashio currents).

 2. Okhotsk Sea type (Okhotsk Sea water that characterized by especially low minimal temperatures in cold intermediate layer and weakly developed warm intermediate layer).

 3. Okhotsk Sea south part type (Okhotsk Sea water featured by high figures of thermohaline parameters in the surface layer dye to penetration of Soya Current water into the Southern Okhotsk Sea region).

 4. Deep straits zone type (transformed water characterized by different thermohaline parameters in surface layer (lower figures of temperature and higher ones for salinity concerning adjacent waters of the sea and ocean) more mighty along vertical section cold intermediate layer and smoother extremes of water masses).

 5. Islands shelf zone type (water discriminated in practice by homogenous vertical distribution of thermohaline parameters).

It was established, that the distinguished water structure types are separated by frontal zones of the various intensity. They are:

 1. Oyashio front – zone of interaction of 1 and 4 types of water structure (inner-structural Kuril front).

 2. Okhotsk Sea Kuril front – zone of interaction between 2 and 4 types of water structures. Here we disclosed abrupt tearing off in the cold intermediate layer of the Okhotsk Sea type of water structure. The front is evident distinctly in the intermediate layers. It divides cold water of cold intermediate layer from the Okhotsk Sea and anomaly of warm water of cold intermediate layer in the zone of Kuril straits.

 3. Soya Current front – linked with intrusion of warmer and saltier water of Soya Current in surface layer that were observed in Southern part of the Okhotsk Sea in 3 type water structure. The front is the zone of contact for 2 and 3 types of water structures.

 4. Kuril straits fronts – linked with circulation around islands with breaches in 1 and 2 circulation around islands with breaks in 1 and 2 fronts under intrusion of Pacific Ocean or the Okhotsk Sea water into the straits zones while running eddy-formation.

 5. Shallow zone fronts – arisen under formation of 5 type water structure (dividing homogenous water in shallows and stratified water of 1 and 2 or 4 types structures).

Fig. 9. Characteristic T, S-curves for Kuril straits (+ - sea, · - ocean)
Fig. 10. Registered water structure types zoning in the Kuril Island region.

The observed picture of hydrological zoning of the Kuril straits aquatic area with adjacent zones of the Okhotsk Sea and the Pacific Ocean and also of distribution of identified water structure types and situation of frontal division is quasi-stationary. Complex water dynamic in Kuril Island region, conditioned by variability of development intensity in Kuril Current and by character of interaction with atmospheric circulation, determines evolution of the frontal division. The fronts become non-steady that is exposed by formation of meanders, eddies and also theirs shifts.

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Hydrochemical characteristics

This Atlas presents the hydrochemical characteristics as the maps of distribution (on different horizons) of the average for multi-years values of the content of 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 hydrological seasons are defined as follows. Winter: January-March. Spring: April-June. Summer: July-September. Autumn: October-December [Antonov et al, 1999].

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Acoustic characteristics

In the Sea of Okhotsk, subarctic structure of water masses are characterized by considerable contrasts in the sound speed field - both inter-seasonal and spatial ones. During the cold period, throughout the sea, very low values of sound speed are observed on the surface (less than 1450-1445 m/s). Just in its southernmost part, till the end of spring, the values of sound speed on the surface increase till 1460-1465 m/s. In summer, the maximal values of sound speed are found in the southern part of the sea (more than 1500 m/s), minimal ones - near the Kuril Islands (less than 1480 m/s). In winter, the vertical distribution of sound speed is characterized by the homogeneous subsurface layer with very low values of sound speed till the depths of 100-200 m, a tachocline with maximal gradients at the depth of 100-200 m (more than 0.1 c^-1), and, as a whole, positive gradients of sound speed in the lower layers. During the warm period, it is formed a sound channel with the minimum of sound speed at the depths of 80-120 m. At that time, from the sea surface till the depths of 60-80 m there is a layer with high negative gradients of sound speed (more than 0.5 c^-1). Deeper it is located a layer with the minimal values and gradients of sound speed, its thickness making 100-200 m. With the depth, the same positive gradients are observed, as in winter. Values of sound speed on the axis of the sound channel make about 1445 m/s, while on the surface they are larger (1460-1475 m/s in June, 1475-1490 m/s in September).

In the structure of the Southern Okhotsk Sea waters formed at the distribution of warm, more saline waters of the Soya Current, the sound speed characteristics differ from those typical for the largest water area, both in the sound speed values on the sea surface, and in the form of the curves of vertical distribution. Vertical structure of the sound speed field here is characterized not only by peculiarities of thermal stratification, but also by non-monotonous vertical distribution of salinity. Vertical distribution of salinity in the surface layer has a maximum preventing the decrease of sound speed values. In August, sound speed on the surface in the southern part of the sea exceeds 1500 m/s. Till the depths of 50-75 m here it is observed a homogeneous layer with the values of sound speed up to 1495 m/s corresponding to the location of a layer of increased values of salinity. Deeper, it occurs quite sharp decrease of sound speed values conditioned by simultaneous decrease of temperature and salinity with the depth reaching the values of 1449-1450 m/s at the depth of 100-200 m corresponding to the location of the sound channel axis. Further on, the sound speed values increase again due to the gradual increase of temperature and salinity with the depth in combination with the growth of hydrostatic pressure. At the depth of 1000 m the values of sound speed make 1475 m/s. Thus, in summer, in the southernmost part of the sea it occurs the maximal range of sound speed variation - from 1495 to 1510 m/s on the surface to 1449-1450 m/s on the sound channel axis. Seasonal changes of sound speed on the sea surface are of almost similar range. In the central part and in the north of the sea the range is about 10-15 m/s less.

Below, the hydrological-acoustical characteristics of the Kuril Island Arc zone are considered in more detail [Moroz and Khrapchenkov, 2001].

According to the above zoning of thermohaline structure of waters of the Kuril straits and the adjacent areas of the Pacific Ocean and the Sea of Okhotsk, each area is characterized by certain types of hydrological structure, which differ from each other by the form of the curve of the vertical distribution of temperature and salinity, as well as numerical values of thermohaline indices of water masses. As far as each hydrological structure of waters is characterized by the proper thermohaline characteristics, it occurs the corresponding hydro-acoustical structure which, in its turn, is characterized by the own types of curves of the vertical distribution of sound speed, depth of location and numerical values of extremums, type and parameters of the sound channel.

Fig. 11. Characteristic profiles of temperature, salinity, sound speed: Pacific type (a), Okhotsk Sea type (b), Southern Okhotsk Sea type (c) and Kuril straits Type (d).

For the Subarctic structure of waters in the Pacific Ocean, the vertical distribution of sound speed possesses a monotonous character in winter and non-monotonous one in summer. During the warm period it is formed a thermal type of sound channel with the distinct asymmetry. The upper part of the channel is conditioned by a seasonal thermocline. Axis location - the temperature minimum in a cold intermediate layer. Further increase of sound speed with the depth is related to the temperature increase in a warm intermediate layer and the hydrostatic pressure increase. With this, it takes place the formation of the so called planar-layered wave channel.

Sound speed field in the Pacific waters structure is non-homogeneous. In a zone of minimal values of sound speed along the islands coast it is distinguished an area characterized by especially low values (to 1450 m/s). This area is related to the Kuril Current flow. Analysis of the vertical sections of the field of sound speed and temperature shows that the sound channel axis corresponding to the location of a nucleus of the cold intermediate layer coincides with the current midstream. On sections of the sound speed field which cross the current flow the lens-like areas are observed, they are contoured by isotachs of minimal sound speed (the same as on the temperature ones - lens-like areas of minimal temperature in the nucleus of the cold intermediate layer). While crossing the Near-Coast Front of the Kuril Current where the temperature changes can reach 5^o at the distance of some hundred meters, the differences of sound speed values make 10 m/s.

In the Okhotsk Sea structure of waters the negative values of minimal temperature which are typical for the cold intermediate layer, condition the appearance of the sharply exhibited under-water sound channel. With this, as for the cold intermediate layer, in the sound speed field it is observed the breakage of the plane-layered wave-guide at the crossing of the Near-Kuril Front of the Okhotsk Sea. Spatial distribution of sound speed is non-homogeneous. In the sound speed distribution on the surface it is observed a decrease of its values in the direction to the shelf of the islands. Spatial picture of the sound speed field here is complicated due to the different-scale non-homogeneities of thermohaline fields related to the observed constant vortex formation. There are lens-like areas with its lower values (the difference makes up to 5 m/s) as compared to the adjacent waters.

In the structure of the Southern Okhotsk Sea waters which is formed at the intrusion of warmer, more saline waters of the Soya Current in the surface layer of the water, profiles of sound speed differ both by the sound speed values and by the form of the curves of vertical distribution and the location of extremums. The form of the vertical curve of sound speed here is determined not only by the temperature profile but also by the non-monotonous vertical distribution of salinity characterizing the structure of the Soya Current waters penetrating into the Southern Okhotsk Sea area. Vertical distribution of salinity in the surface layer has a maximum preventing the decrease of sound speed values. In this relation, the location of sound speed axis is observed somewhat deeper than the location of a nucleus of the cold intermediate layer. Consequently, in the given area the type of sound channel stops to be pure thermal. For the Southern Okhotsk Sea type of the water structure it takes place a maximal range of changes of sound speed values (from 1490-1500 m/s on the surface, to 1449-1450 m/s on the axis of the sound channel).

In the straits zone and on both sides of the Kuril Ridge, as a result of tidal mixing there is formed a considerable number of fronts of different scales. With the front genesis and vortex formation it takes place the change of the location depth of a seasonal thermocline and correspondingly, of a tachocline (sometimes, up to its outcrop to the surface), it is changed the location of a nucleus of the cold intermediate layer, its margins and correspondingly - the sound channel axis and its margins. The brightest structural peculiarities of the sound speed field are found in the current midstream zones in the strait area (as well as in the areas adjacent to the islands). It is observed localization of homogeneous nucleuses of the minimal temperature in the cold intermediate layer coinciding with a zone of maximal current velocities. In the planes of transverse thermohaline sections, these zones are corresponding to the areas bounded by the closed isotherms. In the sound speed field a similar picture is observed - these zones are corresponding to the areas bounded by the closed isotachs. Similar, but more exhibited areas were found as well earlier, with the investigation of such meso-scale non-homogeneities as the eddy formations, frontal and inter-frontal zones in the areas of the Kuroshio-Oyashio Currents, California Current. In this relation, it was ascertained the existence of a special type of sound channel in the ocean which presents itself a three-dimension acoustic wave-guide. Unlike in the known plane-layered wave-guide, here are the zones of not only increased vertical gradients of sound speed, but of horizontal gradients either, which bound the given area on the left and on the right. In the plane of transverse sections - these are the areas bounded by the closed isotachs. In the Kuril straits area it is observed a weakly expressed similarity of the three-dimension wave-guides. Cruise studies of the POI FEB RAS show constant existence of such wave-guides in the studied area.

Thus, in the Kuril Islands area the following peculiarities of hydro-acoustical structure of waters are observed:

Thus, the formation of hydro-acoustical structure of waters in the studied area as a whole is predetermined by peculiarities of hydrological structure of waters. Each area - the Kuril straits zone, adjacent areas of the Pacific Ocean and the Sea of Okhotsk - are characterized both by the definite types of thermohaline structure of waters and by certain structural peculiarities of the sound speed field. In each area, there are proper types of curves of the vertical distribution of sound speed with the corresponding numerical indices of extremums and types of sound channels.

For the Pacific Subarctic structure of waters the formation of sound speed field to the great extent is related to the Kuril Current, where the sound channel axis, as the studies have shown, coincides with the current midstream and a zone of minimal temperature of the cold intermediate layer. The sound wave-guide being formed is of thermal type.

In the Okhotsk Sea structure of waters, the negative values of the minimal water temperature in the cold intermediate layer precondition the formation of the sharply expressed under-water sound channel. It is found that, here, in the sound speed field, as in the nucleus of the cold intermediate layer there is a breakage of the plane-layered wave-guide at the crossing of the Near-Kuril Front in the Sea of Okhotsk.

In the structure of the Southern Okhotsk Sea waters the form of the vertical curve of the sound speed is determined not only by the vertical temperature profile, but also by the non-monotonous distribution of salinity profile due to the intrusion of the warm, more saline waters of the Soya Current. In this relation, the sound channel axis is located somewhat deeper than the nucleus of the cold intermediate layer. The sound channel type stops to be pure thermal. A peculiarity of structure of the sound speed field in the given area is also the maximal range of the sound speed change from the surface to the sound channel axis, as compared to the other areas being considered here.

For the structure of the Kuril straits zone waters, it is typical comparatively small sound speed on the surface, smoothed extremums of the curve of the vertical profile of sound speed and washing-out of the sound channel axis.

In the homogenized waters of the shallow-water zone it is observed the sound channel destruction till its complete vanishing. In the Kuril straits zone and the adjacent areas - both on the Pacific Ocean side and on the Okhotsk Sea side - along with the plane-layered wave-guides there are weakly exhibited three-dimension acoustic wave-guides.

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