V.V.Ivanov1,3, G.I.Shapiro1,2
Features of dense water
cascades off the Arctic shelves
1. School of Earth Ocean and Environmental Science, University of
Plymouth, UK
2. P.P. Shirshov Institute of Oceanology RAS, Moscow Russia
3. Arctic and Antarctic Research institute, St.Petersburg, Russia
Introduction
Cascading (or shelf convection) is a specific type of
buoyancy driven current, in which dense water is formed
over the continental shelf and then descends down the slope
to a greater depth. We present analysis of 24 observed cases
of dense water cascades around the Arctic Ocean, and discuss
their specific features in connection with associated physical
processes. We use numerical parameters, both dimensional
and non-dimensional for quantitative comparison and
contrasting of single cases.
Physical processes
Horizontal density contrast can be produced by temperature,
salinity, or by the combination of both. Over the most part of
the Arctic Ocean ice cover is persistent all year round and the
surface temperature is close to the freezing point (Tfrez). The
source water acquires excessive density mainly due to
freezing and subsequent brine ejection.
Z
Q
Q
brine
SS0
Tfrez
SS1
S D0
S D1
Tfrez
HS
HP
Shelf
Topographic control A thinner homogeneous layer of water
over the shelf (HS) provides a greater salinity response to the
equal amount of ejected brine a thicker layer over the deep
sea (HP).
Unequal rate of brine ejection is caused by inhomogenity in
ice cover and is observed inside the flaw polynyas and
marginal ice zones, MIZ.
Initial salinity distribution can hamper cascading if by the
end of summer the shallow water was essentially fresher than
the deep water (SS0<< SD0).
Data analysis
Possible range of salinity variation in the shelf water is
illustrated by two sections occupied by Russian research
vessels in 1980s west off Novaya Zemlya Archipelago
74°00'N / 50°00'E
73°38'N / 53°55'E
033.78
33.88
32.84
32.42
32.39
31.76
33.84
33.89
32.99
32.84
32.83
31.82
33.97
33.93
33.79
33.18
33.73
32.31
34.38
34.01
34.18
34.11
34.05
33.25
34.09
32.86
34.34
34.51
34.41
33.98
34.35
34.34
34.37
34.43
34.48
Depth, m
34.45
34.46
-5034.52
33.41
34.66
34.65
34.74
34.64
-10034.82
34.71
34.72
34.72
-15034.84
34.79
34.75
34.72
34.85
Salinity, PSU
West Novaya Zemlya shelf
September, 1987
34.72
34.86
34.76
0
20
40
60
80
100
120
Distance, km
32.0
74°00'N / 50°00'E
33.0
34.0
34.4
34.7
34.9
34.41
34.40
34.40
34.40
34.41
34.40
34.40
34.40
34.48
34.45
34.44
34.46
34.61
34.59
34.61
34.60
34.65
34.68
34.72
34.67
34.67
34.69
34.73
34.74
34.78
0
35.1
73°38'N / 53°54'E
34.69
34.68
34.76
Depth, m
-50
-100
34.81
35.01
34.73
-150
Salinity, PSU
West Novaya Zemlya shelf
May, 1980
34.80
0
20
40
60
Distance, km
80
100
120
Broad shelves, encircling the Arctic Ocean interior possess
high potential for the dense water formation. In winter the
water filling various shallow areas is denser than the water at
the same depth over the adjoining slopes:
180
Winter salinity in 0-150m layer
Arctic Ocean Atlas (EWG, 1998)
70
75
80
85
90E
90W
0
30.00
32.00
33.50
34.10
34.30
34.50
34.70
34.90
35.20
However, whether all this dense water finally reaches the
slope producing cascades?
The map below shows 24 confirmed occurrences of dense
water cascades off the Arctic shelves, related to 10 varied
sites:
180
USA
70
8
Russia
9
Canada
75
10
Canadian
Basin
Laptev
Sea
80
0
85
-50
7
90W
-200
90E
6
Kara
Sea
-1000
Nansen Basin
-3000
85
5
-5000
Greenland
4
80
Svalbard
3
2
75
Barents
Sea
1
0
N
Location
1
2
3
4
Bear Island Channel
Storfjord
Central Bank
Westeren Novaya Zemlya
shelf
Franz-Victoria Channel
St.Anna Trough
Severnaya Zemlya shelf
Chukchi Sea shelf
Barrow Canyon
Beaufort Sea shelf
5
6
7
8
9
10
Number of
cases
4
2
2
7
1
2
1
1
2
2
Data source
Bar Kode, 1999
BarKode, 1999
Quadfasel et al., 1992
AARI archive + Barents Sea Atlas
(Matishov et al.,1998)
BarKode, 1999
BarKode, 1999
AARI archive
NODC Atlas (Levitus et al., 1998)
NODC Atlas (Levitus et al., 1998)
Melling and Moor, 1993,1995
What information about cascade can we get from a single
2-D section?
Z
ZA = ZD
D
A
E
ZE
ZC
C
ZB
B
H
A
=
B
C
=
E
X
D
XA
XD
XC
XB
Z=Z( ρA)
X
We identify 5 key points on the density section: (A, B, C, D
and E). Each of these points is associated with a set of five
basic parameters, i.e. T, S, ρ, X, and Z.
Key points and corresponding basic parameters can be
determined directly from the cross-section displaying the
dense water cascade:
73°10'N / 46°30'E
0
74°12'N / 51°05'E
Example 1
-50
Region: West Novaya
Zemlya shelf, Barents Sea
Time: August 1976
Data source: Barents Sea
Atlas, (Matishov et
al.,1998)
Note: there is no neutral
density level at this
section. Hence, B-point is
taken in the deepest
depression.
-100
D
A
-150
-200
E
T-scale
C
2.0
-250
Depth,m
1.0
0.0
-1.0
B
0
-2.0
25
50
75
100
125 Distance,km
70°54'N / 159°25'W
0
71°10'N / 159°02'W
Example 2
A
D
-20
-40
S-scale
34.0
33.9
33.8
-60
33.7
33.6
Depth,m
Region: Barrow Canyon,
Chukchi Sea
Time: March 1982
Data source: NODC Atlas,
(Levitus et al., 1998)
Note: there is no density
minimum at this section.
Hence, C-point is taken in the
deepest depression (no B and
E points).
33.5
33.0
32.5
32.0
0
C
10
20
Distance,km
In 23 cases, available observational data were full enough to
determine the complete set of numerical parameters.
2
9
1.8
ρ0β(Smax-Samb,0)
8
-ρ0α(Tmax-Tamb,0)
1.6
1.4
10
∆ρΤ , ∆ρS [kg/m3]
1.2
1
10
9
0.8
0.6
0.4
4
0.2
7
4
0
9
8
4 10
910
7
4
2
44
4
44
2
4
4
3
3
4
1
2
6
4
3
3
6
2
5
1
1
1
6
6
5
1
1
1
1
-0.2
0
100
200
300
400
500
-ZA(m)
The forcing of Arctic
cascades is depthdependent. In the
upper layer
temperature
contribution to the
density is commonly
much smaller than
that of salinity. In the
deeper layers the
salinity contrast
decreases and
changes sign,
allowing temperature
to prevail and solely
drive the cascade.
5
Corr=0.83
4
log(g'/f2Hc)
The scale (nondimensional) analysis
gives the following
relationship between
reduced gravity (g'), the
Coriolis parameter (f),
depth of the shelf (Hs=ZA), the steepness of the
seabed slope (s), the
thickness of the dense
water pool (Hc), and the
“age parameter”, r.
3
2
2
2.5
3
3.5
log[18s
4
r h]
-0.5 -2
4.5
5
Analysis of the Density ratio shows that cascades deliver
colder and fresher water to the deep ocean in 12 cases. Three
cascades (all in the Barents Sea) initially driven by
temperature contrast, appeared to be warmer and saltier than
the ambient water at ZE depth and experienced strong
mixing.
Conclusions
Average thermohaline contrasts between the cascade and the
ambient water, calculated for all cases are as follows: 1.73°C, 0.36 PSU and 0.40 kg/m3. In the areas of quasisteady polynyas horizontal density contrasts may exceed 1
kg/m3. The average velocity of dense water down slope
leakage varies in the limits 0.3 – 3 cm/s, depending on the
forcing of cascade and its stage. In the majority of the
considered cases, cascades delivered colder and fresher water
to the deep Arctic Ocean.
Acknowledgement
This study was co-funded by the EU INTAS grant 99-1600
References
Shapiro, G.I., Huthnance, J.M. and Ivanov, V.V. (2003). Dense
water overflow off continental shelves. Journal of Geophysical
Research, VOL 108; C12, art. no. 3390
Ivanov, V.V., Shapiro, G.I., Huthnance, J.M., Aleynik, D.L.,
Golovin, P.N., 2003. Dense water cascades around the World
Ocean, Progress in Oceanography,60, 1, 47-98.