JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO. C5, PAGES 2715-2722, MARCH 30, 1983
MesoscaleOceanographicFeaturesAssociatedWith the Central
Bering Sea Ice Edge' February-March 1981
ROBIN
D.
MUENCH
Science Applications, Inc., Bellevue, Washington98005
Midwinter 1981 observationsfrom the central Bering Sea shelf showeda hydrographicstructure
whichwastwo layeredin temperature,
salinity,anddensity.Thisstructure
wasconfined
to a 100-kmwide bandwhichcoincidedapproximatelywith the ice edge.The colder,lower-salinityupperlayer
was continuousin its T-S propertieswith homogeneous
water to the north beneaththe ice. The
warmer,moresalinelowerlayerwassimilarlycontinuous
withwaterto the southnearthe shelfbreak.
A northwestward baroclinic flow of the surface relative to 75 dbar was associated with the layered
structure.The layeredstructureandassociated
flow appearto be regularwinterfeaturesassociated
with the ice edgein this region.Input of low-salinitywaterfrom meltingof ice alongthe edgeis
adequate
to maintainthelayeredstructure,
anditsassociated
baroclinic
circulation,
against
theeffects
of tidal andwindmixing.The resultingobservedoceanographic
featuresweresimilarto thosewhich
have been found in associationwith midshelf and shelf break fronts in other regions and appear to
definea dynamically
controlled
boundarybetweenthe cold,low-salinity
wateron the northernshelf
and the warmer, more saline water near the shelf break.
1.
have presentedsummer current data which supportthe
conceptof this north-northwestward
net flow on the central
shelf,citingvector-averaged
currentspeedsof the orderof 5
INTRODUCTION
This paper presents new oceanographicfield data which
were obtained along the central Bering Sea ice edge in
February-March 1981. These data were more comprehensive than any obtained from this region during winter in the
past and allow a qualitative description of some of the
mesoscaleproperties of temperature, salinity, density, and
currents which were associated with the ice edge.
The conditionsdescribedin this paper were observedon a
portion of the central Bering Sea shelf (Figure 1). Topographically, the central Bering Sea shelf is extremely wide
and flat bottomed.
cm/s. No information is available concerning possible seasonal variability in this circulation.
The only publishedaccountof oceanographic
conditions
associatedwith the winter ice edge on the central Bering
shelf is that of Pease [1980], who describedtemperatureand
salinityconditionsusingdata obtainedin midwinter(early
March) 1979.She describeda low-salinity,low-density'lens'
of waterunderlyingthe ice edgeandattributedthisfeatureto
admixtureof low-salinitywater, derivedfrom meltingof ice
at the edge,into the upperoceaniclayer. Shealsoconstructed a dynamictopographyof the sea surfacewhich qualitatively indicatesthat a baroclinicsurfaceflow was directed
west-northwestward.This flow was approximatelyparallel
Shelf width in a southwestward-northeast
direction is about 600 km, measured from the shelf break
which
occurs
at about
the
coastline or to St. Lawrence
150-m
isobath
to the Alaskan
Island which is continuous with
it. The bottom deepensgradually from the coastline toward
the shelfbreak. The bottom slopeis somewhatless (approxi-
to, and underlay, the ice edge.
mately2 x 10-4) northeast
of aboutthe75-misobathandthe
massif presented by St. Matthew Island than to the south-
west(approximately
3 x 10-4).Bottomslopesaregreaterin
the immediate
locale of St. Matthew
Island.
Section 2 of this paper characterizes the environmental
backgroundon the central Bering Sea shelf. Section 3 briefly
describesthe observationalprogram. Section 4 describesthe
observedoceanographicconditions. Section 5 discussesthe
observationsand qualitatively relates observed conditionsto
the ice edge.
2.
Circulation along the shelf break boundingthe study
regionis northwestwardand parallelto the isobaths.This
patternwas qualitativelydepictedby Takenoutiand Ohtani
[1974] and was also depictedon the more recent schemes
shownin the summarycompiledby Hughes et al. [1974].
The dynamicsof this Bering Slope Current have been
investigated
byKinder
etal. [1975].
Mostrecently,
I•inder
and Schumacher [1981b] depicted this along-slopecurrent
andassigned
to it meanspeedsin the rangeof 5-10 cm/s.
Finally, Kinder and Schumacher[1981b] characterized
BACKGROUND
circulation on the southeasternBering shelf bounding the
studyregionas northwestward,with a midshelfregionof
Oceanography
Information concerning oceanographicprocesseson the
central Bering shelf is limited. Circulation schemesderived
from temperature and salinity characteristicsby Takenouti
and Ohtani [ 1974]and basedupon a survey of previouswork
which was carried out by Hughes et al. [1974] indicate
qualitatively a north-northwestwardnet flow in the vicinity
of the central shelf ice edge. Kitani and Kawasaki [1979]
Copyright 1983by the American GeophysicalUnion.
Paper number 2C0943.
0148-0227/83/002C-0943505.00
2715
weak meancirculationlying betweencoastaland outer shelf
bandsof more vigorousmean flow (Figure 1).
Meteorology
The winter climate of the central Bering shelf is dominated
by northeasterlywinds which are punctuated,at frequent
intervals, by low-pressurecyclonic storm systemswhich
propagate
eastwardthroughtheregion[OverlandandPease,
1982]. Passageof individual storm systemsis manifested
locallyas large speedand directionfluctuationssuperposed
uponthe meanwind field. Dependinguponthe exacttrack
followed by an individualcyclone, storm-associated
winds
MUENCH:
2716
MESOSCALE
175 ø
180 ø
FEATURES
AT THE ICE EDGE
170 ø
160 ø
165 ø
•
T.
ENCE
•
";'
.o
--,
ß
%
ALASKA
...
\ZOo
0 o
"
[U.S.A.)
2-431•
.'.
60 ø
MATTHEWl
.•30,
UNIVAK
I.
w
w
(1-5)
KM
55o-,' ' '
(~10)
'
175ø
170ø
165ø
160ø
Fig. 1. Geographicallocation of the study region (rectangle).Arrows show regionalcurrent features, derived from
Kinder and Schumacher [1981b], with estimated speed ranges (centimeters per second) in parentheses.Dashed line
indicatesapproximate ice edge location at its greatest extent during the 1981 field program. Points labeled BC22 and
BC26 show locations of overwinter 1980-1981 current moorings. Depths are in meters.
on the central shelf may or may not reverse from northerly to
southerly.
3.
Sea Ice
The Bering Sea winter ice cover may be described as
behavingon the regionalscalelike a 'conveyorbelt' wherein
new ice is formed in the northern part of the Bering, is
advected
typically lies somewhere between St. Matthew Island and
the shelf break (Figure 1).
southward
under the influence of the mean north-
erly winter winds, and meltsalongthe ice edge[Muenchand
Ahlniis, 1976; Pease, 1980]. The south-southwestwardmean
speedof ice within this 'conveyorbelt' hasbeenestimatedto
be about 25 cm/s [Pease, 1980]. Though statistics derived
from a rigorousobservationalprogramare not available,the
midwinter thickness of the sea ice as it approachesthe edge
from the north can be characterized as about 0.3 m [Pease,
1980].
The Bering Sea ice edgelocation may fluctuatein midwinter for large distances(approximately 100 kin) over short
time periods (approximately 2 days) as it respondsto local
winds [Muench and Ahlniis, 1976; Niebauer, 1980]. On a
longer time scale, its maximum or mean midwinter extents
may fluctuate as a function of larger-scalemeteorological
parameters[Niebauer, 1980; Walsh and Johnson, 1979a, b;
Overland and Pease, 1982]. Overland and Pease [1982] have
presentedconvincingevidence, in the form of comparisons
between storm tracks and annual ice extent, that the southward extent of the ice cover is correlated with the frequency
of storm passagethrough the region. More frequent storms
disturb the northeasterly wind patterns which tend to extend
the ice seaward;therefore periodsof greater storm activity
are characterized by lesser ice extent. Under these influences, the maximum southward ice extent for a given season
OBSERVATION
PROGRAM
Temperature and Salinity Observations
Sixty-four CTD casts were obtained along the central
Bering Sea shelf winter ice edge on February 26 through
March 11, 1981 (Figure 2). Occurrence of strong southerly
storm-related winds during the early portion of the winter
cruise causeda retreat of the ice edge to approximately 100
km from its initial location.
This retreat allowed the vessel to
proceed farther north than had been possibleon previous
cruises, such as the winter 1979 cruise [Pease, 1980], and to
obtain a comprehensive set of CTD data which extended
acrossthe ice edge from near the shelf break to well north of
the initial ice edge location. Presence of the storm also
allowed observation of temperature and salinity conditions
prior to and following the action of storm winds on the ice
edge region.
The CTD observation program was carried out from the
NOAA vessel Surveyor using a Grundy CTD system which
recorded the data digitally. Using a rosette samplerwith a
pair of deep-seareversingthermometersin conjunctionwith
a Guildline laboratory salinometer, calibration data were
obtainedabout every third cast. The temperatureand salinity data derived from CTD records are believed accurate to
within _+0.02øCand _+0.02%0,
respectively,basedupon intercomparisonbetween the CTD data and the calibrationdata.
A more limited set of CTD data, which are referred to
below in section $, were obtained from the central Bering
shelf in November 1980 using the NOAA vesselDiscoverer.
The data acquisition methods and specificationswere the
MUENCH:
MESOSCALE FEATURES AT THE ICE EDGE
I
N
'-
•
/
/
•
..:'...'
ß•,½,'',..3.,/3,
3/4
'::e•
•.
-
',,
//
'•., •.
ß ..
".
:.1,/6
21
8
5
8••13
••
4•r[ •
177 10 •.
3
(BC22)
-3/5
59 ø
ß
0/0
-. 2/26. 3/11
.• •
_ 58 ø
•2
KM
0
I
50
172 ø
100
170 ø
Fig. 2. Locations of CTD casts occupied in February-March
1981. Numbered casts coincide with those referred to specifically
elsewhere in this paper. Dashed lines (with corresponding dates)
indicate different locations for the ice edge during the cruise. Dashdot lines indicate approximate boundaries of two-layered oceanic
structure, where circled '2' refers to two-layered region and '1'
refers to homogeneouswater. Location of this figure coincideswith
rectangle in Figure 1.
sameas those described above for the February-March 1981
field program.
Current
Observations
Time-series
density were associated with the ice edge within the study
area. The major features of this distribution are illustrated
along a CTD transect which extended from near the shelf
break to the ice edge when it was locatedat its farthest north
point during the cruise (Figure 3). These features included
the following:
1. Water in the northern part of the transect had nearfreezing temperatures(less than - 1.5øC),was low in salinity
(less than 31.9%•), and was vertically homogeneous from
surface to bottom.
•
1 •'4 ø
current observations
were obtained from ear-
ly November 1980 to early June 1981 at 50-m depth at two
locations (Figure 1). The location of mooring BC22 was
59ø10.4'N, 171ø11.4'W. The current meter on this mooring
was at 50-m depth in 76 m of water. Mooring BC26 was
located at 60ø34.2'N, 176ø02.I'W, and the current meter was
at 50-m depth in 119 m of water. Mooring BC22 was
deployedon November 11, 1980, while BC26 was deployed 2
days later. The resulting record lengths were 216 and 218
days, respectively. Mooring BC22 was located in the same
area where the midwinter 1981 CTD survey was conducted.
The current data were obtained using Aanderaa RCM-4
current meters recording at 30-min intervals. The moorings
were of a standard taut wire configuration such as that
described and discussed by Pearson et al. [1981], and the
buoyancy floats were placed about 47 m below the surface to
minimize the effects of wave-induced mooring noise upon
the records.
2. A band approximately 100 km wide, underlying the ice
edgeand extendingto the north and southof it, was strongly
two layered in temperature and salinity. The colder, lowersalinity upper layer was continuous in its temperature and
salinity properties with the water to the north. The warmer,
more saline lower layer was similarly continuous with water
to the south. The depth of the interface between these two
layers varied from less than 30 m to more than 100 m.
3. Water in the southern part of the transect was warmer
(greater than 1.0øC) and more saline (greater than 32.7%0)
than that to the north and showed a nearly vertically
homogeneousstructurejust south of the two-layered region
which gave way to greater stratification in temperature and
salinity as the shelf break was approached.
4. The vertical density structure reflected the temperature and salinity stratification. A two-layered density structure underlay the ice edge, while density was vertically
uniform to the north and weakly stratified to the south. In
the layered structure the lower layer was about 0.2 sigma-t
units denser than the upper layer.
The layered structure which underlay the ice edge was
subjectto small-scalevariability over time periods of a few
days. An example of this variability is illustrated in Figure 4,
which shows the vertical distribution of temperature along
coincident portions of two transects. During the earlier
CAST
18
0
2
3
I
I
OBSERVED CONDITIONS
Distributions of Temperature, Salinity, and DensiLv
The CTD data obtained in February-March 1981 showed
that distinctive distributions of temperature, salinity, and
17
I •'11
lOO
,d
20
19
21
22 23
IIII
I
•
O I
O ,O
'";"
01,,., F,,
50
25
26
I I I
I
27 28
II
I
o.......
•. o •. b:
• --'
Too
. ..:.'.'.v.'.::.:::::!'.'.'.':'.'.'i:.:."."."":::::'"'"""
"' '
/ :.•.'::':'""
.......
150
0
'
I
50
The current records obtained were of high quality, with no
evidence of noise or of the biological fouling which had
adversely affected some of the records obtained in the
southeasternBering shelf region during a previous current
observationprogram [Kinder and Schumacher, 1981b]. The
recorded current speedsand directions are believed, based
upon the instrument design specifications, to be accurate
within _+1 cm/s and -+7.5ø, respectively, at the low current
speedswhich were observed.
4.
2717
I
>32.7
[
•
o• ,••k
I
•
oø/oo
lOO
150
0
..•:':'::
,.__,, ..'...'.:
..:.•:::...:.......
ß.
I
I
IIII
'.•
100
150
I
'-
I I
b b
•
I
!
II
.('" <25.7 ut
..... -:.'::
:..:::.:
...:.:.......
!-.:
v
ß"''.•
"'
:'?'.::"':
'""'
:"'"'0r-r-i--r-r-i
50
KM
Fig. 3. Vertical distributionsof temperature, salinity, and density (as sigma-t) along a transect across the ice edge in FebruaryMarch 1981. Cast locations are shown in Figure 2.
2718
MUENCH: MESOSCALE•FEATURES AT THE ICE EDGE
o- I
-1- 50
I13..
LU
I IIIllll•l
I
...,
I
...,
0 0,, I
•
I
•
'o, -u•
i
TOøC]
•
o
lO
i I I
I
26-27
I
FEB.
lOO
I I
1981
2o
i I I i I!
KM
•
I
_
,
:..'...-•,
..
•'•::•:';'::Z.':.::".".".'"..'.:
:'. .::.T.'..?:•.'..':
:.'-.':.'.:'::,:
I
I
I
I
117 18
0' [/
'i-
20 1•21'
22
1.0• 1MAR.
1981
'•
I
19
I
I
I
I
50-
I-13..
LM
100
_ _
.. ß.... 5•.
•..•...'.½..•..•...::.'.: ...•...'.;' :.;•.•'.
ß...•..-:.../.
•..............-...-....:.-.:........
.....
Fig.4. Vertical
distribution
oftemperature,
attwodifferent
times,
along
twotransects
which
coincided
partiall'y.
Cast
locations are shown in Figure 2.
(February 26-27) occupation of the transect, the bottom
layer was well mixed but considerable stratification was
present in the upper layer. Two days later (March 1) the
lower layer was still well mixed and the upper layer had
become well mixed, with the interface between layers stronger, sharper, and somewhat deeper than it had been at the
earlier date. This modification of the upper layer and the
interface was due to mixing by 10- to 20-m/s south winds
which persistedfrom February 27 to March 1.
A prominent aspect of the observed temperature and
salinity structure was the constancy of its position upon the
shelf when compared with the relatively large fluctuationsin
ice edge location during the field program (Figure 2). During
the strong (10-20 m/s) storm-related south winds which
occurred on February 27 to March 1, the ice edge itself
retreated about 100 km to the north. The response of the
water columnto this wind event is illustratedin Figure4.
The southernboundary of the two-layered structure moved
northwardduringthis period only about 5 km if the boundary
is defined by the point where the + 1.0øCisotherm intersects
the water surface. This boundary showed a similar invariance in location during the March 4-11 period when the ice
edge moved southward under the influence of northerly
winds to virtually the same location it had occupied at the
outsetof the field programon February 26. Use of a different
isotherm as a tracer would have shown a greater excursion,
but still far less than that of the ice edge. Even given that
temperature(and salinity, which showedan identical behavior) is a nonconservative variable and therefore must be used
with caution as a tracer, it is evident that the water column
did not respondto the storm event with lateral motionsof the
same magnitude as those exhibited by the ice edge.
Observed
Currents
The current measurements which were recorded during
winter 1980-1981 were obtained from two widely separated
locations, coincident with the temperature and salinity observationspresentedabove. Only one of the setsof current
measurements(that from BC22) was obtained from the same
regionas the midwinter'
temperature
and salinityobservations (Figure 1). Mooring BC26 was located about 300 km
west of BC22 and was well outside the area included in the
winter 1981CTD field program describedabove; it is included in this discussionbecauseit was located near the ice edge
at the sametime as BC22 and therefore yields informationon
the same ice-edge-related current regime.
This paper is concernedwith current featureswhich relate
to mesoscaleoceanographicprocesses.Therefore only those
current components having time scales longer than tidal
periods will be explicitly considered here. The vectoraveraged currents, over the entire record lengths, were 2.3
cm/s toward
341øT at BC22 and 3.3 cm/s toward
347øT at
BC26. Monthly vector-averaged currents were computed
usingdata which had been processedwith a 35-hourfilter to
remove the tidal signal. The results are shown in Figure 5.
The following points are immediately apparent:
1. Flow was northward throughout the observationperiod at both moorings;i.e., at no time was there a southward
flow componentexcept over shorter-than-monthlyperiods.
2. The flow was strongest, and tended most to parallel
isobaths,during early winter (November-December) and in
spring (May) at both moorings.
3. During midwinter (January-April), currents at both
locations exhibited a tendency for on-shelf flow. This was
most pronouncedin March at mooring BC26, when the mean
flow was entirely on shelf.
The temperature and salinity data obtained in winter 1981
were adequatefor constructionof a dynamic topographyof
the surfacerelative to the 75-dbar level (Figure 6). While the
dynamic method must be used with caution in sucha shallow
continental shelf region because of the possiblepresenceof
frictional, time-varying, and other effects which would vio-
MUENCH' MESOSCALEFEATURES AT THE ICE EDGE
2719
tN
BC22
,,,
i
I
,
,i
I
•
'
'1
I
I'
NOV
DEC
JAN
FEB
MAR
APR
I
MAY
Fig. 5. Monthlyvector-averaged
50-mcurrents
at moorings
BC22andBC26,showing
directions
of isobaths
at each
location. Mooring locationsare shown in Figure 1.
late the assu.
mption of geostrophy, the resultant information
can be useful. This was borne out by Kinder and Schumacher [1981b], Who found good qualitative agreement
between measured currents and those predicted using dynamic topography on the southeastern Bering shelf. The
computed
dynamic
topography
onthecentralBeringshelf
indicates
that a band of northwestward
baroclinic
surface
flow approximately paralleled the ice edge and was associated with the two-layered structure which underlay the edge.
This was in qualitative agreementwith the dynamic tOpography, which showednorthwestwardsurfaceflow, constructed using March 1979 data by Pease [1980]. The computed
winter 1981 baroclinic surface speedsfor this flow, relative
were typically 6-8 cm/s, and time scales were 4-6 days
although some fluctuations appeared to be of longer duration. During early winter the mean flow had been strong
enough that the fluctuations were insufficient to cause reversals to southward flow. In midwinter, however, this was not
the case. The relatively weak mean flow was dominated by
the fluctuating components, with the result that reversals to
southwardflow occurred (Figure 7). A visual inspection of
the filteredtime-seriesrecordsdid not suggestthat any
correlation was present between the current fluctuations at
the two moorings, and no attempt was made to compute
coherency between the two records.
The subtidalflow variability can be summarizedin spectra
to 75 dbar,wereabout7 cm/saci'oss
a 100-km-wide
band. which show the variance energy at each mooring computed
There was also a weak (less than 1 cm/s) southeastward from the 35-hourfiltereddata (Figure 8). The greaterenergy
counterflow well south of the ice edge.
t
The vector-averaged February-March current at mooring
BC22isshown
in Figure6 forcomparison
withthecomputed
I
0
baroclinicflow. Becausethis moøringwas locatedat the
extreme northern end of the CTD transect from Which the
baroclinic flow estimates were derived, and because the
'
KM
•
50
• I
•
3.4 cm/sec
>.17
currentmeterwas at 50-mdepthwhereasthe baroclinic
-
59 ø
-
58 ø
speedswere estimated at the surface, a quantitative comParison was not.attempted. The observed vector-averaged and
computed currents were consistent with each other, however, in that they both flowed north-northwestward.
These computed and observed currents were generally
consistent with
a net north-northwestward
flow on the
centralBeringshelfsimilarto thatdepicted
onthesoutheastern shelfby Kinder and Schurnacher[1981b](Figure 1). The
on-shelf flow observed in the current meter data is of
oO
--
ß
>.14
EAD
uncertain significance given the high short-term flow vail-
23 FEB. - 11 MAR.
ability describedbelow. Such flow would, however, be
consistentwith a northwardnet transportthroughBering
Strait as describedby Coachman and Aagaard [1981].
Subtidal fluctuations in current speed and direction of
shorter-than-monthlytime scaleswere superposedupon the
mean flows. The nature of these fluctuations is illustrated in
a midwinter segment of the current records from both
moorings(Figure 7). Speedsassociatedwith the fluctuations
O/75
I
174 ø
172 ø
db
1981
,
170 ø
Fig. 6. Dynamic topography of surface relative to the 75-dbar
level. Arrow labeled '3.4 cm/s' showsvector-averaged50-m flow for
February-March 1981at that location, derived from mooring BC22.
Dashed lines indicate extremes of ice edg• location during the cruise
period. Location of this figure coincides with rectangle in Figure 1.
2720
MUENCH.'
MESOSCALE FEATURES AT THE ICE EDGE
15
BC22
-15
1
BC26
-15
3
FEB
29
JAN
8
15
13
"
h., i
18
i
23
28
5
MAR
,
,iI
,
-•51
B022
15
B
26
-15
10
I
I
I
i
i
I
i
15
20
25
30
4
9
14
APR
MAR
Fig. 7.
Low-pass-filteredcurrentsfrom mooringsBC22 and BC26 during midwinter 1981. Mooring locationsare
shown in Figure 1.
at the lowest frequencies at BC26 was due to the higher
mean flow speedsthere. The peak at approximately 15 days
was of uncertainorigin but may have been due to a fortnightly tidal signalwhich was apparentin the unfiltered current
speed records (not shown). Finally, peaks are present for
both records in the 4- to 6-day band. These 4- to 6-day peaks
are not significantat the 95% level. They are felt to be real,
however, because they fall in the proper frequency band to
correspond
to subtidalflUctUations
visiblein the time-series
plots (Figure 7).
5.
10
DISCUSSION
It is probablethat the associationbetweenthe two-layered
structureand the ice edge in winter 1981is a common winter
phenomena. Pease [1980] identified portions of a similar
feature in the same region in winter 1979, though the data
-
were
>..
<
not as extensive
as those obtained
in winter
1981.
Alexander and Niebauer [1981] identified portions of a
similar feature on the southeasternBering shelf using temperature and salinity data obtained in the winters of 1975and
5%
1976.Unpublished
temperature
and salinitydataobtained
/
0
i
o.1
FREQUENCY
1 .o
{cpd)
Fig. 8. Variance spectra for low-pass-filteredcurrent speed
recordsfrom mooringsBC22 and BC26. Mooring locationsare
shown in Figure 1.
duringwinter 1979 along transectsacrossthe ice edge in the
southeastern Bering Sea and west of St. Matthew Island
showeda similar structure (J. Newton, personalcommunication, 1981). A similar structure is commonly present, in
winter, shoreward of the shelf break in the southeastern
Bering Sea [Kinder and Schumacher, 1981a].
During the ice-free seasons, the entire portion of the
Bering shelf between about the 50-m isobathand to seaward
of the 100-misobath tends to be two layered in temperature,
salinity, and density. This has been well documentedon the
southeastern Bering shelf by Kinder and Schumacher
[1981a]. They attribute the layering to interactions between
the regionalfreshwater input due to ice melt and terrestrial
runoff, solarinsolation,and tidal mixing. A transectshowing
the vertical distribution of salinity acrossthe central Bering
shelf in November 1980, prior to onset of ice formation,
indicates similar widespread layering (Figure 9). Summer
temperature, salinity, and density data presentedby Kitani
and Kawasaki [1979] along a transect at approximately the
MUENCH:
G^$T
o
Q • O0150 --
•
I
-•
I
•
ß..
I
2721
MESOSCALE FEATURES AT THE ICE EDGE
I
I
LOG^TION$
I
I
I
I
I
I
I
I
ß •
•
.....-v.v' .".' .'- '
•. ß:"/:'.".'.:.•'.
r-.';:-;:""•:•:::
:':':';-'."
"."
."•.:.::.:5:;.".:"'
'"' ß''0
50
KM
Vc•ical dis•bufion of
prior •o ic• formation.
same location from which Figures 3 and 9 were derived
indicatea similar widespreadlayered structure north of the
Pribilofsto the studyarea in concertwith a generalregional
northwestwardflow. While the exact pattern of upper layer
circulation is uncertain, these figures nonethelesssuggest
While a two-layered vertical structure appears on the that ice melt-derived, low-salinity water can cause the
Beringshelfin both the ice-freeandthe ice-coveredseasons, observedupper layer dilution near the edge.
there are sufficient differences between the seasonsto sugA salinitydecreasefrom 32.7 to 32.4%øin approximately
gest that different processesmay be responsible.First, the the upper 50 m of the water column yields, using Schulayering mechanism proposed by Kinder and Schumacher macher et al.'s [1979] method, a buoyant input of about 6 x
[1981a] would have little effect in the ice-covered season 10-4 W/m2. Use of a tidal currentof 15cm/s,computedfrom
becauseby October-November both the freshwater input the BC22 currentrecord, provides(usingthe samemethod)a
shelf break.
and the insolfftion needed to maintain the observed summer
mixingenergy
inputof about100x 10-4 W/m2.Schumacher
stratificationhave virtually ceased.Even given their approximate nature, the 5- to 10-cm/s currents derived above
shouldbe adequate to advect the layered shelf waters away
et al. [1979] estimated that approximately 2% of the tidal
dissipationwas sufficientto mix the water columnin total
water depthsof less than 50 m on the southeasternBering
to the north-northwest.
shelf. Fearnhead [1975] estimated a correspondingvalue of
about !% for the shelf waters surroundingthe British Isles.
Using the values computedabove, 2% of the tidal mixing
A continuous winter source for the
observedlayering is needed to replace the summer mechanisms proposed by Kinder and Schumacher [1981a]. Second, comparisonof the distributionsof salinity shown in
Figures 3 and 9 indicates that the layering was strongly
localized in winter as compared to the summer conditions.
Further, the winter layeringwas associatedwith an appreciable north-northwestward baroclinic surface current; there
is no evidencethat sucha flow is presentin summer.The
remainder of this discussionaddressesthese problems.
In order to maintain a layered structure such as that
observedalongthe Beringice edge,it is necessaryto have an
input of buoyancy large enough to maintain the layering
againsttidal and wind mixing which tend to destroyit. The
buoyancyinputis available,in winter,in the form of lowsalinity water derived from ice which melts along the ice
edge.The averagerate of meltwaterinput alongthe ice edge
can be estimatedusing a mean southwestwardice speedof
about 25 cm/s [Muench and Ahlniis, 1976; Pease, 1980]. A
salinity of about 15%ois assumedfor this ice. While this is
highin i'elationto valuesoften quotedfor Arctic Ocean pack
ice, it is reasonablefor the BeringSeabecauseit is first-year
ice. First-year ice has not been subjectedto the episodic
freezing and thawing events which over a period of several
yearsdecreasethe salt content of multiyear ice to considerably lower values.A meanthicknessfor the ice will be taken
as 30 cm after Pease [1980]. Assuming that the computed
northwestward surface current speed of 7 cm/s is representative of the ice edge region, the meltwater added is
adequateto lower the upperlayer salinityfrom an outershelf
valueof 32.7%0to the observed32.4%0(Figure 3) duringthe
time required for the water to flow from the vicinity of the
energyis about2 x 10-4 W/m2or of thesameorderasthe6
X 10-4 W/m2 which would be requiredto overcomethe
buoyantinputdue to ice meltingand verticallymix the water
column. Hence maintenance'of the two-layered structure
associatedwith the ice edgecan be attributedto the buoyancy flux which is locally inducedby admixtureof meltwater
from the ice. A similar computation was carried out by
Schumacheret al. [1979] for the southeasternBering shelf.
This computation,however, was for the springmelt of the
entire ice cover and its consequentdilution of the upper
layer. Paquetteand Bourke[ 1981] havedescribedsimilarice
edgefrontsin the ChukchiSeaandattributedtheir formation
to the local summer ice melt. The mechanismposed above
for the centralBeringshelfwouldoperate,however,through
the winter as ice melts continually along the ice edge.
The vertical distributions of temperature, salinity, and
densityacrossthe centralBeringshelfice edgebear a strong
resemblanceto those along transectsacross the midshelf
front in the southeastern Bering Sea [Kinder and Schumacher, 1981a]. These characteristicsare typically those of
shelf and shelf break frontal systems. Further, the lack of
appreciableshort-termresponseto strongwind forcing(Figures 2 and 4) suggeststhat an internal, dynamiccontrol on
the layeredstructurewas presentwhichwouldbe consistent
with frontal behavior. It is suggestedthat the oceanic
structure associatedwith the central Bering shelf ice edge
behaves as a front whose structure is maintained by a
balancebetween buoyancyinput due to meltingice and tidal
and wind mixingwhich tend to destroythe õtructure.This
2722
MUENCH'.
MESOSCALE FEATURES AT THE ICE EDGE
front forms a dynamic boundary between the cold, lowsalinity water on the northern shelf and the warmer, more
saline water near the shelf break.
The available data are not sufficientlydetailed to allow
definition of the dynamics associatedwith this Bering shelf
ice edgefront. The evidencesuggests,however,that several
processesmay contribute to frontogenesisand maintenance.
The distributions of temperature, salinity, density, and the
associatedbaroclinic flow along the ice edge are suggestive
of a Margules-typefront such as discussedby Flagg and
Beardsley [1978]. The observed 4- to 6-day current fluctuations are typical of instabilities which can occur in association with these fronts over gently slopingbottom topography
suchas that on the central Bering shelf. The above computations relating front formation and maintenanceto a balance
Flagg, C. N., and R. C. Beardsley, On the stability of the shelf
water/slopewater front southof New England,J. Geophys.Res.,
83, 4623-4631, 1978.
Hughes,F. W.., L. K. Coachman,and K. Aagaard,Circulation,
transport,and water exchangein the westernBeringSea, in
Oceanographyof the Bering Sea With Emphasis on Renewable
Resources, edited by D. W. Hood and E. J. Kelley, pp. 59-98,
Institute of Marine Science, University of Alaska, 1974.
Kinder, T. H., and J. D. Schumacher,Hydrographic structureover
the continentalshelf of the southeasternBering Sea, in The
Eastern Bering Sea Shelf.' Oceanography and Resources,edited
by D. W. Hood and J. A. Calder, pp. 31-52, University of
WashingtonPress, Seattle, 1981a.
Kinder,T. H., andJ. D. Schumacher,
Circulation
overthecon.tinental shelf of the southeastern Bering Sea, in The Eastern Bering
Sea Shelf.' Oceanography and Resources, edited by D. W. Hood
andJ. A. Calder,pp. 53-75, Universityof Washington
press,
Seattle, 1981b.
betweentidal mixing and buoyantinput suggestdynamics Kinder, T. H., L. K. Coachman, and J. A. Gait, The Bering Slope
Currentsystem,J. Phys. Oceanogr.,5, 231-244,1975.
similarto thosediscussed
for shallowshelffrontsby Simp- Kitani K., and S. Kawasaki, Oceanographic structure on the shelf
son and Pingree [1977]. A numerical model of ice edge
frontal systems by Niebauer [1982] suggests, finally, that
local winds play an important role in frontogenesisand front
maintenance.
To summarize, the oceanic structure associated with the
central Bering Sea winter ice edge is two layered in temperature, salinity, and density. This structure can be characterized qualitatively as a frontal system which is maintained by
an input,whichis sufficientto morethanoffsetthe tendency
of tidalandwindmixingto destroythe s•rUcture,
to the
upper layer of meltwater from the ice edge. This frontal
systemcomprisesa dynamically•:ontrolledboundarybetween cold, low-salinity water on the northern shelf and the
wormer, more saline water near the shelf break. It is hoped
edge region of the eastern Bering Sea, 1, The movement and
physicalcharacteristics
of waterin Summer,1978,Far SeasFish.
Res. Lab. Bull., 17, 1-12, 1979.
Muench, R. D., and K. Ahlnfis, Ice movement and distribution in
the Bering Sea from March to June 1974, J. Geophys.Res., 81,
4467-4476, 1976.
Niebauer, H. J., Sea ice and temperature variability in the eastern
Bering Sea and the relation to atmospheric fluctuations, J.
Geophys. Res., 85, 7507-7515, 1980.
Niebauer, H. J., Wind and melt driven circulation in a marginal sea
ice edgefrontal system:A numericalmodel, Cont. ShelfRes., in
press, 1982.
Overland, J. E., and C. H. Pease, Cyclone climatologyof the Bering
Sea and its relation to sea ice extent, Mon. Weather Rev., 110, 513, 19.82.
Paquette,R. G., and R. H. Bourke, Oceancirculationandfrontsas
related to ice melt-back in the Chukchi Sea, J. Geophys.Res., 86,
4215-4230, 1981.
that the currenth!ghlevel of interestin marginalice zone
processes
will leadto acquisition
of newfielddataadequate Pearson,C. A., J. D. Schumacher,and R. D. Muench, Effects of
to define the dynamicsof this complex system.
Acknowledgments. The author wishes to thank James D. Schumacher and H. J. Niebauer for critically reviewing this manuscript.
This work has been funded by the Office of Arctic Programs,Office
of Naval Research, under contract N00014-82-C-0064, and by the
Bureau of Land Management through interagency agreementwith
the National Oceanic and Atmospheric Administration, under which
a multiyear programrespondingto needsof petroleumdevelopment
of the Alaskan continentalshelf is managedby the Outer Continen-
tal ShelfEnvironmental
AssesSment
Program
(OCSEAP)
office.
REFERENCES
Alexander, V., and H. J. Niebauer, Oceanographyof the eastern
Bering Sea ice-edgezone in spring,Limnol. Oceanogr.,26, 11111125, 1981.
Coachman, L. K., and K. Aagaard, Reevaluation of water trans-
ports in the vicinity of Bering Strait, in The EasternBering Sea
Shelf.' Oceanographyand Resources, edited by D. W. Hood and
J. A. Calder, pp. 95-110, University of WashingtonPress,Seattle,
1981.
Fearnhead, P. G., On the formation of fronts by tidal mixing around
the British Isles, Deep Sea Res., 22, 311-322, 1975.
wave-inducedmooring noise on tidal and low-frequencycurrent
observations,Deep Sea Res., 28A, 1223-1229, 1981.
Pea•e, C. •H., Eastern Bering Sea ice processes,Mon. Weather
Rev., 108, 2015-2023, 1980.
•'
Schumacher, J. D., T. H. Kinder, D. J. Pashinski, and R. L.
Charnell, A structural front over the continental shelf of the
eastern Bering Sea, J. Phys. Oceanogr., 9, 79-87, 1979.
Simpson,J. H., and R. D. Pingree, Shallow seafronts producedby
tidal stirring, in Oceanic Fronts in Coastal Processes,edited by
M. J. Bowman and W. E. Esaisas, pp. 29-42, Springer-Verlag,
New York, 1977.
Takenouti, A. Y., and K. Ohtani, Currents and water massesin the
Bering Sea: A review of Japanesework, in Oceanographyof the
Bering Sea With Emphasison RenewableResources,edited by D.
W. Hood and E. J. Kelley, pp. 39-57, Institute of Marine Science,
University of'Alaska, Fairbanks, 1974.
Walsh, J. E., and C. M. Johnson,Interannual atmosphericvariability and associatedfluctuationsin arctic sea ice extent, J. Geophys.
Res., 84, 6915-6928, 1979a.
Walsh, J. E., and C. M. Johnson, An analysis of arctic sea ice
fluctuations, 1953-1977, J. Phys. Oceanogr., 9, 580-591, 1979b.
(Received April 9, 1982;
revised June 21, 1982;
accepted June 21, 1982.)