Journal of Oceanography, Vol. 58, pp. 815 to 823, 2002
Winter Mixed Layer and Formation of Dichothermal Water
in the Bering Sea
TAKAHIRO MIURA *, TOSHIO SUGA and KIMIO HANAWA
Department of Geophysics, Graduate School of Science, Tohoku University,
Aoba-ku, Sendai 980-8578, Japan
(Received 12 October 2001; in revised form 28 February 2002; accepted 9 March 2002)
The temperature minimum layer, called “dichothermal water”, is a characteristic
feature of the North Pacific subarctic gyre. In particular, dichothermal water having
a density of approximately 26.6 sigma-theta ( σθ), which corresponds to the densest
water outcropping in winter in the North Pacific, is seen in the Bering Sea. In order
to clarify the water properties, and the area in which and the process by which the
dichothermal water is formed, a new seasonal mean gridded climatological dataset
with a fine resolution for the Bering Sea and adjacent seas has been prepared using
historically accumulated hydrographic data. Although the waters of the Alaskan
Stream have temperature minimum layers, their temperature inversions are very weak
in climatologies and the core densities of the temperature minimum layers are much
lighter than 26.6σ θ. On the other hand, in the Bering Sea one can see the robust structure of temperature minimum layers, the core density of the dichothermal water being around 26.6 σ θ . In addition, it has been found that the properties of the
dichothermal water observed in the warming season are almost the same as those in
the winter mixed layer. That is, the dichothermal waters are formed in the winter
mixed layer in the Bering Sea. Since these waters are found in the Kamchatka Strait,
i.e., the main exit of the Bering Sea waters, it can be supposed that the dichothermal
waters are exported from the Bering Sea to the Pacific Ocean by the Kamchatka
Current.
1. Introduction
The Bering Sea is the northernmost marginal sea of
the North Pacific Ocean. Although the Bering Sea is spatially rather compact, it is considered that a vigorous water
mass transformation takes place due to an active air-sea
interaction in winter (e.g., Dodimead et al., 1963; Ohtani
et al., 1972; Ohtani, 1973; Favorite et al., 1976). Most of
the inflow to the Bering Sea arrives via the Alaskan
Stream, which transports relatively warm waters in the
surface layer and flows westward along the Aleutian Islands from the Gulf of Alaska. On the other hand, the
Bering Sea flows out into the Pacific Ocean through the
(East) Kamchatka Current, which transports relatively
cold waters in the surface layer and flows along the
Keywords:
⋅ Bering Sea,
⋅ dichothermal layer,
⋅ mixed layer,
⋅ subarctic gyre,
⋅ halocline.
Kamchatka Peninsula. That is, the Bering Sea is the area
where the water mass transformation occurs.
One manifestation of the water mass transformation
in the Bering Sea is the existence of dichothermal water,
that is, subsurface water with a temperature minimum
layer, which is widely distributed in the northwestern
subarctic gyre (e.g., Dodimead et al., 1963; Favorite et
al., 1976). The dichothermal water lies just over the strong
halocline existing around 100–200 m depth in the
subarctic gyre. This strong halocline is also a common
feature of the North Pacific subarctic gyre. Since the
subarctic region is an area where the wind stress curl is
positive, it is there that upwelling of the deeper water
with higher salinity occurs. In addition, the North Pacific
subarctic gyre is the area which receives excessive precipitation (actually, precipitation minus evaporation) (e.g.,
Oberhuber, 1988). Inevitably, in the subarctic region the
strong halocline is formed in the subsurface layer. Since
this strong halocline can inhibit the penetration of the
developing mixed layer in winter, and water temperature
does not contribute much to water density in the lower
* Corresponding author. E-mail: [email protected]
Present address: Frontier Observation Research System for Global
Change, Natsushima, Yokosuka, Kanagawa 237-0061, Japan.
Copyright © The Oceanographic Society of Japan.
815
Fig. 1. Distribution of hydrographic observation stations for four seasons. Numbers of data points used are 8138 (winter), 20778
(spring), 32766 (summer) and 10745 (autumn).
temperature range, relatively lower temperature water in
the winter mixed layer is formed. In the warming season
this lower temperature water remains in the subsurface
as the temperature minimum layer, i.e., the dichothermal
water. This scenario is considered to explain the formation of the dichothermal water (e.g., Ohtani et al., 1972).
Although the scenario mentioned above is plausible,
a thorough description of the formation area and formation process of the dichothermal water has not yet been
published. Therefore, the purpose of the present paper is
to describe the water properties, the formation area and
formation process of the dichothermal water. In order to
do so, seasonal mean climatologies of oceanic condition
with a fine resolution have been newly prepared, using
the raw data archived in the historically accumulated
hydrographic observation dataset.
It is known that temperature minimum layers are
observed in the regions of the Alaskan Stream and the
Gulf of Alaska (e.g., Ohtani et al., 1972; M. Fukasawa,
personal communication). However, our target in the
present study is the dichothermal water, the potential density of which is around 26.6 sigma-theta (σθ). This density corresponds to the densest water outcropping in winter in the North Pacific.
The remainder of the paper is organized as follows.
Section 2 describes the data used and the method of creating a new climatological dataset. Section 3 describes
the oceanic structure observed in climatologies, with spe-
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T. Miura et al.
cial reference to the dichothermal water. Section 4 is devoted to clarifying the relationship between the
dichothermal water and the water in the winter mixed
layer in the Bering Sea. Section 5 presents our conclusion and a discussion.
2.
Data Used and Processing Method for New
Climatologies
Although several climatologies, such as Levitus
(1982), the World Ocean Atlas 1994 (WOA94; NODC,
1994) and the World Ocean Atlas 1998 (WOA98; NODC,
1999), have been released so far, all datasets are heavily
smoothed in space. Actually, half-amplitude scales of filter response functions used in Levitus (1982), WOA94
and WOA98 are approximately 1200 km, 1350 km and
900 km, respectively. These scales are too large to resolve the oceanic structure of the Bering Sea. In the
present study, therefore, a gridded dataset with fine resolution is newly prepared using the raw data of
hydrographic observations.
In the present study we have prepared annual and
seasonal climatologies on grids of 1° × 1° (latitude × longitude). The raw data used are individual profiles of both
temperature and salinity data, which are archived as the
standard depth data of bottle, CTD/STD and XCTD observations in the World Ocean Database 1998 (WOD98;
NODC, 1998). Figure 1 shows the study area and the distribution of observation stations in four seasons. Here,
Fig. 2. Partition of four sub-regions used in processing the
data to create seasonal mean climatologies on 1° × 1°
(lat. × lon.) grids.
winter is defined as three successive months of January
through March, and so on. It can be seen that both the
data number and coverage are relatively good in spring
and summer, while those in autumn and winter are small
and unevenly distributed in space. Therefore, caution
should be exercised in respect of this point when interpreting climatologies.
Prior to the averaging process, the study area is divided into four sub-regions in order to avoid creating artificial water masses due to the averaging process, especially around the Aleutian Islands. Actually, the four subregions, i.e., the Bering Sea, the Pacific Ocean, the Sea
of Okhotsk and the Arctic Ocean, are divided by islands
and peninsula as shown in Fig. 2. In each of the four subregions, all the data are then smoothed and gridded on
1° × 1° grids on the 24 standard levels from the sea surface to 1500 m, using a Gaussian filter with an e-folding
scale of 50 km and a window of 100 km per side. The
filtering removed fluctuations with scales less than 190
km.
The 24 standard levels are the same as those found
in WOA98: 0, 10, 20, 30, 50, 75, 100, 125, 150, 200,
250, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200,
1300, 1400, 1500 m. Using these standard level data, an
analysis dataset was prepared having a vertical resolution of 10 m from the sea surface to 150 m and of 25 m
from 150 to 1500 m using a linear interpolation scheme.
Although no comparison between the present
climatologies and previous ones is shown here, it was
found that the present climatologies can reveal a fine oceanic structure and therefore it is considered that the
present one is suitable for our purpose.
Fig. 3. Yearly mean dynamic topography (dynamic meters) at
200 db referred to 1000 db. Alphabetical symbols from A to
F denote the boxes along the flow pattern, where T-S diagrams are shown in Fig. 7.
3.
General Description of Oceanic Structure in the
Bering Sea and Adjacent Areas
3.1 Circulation field
First, the circulation field is grossly observed using
the annual mean map of dynamic topography at 200 db
surface referred to 1000 db, as shown in Fig. 3. The reader
should note that this figure is drawn in each of the four
sub-regions and combined to a single figure. In the present
study, the same procedures are adopted for all the horizontal distribution maps of variables.
Although density stratification is very weak in the
subarctic region and therefore current has a deep structure (e.g., Reed, 1995), major currents and circulation
patterns are observable in Fig. 3. That is, two large cyclonic gyres are seen in the western part of subarctic gyre
and in the Gulf of Alaska. The former is called the Western Subarctic Gyre (hereafter WSAG) and the latter corresponds to the Alaskan Gyre (Dodimead et al., 1963). In
addition, a small cyclonic gyre is seen around 50°N and
175°W. However, it is not clear whether this is a permanent feature or a transient eddy. In the area south of 50°N,
dynamic height contours run west to east, representing
the Subarctic Current and the North Pacific Current. Contours also run along the Aleutian Islands and along the
Kamchatka Peninsula, which represent the Alaskan
Stream and the (East) Kamchatka Current.
It can also be seen that a part of the Alaskan Stream
waters enters the Bering Sea through several passes: the
Amukta Pass (172°W), the Amchitka Pass (180°) and the
Near Strait (172°E). These inflow routes to the Bering
Dichothermal Water Formation in the Bering Sea
817
Sea coincide with previously published studies (e.g.,
Favorite et al., 1976).
In the Bering Sea, although Stabeno and Reed (1994)
revealed an eastward current along the north slope of the
Aleutian Islands based on the observation of satellitetracked drifters, this feature cannot be seen clearly. Since
the eastward current reported by Stabeno and Reed is
narrow (approximately 20 km), it is likely that our climatology smoothed it out. In the central part of the Bering
Sea the dynamic topography pattern suggests the existence of a cyclonic gyre. Since no closed contour exists
and the contour interval is relatively wide, this pattern in
the Bering Sea shows that cyclonic circulation is very
weak in speed.
3.2 North-south and west-east vertical sections in the
Bering Sea
Figure 4 shows vertical sections of potential temperature, salinity and potential density along 180° in winter and summer. In these cross sections, the position of
the Aleutian Islands is approximately 52°N and the shallower portion near 54°N is part of the Bowers Ridge.
As mentioned in Introduction, since salinity contributes dominantly to density, both features of salinity and
potential density are very similar. Below approximately
200 m in winter salinity and potential density sections,
two doming (convex) structures are seen around 50°N and
56°N. The former doming structure corresponds to the
eastward flowing Subarctic Current in the south and the
westward flowing Alaskan Stream in the north. The latter doming reflects the existence of cyclonic circulation
in the Bering Sea. In summer sections, although the former
doming structure is situated at the same latitude, the latter shifts northward by 2 degrees latitude (around 58°N)
and its amplitude is smaller. This means that the cyclonic
circulation in the Bering Sea becomes weaker and its
center shifts northward. The halocline and pycnocline
exist around 200 m in the southern part and 250 m in the
northern part. Central values of halocline and pycnocline
can be read as roughly 33.6 psu and 26.7σθ, respectively.
In the surface layer there are two cores of salinity minimum in both winter and summer sections: one is around
49°N and the other is over the continental shelf.
On the other hand, the temperature structure is quite
different from those of salinity and potential density in
both winter and summer sections. In winter, cold water
of temperature less than 3°C covers the surface layer in
the Bering Sea. Waters with temperature less than 0°C
appear especially over the continental shelf. Below this
layer, the temperature maximum layer with a temperature of about 3.5°C lies centered at 350 m. On the Pacific
side the contours rise northwards. In summer, a seasonal
thermocline is formed around 50 to 100 m throughout the
entire study area. Below the seasonal thermocline, a tem-
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T. Miura et al.
Fig. 4. Vertical sections of (a) potential temperature (°C),
(b) salinity (psu) and (c) potential density (σθ) along 180°.
Winter (left panels) and summer (right).
perature minimum layer with temperature less than 3.5°C
is seen in the Bering Sea. This water is the dichothermal
water, which is our target in the present study, and its
lowest temperature is less than 2.5°C. The thickness of
this structure is about 150 m, and salinity and potential
density at the central portion of the water can be read as
33.2 to 33.3 psu and 26.5 to 26.6σθ. That is, this water is
located just above the halocline and pycnocline. Again,
below the dichothermal water, a temperature maximum
layer exists as in the winter section.
Around 50°N in the Pacific region, another temperature minimum layer with temperature less than 3.5°C is
seen at 200 m depth. Since the surface temperature at this
region in winter is from 3°C to 3.5°C, there is a possibility that this temperature minimum layer is formed there
in winter, independently of the dichothermal waters in
the Bering Sea. However, this latitudinal belt corresponds
to the ridge between the Subarctic Current flowing east-
Fig. 5. As in Fig. 4 but along 57°N.
Fig. 6. As in Fig. 4 but along 49°N.
ward and the Alaskan Stream flowing westward, so there
is another possibility that this temperature minimum layer
is advected by one of these currents. This issue is beyond
the scope of the present paper, and further clarification is
needed using appropriate observational data.
The above description for the summer section is quite
consistent with that given by Roden (1995), who described
the oceanic structure in the same area using the data of
WOCE-WHP P14 line obtained in July 1993. This means
that the present climatologies retain the major characteristic features of the Bering Sea.
As in Fig. 4, Fig. 5 shows vertical sections of potential temperature, salinity and potential density along 57°N
in winter and summer. As a gross feature, salinity and
density structures indicate the existence of a northward
flow in the eastern part and a southward flow off the
Kamchatka Peninsula. In the winter temperature section,
the top of the temperature inversion is seen around 200
m. Surface temperature is lower in the west (less than
1°C) and over the continental shelf (less than 0°C). In
the summer section, a strong seasonal thermocline de-
velops in the upper 100 m. Below this seasonal
thermocline, the dichothermal water occupies with the
thickness of about 150 m. The properties of the core layer
of the dichothermal water are a salinity of 33.2–33.3 psu
and a density of 26.6–26.7σθ through the entire section.
On the other hand, temperature is 2.5°C at 180° and less
than 1°C at the westernmost part. Below the dichothermal
water, again a temperature maximum layer with temperature of 3.5°C is found through the entire section.
3.3 West-east vertical section in the Pacific Ocean
As in Fig. 4, Fig. 6 shows vertical sections of potential temperature, salinity and potential density along 49°N
in winter and summer. Again, structures of salinity and
potential density are almost the same in both winter and
summer sections. Halocline and pycnocline are seen
around 100–200 m through the entire section. The bending toward a deeper layer of halocline and pycnocline at
the westernmost part represents the existence of the
Kamchatka Current. Below the halocline and pycnocline
there are two doming structures: one is around 165°E and
Dichothermal Water Formation in the Bering Sea
819
Fig. 7. T-S diagrams at selected boxes (see Fig. 3 and text).
the other around 172°W. These are manifestations of the
WSAG and a small cyclonic gyre, the existence of which
is already pointed out in Subsection 3.1. There is no doming structure in the eastern half of the section. This is
because the 49°N line does not cut the central part of the
Alaskan Gyre (see Fig. 3). These sections also show that
the development of a winter mixed layer is prohibited by
the strong halocline and pycnocline.
Similar to the 180° and 57°N sections (Figs. 4 and
5), the temperature structure is quite different from those
of salinity and potential density. In the westernmost part
of the winter section, cold waters with temperature less
than 3°C can be seen in the surface layer, while the eastern part contains warm waters with temperature greater
than 4°C. The former corresponds to the water transported
by the Kamchatka Current and the latter is the waters of
the Subarctic Current. Between these areas, the vertical
temperature gradient is very weak. Again, a temperature
maximum layer is seen in the western half of the section,
from the west to approximately 175°W. In the summer
section, as in salinity and density sections, a seasonal
thermocline develops around 100 m. In the Kamchatka
Current region, the dichothermal structure appears
centered at 150 m. The properties of the core layer of this
water are temperature less than 2.5°C, salinity of 33.3
psu and potential density of 26.7σ θ. In the summer section, around the International Date Line, there are isolated regions showing temperature inversions. As mentioned in the previous subsection, it is not clear whether
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T. Miura et al.
these temperature minimum layers are formed there or
advected.
3.4 Temperature-salinity diagrams along the circulation
pattern
Figure 7 shows temperature-salinity (T-S) diagrams
at boxes labeled A through F in Fig. 3. As judged from
the dynamic height distribution, these boxes are selected
along the circulation path from upstream to downstream.
Box A is situated at the transition area from the Alaskan
Gyre to the Alaskan Stream. Box B is placed at one of
entrances of the Alaskan Stream waters to the Bering Sea,
and Box C is over the Bowers Ridge in the Bering Sea.
Boxes D to F are arranged from upstream of the
Kamchatka Current to downstream. For Boxes C through
F, 9-grid data (3° × 3° box) are plotted on the diagram,
and for Boxes A and B, 8-grid data (2° × 4° box) are plotted. Boxes A and B are set taking into account that the
Alaskan Stream is narrow in width.
Evolution of the temperature minimum layer, i.e., the
dichothermal water, can be clearly observed in these diagrams. At Boxes A and B in the Alaskan Stream region,
although very weak temperature inversions are certainly
seen, most of the surface water does not decline below
3°C even in winter. In addition, the densities of temperature minimum layers are much lighter than 26.6σθ.
On the other hand, at Boxes C through F in the Bering
Sea, the temperatures of surface waters descend below
3°C in winter, and the temperatures in the uppermost lay-
Fig. 8. Horizontal distribution of water properties averaged in the winter mixed layer. (a) Potential temperature (°C), (b) salinity
(psu), (c) potential density (σ θ) and (d) winter mixed layer depth (m).
ers increase in the warming season. Therefore, from spring
to autumn, temperature minimum layers are formed robustly, and temperature maximum layers are formed
throughout the whole year below them. As the Box goes
downstream, temperatures at the temperature minimum
layers decrease from about 2.5°C at Box C to 1°C at Box
F (see the panels for summer). Salinity and potential densities of the temperature minimum layers are 33.2–33.3
psu and 26.6–26.7σθ, respectively.
We may speculate why core temperatures of the
dichothermal layers decrease toward downstream. First,
vertical mixing might be very weak even in the surface
layer. As a result, core temperatures of the dichothermal
waters are well retained over time. In addition, since the
circulation field is also weak in the Bering Sea, as mentioned above, the water column has to experience winter
more than once. Therefore, at the downstream side, the
core temperatures of the dichothermal waters decrease
more than those upstream. This issue will be discussed in
an accompanying paper (Miura et al., 2002), the purpose
of which is to reconstruct the formation of the
dichothermal waters using a Lagrangian-type, one-dimensional numerical model.
In conclusion, findings on the dichothermal layer
based on these T-S diagrams can be summarized by stating that the dichothermal waters are formed robustly in
the Bering Sea and as waters flow downstream, core temperatures of the dichothermal water decrease from 2.5°C
at the Bowers Ridge to about 1°C at the Kamchatka Strait.
In the next section we examine the relationship between the dichothermal water and winter mixed layer
throughout our study area.
4.
Relationship between Dichothermal Water and
Winter Mixed Layer
In this section, using the maps of horizontal distributions of potential temperature, salinity and potential
density, we examine whether the dichothermal water is
formed in the winter mixed layer.
Figure 8 shows the horizontal distribution of potential temperature, salinity and potential density in the winter mixed layer, and the mixed layer depth. Here the bottom of the mixed layer is defined as the depth with density heavier by 0.125σθ than that at the sea surface. Concentrated contours of mixed layer temperature and salinity (Fig. 8) lie west-east in the 40–50°N latitudinal bands
where the North Pacific and Subarctic Currents flow eastward, and along the coast of Gulf of Alaska and the edge
of continental shelf in the Bering Sea. In the former region, however, contours of potential density lie rather
north to south. That is, the subarctic frontal region is not
an especially remarkable front of density. In the Bering
Sea, although temperatures decrease from the southeastern side (about 3°C) to the northwestern side (about 0°C),
salinities and densities are relatively homogeneous at
33.1–33.2 psu and 26.4–26.6σθ, respectively. Mixed layer
depths are shallower (less than 100 m) in the Gulf of
Alaska and over the continental shelf in the Bering Sea.
Dichothermal Water Formation in the Bering Sea
821
Fig. 9. Horizontal distribution of water properties at the core layer of dichothermal water in summer. (a) Potential temperature
(°C), (b) salinity (psu), (c) potential density (σ θ) and (d) depth of the core layer of dichothermal water (m).
In the area south of the Subarctic Current, the rest of the
Bering Sea and part of the Sea of Okhotsk, mixed layer
depths exceed 140 m.
Figure 9 shows the distribution of water properties
at the temperature minimum layer, that is, central depth
of the dichothermal water, and the depth in summer. Areas having no temperature minimum layer are shaded, i.e.,
no dichothermal water is present there. It is found that
the dichothermal water distributes in the Bering Sea, the
Sea of Okhotsk and the WSAG, and along the northern
side of the Subarctic Current. Although the depths of the
temperature minimum layer are less than 100 m in the
Sea of Okhotsk and greater than 140 m along the Subarctic
Current and the slope region in the Bering Sea, they are
about 100–120 m in the major area. In the Bering Sea, in
general, temperatures decrease from the east (3°C) to the
west (1°C). On the other hand, salinity and potential density are very homogeneous at 33.2–33.3 psu and 26.5–
26.6 σθ, respectively.
Comparing the water properties of Figs. 8 and 9, we
notice the differences are very small. It can therefore be
concluded that the dichothermal water is the water formed
in the winter mixed layer.
5. Conclusion and Discussion
In the present study, in order to clarify the water properties and the area in which and the process by which the
dichothermal water is formed, a new seasonal mean
gridded climatological dataset with a fine resolution for
822
T. Miura et al.
the Bering Sea and adjacent seas was prepared, using historically accumulated hydrographic data. The particular
focus was the dichothermal water having a density of
approximately 26.6σθ, which corresponds to the densest
density outcropping in winter in the North Pacific.
Although waters of the Alaskan Stream had weak
temperature minimum layers, no robust feature of temperature minimum layers existed in the climatologies,
while waters in the Bering Sea had a firm structure at
densities around 26.6σ θ. In addition, it was found that
properties of the dichothermal water observed in warming season were almost the same as those in winter mixed
layer. That is, the dichothermal water having a density of
approximately 26.6σθ is formed in the winter mixed layer
at the Bering Sea. Since this water was found in the
Kamchatka Strait, i.e., the main exit of the Bering Sea
waters, it can be considered that the dichothermal water
was exported from the Bering Sea to the Pacific Ocean
by the Kamchatka Current.
In the textbook by Tomczak and Godfrey (1994), it
is stated that the water of the temperature minimum in
the Bering Sea originates on the shelf area of the Bering
Sea during winter as a result of convection under the sea
ice. For the sea area adjacent to the shelf region, this possibility might exist to some degree in the formation of
temperature minimum layers. However, for the majority
of the Bering Sea the present results show that the above
statement should be revised, as that the temperature minimum layer is formed in the winter mixed layer process.
As mentioned in Introduction, since the subarctic
region has a strong halocline everywhere, the
dichothermal waters might be formed everywhere. Actually, Fig. 9 shows the existence of the dichothermal waters in the Sea of Okhotsk, the region of WSAG, the sea
area along the Kuril Islands and the region along the northern part of the Subarctic Current. Except for those in the
Sea of Okhotsk, Fig. 9 seems to suggest that the
dichothermal waters formed in the Bering Sea are a major contributor in forming their structure. However, in
future, this hypothesis should be examined.
Although we found that the formation process is one
of the wintertime mixed layer in the Bering Sea, we need
a quantitative assessment between heat and freshwater
budgets and the resultant water mass transformation. In
our accompanying paper (Miura et al., 2002), we try to
examine the formation of the dichothermal water using a
Lagrangian-type, one-dimensional numerical model under realistic surface flux conditions.
Acknowledgements
The authors wish to express their sincere thanks to
the members of the Physical Oceanography Laboratory,
Tohoku University for their useful comments. Dr. Jiro
Yoshida, the editor of the journal, and two anonymous
reviewers gave useful comments. This study was conducted as part of the Subarctic Gyre Experiment (SAGE),
which was financially supported by the former Japanese
Science Technology Agency and the present Japanese
Ministry of Education, Culture, Sports, Science and Technology.
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