Major pathways of Atlantic water in the northern North Atlantic and

Nordic Seas toward Arctic

Kjell Arild OrvikGeophysical Institute, University of Bergen, Bergen, Norway

Peter NiilerScripps Institution of Oceanography, La Jolla, CA, USA

Received 27 February 2002; accepted 22 May 2002; published 1 October 2002.

[1] The major pathways of near-surface Atlantic water inthe northern North Atlantic and Nordic Seas are identifiedas current speeds above 30 cm/s, using 1014 Lagrangiandrifters combined with previously published hydrography.The inflow over the Scotland-Greenland ridge andestablishment of the two-branch Norwegian AtlanticCurrent (NwAC) are described in light of the circulationin the northern North Atlantic. The western branch of theNwAC appears as a jet in the Polar Front, topographicallyguided from the Iceland-Faroe Front, through the NordicSeas toward Fram Strait. The eastern branch starts as ashelf edge current above the Irish-Scottish continentalshelf, and after passing through the Faroe-ShetlandChannel, it continues northward along the Norwegianshelf edge toward the Arctic, with a branch bifurcating intothe Barents Sea. The NwAC appears to maintain its two-branch structure throughout the Nordic Seas, with theAtlantic water confined to a 200 –600 km widewedge. INDEX TERMS: 4512 Oceanography: Physical:

Currents; 4532 Oceanography: Physical: General circulation; 4528

Oceanography: Physical: Fronts and jets; 4536 Oceanography:

Physical: Hydrography. Citation: Orvik, K. A., and P. Niiler,

Major pathways of Atlantic water in the northern North Atlantic

and Nordic Seas toward Arctic, Geophys. Res. Lett., 29(19), 1896,

doi:10.1029/2002GL015002, 2002.

1. Introduction

[2] In a global warming perspective, the inflow of warmand saline water from the northern North Atlantic into theNordic Seas (Norwegian, Greenland and Iceland Seas), andits extension and flow northward toward higher latitudes, isof great importance. This study emphasizes the near-surfacepathways of Atlantic Water (AW) in the northern NorthAtlantic and Nordic Seas (Figure 1), and is based on 1014Lagrangian drifters released during 1989–2001. From asubset of the data used here, previous studies have beenpublished for the near-surface circulation in the followingareas: the Nordic Seas [Poulain et al., 1996], Icelandicwaters [Valdimarsson and Malmberg, 1999], and south ofthe Iceland Faroe Ridge [Fratantoni, 2001]. Poulain et al.[1996] revealed for the first time a two-branch structure ofthe Norwegian Atlantic Current (NwAC) in the southern

Norwegian Sea. Orvik et al. [2001] identified these twobranches as an eastern branch which acts as a nearlybarotropic shelf edge current, and a western branch whichis a topographically steered jet in the Polar Front (thetransition zone between Atlantic and Arctic water in theNordic Seas).[3] This study is based on about twice the number of

observations available to earlier studies, and will represent asynthesis and extension of previous and recent findings inthe northern North Atlantic, and the Nordic Seas. To thedrifter data we add hydrography, and this leads to somerevision of previous conclusions, particularly in the NordicSeas; [e.g. Poulain et al., 1996]. A primary purpose of thispaper is to substantiate the overall circulation pattern inFigure 1, where schematics of major pathways of near-surface AW are shown superimposed on the sea surfacetemperature (SST) obtained from an AVHHR image.Repeated references will be made to this figure, where theestablishment and pathways of the two-branch NwAC isemphasized in light of the inflow pattern and connectionwith the northern North Atlantic. Only the overall circu-lation pattern obtain during the 1989–2001 observationperiod is described here. Variability on seasonal and annualtimescales is not taken into account.

2. Data and Data Processing

[4] The bulk of the data set was compiled using theSVP Lagrangian drifters, drogued at 15 m depth. Theirdata collection, transmission and water following charac-teristics are described by Niiler [2001]. The complete dataset is described by Reverdin et al. [2002]. A compositeplot of the drifter data density and deployment sites isshown in Figure 2. The figure shows a high data densityall over the northern North Atlantic, in particular near theGulf Stream and its eastern and northern extensions. Forthe Nordic Seas, the data density is high around Iceland,the Scotland-Greenland inflow area and in the easternNorwegian Sea, covering the AW. The major northwardpathways of AW are identified by selecting the fastestmoving drifters, with 24-hour average speed greater than30 cm/s, and then plotting their locations according totheir east–west and north–south velocity components(Figures 3a–3b). Uniformly colored observations showmajor pathways, while mixed color patterns suggest eddyfields. The strong currents are defined by many differentdrifters that have entered regions of strong flow, and are

GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 19, 1896, doi:10.1029/2002GL015002, 2002

Copyright 2002 by the American Geophysical Union.0094-8276/02/2002GL015002$05.00

2 - 1

not necessarily continuous pathways in which any partic-ular drifter remains over a great distance.

3. Results

3.1. Northern North Atlantic Pathways Toward theNordic Seas

[5] The strong currents shown in Figure 3 clearly illus-trate the well-known flow fields related to the westernboundary current system of the Gulf Stream with a bifurca-tion of the eastward flowing Gulf Stream into its continu-ation as the Azores Current and the northward flowingNorth Atlantic Current (NAC). The core of the NAC can betraced northward east of Newfoundland (Flemish Cap) intothe ‘‘northwest corner’’ where it retroflects in an almostcomplete circle before separating from the western boun-dary at about 50–52�N. The core of the NAC continueszonally eastward toward a gap in the Mid-Atlantic Ridge[Carr and Rossby, 2001]. Farther east it splits into twomajor northeastward flowing branches; one through theIceland Basin and the other one through the Rockall

Trough. These two branches form the major northwardpathways of AW in the northern North Atlantic [Fratantoni,2001].[6] The swift-flowing branch that continues northeast-

ward through the Rockall Trough, upon encountering theIrish-Scottish shelf, appears to be constrained as a topo-graphically trapped shelf edge current. This current increasesits speed along the Scottish slope toward the Faroe-ShetlandChannel [Burrows et al., 1999], where it enters the Norwe-gian Sea. Through the Iceland Basin, the flow appears to beconcentrated in a wider, eddy structured western branch(Figures 3a–3b), which continues northeastward towardsoutheastern Iceland. In this area the flow appears tobifurcate into a northward and a southward branch. Its majornorthward part crosses the Iceland-Faroe Ridge close toIceland through the ‘‘western valley’’, and after passingthe ridge turns eastward and forms the Iceland-Faroe frontaljet [Perkins et al., 1998].[7] The bifurcation of the flow southeast of Iceland and a

return flow of AW around the Reykjanes Ridge, consists ofa strong current southward along the eastern slope, and

Figure 1. Schematic of the major pathways of near-surface Atlantic water in the northern North Atlantic and Nordic Seas(dark arrows) as derived from near-surface Lagrangian drifter data, in context of superimposed sea surface temperaturefrom AVHHR image in March, 1991.

2 - 2 ORVIK AND NIILER: MAJOR PATHWAYS OF ATLANTIC WATER

subsequently northeastward flow along the western slope ofthe ridge. It then continues westward over the DenmarkStrait and bounds the southwest flowing East GreenlandCurrent over the continental shelf, as a concentrated jet on

the seaward rise. The observations also show a distinctwestward flow across the Reykjanes Ridge close to Iceland,which continues toward the Denmark Strait. Only fewdrifters released south of Iceland enter the Nordic Seas westof Iceland, with subsequent small velocities west and northof Iceland. These and other data illustrate the sporadic andvariable inflow pattern through the Denmark Strait andnorth of Iceland [Perkins et al., 1998]. Our synthesis ofthe drifter data in Icelandic waters with emphasis on thecirculation east of Iceland and around the Reykjanes Ridge(Figure 1) agrees with the results of Valdimarsson andMalmberg [1999], and Perkins et al. [1998].

3.2. Nordic Seas Pathways Toward the Arctic

[8] According to section 3.1, the AW enters the NordicSeas through two major pathways: the Faroe-ShetlandChannel and over the Iceland-Faroe Ridge. Figures 3a–3bshows that the fastest flow in the Iceland-Faroe Front moveseastward with a meandering and unstable structure [Readand Pollard, 1992]. This branch maintains its properties as afrontal jet and continues farther northeastward into theNorwegian Sea as the western branch of the NwAC, afterpassing to the north of the Faroes. This current then tends tofollow the topographic slope of the Vøring plateau [Poulainet al., 1996] toward Jan Mayen. As illustrated schematicallyin Figure 1, the observations show that the major pathwayturns northeastward along the slope of the Mohn Ridge.This pathway then appears to turn northward west of BearIsland and continues along the Knipovich Ridge toward theFram Strait.[9] A hydrographic section across the Lofoten basin

along 71�N [Mauritzen 1996, Figure 9] shows the AW asa 600 km wide, 800 m deep slab, with a distinct Polar Front

Figure 3a. Distribution of the fastest drifters with currentspeed exceeding 30 cm/s as north–south components.Uniformly colored observations show major pathways,while mixed color patterns indicate eddy fields.

Figure 3b. Distribution of the fastest drifters with currentspeed exceeding 30 cm/s as east–west components.Uniformly colored observations show major pathways,while mixed color patterns indicate eddy fields.

Figure 2. Data density as 24-hour average observations in2� latitude by 6� longitude bins computed from 1014Lagrangian drifters. Release locations are indicated as blackdots for the 966 SVP-drifters drogued at 15 m depth, andblue dots for the 48 Meldrum drifters at 50 m.

ORVIK AND NIILER: MAJOR PATHWAYS OF ATLANTIC WATER 2 - 3

over the Mohn Ridge. Along a 73.5�N section [Mauritzen1996, Figure 11] the AW is shallower and extends about300 km offshore with a distinct Polar Front over theKnipovich Ridge. The hydrographic sections agree withthe SST field in Figure 1, which in combination with drifterobservations substantiate this pathway as a topographicallysteered jet in the Polar Front. The deeper AW in the Lofotenbasin coincides with a stronger eddy-like drifter pattern(Figures 3a–3b).[10] North of the Faroes the fastest current bifurcates and

partly continues into the Faroe Shetland Channel, along itswestern slope. Then it retroflects and merges with theAtlantic inflow along the eastern slope of the Faroe-Shet-land Channel [Poulain et al., 1996]. This inflow appears asa nearly barotropic current [Burrows et al., 1999]. The flowpartly branches into the North Sea and continues northwardas the eastern branch of the NwAC along the Norwegianshelf edge toward the Arctic, with a branch bifurcating intothe Barents Sea.

4. Discussion and Concluding Remarks

[11] In this study the major northward pathways of near-surface AW in the northern North Atlantic and the NordicSeas are identified, using the currents from Lagrangiandrifters drogued at 15 m depth, whose speeds exceed 30cm/s. Combined with hydrography, these observations showthe circulation pattern illustrated in Figure 1. The westernbranch of the NwAC appears as a jet in the Polar Front,topographically guided from the Iceland-Faroe Front,through the Norwegian Sea toward the Fram Strait, wherethe AW is subducted [Mauritzen, 1996]. This northwardextension of the western branch of the NwAC is in agree-ment with van Aken et al. [1995], revealing a northwardflowing frontal jet over the Knipovich Ridge. Also from avolume flux perspective this understanding of the westernbranch of the NwAC is substantiated, e.g. in the Iceland-Faroe Front the volume transport is estimated to be 3.5 Sv(1 Sv = 106m3 s�1) [Perkins et al., 1998], in the southernNorwegian Sea 3.4 Sv [Orvik et al., 2001], and over theKnipovich Ridge 3 Sv [van Aken et al., 1995].[12] In a broad sense, the two-branch structure of the

northward flowing AWappears to be established as the zonal,eastward flowing NAC splits into two northward branches(Figure 1). Both branches show topographic steering: theeastern branch flows through the Rockall Trough where atopographic trapped shelf edge current appears to be estab-lished upon reaching the Irish-Scottish shelf, and the westernbranch flows through the Iceland Basin toward Iceland. Theobservations then show that the AW enters the Nordic Seasmainly through the Iceland-Faroe gap close to Iceland andthrough the Faroe-Shetland Channel. Thus, the establishmentof the two-branch NwAC in the Norwegian Sea-after cross-ing the Scotland-Greenland Ridge, can be interpreted as anextension of the two major branches in the northern NorthAtlantic. The NwAC appears to maintain its two-branch

structure throughout the Nordic Seas toward the Fram Strait.The eastern branch consists of a nearly 3500 km long, nearlybarotropic shelf edge current and the western branch is a jetin the Polar Front, guided along the dominant topographicfeatures from the Iceland-Faroe Front into the Fram Strait.The extension of AW in the Nordic Seas is then confined tothe 200–600 km wide strip between these two majorpathways.[13] In this study, we have shown for the first time the

western branch of the NwAC as a continuous jet in the PolarFront, extending from the Iceland-Faroe Ridge to the FramStrait. The topographic guidance of the frontal jet indicatesa deeper current along its pathways [Svendsen et al., 1991],including the slopes of the Vøring Plateau, the Mohn Ridgeand the Knipovich Ridge.

[14] Acknowledgments. Thanks are due to Sharon Lukas for per-forming data processing and providing figures, and to Øystein Skagseth andAlastair Jenkins for constructive criticism of the manuscript.

ReferencesBurrows, M., S. A. Thorpe, and D. T. Meldrum, Dispersion over the Heb-ridean and Shetland shelves and slopes, Cont. Shelf Res., 19, 49–55,1999.

Carr, M.-E., and T. Rossby, Pathways of the North Atlantic Current fromsurface drifters and subsurface floats, J. Geophys. Res., 106, 4405–4419,2001.

Fratantoni, D. M., North Atlantic Surface Circulation During the 1990’sObserved with Satellite-Tracked Drifters, J. Geophys. Res., 102, 22,067–22,093, 2001.

Mauritzen, C., Production of dense overflow water feeding the North Atlan-tic across the Greenland-Scotland Ridge, Deep Sea Res., 43, 769–805,1996.

Niiler, P. P., The world ocean surface circulation. Ocean Circulation andClimate, Volume 77, edited by G. Siedler, J. Church, and J. Gould,Academic Press, San Diego, 193–204, 2001.

Orvik, K. A., Ø. Skagseth, and M. Mork, Atlantic inflow to the NordicSeas: Current structure and volume fluxes from moored current meters,VM-ADCP and SeaSoar-CTD observations, 1995–1999, Deep Sea Res.,48, 937–957, 2001.

Perkins, H., T. S. Hopkins, S.-A. Malmberg, P.-M. Poulain, and A. Warn-Varnas, Oceanographic conditions east of Iceland, J. Geophys. Res., 103,21,531–21,542, 1998.

Poulain, P.-M., A. Warn-Varnas, and P. P. Niiler, Near-surface circulation ofthe Nordic seas as measured by Lagrangian drifters, J. Geophys. Res.,101, 18,237–18,258, 1996.

Read, J. F., and R. T. Pollard, Water Masses in the Region of the Iceland-Faeroes Front, J. Phys. Oceanogr., 22, 1365–1378, 1992.

Reverdin, G., P. Niiler, and H. Valdimarsson, North Atlantic Ocean surfacecurrents, J. Geophy. Res., 2002(accepted).

Svendsen, E., R. Sætre, and M. Mork, Features of the northern North Seacirculation, Cont. Shelf Res., 11, 493–508, 1991.

Valdimarsson, H., and S.-A. Malmberg, Near-surface circulation in Icelan-dic waters derived from satellite tracked drifters, Rit Fiskideildar. J. Mar.Res. Inst-Reykjavik, 16, 23–39, 1999.

van Aken, H. M., B. Gereon, and M. Hankel, The anatomy of the ArcticFrontal Zone in the Greenland Sea, J. Geophys. Res., 100, 15,999–16,014, 1995.

�����������K. A. Orvik, Geophysical Institute, University of Bergen, N-5007

Bergen, Norway.P. Niiler, Scripps Institution of Oceanography, La Jolla, CA 92093-0230,

USA.

2 - 4 ORVIK AND NIILER: MAJOR PATHWAYS OF ATLANTIC WATER