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]
|
12
% of probability |
Month |
I |
II |
III |
IV |
V |
VI |
VII |
VIII |
IX |
X |
XI |
XII |
Height, m |
10 |
10 |
12 |
8 |
8 |
8 |
8 |
9 |
8 |
10 |
13 |
12 |
|
Reduced to 3% of probability |
Height, m |
13,0 |
13,0 |
15,6 |
10,4 |
10,4 |
10,4 |
10,4 |
11,7 |
10,4 |
13,0 |
16,9 |
15,6 |
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.
<<<Up>>>
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 ‰ ).
Horizontal distribution of the water temperature
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.
Vertical distribution of temperature
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°.
Horizontal distribution of salinity
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.
Vertical distribution of salinity
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
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...-1o
in the north-east (shelf) up to 3.0-3.5o 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 4.0o.
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 1.5o) 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.
Waters circulation and currents
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 60o 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
55o 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.
Tides
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.
Ice conditions
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 57-58o 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:
- a relatively thin (up to 30 m) upper homogeneous layer with
the maximal values of sound speed;
- large local negative gradients (up to 1 s-1) in
the under-laying layer till the depths of 50-80 m;
- minimal gradients in the cold intermediate layer, which thickness
ranges from 50 to 100 m in the center of the sea up to 150-200
m in the western part;
- positive gradients in a permanent tachocline at the depth
of 150-250 m (about 0.1 s-1) which value gradually
decreases towards the bottom.
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:
- A subsurface zone where the main changes of the sound speed
values occur due to seasonal motion of hydrometeorological characteristics.
During the cold period of a year due to the cooling and convection
here it is formed a quite homogeneous layer of the waters with
the positive vertical gradient of sound speed where the sound
speed values by the vertical differ not more than for 2-3 m/s.
Thickness of this layer ranges from 50-90 m in the open ocean
up to 100-250 m in the bays of Kamchatka Peninsula. As usual,
in the bays the sound speed values are 4-5 m/s less than in
the open ocean. During the warm period of a year, in this zone,
as a result of warm interaction of the ocean and atmosphere
it is formed a sub-surface quasi-homogeneous layer of waters,
which thickness is maximal in the open ocean, (but it doesn’t
exceed 25-30 m), and is minimal in the anticyclone eddies -
less than 10 m. Under this layer up to the depth of 40-50 m
it is formed a seasonal tachocline, where the negative vertical
gradient of sound speed can reach 4 c-1. Values of
sound speed in the sub-surface layer in the open ocean and in
the bays of the peninsula differ considerably, for instance
in the Avachinsky Bay in September the difference makes 10-15
m/s. As to the range of seasonal fluctuations of sound speed
on the ocean surface, it reaches 40-50 m/s both in the bays
and in the open ocean.
- A zone coinciding with the cold intermediate layer where the
positive gradient of sound speed is minimal. In the open ocean
this zone is minimal or practically absent, and near the continental
slope and in Kamchatka Current it varies from 100 up to 300
m.
- A zone of constant tachocline which thickness varies from
50-75 m in the open ocean up to 100-150 m in the anticyclone
eddies and above the continental slope. Here, the vertical gradient
of sound speed makes 0.2-0.5 s-1.
- A zone coinciding with the warm intermediate layer located
at the depth from 150 up to 400 m in the open ocean and from
250-450 up to 500-650 m in the bays of Kamchatka Peninsula.
For this zone it is typical a positive vertical gradient of
sound speed somewhat lower than in the previous zone.
- A deep water zone situated deeper than 450 m in the open ocean
and deeper than 650 m along the coast of Kamchatka Peninsula.
For this zone it is typical a small positive vertical gradient
of sound speed close to the hydrostatic one, as well as very
small variability of the acoustic properties.
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:
- for the upper layer of 0-50 m during the summer-autumn period
a mean lateral difference is ~ 5-10 m/s for 10 miles, in the
anticyclonic eddies it increases up to 15 m/s for 10 miles,
at a level of 20 m - up to 26 m/s for 10 miles;
- in a layer of 50-100 m the maximal horizontal gradient of
sound speed doesn’t exceed 5 m/s for 10 miles;
- in a layer of 150-300 m a maximal horizontal gradient of sound
speed is ascertained transverse the jet of the Kamchatka Current
(5-10 m/s for 10 miles). In the anticyclonic eddies a similar
gradient of sound speed is found practically to the depth of
500 m.
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