ICES Statutory Meeting 2001
ICES CM 2001/W:10
Circulation of Atlantic water in the northern North Atlantic and Nordic Seas.
Kjell Arild Orvik# and Peter P. Niiler##
#)
Geophysical Institute, University of Bergen, N-5007 Bergen, Norway
Scripps Institution of Oceanography, La Jolla, CA 92093-0230
##)
Abstract
Pathways of the surface water in the northern North Atlantic to the Nordic Seas are presented
using 999 near-surface Lagrangian drifters. Based on the drifter data in the northern North
Atlantic, the inflow of Atlantic water to the Nordic Seas over the Scotland-Greenland ridge
and the establishment of the two-branch Norwegian Atlantic Current are discussed. This twobranch current appears to have a topographic trapped eastern branch and a western branch as a
frontal jet, also topographically guided. The origin of the drifters that pass through the Svinøy
Section which cuts through the Atlantic inflow just to the north of the Faroe-Shetland channel,
is presented. Downstream from the Svinøy Section, the eastern branch is stable and extends to
the Arctic. The western branch follows the western slope of the Vøring Plateau toward Jan
Mayen where it turns northeastward along the eastern slope of the Mohn Ridge. Farther
northward into the Fram Strait the drifters and hydrography still reveal the maintenance of a
two-branch Norwegian Atlantic Current
1. Introduction
The inflow of warm and saline water from the northern North Atlantic into the Nordic
Seas (Norwegian, Greenland and Iceland Sea), and its extension northward to higher latitudes,
are important factors for climate and biological production in Northern Europe. From a global
warming perspective, it is suggested that an increase of greenhouse gasses in the atmosphere
will effect dramatic changes in this circulation pattern and possibly passing a threshold with a
circulation breakdown. Therefore a comprehensive understanding of the circulation pattern of
this northward flowing Atlantic water is of great importance.
In this study we will emphasize the near-surface circulation pattern in the
northern North Atlantic and the Nordic Seas (Fig. 1), based on 999 Lagrangian surface drifters
released in the area. Poulain et al, 1996, studied the near-surface circulation in the Nordic Sea
and their Lagrangian drifter measurements composed the first basin-scale, near-surface
velocity data of the Nordic Seas. They also revealed for the first time, the two-branch
structure of the Norwegian Atlantic Current (NwAC). This study is an extension of their
findings, now based on a much larger number of drifters in combination with moored current
observations and hydrography section. We also strive to improve the understanding of the
circulation pattern in the northern North Atlantic and its connections with the Nordic Seas
through the establishment of the two-branch NwAC. Thus, the major pathways of the
northward flowing Atlantic water in the northern North Atlantic and the Nordic Seas will be
discussed. The establishment of the two-branch NwAC will also be studied in the light of the
Svinøy section, where we have established an array of moored current meters for monitoring
purposes of the Atlantic inflow (Orvik et al., 2001).
2. Data and data processing
The drifter used in this study is the WOCE/TOGA Lagrangian drifter described by
Sybrand and Niiler (1991). Its flotation element is a small fiberglass sphere, which houses an
Argos transmitter, drogue-presence sensor, and a thermistor for measuring sea-surface-
temperature (SST). Attached to the surface float is a thin polyurethane coated wire tether and
a large holey sock drogue, centered at a standard depth of 15 m (except for the Meldrum data
which were drogued at 50 m depth). For a majority of he drifters, locations are measured by
ARGOS satellite to an accuracy of 0.5 km, every 2-hour.
Procedures for processing of data are described in detail by Hansen and Poulain
(1996). Firstly, drifter data are filtered for drogue loss, but because drogue attachments are
determined, number of observations is increased by performing a statistical model on the
effect of wind on drifter motion without drogue. So a combination of drogged and windcorrected undrogged drifter velocities are used. In this study, the final data set generated
consists of daily averaged velocities.
A composite plot of the 999 drifter trajectories used in this analysis is shown in Figure
2, where deployment sites are indicated. The figure shows a high data density all over the
northern North Atlantic, particularly near the Gulf Stream and its eastern and northern
extensions. For the Nordic Sea (Norwegian, Greenland and Iceland Seas) the data density is
high around d Iceland, the Scotland-Greenland inflow area and in the eastern Norwegian Sea.
Thus the data coverage is suitable for a study of the flow field of Atlantic water.
We attempt to identify the major northward pathways of Atlantic water in terms of the fastest
moving drifters, defined as speed >30 cm/s (Fig 3a –b) and corresponding lower speed and
fluctuating areas as <30 cm/s (Fig 4).
To describe the circulation patterns quantitatively in a Eulerian framework, the drifter
velocity measurements are grouped into spatial bins of size 1olatitude x 1odeg longitude. This
box size was chosen to provide a reasonable and improved spatial resolution of major current
features compared with earlier studies (e.g. Poulain et al., 1996). We also ensure that most
boxes contain sufficient data to form statistical reliable values of mean current (vector) and
variance (standard error ellipses) in Fig 5, as well as eddy kinetic energy (EKE) in Fig 6. Only
boxes containing a minimum of 5 independent velocity measurements were retained, based on
specific judgments i.e. a 2-day decorrelation timescale. Consequently it may cause a larger
standard error of the box-averaged velocity estimates in some regions
3. Results
3.1 Northern North Atlantic pathways toward the Nordic Seas.
The major pathways are revealed by showing daily averaged drifter observation above
30 cm/s. The drifters clearly illustrate the classical flow field related to the western boundary
current system of the Gulf Stream: 1) the bifurcation of the eastward flowing Gulf Stream
into its continuation as the Azores Current and the northward flowing North Atlantic Current
(NAC). The NAC flows northward east of New Foundland (Flemish Cap) into an area known
as the Northwest Corner where it retroflects in a almost complete circle before separating
from the western boundary about 50-52oN. It then in a broad feature flows eastward along the
Arctic front (Rossby, 1996; Carr and Rossby, 2001) through a gap connecting the MidAtlantic Ridge and Reykjanes Ridge (Charlie Gibbs Fracture Zone) and then farther east it
splits into two major northeastward flowing branches; through the Iceland Basin and the
Rockall Trough. These two branches form the major northward pathways of Atlantic water in
the northern North Atlantic.
The eastern branch continue northeastward through the Rockall Trough and upon
reaching the eastern slope of the Rockall Trough and the Irish-Scottish shelf, it is transformed
into a topographic trapped shelf edge current. This eastern boundary current then flows along
the Scottish slope toward the Faroe-Shetland Channel where it enters the Norwegian Sea
along its eastern slope.
2
The more wide and eddy structured western branch ( Figs 3) continues northeastward
through the eastern Iceland Basin toward Iceland and according to the drifter observation its
major part passes the Iceland-Faroe Ridge close to Iceland. Due to hydrography this pathway
is related to tilting density surfaces rising toward west, indicating a geostrophic current. After
passing the ridge, it turns eastward and forms the Iceland-Faroe frontal jet.
In spite of a large number of drifters deployed south of Iceland, the observations show
small and variable currents over most of the Reykjanes Ridge (Fig 4), but with a significant
westward flow close to Iceland, which continues into the Denmark Strait. However, along the
eastern slope of the Reykjanes Ridge the drifters show a striking southwestward flow and a
similar northeastward flow along the western slope of the ridge Fig 3 and Fig 5. This indicates
a southwestward return flow of Atlantic water along the Reykjanes Ridge that retroflects near
its southern tip and continues along the western slope of the Reykjanes ridge toward the
Denmark Strait in a similar pattern as the deep overflow (Dickson et al., 1990). There, most of
the drifters again retroflect toward south-southwest forming the East Greenland Current as a
strong western boundary current in an extension as a cyclonic circulation in the Labrador Sea
with speed observed above 100 cm/s. Just a few drifters flow into the Nordic Seas west of
Iceland. They show small velocities west and north of Iceland, and thus a small and variable
inflow of Atlantic water through the Denmark Strait seems reasonable.
3.2 Nordic Sea pathways.
The Atlantic water enters the Norwegian Sea mainly through two pathways: through
the Faroe-Shetland Channel (SST: 8-9oC ), and over the Iceland-Faroe Ridge (SST: 6-7 oC)
(Hansen and Østerhus, 2000). Fig. 3 shows that the fastest near-surface drifters crossing the
Iceland- Faroe Ridge close to Iceland, indicating a major pathway of Atlantic water in that
area. After crossing the ridge, the drifters turn eastward along the jet of the Iceland-Faroe
front, with its meandering and unstable structure (Read and Pollard, 1992; Allen et al., 1994).
This branch as a frontal jet continues farther northeastward after passing to the north of the
Faroe Islands into the Norwegian Sea. It passes through the Svinøy section as the western
branch of the Norwegian Atlantic Current (NwAC), where it also has been identified as a
frontal jet (Orvik et al., 2001). Downstream from the Svinøy section the drifters tend to follow
the topographic slope of theVøring plateau (Poulain et al., 1996) and farther toward Jan
Mayen. Figs 2, 3 and 5. Then the major pathway turns northeastward, topographically guided
along the eastern slope of Mohn Ridge. This pathway then appears to turn northward west of
the Bear Island and continues toward north into the Fram Strait, still as a western branch of
the NwAC.
The drifters in the Iceland-Faroe frontal jet that continues eastward along the northern
slope of the Faroe Plateau, bifurcate and partly continue into the Faroe Shetland Channel,
along its western slope. Then they retroflect and merges with the Atlantic inflow along the
eastern slope of the Faroe Shetland Channel (Poulain et al., 1996). This inflow appears as a
barotropic current and has temperature and salinity characteristics of 9oC and 35.4 (Gould et
al., 1985; Dooley and Meincke, 1981; Burrows et al., 1999). The drifters partly branches into
the North Sea and continues northwards through isobathic confluence towards the Svinøy
section as the eastern branch of the Norwegian Atlantic Current (NwAC). Farther northward
the drifter pathways split into a minor coastal and a major shelf edge branch (Poulain et al.,
1996) as it flows northward along the Norwegian shelf edge into the Arctic, after a bifurcation
into the Barents Sea. This eastern branch of the NwAC appears as an about 3500 km long
eastern boundary current, trapped along the Norwegian shelf edge from Shetland into the
Arctic west of Svalbard at 80oN. Occasionally, the drifter observations exceed current speed
of 100 cm/s in this eastern branch of the NwAC.
3
In Fig. 7 the drifter passing through the Svinøy section are tracked, and their release
positions are indicated. They show that the different branches of the Atlantic inflow into the
Nordic Seas merge in the Svinøy section. Even drifters released south and west of Iceland are
tracked; indicating that also the Atlantic inflow through the Denmark Strait passes through the
Svinøy section. The drifter tracks confirm that the Svinøy section is a key area for monitoring
the Atlantic inflow to the Nordic Sea.
In Poulain et al. (1996), the two-branch NwAC was introduced for the first time. In
Orvik et al. (2001), these two-branches of the NwAC were identified as an eastern branch
with properties as a barotropic shelf edge current, and a western branch as a jet in the Polar
front about 300 km farther offshore. Fig 8 shows a hydrographic section across the Lofoten
basin (Gimsøy section) and the Bear Island section (along 75oN). The Gimsøy section across
the Lofoten basin shows the Atlantic water as a 600 km wide and 800m deep wedge. In the
Bear Island section the Atlantic water has an extension of 300 km and is shallower. The most
striking feature, is that in both sections the transition zone between the Atlantic and Arctic
water appears as a distinct front, the Polar front. In the Gimsøy section the Polar front is
located over the eastern slope of the Mohn Ridge, while in the Bear Island section it appears
over the steepest slope toward the deep Greenland Sea, at about 2500m depth. In both sites,
the Polar front coincides with the major drifter pathway in the Norwegian Sea. This evidently
shows that the western branch of the NwAC is a jet in the Polar front, topographically guided
from the Iceland-Faroe front to the Fram Strait. These findings also coincide with Mauritzen
(1996), who found the Polar front over the eastern Mohn Ridge along a 71oN hydrographic
section, and over the steep slope of about 2500 m along a 73.5oN section. Even in the Fram
Strait, a hydrographic section along 79oN (Fahrbach et al., 2001), shows a distinct subsurface
Polar front, and presumably an associated frontal jet. Mauritzen (1996) also showed that the
Atlantic water was deeper in the Lofoten basin, down to 900 m. This deepening of the
Atlantic water coincides with the drifter patterns in the Lofoten basin, which according to Figs
3, 5 and 6, reveal a stronger eddy field in that area.
4. Discussion and concluding remarks
The major northward pathways of near-surface Atlantic water in the northern North
Atlantic and the Nordic Seas are identified in terms of current speed above 30 cm/s. These are
schematically shown in Fig. 9, superimposed on the sea surface temperature (SST) from an
AVHHR image in March. In the northern North Atlantic the zonally east flowing NAC splits
into two major northeastward flowing branches. Both branches show a topographic guidance:
the eastern branch through the Rockall Trough where its major part transforms into a
topographic trapped shelf edge current upon reaching the Irish-Scottish shelf, and the western
branch flowing northeastward in the eastern Iceland Basin toward Iceland. The most striking
feature in the northern North Atlantic is the pathway of Atlantic water around the Reykjanes
Ridge, a southward flow along the eastern slope and then a northeastward flow along the
western slope of the ridge, toward the Denmark Strait, where it merges with the East
Greenland Current.
The drifters also reveal that the Atlantic water enters the Nordic Seas mainly through
the Iceland-Faroe gap close to Iceland and through the Faroe-Shetland Channel. Then the
establishment of the two-branch NwAC is demonstrated as well as the applicability of the
Svinøy section as a site for monitoring the Atlantic inflow. The structure of the eastern branch
as a barotropic shelf edge current extending into the Arctic west of Svalbard is well known.
However, what we have shown for the first time is the pathway of the western branch, farther
downstream from the Svinøy section. This two-branch pattern of the NwAC was shown by
Poulain et al. (1996), and the identification of the western branch as a frontal jet was shown in
Orvik et al. (2001). Here we have shown that this western branch appears as a topographically
4
guided jet in the Polar front, all the way from the Iceland-Faroe front and into the Fram Strait,
where the two-branch structure still maintains. The topographic guidance of the frontal jet
mirrors a deep current along the steepest topography (Svendsen et al., 1991) of the Vøring
Plateau and Mohn Ridge. This is because the pressure field related to a topographically
steered deep current acts as an artificial sloping bottom on the Atlantic water. A distinct
topographic steered deep current is observed in the Svinøy section (Orvik et al., 2001) and
also along the 2000 m isobath of the western slope of the Vøring Plateau, in the site of OWS
“Mike” (Østerhus, personal communication). To our knowledge, the understanding of deep
currents in the Mohn Ridge area and the slopes of the Norwegian and Greenland Seas are
limited. The extensive topographic steering of the jet in the Polar front justifies their
existence.
Acknowledgement
This study was performed during one of the author’s sabbatical stay at Scripps. Thanks are
due to Sharon Lukas for performing the data processing and providing the figures. The
hydrographic sections provided by Kjell Arne Mork at the Marine Research Institute are also
appreciated.
References
Allen, J. T., Smeed, D. A., Chadwick, A. L., 1994. Eddies and mixing at the Iceland-Faroes
Front. Deep-Sea Research I, Vol. 41, No.1, 51-79.
Burrows, M., Thorpe, S.A., Meldrum, D., T., 1999. Dispersion over the Hebridean and
Shetland shelves and slopes. Continental Shelf Research, 19, 49-55.
Carr, M.-E., 2001. Pathways of the North Atlantic Current from surface drifters and
subsurface floats. Journal of Geophysical Research, Vol. 106, No. C3, pp 4405-4419, March
15, 2001
Dickson, R. R., Gmitrowicz, E. M., Watson, A.J, 1990. Deep-Water renewal in the northern
North Atlantic. Nature, Vol 344, 26 April 1990
Dooley, H. D., Meincke, J., 1981. Circulation and water masses in the Faroe channel during
Overflow 73. Deutsches Hydrographisches Zeitschrift, 34, 41-55.
Farbach, F., Meincke,J., Østerhus, S., Rohardt,G., Schauer,U., Tverberg,V., Verduin, J., 2001.
Direct measurements of volume transports through Fram Strait. Polar Research, Sverdrup Vol
(in press)
Gould, W. J., Loynes, J., Backhaus, J., 1985. Seasonality in slope current transport N.W. of
Shetland. ICES Stat. Meeting 1985, C. M. 1985/C:7
Hansen, B., Østerhus, Ø., 2000. North Atlantic-Nordic Seas exchanges. Progress in
Oceanography 45,109-208
Hansen, D.V., Poulain, P.M., 1996. quality control and interpolations of WOCE/TOGA drifter
data, J. Atmos. Ocean. Tech., 13, 900-909, 1996
Mauritzen, C., 1996. Production of dense overflow water feeding the North Atlantic across the
Greenland-Scotland Ridge. Deep-Sea Research, 43 (6), 769-805.
5
Orvik, K.A., Skagseth, Ø., Mork, M., 2001. Atlantic inflow to the Nordic Seas: current
structure and volume fluxes from moored current meters; VM-ADCP and Sea Soar-CTD
observations, 1995-1999. Deep- Sea Res. I 48 (4), 937-957.
Poulain, P.-M., Warn-Varnas, A., Niiler, P.P., 1996. Near-surface circulation of the Nordic
seas as measured by Lagrangian drifters. Journal of Geophysical Research, Vol. 101, No. C8,
pp 18237-18258, August 15, 1996
Rossby, H., T., 1996. The North Atlantic Current and surrounding water ‘at the crossroads’,
Rev. Geophys. 34, 463-481, 1996
Read, J. F., Pollard, R.T., 1992. Water Masses in the Region of the Iceland-Faeroes Front.
Journal of Physical Oceanography, 22, 1365-1378.
Sybrand, A..L., Niiler, P.P., 1991. WOCE/TOGA Lagranian drifter construction manual SIO
Ref.91/6, WOCE Rep. 63, 58 pp., Scripps Institution of Oceanography, La Jolla, CA, 1991
Svendsen, E., Sætre, R., Mork, M., 1991. Features of the northern North Sea circulation.
Continental Shelf Research, Vol 11, No 5, pp 493-508.
List of figures.
Figure 1. Bathymetry of the northern North Atlantic and the Nordic Seas with the Svinøy -,
Gimsøy -, and Bear Island sections
Figure 2. A composite diagram of all drifters trajectories used in the analysis with release
locations indicated as blue dots. The red colored tracks show the 951 SVP drifters drogued at
15 m, and light blue show the 48 Meldrum drifters drogued at 50 m.
Figure 3. Distribution of the fastest drifters in terms of current observations exceeding 30 cm/s
a) north-south components b) east-west components
Figure 4. Distribution of the slowest drifters in terms of current observations less than 30 cm/s
Figure 5. Eulerian statistics in 1O latitude by 1O longitude bins computed from combined
drogued and wind corrected undrogued observations in term of mean velocity vectors with
standard uncertainty ellipses
Figure 6. Eulerian statistics in 1O latitude by 1O longitude bins computed from combined
drogued and wind corrected undrogued observations in term of eddy kinetic energy (EKE)
Figure 7. A diagram of all drifter trajectories passing through the Svinøy section
Figure 8a. Salinity, potential temperature and potential density (σθ) in the Gimsøy-NW
section, June 2001 (with courtesy from the Marine Research Institute, Bergen)
Figure 8b. Salinity, potential temperature and potential density (σθ) in the Bear Island-W
section, May/June 2001 (with courtesy from the Marine Research Institute, Bergen).
Figure 9. Schematic of the major pathway of near-surface Atlantic water in the northern North
Atlantic and Nordic Seas (dark arrows) as derived from near-surface Lagrangian drifter data,
superimposed on the sea surface temperature from an AVHHR image in March
6
Figure 1. Bathymetry of the northern North Atlantic and the Nordic Seas with the Svinøy -,
Gimsøy -, and Bear Island sections
7
Figure 2. A composite diagram of all drifters trajectories used in the analysis with release
locations indicated as blue dots. The red colored tracks show the 951 SVP drifters drogued at
15 m, and light blue show the 48 Meldrum drifters drogued at 50 m.
8
Figure 3a. Distribution of the fastest drifters in terms of current observations exceeding
30 cm/s; north-south components
9
Figure 3b. Distribution of the fastest drifters in terms of current observations exceeding
30 cm/s; east-west components
10
Figure 4. Distribution of the slowest drifters in terms of current observations less than 30 cm/s
11
Figure 5. Eulerian statistics in 1O latitude by 1O longitude bins computed from combined
drogued and wind corrected undrogued observations in term of mean velocity vectors with
standard uncertainty ellipses
12
Figure 6. Eulerian statistics in 1O latitude by 1O longitude bins computed from combined
drogued and wind corrected undrogued observations in term of eddy kinetic energy (EKE)
13
Figure 7. A diagram of all drifter trajectories passing through the Svinøy section
14
Depth [m]
500
35
34.88
1000
34.9
1500
2000
2500
S
-0.5
-0.8
-0.9
2000
-1
-1.05
θ
28
Depth [m]
500
1000
27 .7
27.8
27.9
28 .05
28 .03
28 .06
1500
2000
7
4
1000
2500
6
5
0
1500
34.90 5
34 .91
34 .
91 3
34.91 2
34 .91
500
Depth [m]
35.1
28 .07
28.07 7
28 .0
8
28.082
2500
σ
800
700
600
500
400
Distance [km]
300
200
Figure 8a. Salinity, potential temperature and potential density (σθ) in the Gimsøy-NW
section, June 2001 (with courtesy from the Marine Research Institute, Bergen)
15
100
θ
0
34 .88
1000
34.9
1500
3
2000
34 .91 2
2500
34 .91
-0 .5
3
-0.8
4
0
-0 .5
1000
1500
-0 .9
2000
500
-0 .8
-0 .9
-1
2500
Depth [m]
S
5
0
34 .913
34 .91 2
2
500
Depth [m]
34 .91
34 .9
1
Depth [m]
500
35 .1
35
θ
-1 .05
28 .03
1000
28 .07
1500
2000
28 .08
28 .077
28 .08 2
2500
900
28
28 .05
28.06
800
27 .7
27 .9
700
600
28 .08 4
500
400
Distance [km]
σ
300
200
100
Figure 8b. Salinity, potential temperature and potential density (σθ) in the Bear Island-W
section, May/June 2001 (with courtesy from the Marine Research Institute, Bergen).
16
θ
0
Figure 9. Schematic of the major pathway of near-surface Atlantic water in the northern North
Atlantic and Nordic Seas (dark arrows) as derived from near-surface Lagrangian drifter data,
superimposed on the sea surface temperature from an AVHHR image in March
17