On the question of accumulation of ice-melt water south of the ice in the Chukchi Sea.

DUATI SCHOOt

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THESIS

OF

ON THE QUESTION OF ACCUMULATION
ICE-MELT WATER SOUTH OF THE ICE IN THE
CHUKCHI SEA

by

Robert Glenn Handlers

March 1977

R.G. Paquette
Thesis Advisors: R.H. Bourke

Approved for public release; distribution unlimited.

A report submitted to
Director, Arctic Submarine Laboratory
Naval Ocean Systems Command
San Diego

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4. TITLE (and Subtltlai

On the Question of Accumulation of
Ice-Melt Water South of the Ice in
the Chukchi Sea

»• T/YPE OF REPORT a PERIOD COVERED

Final

1 July 1976-30 Sept 197'

S. PERFORMING ORG. REPORT NUMBER

7. AUTHORS

Robert Glenn Handlers in conjunction
with Robert G. Paquette and
Robert H. Bourke

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Naval Postgraduate School
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Arctic Submarine Laboratory, Code 90
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March 1977

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16. SUPPLEMENTARY NOTES

19. KEY WORDS (Canfinwa an ravmraa alma II nmcaaamry and Identity my mloak nummmt)

Marginal Sea-Ice Zone MIZPAC Physical Oceanography

Arctic Ocean Ice Circulation

Chukchi Sea Currents Ice Melt

20. ABSTRACT (Contlrmo on ravaraa mlda II i

tarry and idmntttr my mloak mammae)

The processes controlling the distribution of melt water
from the retreating ice edge in summer in the Chukchi Sea
were examined in order to provide evidence of the flow
regime. Current and salinity data from the National Oceano-
graphic Data Center (NODC) files and from four MIZPAC cruises
were utilized in this work. An increase in melt-water
content towards the ice in the approximately 30 km wide

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(20. ABSTRACT Continued)

ice-melt zone as well as an abrupt salinity decrease were
observed. This effect was presumed to be due to scattering
of ice from a diffuse ice margin accompanied by melting.
North of the ice edge the fresh-water content was greater2
than that of southerly water by an amount (150-200 gm/cm )
equivalent to the thickness of the ice cover. These
findings together with an independent comparison of transport
times and ice satellite data provide good evidence that the
current flows faster than the ice retreats during summer
in the eastern half of the Chukchi Sea.

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On the Question of Accumulation
of Ice-Melt Water South of the Ice in the

Chukchi Sea

by

Robert Glenn Handlers
Lieutenant, United^States Navy
B.S., The Pennsylvania State University, 1969

Submitted in partial fulfillment of the
requirements for the degree of

MASTER OF SCIENCE IN OCEANOGRAPHY

from the

NAVAL POSTGRADUATE SCHOOL
March 1977

o I

NAVAL POSTGRADUATE SCHOOL
Monterey, California

Rear Admiral Isham Linder Jack R. Borsting

Superintendent Provost

This thesis is prepared in conjunction with research
supported in part by the Arctic Submarine Laboratory,
Naval Undersea Center, San Diego, under Project Order
No. 00610.

Reproduction of all or part of this report is
authorized.

Released as a Technical Report by:

ABSTRACT

The processes controlling the distribution of melt water
from the retreating ice edge in summer in the Chukchi Sea
were examined in order to provide evidence of the flow re-
gime. Current and salinity data from the National Oceano-
graphic Data Center (NODC) files and from four MIZPAC cruises
were utilized in this work. An increase in melt-water con-
tent towards the ice in the approximately 30 km wide ice-
melt zone as well as an abrupt salinity decrease were ob-
served. This effect was presumed to be due to scattering
of ice from a diffuse ice margin accompanied by melting.

North of the ice edge the fresh-water content was greater

2
than that of southerly water by an amount (150-200 gm/cm )

equivalent to the thickness of the ice cover. These findings

together with an independent comparison of transport times

and ice satellite data produce good evidence that the

current flows faster than the ice retreats during summer

in the eastern half of the Chukchi Sea.

TABLE OF CONTENTS

I. INTRODUCTION 10

II. GENERAL OCEANOGRAPHY 14

III. METHODS 21

IV. RESULTS 27

V. CONCLUSIONS 40

BIBLIOGRAPHY 41

INITIAL DISTRIBUTION LIST 42

LIST OF TABLES

TABLE I. Comparison of the mean current speed of the
upper 10 m and the mean speed over the
remainder of the water column 39

LIST OF FIGURES

FIGURE

1. Vertical salinity sections from MIZPAC

1974, Stations 22, 23, and 24 12

2. STATEN ISLAND current measurements, July,

1968 (from Coachman, et al., 1976) 17

3. OSHORO MARU current measurements, July,

1972 (adapted from Coachman, et al., 1976) 18

4. Upper level flow patterns (adapted from
Coachman, et al., 1976) 20

5. Transport streams in the Southern Chukchi
Sea based on the assumed upper-level flow
pattern of Coachman, et al. (1976) 24

6. Time of transport for streams I, II and
III from Bering Strait northward to

70°N latitude 25

7. Monthly plot of codified surface salinities

for August 29

8. MIZPAC 1974 symbolic plot of salinities 31

9. MIZPAC 1975 integrated fresh-water content 33

10. MIZPAC 1974 integrated fresh-water content 35

11. Mean weekly position of the ice edge from

June to August for the years 1972 to 1975 36

ACKNOWLEDGMENT

I gratefully acknowledge the assistance of my thesis
advisors, Drs . R.G. Paquette and R.H. Bourke. Their
interest and suggestions have been most valuable in the
preparation of this thesis. I also wish to thank the faculty
and staff of the Naval Postgraduate School for their help
and guidance throughout my graduate work. Finally, I would
like to express my appreciation to my wife, Paula, for her
help and typing assistance throughout this work.

I. INTRODUCTION

During the course of investigation in the marginal sea-
ice zone as a part of the MIZPAC program, interest has
been generated in the processes controlling the distribution
of melt water from the retreating ice edge in summer in the
Chukchi Sea. This thesis investigates the possibility that
the distribution of ice-melt can provide evidence of the
flow regime in the Chukchi Sea. Paquette and Bourke (1976) ,
working with MIZPAC 1974 data, found a zone of dilute water
south of the ice and hypothesized that this dilute water
could be the signature of ice-melt water which was being
carried northward more slowly than the ice was retreating.
This study brings to bear appropriate historical data in an
effort to determine the general validity of this hypothesis.
In the end it will be shown that the mean current flows
northward faster than the ice retreats, which implies that
the greater part of the ice-melt water is pushed northward
under the ice. Over considerable portions of the ice margin,
the dilute water is confined to a narrow band close to the
ice. However, in the 1974 and 1975 MIZPAC data, there were
sizable boluses of fresh water well to the south of the ice

MIZPAC refers to Marginal Sea-Ice Zone Pacific, an
investigation of the Pacific Marginal Sea-Ice Zone under
the general direction of the Arctic Submarine Laboratory,
Naval Undersea Center, San Diego.

10

which is explained as a phenomenon localized in time and
space.

To test the hypothesis of the existence of an ice-melt
zone, the following analyses were undertaken. Initially,
a number of vertical salinity profiles were examined for
evidence of a more-or-less sharp transition in the near-
surface waters from high to low salinity south of the ice.
Figure 1 consists of three salinity profiles taken from
MIZPAC 1974 data along a south-north section approaching
the ice. It can be seen that the isohaline water at
Station 22, approximately 100 km from the ice, is water
from the southern Chukchi Sea which has experienced no
dilution by ice-melt water. The low salinity values of
Station 24, located within about 10 km of the ice edge,
clearly shows the dilution caused by the melting ice.
Station 23, approximately 45 km outside the ice, shows that
marked dilution of near-surface waters can occur at distances
remote from the retreating ice edge. Several examples of
sections taken from the MIZPAC data showed this marked
near-surface salinity reduction, but few profiles encom-
passed a sufficient number of stations which showed the
progressive dilution from well outside the ice to within
the ice. Therefore, further evidence was sought.

Next, all the summer Chukchi Sea data in the National
Oceanographic Data Center (NODC) files up to 1972 and the
data from four MIZPAC cruises were examined for additional

11

27

28

29

SALINITY (%.)

30 31

32

3:

u

f

l

I \ 1

l

•

-

t

\ 23

v:

22

10

X !

~20

s

N '

I
a.
a

30

40

1

1

t
•

\\
V

\

FIGURE 1. Vertical salinity sections from MIZPAC 1974,
stations 22, 23, and 24.

12

evidence of salinity dilution by ice melt. Surface salinity
and five-meter-depth salinities were classified into four
salinity groups and plotted by months. These results indi-
cated low-salinity water in the vicinity of the ice and on
the eastern side of the Chukchi Sea. The low salinities
near the coast and in Kotzebue Sound raised the suspicion
that the salinity change near the ice was not entirely due
to ice melt, i.e., some low-salinity water may have had its
origin in Bering Strait or in the Kobuk River. It was also
considered possible that some ice-melt water had been mixed
downward and was not being found in the plots of the near-
surface salinities.

To examine the latter possibility, the fresh water con-
tent within a water column was obtained by integration for
all MIZPAC stations and the results plotted. These plots
showed that a rapid increase in fresh water content occurred
near the ice edge and beyond it, and that the dilution
could be ascribed to melting from an appropriate thickness
of ice.

An attempt was made to compare the observed rate of ice
retreat from satellite data with the northward rate of
flow using historical water velocity data and computed
transport rates. This analysis provided an independent
verification of the conclusion inferred from the previous
analyses that the near-surface waters indeed flow northward
faster than the ice retreats.

13

II. GENERAL OCEANOGRAPHY

An understanding of the role of ice-melt water in
the Chukchi Sea depends upon several factors, and upon the
answers to the following basic questions: Does northward-
flowing water travel as fast as the ice retreats? How can
one describe background water properties so that the pre-
sence of ice melt may be seen as a deviation from the back-
ground? Are there sources of fresh water other than ice
melt, which may also contribute to the deviation? To
answer these questions it is necessary to review a limited
amount of the general oceanography, and in particular that
which concerns the northward flow of the waters from Bering
Strait, whose properties vary with position in the Strait
and with time. Previous studies (Paquette and Bourke,
1976; Zuberbuhler and Roeder, 1976) have suggested that the
rate of ice melt is influenced by the speed and heat content
of the northward- flowing water. Seasonal differences,
therefore, affect the rate at which melt water is provided.
Another seasonally influenced parameter is the position of
the ice edge which is also important to the problem. Unfor-
tunately, most of the historical data do not give the position
of the ice edge and it must be inferred indirectly.

The following section will rely heavily on the work of
Coachman, Aagaard and Tripp (1976) , who will henceforth
be designated as CAT. The Chukchi Sea is a shallow

continental-shelf sea with an average depth of about 45 m.
Ice normally covers the Chukchi Sea from November to
June, and then melts back fairly rapidly in an irregular
fashion to a northern limit of about 73° N latitude, achieved
in mid-September. Bering Strait is a narrow, shallow (30-
50 m) passage between Alaska and Siberia which connects the
Bering and Chukchi Seas. The dominant feature of the
Bering Strait, as demonstrated by CAT, is the general north-
ward direction of the mean flow, at least during the summer.

The following specific flow and water property char-
acteristics through Bering Strait are presented here as
they are necessary for the transport calculations used later
in this study. In the eastern part of Bering Strait CAT
found in mid-summer (August) a sharp pycnocline at 10-15 m.
which typically separates a surface layer of warm (9-
10°C) , fairly low-salinity (30-31°/oo) water from a deeper,
colder (1-4°C) , more saline (31-32.7 /oo) water. The western
part contains relatively uniform cold (1-4°C) and saline
(32.7-33 /oo) water. Earlier in the summer less dilution
in the eastern part of Bering Strait is observed. Concurrent
with the arrival of Yukon River water in the eastern Bering
Strait in August, salinities decrease to approximately
29 ^oo. Higher salinities return during the month of October,
CAT also report that strong velocity shears occur between
these water masses, e.g., the flow in the upper layer in
the eastern part of the Strait commonly exceeds 100 cm/sec

15

while the deeper-lying water has intermediate speeds of
30-60 cm/sec. The flow through the western channel is
slower and more uniform.

CAT identify three water masses in Bering Strait,
arranged laterally from east to west, and dif f erentiable
on the basis of salinity: Alaskan Coastal, Bering Shelf,
and Anadyr Waters. Within a short distance north of Bering
Strait, the Anadyr and Bering Shelf Waters are combined to
form Bering Sea Water. Bering Sea Water dominates the
central and western part of the southern Chukchi Sea and
has a median salinity of 32.5 ^oo« The Alaskan Coastal
Water, according to CAT, lies to the east of the Bering Sea
Water and is joined by effluent from Kotzebue Sound. They
state that there is no way to distinguish between Alaskan
Coastal and Kotzebue Sound Waters because of similar tem-
perature and salinity characteristics. CAT indicate the
temperature and salinity properties of the Alaskan Coastal
Water are variable, but in general, the salinities range
between 30.5 /oo and 32.0 /oo.

The general current pattern through Bering Strait and
in the southern portion of the Chukchi Sea in mid-summer
can be inferred from individual current measurements taken
during the STATEN ISLAND 1968 and OSHORO MARU 1972 cruises
(Figures 2 and 3) . These measurements demonstrate the
general northward direction of flow and the variability of
the currents with depth. A schematic of the surface flow
pattern for the Chukchi Sea, as constructed by CAT from

17?'

!9*

I7G

Vwf//

VV ALASKA

^V^V*UQ^-

0 - 10m —

IU - 30m --■

m- bottom

0 50

■ i i i i i

FIGURE 2. STATEN ISLAND current measurements, July, 1968
(from Coachman, et al., 1976).

17

72*-

178° 176* 174* 172' 170° 168* 166* 164* 162* 160* 158* 156*

-72*

7cr

68*

DEPTH IN METERS

178° 176* 174* 172" 170° 168° 166* 164" 162° 160* 158* 156*

FIGURE 3. 0SH0R0 MARU current measurements, July, 1972
(adapted from Coachman, et al., 1976).

18

historical current measurements (Figure 4) , indicates that
the current flows more rapidly on the eastern side of the
Chukchi Sea with an initial speed of 150 cm/sec. As the
water proceeds northward, the average speed decreases to
20-30 cm/sec, while over the south-central Chukchi Sea,
the average speed is 15-25 cm/sec.

19

- 72°N

174°

FIGURE 4.

170'

166°

162°W

Upper level flow patterns (adapted from
Coachman, et al., 1976).

20

III. METHODS

All of the summer data in the Chukchi Sea from 1922 to
1972 on file at NODC and the MIZPAC 1971, 1972, 1974 and
1975 data were examined to find evidence of the general
presence of dilute water south of the ice. Computer pro-
grams were used to retrieve stored data from magnetic
tapes and to create monthly plots of MIZPAC and NODC surface
and five-meter-depth salinities. Originally, an attempt
was made to contour salinities on the NODC plots, but the
results were too noisy. However, when the salinities were
grouped into ranges and plotted as coded symbols, the noise
was eliminated to a great extent.

NODC data were combined by months (July-September)
regardless of year while salinity plots were created for
each of the MIZPAC cruises. In an attempt to further reduce
the noise in the NODC surface salinity plots, it was assumed
that the noise might be due to year-to-year variability
in the position of the ice edge. Accordingly, the monthly
plots were separated into severe and not-so-severe years
using Barnett's severity index (1976). This severity covers
the time period from 1955 to 1972; hence, monthly plots
were only presented for this time period. There was only
a slight improvement in consistency as a result of this
breakdown; therefore, the data were recombined and analyzed.

21

As the surface and five-meter-depth salinity plots showed
essentially the same information, only the surface plots
were used for analysis.

The integrated fresh-water content was then computed

for the MIZPAC data by numerically integrating the following

SR— S
equation: W = P / ep dz . This calculation yields the

Z 0 SR ,

weight in grams of fresh water in a water column of 1 cm

area and height z, measured to the surface, as compared to

a similar column of water with a reference salinity (SR) of

33.33°/oo. The latter choice is a rough median value for

the higher salinities found near the bottom south of the

3

ice. The density of the water (p) is equal to 1.03 gm/cm

and S represents the average salinity in the depth incre-
ment, dz. These fresh-water contours were based on a
reference depth of 30 m, a compromise so as not to exclude
too many shallow-water stations , but not to exclude too
much fresh water from the near-bottom layers.

The fresh-water content of a water column suffers one
serious difficulty as a diagnostic tool: it is not
necessarily a conservative parameter, even in the absence
of mixing. If an upper, fresh-water-rich layer converges
or diverges to a greater extent than a lower layer, a
change in fresh water content will result. For example,
if a dilute water column of 30 m depth were concentrated
into an upper layer of 10 m, the fresh-water content would
decrease. It will be shown that the assumption of negligible

22

convergence or divergence is true in the vicinity of the
ice edge, but is not necessarily tenable south of the ice.

In an attempt to predict both the time- varying water
properties north of Bering Strait and the velocity of the
water at the ice, the water speeds and time of transport
were computed for the waters issuing from four vertical
slices through Bering Strait. These four streams (desig-
nated I, II, III, and IV from east to west) were selected
to correspond roughly to the direction of the upper-level
flow pattern (CAT, 1976) . Transport values through Bering
Strait were calculated for each stream using STATEN ISLAND
1968 current measurements and the cross-sectional areas of
each stream. The resulting transports for streams I-IV
were 0.630 Sv, 0.270 Sv, 0.340 Sv and 0.390 Sv, respectively,
yielding a total transport through Bering Strait of 1.63 Sv
which is in close agreement with Coachman and Aagaard's
(1974) value of 1.5 Sv.

Streams I, II, and III were then separated into trape-
zoidal segments, as shown in Figure 5. Transport times
within these streams, north to 70° latitude, were calculated
using trapezoidal integration (Figure 6) . Velocities in
the vertical were assumed uniform from top to bottom.
Transport times rather than actual current speeds were
used to monitor the flow rate in the southern Chukchi Sea
as current data in this region are extremely sparse. Cal-
culations were discontinued at 70° latitude because the flow

23

72«N

70<

68°

66*

174°

170

iee4

162°

FIGURE 5. Transport streams in the southern Chukchi Sea
based on the assumed upper-level flow pattern
of Coachman, et al. (1976) .

24

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25

pattern separates into northeast and northwest branches
at approximately 69° latitude and becomes extremely variable
beyond 70° latitude (CAT, 1976) . Stream IV was eliminated
from the calculations because of uncertainty of the flow
pattern in the western Chukchi, and also because it was to
the west of the area encompassed by the MIZPAC cruises.

An attempt was made to apply transport times to salini-
ties and fresh-water contents in Bering Strait to extrapo-
late them to the vicinity of the ice margin as "background"
values which might be expected there. Both attempts failed,
the fresh-water contents because they were not conservative
with respect to divergence and convergence in the southern
Chukchi Sea, and the salinities because of intrusion of
low-salinity water from Kotzebue Sound and possibly low
salinities in the unsampled shallows of the eastern Bering
Strait.

26

IV. RESULTS

Vertical salinity profiles from the MIZPAC cruises
indicated that marked dilution of the near-surface waters
occurred well outside the ice margin with increasing dilu-
tion observed as the water approached the ice edge (Figure
1 in Section 1) . The position of the initial dilution,
which was assumed by Paquette and Bourke (1976) from MIZPAC
1974 data to be the initial signature of ice-melt water,
occurred from 50 to 100 km outside the ice edge. The ice-
melt signature appears to be a widespread phenomenon south
of the ice and its position appears to vary with the location
of the ice edge in the Chukchi Sea, i.e., the ice-melt
signature shifts progressively northward with the retreat
of the ice. The distance of the ice-melt signature from
the ice edge is variable in time and space because the ice
retreats in an irregular pattern across the Chukchi Sea
and its rate of retreat varies from year to year. Dilution
from ice melt can occur at distances as great as 100 km
from the ice due to fluctuations in the position of the ice
margin in response to wind shifts (time scale of a few days)
and to melting of stray ice floes or isolated patches of
ice.

The monthly surface salinity plots from the combined
NODC data also showed that the low salinities in summer
were concentrated in the northern portion of the Chukchi Sea.

27

Only the results from the August data are discussed as the
results from July and September are similar. Salinities
less than 30 9/00, shown in Figure 7 as |'s and x's, are
found clustered in a narrow band at a latitude where the
ice edge might be expected to be found in August. The few
moderately high salinities in this region ( I's) were
probably taken at stations fairly remote from the ice edge.
(Ice concentration data are not present in the NODC data.)
To the south of this band of dilute water, Figure 7 shows
no evidence of dilution by ice-melt within the central
Chukchi Sea. Near-surface salinities are generally every-
where greater than 31.5Q/oo (A's) which is undiluted
Bering Sea Water as identified by CAT (1976) . The zone of
moderate salinity water north of Cape Lisburne is Alaskan
Coastal Water. The NODC data show that another low-salinity
zone is present during the summer in the vicinity of Kotzebue
Sound. This is from the discharge of the Kobuk River which
peaks in August (USGS, 1973) .

The MIZPAC salinity plots are able to confirm the
results suggested by the NODC monthly plots in two important
ways. Because the location of the ice edge is a known
feature of the MIZPAC cruises, it is possible to accurately
assess the distribution of ice-melt water found outside the
ice margin. Also the MIZPAC cruises penetrated well into
the ice, usually 10 n mi but occasionally as far as 30 n mi
behind the ice front, providing information on the amount
of dilution occurring under the ice.

28

i r

i r

CHUKCHI SEA

♦*♦ .

♦ J* ♦*

*■ f ■ * ■

¥ ■

CAPE LISBURNE

70°N

175° 170° 165° 160° 155°W

FIGURE 7. Monthly plot of codified surface galinities for
August. Salinities less than 29 /oo - circle,
29-30 °/oo - x , 30-31.5 °/oo - square ,
greater than 31.5 °/oo - diamond .

29

The MIZPAC 1974 cruise data, taken during the last
half of July, show that undiluted Bering Sea Water covers
most of the central Chukchi Sea south of Point Hope (Figure
8) . Initial dilution by ice melt occurs as far as 50 km
outside the ice edge in the central Chukchi, but occurs
less than 10 km from the ice northwest of Icy Cape. This
large variation in width of the melt-water area is most
likely due to one or more of three inter-related processes:
the variation in speed of the northward-flowing surface
waters, the irregular scattering southward of diffuse ice,
or large eddies.

The ice penetrations indicate that low-salinity water
extends well behind the ice edge. In general, the 29 ^oo
salinity contour is fairly coincident with the ice margin.
Salinities at the northern extremities of the ice penetra-
tions were typically 26 to 27 ^oo, but values less than
20"°/oo were not uncommon. However, salinities as low as
20"°/oo generally did not extend to depths as deep as 5 m.
Data from the August MIZPAC 1975 cruise, which covered
nearly the same area as the 1974 cruise, show nearly the
same features as the salinity distribution described above.
High salinities associated with Bering Sea Water were found
over most of the central Chukchi Sea. Initial dilution by
ice melt occurred outside the ice edge while salinities
rapidly decreased from the ice edge northward to the limits
of ice penetration. The 1975 data did show an exception to

30

175°
FIGURE 8.

170'

165'

160

70°N

65°
155°W

MIZPAC 1974 symbolic plot of salinities. The
dashed line represents the 1 OKTA ice edge.
Salinities less than 29 °/oo - circle,
29-30 °/oo - x , 30-31.5 °/°° - square,
greater than 31.5 °/oo - diamond .

31

this pattern in the bight north of Cape Lisburne where the
29 ^oo contour extended well south of the ice edge (Figure
9) . It is possible that diluted melt water was trapped
in the gyre north of Cape Lisburne observed during this
cruise (Zuberbuhler and Roeder, 1976) and previously
reported in this area by CAT (1976) .

This salinity distribution leads one to the conclusion
that the net northward flow rate of the water must be
greater than the rate of ice retreat. If the water were
moving slower than the rate of ice retreat, one would expect
an accumulation of low-salinity water well outside the ice
with little or no salinity gradient. The fact that low
salinities are generally found close to the ice and decrease
rapidly behind the ice margin indicates that near-surface
waters are sweeping the dilute melt water under the ice
faster than the ice melts back. Local exceptions principally
due to eddies and gyres can retard the net northward flow.
The fact that there is fairly generally an area of decreasing
salinity a few tens of km south of the ice, at least in the
eastern Chukchi Sea, may be explained as due to the frequent
tendency of the ice from the ice margin to scatter southward
as it melts .

The fresh water accumulated within the water column,
as represented by integrated fresh-water contours calculated
from the MIZPAC data, generally showed a marked increase
in the immediate vicinity of the ice edge. In interpreting

32

175

FIGURE 9.

170

165°

160c

70°N

155°W

MIZPAC 1975 integrated fresh-water conteng.
The dotted lines represent the 29 and 30 /oo
salinity contours. The dashed lines indicate
the ice boundaries in OKTAs. Contours of
integrated fresh-water content in gm/cm2 are
shown in solid lines.

33

these gradients of accumulated fresh water it was initially
assumed that convergence and divergence effects were
negligible. Figures 9 and 10, based on the MIZPAC 1975

and 1974 data, generally show an increase in fresh-water

2

content from 150 gm/cm found 30 to 50 km outside the ice

2
to more than 300 gm/cm behind the ice margin. This 150-200

2

gm/cm increase is equivalent to what one might expect

from the melting of the 1.5 to 2 m of ice present. It is

2
also seen that the 150 gm/cm contour corresponded closely

with the marked salinity decrease associated with the ice-
melt zone. It appears, therefore, that the assumption of
negligible convergence and divergence in the vicinity of
the ice edge is tenable. The distribution of fresh-water
content leads one to the same conclusions regarding the
relative rates of ice retreat and current flow as inferred
from the salinity plots, namely that ice-melt water is being
swept under the ice as no plateau of high ice-melt water
content is found south of the ice. Therefore, the current
in general must be moving faster than the retreat of the
ice with all of the ice-melt water eventually being pushed
under the ice.

Ice-retreat data for the southern Chukchi in early
summer obtained from National Environmental Satellite
Service ice charts (FLEWEAFAC, 1976) is shown in Figure 11
for the period 1972 to 1975. When compared with mean water
transport times calculated from measured transport rates

34

175
FIGURE 10.

170

165

160'

70°N

155°W

MIZPAC 1974 integrated fresh-water content.
The dotted lines represent 29 and 30 /oo
salinity contours. The dashed line indicates
the 1 OKTA ice edge. Contours in integrated
fresh-water content in gm/cm2 are shown as
solid lines.

35

(n.) aaniuvT

36

through Bering Strait (Figure 6) , it is clear that indeed
the mean current does flow faster than the mean rate of
ice retreat. These calculations indicate that, even in
the southern Chukchi Sea, water transport velocities in
each stream are greater than the rate at which ice melts
back based upon weekly positions of the ice edge. Several
exceptions are noted, however, north of 69° latitude.
North of this latitude the speed of the current in the
central Chukchi (transport streams II and III) was approxi-
mately the same as or somewhat slower than the rate of ice
retreat. There was also a period of time from late May to
early June 1972 when the ice in the central Chukchi Sea
retreated more rapidly than the computed velocity. In
these cases it could be that the northward flow in the
central Chukchi was retarded by gyre-like motion. However,
it is likely that the surface current, even in these excep-
tional cases, was flowing faster than the ice retreated
because there is good reason to believe that the upper
layer flows more rapidly than the mean of the water column.
Surface current speeds in the Chukchi Sea are expected
to be faster than the mean speed because the homogeneous
waters in Bering Strait become abruptly layered north of
the Strait (NAVOCEANO, 1958) , and hence transport velocities
in the upper layer must increase if the total transport is
to be conserved. This conclusion may be fortified by inspec-
tion of the relative velocities in the upper 10 m of water

37

column as compared to the average of the remainder of the
water column. Such a comparison was made for all of the
current measurements in the cruises listed in Table I.
Here, current magnitudes were regarded as more appropriate
than northerly components because of the oscillatory nature
of the currents. The results of Table I verify that the
upper 10 m of the water column, which contained most of
the fresh water, travels faster than the lower half of the
water column by 15 to 50 percent. Therefore, the compari-
son of transport velocity and observed ice retreat rate
reinforce the previous conclusion that the northward flow
of near-surface water in the Chukchi Sea exceeds the rate
at which the ice edge is melted back.

38

TABLE I. Comparison of the mean current speed
of the upper 10 m and the mean speed
over the remainder of the water column

Cruise Location Top (cm/sec) Bottom (cm/sec)

OSHORO MARU 1972 Bering Strait 51.5 43.8

OSHORO MARU 1972 69° latitude 15.8 10.9

STATEN ISLAND 1968 Bering Strait 59.9 49.0

STATEN ISLAND 1968 67° latitude 35.8 30.8

39

V. CONCLUSIONS
The following conclusions resulted from this study:

Good evidence was found that the current flows
northward faster than the ice retreats in the summer months
in the eastern half of Chukchi Sea. An earlier hypothesis
of an ice-melt zone moving more slowly than the ice edge
retreats was not substantiated.

A band of water approximately 30 km wide exists
south of the ice in which the melt-water content increases
toward the ice. The cause of this band of dilute water is
believed to be the scattering southward of ice from a diffuse
ice margin accompanied by melting. This band varies in width
in response to local phenomenon.

North of the ice edge the fresh-water content of
the water column is found to be greater than that of more
southerly water by an amount equivalent to the approximate
thickness of the ice cover.

40

BIBLIOGRAPHY

1. Barnett, D. G. 1976. A practical method of long
range ice forecasting for the north coast of Alaska,
Part I. Fleet Weather Facility, Suitland, Md. 16 pp.

2. Coachman, L. K. and K. Aagaard. 1974. Physical oceano-
graphy of arctic and subarctic seas. In: Marine
Geology and Oceanography of the Arctic Seas, Chpt. 1,
Herman, Y. ed. , Springer-Verlag, New York. 72 pp.

3. Coachman, L. K. , K. Aagaard and R. B. Tripp. 1976.
Bering Strait: The regional physical oceanography.
Seattle, University of Washington Press. 192 pp.

4. Fleet Weather Facility, Suitland (FLEWEAFAC) . 1976.
Western Arctic sea ice analysis 1972-1975.

5. Paquette, R. G. and R. H. Bourke. 1976. Oceanographic
investigation of the marginal sea- ice zone of the
Chukchi Sea — MIZPAC 74. Dept. of Oceanography, Naval
Postgraduate School, Monterey, Tech. Rpt. NPS-58PA76051.

6. U.S. Geological Survey (USGS) . 1973. Water resources
data for Alaska (1973). U.S. Govt. Print. Off., Wash.,
D.C.

7. U.S. Naval Oceanographic Office (NAVOCEANO) . 1958.
Oceanographic atlas of the Polar Seas, Part II, Arctic.
H. 0. Pub. No. 705. 149 pp.

8. Zuberbuhler, W. J. and J. A. Roeder. 1976. Oceanography,
mesostructure, and currents of the Pacific marginal
sea-ice zone — MIZPAC 75. Master's Thesis, Naval
Postgraduate School, Monterey. 203 pp.

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169590
Handlers

On the question of
accumulation of ice-
melt water south of the
ice in the Chukchi Sea.

Handlers

On the question of
accumulation o* fee-
melt water south of the
fce 'n the Chukchi Sea

169590

4