Bering Sea

Physical and geographical characteristics and hydrometeorological conditions

The Bering Sea is located in the Northern Pacific between the Asian and North American continents in the west and east, and the Aleutian and Komandorskie Island Arc in the south. In the north it links with the Arctic Ocean through the Bering Strait, in the south - with the Pacific Ocean through the numerous straits of the Komandorskie-Aleutian Island Arc. The Bering Sea is related to the semi-closed marginal seas of the mixed continent-ocean type. Occupying the territory between the parallels of 66° 30’ and 51° 22’ N and meridians of 162° 20’ E and 157° W the sea area is 2315 thousand km², its volume makes 3796 thousand km³, the sea is the third in the World Ocean by size. Average sea depth makes 1640 m and the maximal one - 4420 m (in Kamchatka Strait). The overall length of the coastal line of various configurations is 13340 km.

Main morphological zones in the bottom relief of the Bering Sea are the shelf, island shoals, continental slope and deep-water basin; they are well expressed on a bathymetric map. Main peculiarity of the relief is predominance of two main bathymetric levels - the shelf zone (0 - 200 m) and the abyssal basin with the depths of more than 1000 m, which are distinctly localized in the north-eastern and south-western parts of the sea and are comparable according to the occupied territory. Vast continental shoal 600-1000 km wide presents itself a plain under-water valley in which bounds specific hydrological and bio-hydrochemical processes are developed and the isolated water masses are formed. Continental shoal near the coast of Kamchatka and Komandorskie-Aleutian Island Arc is narrower and its relief is more complicated. Comparatively narrow continental slope (200-3000 m) almost in all its extension passes into a deep-water ocean floor with the abrupt scarps and it is strongly dissected. The bottom of the deep-water zone is comparatively even. Here, in the south-western part of the sea, there are two underwater ridges (Shirshov Ridge and Bowers Ridge) which are not a significant obstacle for the water exchange. The straits linking the sea with the Pacific Ocean play an important role in formation of hydrological mode and general water circulation. Total length of their transverse cross-section conditioning water exchange through the southern boundary makes 731 km. The deepest-water straits are located in the western part of the island arc. Here their depth reaches 760-4400 m at the width of 125-360 km (the Straits of Kamchatskiy, Blizhniy/Near, Amchitka, and Buldir). The other straits are relatively shallow-water (up to 100 m). The Bering Strait, in the north of the sea, is 42 m deep and 85 km wide; it doesn’t play a significant role in the external and internal water exchange of the basin.

In this region, the climate is of monsoon type, its peculiarity being seasonal variation of the dominant winds and obvious differences in the motion of meteorological elements in the course of a year. Due to the vast territory of the sea, here it is also observed considerable climate distinction between the particular sea areas. As a whole, to the north of 55° N the climate, especially in the coastal zone, is more severe, continental type. Southward, where the stabilizing influence of the ocean increases, it is more moderate, marine type. Here it is observed less amplitude of the air temperature variation, greater values of precipitation and cloudiness. With this, at the expense of non-regular cooling, the western areas of the sea are colder than the eastern ones. Main climate background is formed by circulation factors, i.e. by the air masses transport, not by the radiation components. During a year, the Bering Sea is under the influence of three main atmospheric bar formations making effect on the formation and transport of the air masses and the distribution of meteorological elements over its water area: the Aleutian minimum, North-Pacific maximum, and Siberian winter anticyclone. Their location and intensity greatly change from season to season, but during the cold period they are the most contrast. In winter, over the largest part of the water area the strong winds of the north and north-eastern direction predominate; they transport the cold arctic air from the polar basin and continent. At that time, in the south-eastern part of the sea there are the south and south-western winds, and in the southern part the western and eastern bearings predominate. In summer period, over all water area of the sea the winds of southern bearings predominate. Average annual values of the wind velocity in the coastal areas make 6-8 m/s, and in the open areas of the sea - 8-12 m/s, while growing from the north to the south. Relatively high here it is the repeatedness of deep cyclones during the cold period and the related storms (from 5-10 to 15-20 days a month). The winter storms are especially dangerous for navigation, they are accompanied, mainly in the southern area of the sea, by the development of strong wave (wave height exceeding 10 m) and the vessel icing. Maximal velocities of the storm winds reach 38-45 m/s in winter, in summer - up to 37 m/s.

Maximal wave heights in the Bering Sea by the vessel observation data for the period from 1965 to 1989 [Polyakova A.M. et al, 2002]

The coldest months are January and February, and the warmest ones - July and August. Average monthly values of the air temperature in the cold months are +1...-4° in the south-western and southern parts of the sea, and -15...-20° in the north (here, in the near-continental areas it may decrease up to -40...-50°). In the warm months, when it takes place the change of the pressure systems, the air over the sea is warmed up to 4-13°. As a whole, the sea is characterized by the negative annual sum of heat balance on its surface (with the exception of the southernmost areas) and the predominance of the precipitation over evaporation [Demenitskaya et al, 1974; Dobrovol'skii and Zalogin, 1982; Korobko, 1957; Rumiantsev, 1959; Stabeno et al, 1999; Terziev et al, 1999].

At present, the Bering sea is one of the most productive marine ecosystems in the world. Here it is concentrated up to 40 % of the total amount of the USA fisheries.

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Hydrological characteristics

Hydrological mode of the described area is generally conditioned by its geographic position, climate, water exchange with the Pacific Ocean, and bottom topography. Near the coastline the considerable meaning is acquired by the continental discharge, tides and the coastline configuration. 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: [Bogdanov et al, 1991; Demenitskaya et al, 1974; Dobrovol'skii and Zalogin, 1982; Stabeno et al, 1999; Takenouti and Ohtani, 1974; Terziev et al, 1999], 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 ‰ ).

Observed characteristics of the field of the water temperature horizontal distribution on the surface and on deep horizons are formed and continuously changed under the influence of physical processes of different scale and intensity occurring on the surface and in the water depths. Fluctuations of these characteristics are most distinctly manifested in the surface active layer, where it is distinctly traced their short-period and diurnal variability, seasonal intra-annual and inter-annual climatic motion, non-periodic fluctuations of different nature. Physics of these processes and regional peculiarities of thermal regime of the water area are comparatively well studied, and assimilation of the long-term hydrological observations data gives the opportunity to construct the generalized schemes of spatial distribution of temperature on different levels for all months of a year.

In all seasons, except summer, sea surface temperature (SST) generally increases from north to south. With this, the temperature field, as the spatial distribution of the amplitudes of its interannual fluctuations is considerably inhomogeneous in zonal and meridional directions. The value of the interannual fluctuations of this parameter on the sea surface changes from 12-14° in the near-continental areas up to 4.5-6° in the marine area of the Aleutian Ridge. At the depth of 50 m these values, as a rule, are 2-4 times less than the surface ones, and at the depth of 100-150 m they don’t exceed 1.5-2.3°. In summer, at the expense of intensive vertical mixing of waters in the area of the Aleutian Straits the temperature on the surface as a whole is lower, than in the north-west of the water area. In October, the temperature background starts to approach the winter state and the formation of the ice cover in the north-western part of the sea begins. In winter and in spring, in the area of the Bering Sea shelf break, near the ice edge, it is observed the considerable contrast in the temperature field for the northern and southern parts of the sea. In this period, the SST varies from 0 to -1.5° in the north, up to 3-4° in the south. The lowest temperatures (-1.4...-1.6°) are observed in the shallow water bays and inlets intruding into the land, and in the areas covered with ice. On the maps of SST well seen are the areas of supply of the warmer Pacific waters in the south of the area, and the area of the cold waters propagation near Kamchatka coast. In May, water temperature starts to increase, in August it reaches 9-12° in the most part of the water area, and 4-7° - in the north. August is the time for maximal heating of the surface waters in the most part of the water area. At that time, in the coastal shallow-water areas the water temperature is usually higher than in the open sea (11-14°), and the lowest temperatures are observed near the Bering Strait. September-October is the period of the fall cooling of the surface waters and the characteristics of the temperature field gradually pass to the winter type. Period of the greatest cooling terminates in April.

Seasonal changes of the water temperature in the open part of the sea occupy the upper layer up to the depths of 250-300 m, deeper they are practically absent. Below 400-500 m in all places it is observed monotonous decrease of temperature with the depth from 3.3-3.7° to 2.7-2.9° at the level of 1000 m in all seasons. By the average for-many-years data, on the level of 200 m in the deep-water Aleutian and Komandorskiy/Kamchatka Basins the temperature values in characteristic months in all places increase from the north-west to the south-east approximately from 1.0 to 4.0°. Its spatial distribution on this level is more inhomogeneous than on the surface. Significant role in formation of the temperature field on the deep levels is plaid by the processes of the water exchange with the ocean through the deep-water straits in the south-western part of the sea. By the available data the values of the water temperature on 2000 m level vary in the bounds of 1.80 to 1.95°, and on the level of 3000 m - from 1.56 to 1.70°.

The above data manifest the most common characteristics of the large-scale distribution and variability of the water temperature, which as well can vary from year to year (climate shifts) and be precised in the course of new data accumulation. In practical aspect, for instance, for fishery oceanography and for the estimation of the regional ecosystem state, important are not only general, background characteristics of marine environment, but also the factual distributions of parameters in particular regions. The investigation results have shown that a significant role in formation of smaller, meso-scale inhomogeneities of the temperature field on the surface levels is plaid by the frontal zones and eddy formations, which occur in the coastal zone, on the shelf, in the deep-water basin, and recently have been the object of subtle investigation. With this, except the traditional contact methods of their identification it is widely used regular monitoring remote satellite observations. At present, the maps of temperature and other parameters of the sea surface are well accessed through the Internet in the real time scale.

According to the available classification, the Bering Sea is located in the area of the subarctic structure of waters characterized by monotonous decrease of salinity with the depth and by non-monotonous change of temperature. In all water area of the sea, excepting the shallow-water area and the area of the Aleutian Ridge, during all seasons on the vertical profiles of temperature and sections it is traced a cold subsurface layer (CSL) and a warm intermediate layer (WIL). Core of the cold subsurface (intermediate) layer is distinctly expressed just in the warm period of time beyond the bounds of the shelf zone. In a cold period of time, in the process of cooling, in the active sea layer the convection develops, seasonal thermocline vanishes and the upper margin of the cold layer is thinning towards the surface. Below the core of this layer the water temperature increases again and, reaching the local maximum in the core of the warm intermediate layer, it monotonously decreases up to the bottom. The depth of the lower boundary of the active layer below which the seasonal motion of temperature is almost not traced, changes beyond the shelf zone from 100-150 m in the eastern part of the sea to 200-300 m in the western part. Thickness of the upper mixed quasi-homogeneous layer where the vertical gradients of temperature are less than 0.01°, during the period from June to November increases from 10-30 m up to 30-75 m, and in the coastal zone an isothermal process propagates up to the near-bottom horizons. In January-March when seasonal thermocline is destroyed the thickness of this layer in all places increases up to 100-250 m, the least values (75-100 m) being observed in places of the Pacific waters intrusion to the sea in the south of the region.

Core of the CSL, as an independent structural element, is formed with the surface layer starting to heat. Minimum values of the core temperature and depth are different in different areas of the water area and are changing in seasonal motion. Minimal temperature values (+1...-1°) and the least depths of the core occurrence (30-50 m) in the period from June to October are observed on the Bering Sea shelf in the north-eastern part of the sea. In the central part of the deep-water basin in autumn the CSL core with the temperature of 2-3° deepens up to 100-150 m. The largest values of the core temperature (3.0-3.5°) are observed near the Aleutian Islands.

Warm intermediate layer by its origin is related to the transformation of waters incoming from the Pacific Ocean (mainly, through the Blizhniy/Near Strait) and their cooling on the surface, as a result of winter convection. Depth of the WIL core changes from 250-500 m, and the temperature values - from 3.4 to 4.0°. Under the WIL, up to the depths of 1200-2000 m it is located a layer of the major thermocline with the monotonous decrease of temperature up to 1.8-1.9°. Below it the deep waters occur, where the temperature decreases up to 1.5-1.7°. They occupy the largest part of all sea volume.

The curves of the temperature vertical distribution, especially in the bounds of the upper 200-300 m layer, significantly vary in a wide range of the time scale. For instance, the value of diurnal variations of the water temperature in some areas (in the fall of depths and near the coast) in the warm period can reach 3-7°.

Main large-scale peculiarities of salinity field are conditioned by the characteristics of the water balance on the surface of the Bering Sea (predominance of precipitation over evaporation, influence of the ice formation processes and ice melting), continental discharge in the coastal areas, as well as by the colder Pacific waters supply through the straits, their transport by currents and transformation. Salinity of the surface layer of waters, as a whole, decreases from south to north from 33.0-33.3‰ (south-western and central part of the sea) to 31-32‰ for all seasons. In summer and at the beginning of autumn, water salinity on the surface is less than in winter, but larger, than in spring. In winter it decreases at the expense of the ice formation and decrease of the land discharge, and in spring it sharply decreases. Maximal values of salinity (33.2-33.3‰) on the surface are observed in the area of Blizhniy/Near Strait, in the western part of the deep-water basin and near the middle part of the Aleutian Islands, and the minimal ones (up to 20-25‰) - in the continental bays and inlets of the coastal band. Freshening of the coastal waters reaches its maximum in July. In particular months on the margin of the eastern Bering Sea shelf and in the coastal areas the zones of maximal horizontal gradients of this parameter are distinctly expressed as salinity fronts.

With the depth, both in the surface layer, and in the lower layers, the salinity is continuously increasing in the bounds of all sea area, but the main features of its spatial distribution up to the levels of 50-75 m stay almost unchanged. Below the level of 100 m the horizontal gradients of salinity field are smoothed. On level of 200 m the background values of spatial changes of salinity don’t exceed 0.5-0.6‰, and the general character of its distribution is related to the circulation processes. On levels of 500-1000 m salinity values increase from the north-east to the south-west (from 33.85 to 34.15‰ and from 34.20 up to 34.50‰ correspondingly) which is related to the peculiar features of the Pacific waters distribution and transformation in the bounds of the deep-water basin. In the lower layers the range of spatial changes of salinity becomes narrower from 34.50-3465 ‰ (level 2000 m) up to 34.60-34.65 ‰ (3000 m).

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

Unlike the temperature curves, the salinity profiles are almost similar for all seasons of a year and as a whole they are characterized by monotonous decrease of salinity with the depth. Seasonal changes are generally exhibited in the bounds of the upper active 75-150 m layer. With the start of the development of winter convection mixing accompanied by the ice formation on the vast water areas, the values of the vertical gradients of salinity in this layer are decreasing and in the thickness of the active layer an upper quasi-homogeneous layer is formed. In winter and in spring here the layers with the inverse distribution of salinity also occur. On the surface of the active layer the largest intra-annual fluctuations of salinity occur. Their values are generally 0.5-0.8‰ in the deep-water part and 1-2‰ in the shallow-water part of the Bering Sea. In the bays and inlets of the continental part of the coast they are considerably higher and can reach 3-7‰ and even 10-15‰. With the depth these fluctuations extinguish up to 0.3-0.5‰ (1.0-1.2‰ on the shelf) on the lower boundary of the active layer. Below 150 m the intra-annual fluctuations of salinity already don’t penetrate from the surface. In the main halocline which lower boundary is located at 700-1100 m, these fluctuations are related to the circulation peculiarities and to the intra-water exchange processes. Short-period changes of salinity on different horizons are conditioned by the interaction of a wide spectrum of hydrometeorological and dynamic processes in the water thickness and on the sea surface. The value of minimal diurnal fluctuations of salinity (0.1‰) is observed on the Eastern Bering Sea shelf. In the area of some straits and frontal zones it increases up to 0.2-0.4‰ and 1.0-1.9‰, correspondingly, and reaches maximal values in bays and near-mouth areas (3.0-3.6‰). During cold period these fluctuations are exhibited, generally, in a layer of a jump of hydrological characteristics.

Water masses are usually suggested to be comparatively large volumes of waters formed in certain areas and possessing almost constant and continuous distribution of characteristics in the origin area and in the areal of distribution during a long period of time. Water masses form the main components (layers, extremums) of the vertical structure of the water thickness. On the margins between the water masses the frontal zones are formed in which the horizontal gradients of temperature, salinity and other characteristics are sharpened. As it was mentioned above, the main water mass of the Bering Sea is characterized by the subarctic structure, which main peculiar feature is the presence of the cold and warm intermediate layers, making the independent water masses - intermediate Bering Sea water and the intermediate Pacific water. As a whole, all water thickness of the deep-sea in summer is distinctly separated into four layers: surface layer, cold intermediate layer, warm intermediate layer, and abyssal layer. On the Eastern Bering Sea shelf at that period of time only two water masses are distinguished: surface (higher values of temperature and lower values of salinity) and the near-bottom (higher values of salinity and lower values of temperature) ones.

Surface water mass (SWM) is formed during the warm period as a result of radiation heating and freshening of the upper layer waters. A layer occupied by the SWM is 20-50 m thick; it is characterized by the temperature of 7-10° on the surface and 4-6° on the lower boundary and by salinity of about 32-33‰. The largest thickness of this layer is observed in the open part of the sea, and the minimal values of salinity (<31‰) - in the coastal zone.

Intermediate Bering Sea water mass (IBWM) is formed as a result of the autumn-winter convection mixing and subsequent summer heating. The depth of the IBWM core occurrence increases from 50 m on the Eastern Bering Sea shelf up to 100-150 m in the central and southern parts of the deep-water basin. Salinity values vary in a seasonal motion in the bounds of 32-34‰. Temperature values in the core increase from +1...-1^o in the north-east (shelf) up to 3.0-3.5^o near the Aleutian Islands. In the deep-water part of the sea the horizontal distribution of salinity in the IBWM core is more homogeneous than on the shelf.

Intermediate Pacific Water Mass (IPWM) is identified in a layer of 100-650 (900) m for all seasons. In different areas its core occurs at the depth from 250 m up to 500 m. Salinity in the IPWM core varies from 33.4 to 34.0‰, and the temperature - from 3.4 to 4.0^o.

Deep Water Mass (DWM) is formed as a result of the waters passing through the straits from the Pacific Ocean and their subsequent transformation. It occupies most of the sea water area and it is characterized by weak spatial-temporal variability of hydrological characteristics. Its upper boundary is located at the depths of 800-1200 m, and the core with the maximal values of salinity (about 34.7‰) and the temperature values minimal for the DWM (about 1.5^o) are located at the bottom of the deep water basin.

In some locations, especially on the basin periphery, it is observed some modification of the main water masses, the vertical layering of waters extinguishes and new isolated water masses appear.

The field of total currents in particular areas of the sea is formed as a result of composition of different types of the water motions of different spatial-temporal scale: relatively constant non-periodic currents, variation of seasonal and synoptic scale, tidal, inertial and surge phenomena. Depending on the time scale accepted for the averaging, their characteristics can differ in this or that point of spatial coordinates. The existing schemes of the seawaters circulation are based either on uncoordinated data of in situ observations, or they are got by calculation methods.

Main peculiar feature of the circulation system of the Bering Sea is a cyclone vortex of the general motion of waters (counter clock-wise) in the most part of the water area. To the north of 60^o N, on the eastern Bering Sea shelf it is observed less significant anti-cyclone vortex. These links of a circulation chain are formed, first of all, at the expense of the Pacific waters being continuously supplied through the straits of the Komandorskiye-Aleutian Island Ridge and due to the wind effect on the sea surface. The main flux of the Pacific waters, 200 miles wide, enters the sea between the Aleutian and Komandorskie Islands and moves to the north, east and north-east while forming separate branches and local vortices. In the south and south-east, through the Aleutian straits on the Pacific Ocean side, the branches of the Alaskan Stream are penetrating, which also impose significant effect on the sea waters circulation as a whole. To the north of 55^o N the main flux deviates to the north-west and follows to the Koryak coast of the Asian continent. The main transport of waters near the western edge of the eastern Bering Sea shelf is carried out by the current named the Transverse or Bering Slope Current. Mean velocity of this current is 5-10 cm/s, the maximal one - 10-15 cm/s (near the Koryak coast). While approaching the Asian continent the Transverse Current is gradually deviating to the west and branching into two fluxes. Along the coast, most part of the waters turns to the south while giving the start to the cold Kamchatka Current which discharges the Bering Sea waters to the Pacific Ocean. The velocity of this current makes about 15 cm/s (maximal average diurnal values reach 40-80 cm/s, and in the Strait of Kamchatka - 90-120 cm/s). Another branch turns to the north-east giving the start to the Navarin Current which over-bending the off-shore part of the Gulf of Anadyr, while forming there a cyclone vortex, and transports the waters to the northern part of the sea - to the Bering Strait and Norton Bay. Velocity of non-periodic currents in the Gulf of Anadyr changes from 5 to 22 cm/s, in the Shpanberg Strait it makes 5-10 cm/s, and in the Chirikov Strait it can reach 50 cm/s. In the east of the sea, in the median part of the Bering Sea shelf and in the central part of the deep water basin the velocities of constant currents are relatively small (2-6 cm/s). On the periphery of these areas, on the continental slope and by the under-water rises they are somewhat growing (up to 10-15 cm/s) Maximal velocity values of these currents are observed in the narrow parts of the straits, near Kamchatka and Koryak coast (up to 25-50 cm/s). By the available data, at definite synoptic situations the velocity of non-periodic currents in some areas can reach 80 cm/s.

Main features of inter-annual variability of constant surface currents are determined by the large-scale peculiarities of the atmospheric circulation in different seasons, leading to the changes of the wind mode over the water area. Schemes given in the atlas provide general notion on the character of these changes in the areas free of the ice cover and provided by the observation data to conduct calculations. They show that the general cyclone character of the waters motion in the bounds of the deep water sea basin is preserved for the largest period of a year, and the maximal values of currents velocity in particular branches of circulation are observed in autumn and winter. In the area of the Eastern Bering Sea shelf it is observed significant reconstruction of the current field from summer to winter. In relation to the predominance of the winds of northern bearings, in autumn and winter the waters removal from the Bering Sea to the Pacific Ocean in the south-western part of the Komandorskiye-Aleutian Ridge is obviously increasing at that time. The given diagrams are plotted by the average long-term data and they characterize the background peculiarities of the current field. Observed values of the current velocity constantly vary and can increase 4-5 times in the surface layer under the influence of the local atmospheric pressure systems. General cyclone character of the waters motion in the deep-water basin is preserved up to the level of 2000 m. In this area at the depth of 100 m the velocities of constant currents don’t exceed 5 cm/s (up to 10 cm/s on the periphery). In the lower layers, in some areas of the sea, on the background values of 2-7 cm/s the velocities of up to 10-15 cm/s occur.

Eddy formations and meanders of currents are referred to those peculiarities of the waters dynamics in the Bering sea which are smaller in scale but more important in a hydrological sense. Eddy formations are distinctly exhibited in satellite images where they are seen as the localized anomalies of sea level deviations or of the temperature field anomalies. Eddies of the horizontal scales of 10-200 km and velocity of 20-30 cm/s (more than 40 cm/s in Kamchatka Current) are usually observed in the areas of the depth deeper than 150 m and can exist for a long time.

Significant input to the fluctuations in velocity and direction of currents of the Bering Sea is done by the reverse periodic currents. These currents are relatively weak in the open part of the sea where they possess a rotational character, but in the straits, near the islands, by the continental coast and shoals, their velocities make 1-2 m/s, and in some shallow water straits they reach 4-6 m/s.

Tidal phenomena are mainly conditioned by peculiarities of distribution of the Pacific Ocean tidal wave in the Bering Sea. They induce considerable fluctuations of the sea level, velocities and direction of currents. By the character of the level fluctuations here all types of tides occur: semidiurnal, irregular semidiurnal, irregular diurnal and diurnal. On the largest water area the non-regular diurnal tides predominate. The smallest tidal amplitudes observed in the north, in the Bering Sea area (up to 0.5 m), and the largest ones - in the Bristol Bay (more than 8 m). In the other areas near the continental coast and islands the largest value of tide doesn’t exceed 1.5-2 m.

The Bering Sea is the northernmost of the Far Eastern Seas and the most severe by its climate characteristics and ice conditions. In winter and spring about half of its water area is covered by the pack and drifting ice. Almost all ice mass is formed and melts immediately in the sea basin. As a whole, the duration of the ice period with regard to the winter being severe or not makes 80-252 days for warm winters, 120-294 - for moderate ones, and 170-365 - for severe winters. Correspondingly, different for different years are the areas of the ice cover and the time of the ice cover of sea-ice area maximum. In warm winters the ice covers about 20% of the sea area, and the sea-ice maximum falls to the end of February. In moderate and severe winters the ice covers correspondingly up to 37 and 56% of the area, and the time of the sea-ice maximum shifts to the first half of April. During this month, the ice edge passes from the Bristol Bay across the Pribilof Islands and farther to the west along 57-58^o N. Further on, in the central part of the basin it is gradually sinking to the south, towards the Komandorskie Islands, and passes along the coast up to the southern termination of the Kamchatka Peninsula. The southern part of the sea doesn’t freeze all the year round. The ice formation process begins in the north-western part of the Bering sea, where the ice appears in October and is gradually removed to the south. In the Bering Strait, in the Gulf of Anadyr and Norton Sound the ice can be seen as early as September. In November-December the floating ice appears near Kamchatka Peninsula and Komandorskie Islands. In winter all northern part of the sea is filled in by heavy, impassable ice of up to 6 m thick. Influenced by wind and currents the ice fields are in motion. As a result of periodic compression and extension in the ice cover the hummocks appear of up to 20 m high, as well as the ice leads and lanes. Some part of the ice is removed to the north, to the Chukchi Sea.

In the second half of April the process of the sea clearance starts. The ice edge is quickly shifting to the north and in June-July the sea is completely cleared of ice. But in the western half of the Bering Strait the ice can be found during the whole year period. The characteristics of the ice mode in bays, inlets and some straits are strongly influenced by the winds of the surge direction and the reverse one. On the basis of the long-term observations, statistical characteristics of the ice distribution throughout the water area, the variability of various parameters of the ice cover are quite well studied and described in detail in papers.

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Hydrochemical characteristics

The given version of the atlas presents the hydrological characteristics as the maps of distribution (at different levels) of the average long-term values of the following contents: 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 used data source (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

The most important hydroacoustical characteristics of the water structure is the sound speed. It is known that the value of sound speed in the seawater generally depends on three factors: temperature, salinity, and hydrostatic pressure (note, that the extent of the influence of each of the given factors is different).

The vertical structure of the sound speed field in the Bering Sea in summer and autumn is characterized by the following peculiar features:

In winter, in the south-western and northern parts of the Bering Sea when the wind and convection mixing increases it is formed a homogeneous layer with the minimal values (and gradients) of sound speed up to the depth of 100-200 m. At that time on the sea surface the sound speed values are less than 1450 m/s, and the minimal values (less than 1445 m/s from the surface to the bottom) are observed in the Gulf of Anadyr. In the deep part of the sea, the waters vertical structure is formed to a great extent under the influence of the Northern Pacific waters advection through the straits, so in winter, here the values of sound speed are higher than 1455 m/s.

Boundaries of the subarctic waters interaction with the waters from the central part of the Bering Sea are well traced by the character of the horizontal distribution of sound speed. This is expressed by the following: for the most period of the year the isotachs on the sea surface are parallel to the coast in the south-western part and are practically latitudinal in the central part of the sea. With this, the horizontal gradients in the transitional zones reach 0.6-0.4 m/s per mile during the whole year, that is several times higher than the background values (0.10-0.09 m/s per mile). Mixing of the waters different in their characteristics leads to the non-homogeneity of the sound speed field in the area of the underwater sound channel (USC). For the coastal shelf waters it is typical the USC axis depth changing from 0 m in winter to 50-60 m in spring. In the continental slope area, in the south-western part during the cold period it takes place sinking of the cold and higher salinity shelf waters to under warmer and more saline transformed subarctic waters. As a result, in summer here the depth of the USC axis location (sound speed is 1450-1455 m/s) increases up to 150 m and more. In the central and southern parts of the sea the USC width reaches 2500 m, and the sound speed values in the channel axis increase up to 1456-1468 m/s. With this, the axis of the sound channel rises to the depth of 100-125 m. At that time the maximal vertical gradients of sound speed are observed in a seasonal thermocline (more than 1 s^-1), and the thickness of a homogeneous layer with the sound speed of 1475-1485 m/s increases up to the maximal values. Maximal amplitude of seasonal changes (up to 35-40 m/s) is found on the sea surface.

Below, we shall consider in detail the characteristics of sound speed field for the area of the whole eastern coast of Kamchatka Peninsula [Moroz and Khrapchenkov, 2001]. By the vertical distribution of the sound speed values in this area we can distinguish the following main zones:

The vertical structure of the sound speed field on the shelf differs from the structure of the deep-water areas by the absence of the last three zones. On the shelf, vertical gradient of sound speed field is conditioned by the influence of the vertical circulation in the upwelling zones, by the autumn-winter convection, the surface waters freshening by the continental discharge and sediments. In winter, as a result of cooling of the surface water layer the minimal values of sound speed occur on the surface. The homogeneous layer with the sound speed values of 1443-1445 m/s at that period of time propagates up to the depths of 100-150 m, further on, up to the constant tachocline the sound speed gradients reach 0.05 s^-1. In the tachocline at the depths of 350-450 m the mean gradient of sound speed reaches 0.1 s^-1 and doesn’t vary during the whole year, and its location by the vertical under the influence of the eddies can change for 100 m. In April-May, due to warming of surface layer it is formed a seasonal tachocline which is located near the surface till July, and its thickness reaches 40 m. Gradients of sound speed for this time period vary from 0.01 in April up to 0.72 s^-1 in July. Thus, in April-May the sub-surface minimum of sound speed originates. In May it is located at the depth of 100 m, then its depth varies in the bounds of 40 to 110 m. It is formed a lens of cold waters in which the values of sound speed don’t exceed 1455 m/s, and its thickness varies from 100 to 200 m. Till autumn the values of sound speed in a cold lens somewhat increase - from 1448 to 1455 m/s (minimal ones), in December the minimum of sound speed decreases to the depth of 150 m.

During the cold period, in the all considered area it is formed a sound channel propagating from the ocean surface to the bottom, and the minimum of sound speed is located on the surface. During the warm period, in the course of formation and existence of the sub-surface sound channel its width changes from 400 to 1500 m, increasing in September up to 2000 - 2500 m. Near the water surface at that time it is formed a homogeneous layer up to 20 m thick, where sound velocities exceed 1490 m/s. Till December the width of the sound channel decreases up to 450 m at the simultaneous formation of the sub-surface sound channel up to the depths of 100-150 m.

Horizontal distribution of sound speed on the surface, on the axis of the underwater sound channel, as well as on deeper horizons is practically similar. Isolines of sound speed are located parallel to the coast. With this, the minimal values both on the ocean surface and on the channel axis are observed along the continental slope, where the thickness of the cold intermediate layer is maximal. In the anticyclonic eddies the isolines of sound speed form a ring and simultaneously the horizontal gradients of sound speed increase. At a definite distance from the coast where the thickness of the cold layer decreases till the minimum it occurs a specific frontal zone till the depth of 400-500 m manifesting itself in all oceanographic characteristics. The most considerable differences in the sound speed field (up to 25-30 m/s) here are observed in summer-autumn period at the depths of 25-40 m. At the beginning of spring they don’t exceed 4-6 m/s. At the axis of the underwater sound channel the values of sound speed are changing not so considerably, not more than for 6 m/s. At the depths of 200-300 m these changes are reaching 10-15 m/s. Deeper than 500 m the horizontal changes in the sound speed field are insignificant.

Seasonal variations of sound speed occur generally in the upper layer of the sea and ocean: in the coastal area in a layer of 500 m, and off the coast - in a layer of 100 m. With this, the maximal amplitude of seasonal variations on the surface is found on the shelf and on the eastern boundary of the Kamchatka Current - up to 50 m/s. In the current itself the amplitude of seasonal variations don’t exceed 40 m/s, and inside anticyclone vortices it is even less - 30 m/s. With the depth the maximum amplitude of seasonal fluctuations is shifting to the continental slope side, where at the depth of 100 m it doesn’t exceed 10 m/s. Deeper than 200 m the amplitude of sound speed is increasing again up to 20 m/s, but this time at the expense of the anticyclonic eddies. Thus, the maximal gradients of sound speed in the studied area are observed in a layer of 0-300 m:

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