Rapp. P.-v. Réun. Cons. int. Explor. Mer, 188: 59-65. 1989
Sea-ice thickness distribution in the Irans Polar Drift Stream
Peter W adham s
Wadhams, Peter. 1989. Sea-ice thickness distribution in the Trans Polar Drift Stream.
- Rapp. P.-v. Réun. Cons. int. Explor. Mer, 188: 59-65.
During Jun e-Ju ly 1985 ice thickness data from upward-looking sonar were obtained
by a British submarine in the Greenland Sea, the Fram Strait, and northward into the
Eurasian Basin of the Arctic Ocean. A preliminary analysis of the data shows that the
mean ice thickness in a test area of the Trans Polar Drift Stream is almost identical to
values found during a late winter cruise in 1979 and an autumn cruise in 1976. The
data show a radical change in ice properties between the marginal ice zone of the
Fram Strait and the central Arctic Basin, with more pressure ridges and fewer leads in
the interior zone.
Peter Wadhams: Scott Polar Research Institute, University o f Cambridge, Lensfield
Road, Cambridge CB2 1ER, England.
Introduction
The Trans Polar Drift Stream is one of the two main
surface current systems of the Arctic Basin and is the
source of most of the ice which is transported through
the Fram Strait into the G reenland Sea. The Fram Strait
itself handles 90 % of the heat exchange and 75 % of the
mass exchange between the Arctic Ocean and the rest
of the World Ocean (Aagaard and Greisman, 1975),
and therefore it is of great importance to have a good
understanding of the ice transport through the Strait.
The present state of knowledge has been reviewed by
Vinje and Finnekåsa (1986), who show that while the
annual mean ice drift can be obtained quite well from
satellite and drift-buoy data, the mean thickness of the
ice passing through the Fram Strait is harder to esti­
mate. Direct drilling has taken place in the Strait and, as
part of the Greenland Sea Project (AWI, 1987), up­
ward-looking sonars will be moored there for periods of
up to several years. Nevertheless, at present the main
sources of data on thickness distribution along-stream
are the results of submarine cruises, specifically the
October 1976 cruise of HMS "Sovereign" (Wadhams,
1981) and the A p ril-M a y 1979 cruise of the same vessel
(Wadhams, 1983). We here present preliminary results
from a new dataset obtained during late June and early
July 1985 by another British submarine.
D a ta reduction
The profiles were obtained by an Admiralty pattern 45
kHz sonar of narrow beam (less than 5°), and were
recorded on chart rolls which were digitized using a
curve follower. A stage of pre-processing, additional to
that required for previous cruises, was necessitated by
the rather large porpoising motion of the submarine. To
remove this, the record was digitized twice, the first
digitizing being the envelope of the sonar record, and
the second being only polynyas, cracks, and other
points where the open-water surface could be seen.
These were frequent because of the open-ice conditions
which occur in the summer season. The second, piecewise, digitized profile was fitted to a polynomial and the
resulting curve subtracted from the first digitized profile
to yield a valid profile of ice draft. The draft record was
not corrected for the effect of beamwidth as described
by Wadhams (1981), since the narrow beam leads us to
expect only a small effect tending to overestimate the
true m ean draft. The data points were interpolated to
1.5-m intervals, and the record was then divided into
50-km sections which were subjected to statistical analy­
sis.
M ean ice drafts
The statistic of greatest interest is the mean ice draft.
Figure la shows the mean draft of each 50-km section
plotted in the position of the centroid of the track sec­
tion concerned. This treatment was necessary owing to
the extremely convoluted nature of the submarine
track. The submarine spent periods in two regions: the
first, in the Fram Strait, yielded six sections of data in
close proximity, while the second, a box bounded by
83°30' and 84°30'N, 0° and 10°E, is shown in an ex­
59
3«
» 4 93
• 5 44
6 03*
5 45
•5-2 4
• 400
2-29«
*309
PlTSBtRGEN
2 54
2 50 •
2 83
O
10'
0*
10°
Figure la. Mean ice drafts from 50-km track sections of the Ju ne-Ju ly 1985 dataset. Each spot marks the centroid of the section
concerned. The northernmost box is shown in expanded form in Figure 2. The ice edge is taken from a NOAA-9 AVHRR image
for 30 June 1985.
panded form in Figure 2. The overall dataset consists of
61 sections representing 3050 km of profile.
The data can be conveniently divided into three re­
gions: the marginal ice zone (MIZ) of the Fram Strait,
60
south of 80°N; the Eurasian Basin from 80° to 83°N; and
the northernmost box, which can be regarded as a “test
area” for the Trans Polar Drift Stream on account of its
intensive sonar coverage.
10ÖW
o
Figure lb. Mean ice drafts from 50-km sections obtained by HMS "Sovereign”, A pril-M ay 1979 (after Wadhams. 1983).
The marginal ice zone yields results which are not
dissimilar to those obtained by HMS “Sovereign” in
A p ril-M a y 1979. A comparison with Figure lb, from
Wadhams (1983), which shows the 1979 results, reveals
few differences. The mean ice draft is usually less than 3
m in the zone near the ice edge, while the higher figures
of 3.47 and 3.61 m are found somewhat farther west in
the ice stream as in the case of the 1979 data. An
anomalous value is the 4.87-m section, which may rep­
resent a massif of heavily deformed multi-year ice drift­
ing through the Strait. Similarly the 1.15-m section re­
fers to a track which included a considerable length of
record obtained from outside the ice edge, although the
track centroid was inside. In the M IZ the mean draft
can be expected to vary rapidly with distance from the
ice edge, on account of the break-up and melting which
occur there, so taking an overall average is not a very
meaningful exercise. Nevertheless, we find that the
overall mean draft of sections south of 80° in the 1979
data is 2.62 m, while in the 1985 data it is 2.66 m. The
difference is not significant, given the expected var­
iability across the MIZ. We note also that the southern­
most sections, at 77°N, are no thinner than those farther
north in the Strait.
The Eurasian Basin fro m 80° to 83°N yields results
which show some differences from 1979. In 1979 (Fig.
lb) the tentative depth contouring at the Greenwich
meridian gave mean thicknesses between 3 m and 4 m.
8 4° 3 0 ' Nf-----
• 4 40
*4-6 2
5 -3 9 *
3 ^ 4-74
“ * 3 -8 7
• 4-6 8
T5
4 - 15«
• 5-17
,
• 5-30
• 4-6 2
• 4-0 7
84° N
5-6 1 •
• 5-3 6
4 -7 5 * « 3 - 4 7
« 5 -7 0
• 5-83
5-12«
4 - 8 7 / 5*39
4 93
4 -9 0 » * 4 -4 7
3 -7 3 *
• 3-4 7
• 4-02
• 4-69
3-5 7* • 5 3 6
• 6-4 7
• 5-69
5 E
1
J
1 Cf E
Figure 2. Mean ice drafts from 50-km track sections with centroids in the region 83°30'N—84°30'N 0°—10°E. Overall averages
are for the three densely sampled quadrants.
while thicker ice was found only farther to the west,
beyond 7°W. The interpretation of this result was that
the thicker contours represent ice which has moved
southwards in the western part of the Drift Stream and
has become heavily deformed by convergence as it en­
counters the downstream land boundary of northern
Greenland. The heavily deformed ice is then squeezed
to the southeast, rounds Nordøstrundingen (the north­
eastern cape of Greenland) and enters the western side
of the Fram Strait. This effect was predicted in the
model of Hibler (1979), was detected in the 1976 "Sov­
ereign” data (Wadhams, 1981), and was discussed in the
light of other datasets by Wadhams (1986). There is
some evidence from the 1985 data (Fig. la) that the
heavily deformed ice is present farther to the east than
in 1979 (it was not worth while attempting to contour
the 1985 data because the locations of the centroids lie
in a line rather than being spread over an area). The
overall mean draft of the nine sections lying between 80°
and 83°N in Figure la is 4.37 m, while that of the
sections from 80° to 83° in Figure lb , excluding the two
nearest the ice edge, is 4.13 m. The difference is not
highly significant, but it does suggest that it would be
worth running ice models with 1979 and 1985 wind field
data to see if a difference in mean draft is predicted.
The D rift Stream test area is of special interest, be­
cause the very large dataset of Figure 2 (36 sections)
permits a good comparison with the 1976 and 1979 data.
We find a remarkable constancy from year to year.
Averaging the data of Figure 2, we find overall mean
62
drafts in three quadrants of the region, of 4.73 m, 4.91
m, and 4.93 m, which is reasonably consistent. The
weighted mean draft of all 36 sections is 4.85 m. If we
now examine the October 1976 “Sovereign" data (Wad­
hams, 1981) we find that four 100-km sections of track
were contained within this box (sections 29—32 of Wad­
hams, 1981, Fig. 4), having mean drafts (Wadhams,
1981, Table 3) of 4.46, 4.51, 4.55, and 4.86 m, a
weighted average of 4.60 m. If we examine the points in
1979 (Fig. lb) which lie north of 83°30' we find only
three sections, with means of 3.85 m. 4.19 m. and 5.96
m, a weighted average of 4.67 m. Inclusion of the five
additional sections lying north of 82°30' changes the
weighted average to 4.75 m. The three sets of results
were obtained in different years, and in different sea­
sons of the year, varying through late winter to early
summer to late autumn. The thickness of the snow
cover, the thickness of ice in leads, and the composition
of the ice cover itself might all be expected to be differ­
ent, yet the mean draft is little changed. In fact a beamwidth correction to the 1985 data would probably re­
duce mean drafts by 0.1 —0.2 m, thus making the agree­
ment even better. It was already rem arked in Wadhams
(1981) that this region of the central Arctic Basin has
remarkably homogeneous ice conditions, with no signif­
icant variation from here to the North Pole. Now we see
that the mean ice draft is stationary as well as homoge­
neous within the limits of error of the observations. This
suggests that self-adjusting feedback mechanisms may
be at work in the ice cover; they compensate, say, for
0 9
P ro b a b il it y
Density
m -1
0 6
0 5
0 4
0 2
3
2
4
6
5
7
8
Figure 3. Probability density functions of ice draft in the range 0—8 m for two 50-km sections of ice profile from the MIZ and
central Eurasian Basin,
melting in leads by partially closing the lead systems,
maintaining the overall mean draft as a constant regard­
less of season.
It should be noted that a further test of this stationarity will be possible from a new dataset (May 1987)
collected by the author in HMS “Superb” from similar
regions of the Arctic.
Ice draft distribution
Further information about the relative fractions of dif­
ferent ice types present in the ice cover can be obtained
by considering the probability density function (pdf) of
ice draft. Figure 3 shows two pdfs of 50-km track sec­
tions from quite different parts of the Arctic. One sec­
tion is from the marginal ice zone, at 78°27'N 2°55'W
Table 1. Percentages of ice cover in 1° latitude ranges found in four depth categories.
7 7 -7 8 °......................
78 -7 9 °......................
79 -8 0 °......................
80 -8 1 °......................
8 1 -8 2 ° ......................
82 -8 3 °......................
83 -8 4 °......................
84 -8 5 °......................
With thin ice removed (%)
Ice cover (%)
Latitude range
<0.5 m
0 .5 -2 m
2—5 m
>5 m
0 .5 -2 m
2 -5 m
>5 m
25
33
25
45
20
6
7
11
23
20
20
11
11
8
11
10
37
33
34
22
36
35
42
40
15
14
21
22
33
51
40
39
31
30
27
20
14
9
8
11
49
49
45
40
45
37
45
45
20
21
28
40
41
54
43
44
63
Table 2. Frequency and mean widths of polynyas in 1° latitude
ranges.
Latitude
range
7 7 - 7 8 ° . .. .
7 8 - 7 9 ° ....
7 9 - 8 0 ° ....
8 0 - 8 1 ° ....
8 1 - 8 2 ° ....
8 2 - 8 3 ° ....
8 3 -8 4 ° ... .
8 4 - 8 5 ° ....
Polynyas
per km
Mean width
(m)
Track composed
of polynyas (%)
1.16
1.60
1.46
0.83
0.61
0.27
0.70
0.69
76.6
103.1
85.5
129.5
116.0
155.6
85.6
109.5
8.9
16.5
12.5
10.7
7.0
4.2
6.0
7.6
(mean draft 1.91 m), while the other is from the north­
ernmost part of the test area, at 84°21'N 1°56'E (mean
draft 5.51 m). The M IZ section contains a large amount
of open water or very thin ice, and then has a second
peak in the draft range 1.5—3 m, representing unde­
formed first-year and multi-year ice. There is relatively
little ice beyond 3 m, where most ice present would be
in the slopes and crests of pressure ridges, and almost
none beyond the limit of the plot at 8 m. The interior
section has much less thin ice, almost no ice in the depth
range 1—2 m representing undeformed first-year ice
(which reaches a maximum thickness of 2 m after one
winter’s growth), only moderate amounts in the depth
range 2 - 3 m representing undeformed multi-year ice,
but a large quantity of ice at greater depths, implying a
very much heavier degree of pressure ridging than in the
M IZ section.
This change in the nature of the ice cover can be seen
more clearly if we divide the drafts into the four ranges
0 —0.5 m (open water and young ice), 0.5—2 m (firstyear ice), 2—5 m (mainly multi-year ice and some de­
formed ice), and 5 m upwards (entirely deformed ice).
If we also consider the track sections in 1° increments of
latitude, we obtain m ean values for the percentage of
the ice cover in each category, given by Table 1.
In this table the figures for the 80—81° and 82—83°
latitudes are the least reliable since these latitudes each
included only 100 km of ice profiling. It can be seen
from the left-hand part of the table that there is a radical
change in ice composition which begins north of the
Fram Strait at 80°N and which is complete by 82°N. In
the Fram Strait M IZ the typical ice composition is
2 5 - 3 3 % open water or thin ice, 20—23 % undeformed
first-year ice, 33—37% undeformed multi-year or de­
formed first-year ice, and only 14—21 % thick deformed
ice. In the central Eurasian Basin the open water/young
ice percentage has dropped to 7—11, there is only
10—11 % undeformed first-year ice, and the fraction of
undeformed multi-year plus deformed first-year ice re­
mains almost unchanged at 40—42 % , while the percent­
age of thick deformed ice has risen to 39—40.
Since the percentage of open water or young ice is a
function of the temporary state of stress of the ice cover
64
or, in the M IZ, of the track of the submarine relative to
the ice edge, we have renormalized the table by remov­
ing the first depth category. The results are shown on
the right-hand side of Table 1. They again show a very
clear picture. With the leads excluded, the remaining
ice in the M IZ is 27—31 % undeformed first-year,
45—49 % multi-year or deformed first-year, and
20—2 8 % thick deformed ice. In the central Eurasian
Basin there is only a third as much undeformed firstyear ice, about the same amount of multi-year or d e­
formed first-year, but more than twice as much thick
deformed ice.
Polynyas
We expect the ice cover to be fairly open in summer,
and indeed this was found to be the case. An analysis
was carried out to detect continuous stretches of ice
with drafts less than 1 m, each such stretch being d e­
fined as a polynya. We found mean numbers of poly­
nyas per km of track with corresponding mean widths,
given by Table 2.
There is some variability in these results, but if we
compare the M IZ (south of 80°) with the central E u r­
asian Basin (north of 83°) there are clearly fewer poly­
nyas in the central Basin - about half as many as in the
M IZ - and they occupy only 4 - 8 % of the ice cover
instead of 8 —17 %.
Pressure ridges
Independent pressure ridges were identified using the
same Rayleigh criterion as in the earlier datasets (Wad­
hams. 1981, 1983). Considering only ridges with drafts
exceeding 9 m, to avoid danger of confusion with un­
deformed multi-year ice, we find overall averages in the
various latitude ranges, given by Table 3. The volume of
ice contained in ridges (per unit area of ice cover) is
proportional to the number of keels per km and to the
square of the mean draft, so we have added a function
proportional to this quantity to the table.
Table 3. Frequencies (n), mean drafts (n), and relative volumes
of ridges deeper than 9 m in 1° latitude ranges.
Latitude
range
7 7 -7 8 ° ...
7 8 -7 9 '. ..
7 9 -8 0 ° ...
8 0 -8 1 ° ...
8 1 -8 2 ° ...
8 2-8 3 °..
83 -84 °..
8 4-8 5 °..
Ridges per km
(draft >9 m)
Mean draft
(m)
Relative volume
of ridged ice
(l* n)
0.71
0.47
0.54
0.61
0.99
1.23
1.18
1.11
11.97
11.48
11.85
12.42
12.29
12.24
12.25
12.54
101
62
76
94
149
184
177
175
With the exception of the 7 7 -7 8 ° range, which ap­
pears to be anomalous, there is a continuous increase in
the volume of ridged ice with increasing latitude, until a
steady value is reached beyond 82°N. The main cause of
the change is an increase in pressure ridge frequency, by
a factor of more than two, with a smaller but still signif­
icant tendency towards an increase in mean ridge depth.
The result is that in the central Eurasian Basin the
volume of deformed ice per unit area of ice cover is
about three times that found in the Fram Strait M IZ
region. There are two possible reasons for this: melting
of ice keels near the ice edge; and a mixing in the Fram
Strait between ice from the Trans Polar Drift Stream
and younger ice which has moved west across the north
of Svalbard from the Soviet Arctic. The problem can be
resolved by a deeper analysis of the statistical properties
of the ice; this will be the subject of a future paper.
Con clusions
These preliminary results represent highlights of the
1985 dataset. The most interesting finding is that within
the central part of the Trans Polar Drift Stream the
mean ice draft does not appear to vary significantly with
season or from year to year. This result, if confirmed by
the most recent 1987 dataset, represents a real challenge
to thermodynamic and dynamic models.
5
Rapports et Procès-Verbaux
A c k n o w le d g e m e n t s
I am very grateful to Lt-Cdr Ian Williams, R. N., repre­
senting Flag Officer Submarines, Ministry of Defence
(Navy), for the release of this dataset. I thank the Office
of Naval Research and the Natural Environment R e­
search Council for financial support; Ruth Weintraub
for computing; Mercedes Cowan for research assist­
ance; and Robert Southern for diagrams.
R e fe r e n c e s
Aagaard, K., and Greisman, P. 1975. Towards new mass and
heat budgets for the Arctic Ocean. J. geophys. Res., 80 (27):
3821-3827.
AWI. 1987. Greenland Sea Project. An International Plan of
the Arctic Ocean Sciences Board. 2nd ed. Alfred Wegener
Institute for Polar and Marine Research, Bremerhaven,
April 1987. 47 pp.
Hibler, W. D. III. 1979. A dynamic thermodynamic sea ice
model. J. phys. Oceanogr., 9 (4): 815-846.
Vinje, T., and Finneksåsa, O. 1986. The ice transport through
the Fram Strait. Norsk Polarinstitutt, Oslo, Skrifter nr. 186,
39 pp.
Wadhams, P. 1981. Sea-ice topography of the Arctic Ocean in
the region 70°W to 25°E. Phil. Trans, roy. Soc., London,
A302 (1464): 45-85.
Wadhams, P. 1983. The sea ice thickness distribution in Fram
Strait. Nature, Lond., 305 (5930): 108-111.
Wadhams, P. 1986. The seasonal ice zone. In The geophysics of
sea ice, pp. 825-991. Ed. by N. Untersteiner, Plenum Press,
New York. NATO ASI Series: Series B; Physics, Vol. 146.
65