Major pathways of Atlantic water in the northern North Atlantic and Nordic Seas toward Arctic

Authors


Abstract

[1] The major pathways of near-surface Atlantic water in the northern North Atlantic and Nordic Seas are identified as current speeds above 30 cm/s, using 1014 Lagrangian drifters combined with previously published hydrography. The inflow over the Scotland-Greenland ridge and establishment of the two-branch Norwegian Atlantic Current (NwAC) are described in light of the circulation in the northern North Atlantic. The western branch of the NwAC appears as a jet in the Polar Front, topographically guided from the Iceland-Faroe Front, through the Nordic Seas toward Fram Strait. The eastern branch starts as a shelf edge current above the Irish-Scottish continental shelf, and after passing through the Faroe-Shetland Channel, it continues northward along the Norwegian shelf edge toward the Arctic, with a branch bifurcating into the Barents Sea. The NwAC appears to maintain its two-branch structure throughout the Nordic Seas, with the Atlantic water confined to a 200–600 km wide wedge.

1. Introduction

[2] In a global warming perspective, the inflow of warm and saline water from the northern North Atlantic into the Nordic Seas (Norwegian, Greenland and Iceland Seas), and its extension and flow northward toward higher latitudes, is of great importance. This study emphasizes the near-surface pathways of Atlantic Water (AW) in the northern North Atlantic and Nordic Seas (Figure 1), and is based on 1014 Lagrangian drifters released during 1989–2001. From a subset of the data used here, previous studies have been published for the near-surface circulation in the following areas: the Nordic Seas [Poulain et al., 1996], Icelandic waters [Valdimarsson and Malmberg, 1999]], and south of the Iceland Faroe Ridge [Fratantoni, 2001]. Poulain et al. [1996] revealed for the first time a two-branch structure of the Norwegian Atlantic Current (NwAC) in the southern Norwegian Sea. Orvik et al. [2001] identified these two branches as an eastern branch which acts as a nearly barotropic shelf edge current, and a western branch which is a topographically steered jet in the Polar Front (the transition zone between Atlantic and Arctic water in the Nordic Seas).

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 temperature from AVHHR image in March, 1991.

[3] This study is based on about twice the number of observations available to earlier studies, and will represent a synthesis and extension of previous and recent findings in the northern North Atlantic, and the Nordic Seas. To the drifter data we add hydrography, and this leads to some revision of previous conclusions, particularly in the Nordic Seas; [e.g. Poulain et al., 1996]. A primary purpose of this paper is to substantiate the overall circulation pattern in Figure 1, where schematics of major pathways of near-surface AW are shown superimposed on the sea surface temperature (SST) obtained from an AVHHR image. Repeated references will be made to this figure, where the establishment and pathways of the two-branch NwAC is emphasized in light of the inflow pattern and connection with the northern North Atlantic. Only the overall circulation pattern obtain during the 1989–2001 observation period is described here. Variability on seasonal and annual timescales is not taken into account.

2. Data and Data Processing

[4] The bulk of the data set was compiled using the SVP Lagrangian drifters, drogued at 15 m depth. Their data collection, transmission and water following characteristics are described by Niiler [2001]. The complete data set is described by Reverdin et al. [2002]. A composite plot of the drifter data density and deployment sites is shown in Figure 2. The figure shows a high data density all over the northern North Atlantic, in particular near the Gulf Stream and its eastern and northern extensions. For the Nordic Seas, the data density is high around Iceland, the Scotland-Greenland inflow area and in the eastern Norwegian Sea, covering the AW. The major northward pathways of AW are identified by selecting the fastest moving drifters, with 24-hour average speed greater than 30 cm/s, and then plotting their locations according to their east–west and north–south velocity components (Figures 3a 3b). Uniformly colored observations show major pathways, while mixed color patterns suggest eddy fields. The strong currents are defined by many different drifters that have entered regions of strong flow, and are not necessarily continuous pathways in which any particular drifter remains over a great distance.

Figure 2.

Data density as 24-hour average observations in 2° latitude by 6° longitude bins computed from 1014 Lagrangian drifters. Release locations are indicated as black dots for the 966 SVP-drifters drogued at 15 m depth, and blue dots for the 48 Meldrum drifters at 50 m.

Figure 3a.

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

3. Results

3.1. Northern North Atlantic Pathways Toward the Nordic Seas

[5] The strong currents shown in Figure 3 clearly illustrate the well-known flow fields related to the western boundary current system of the Gulf Stream with a bifurcation of the eastward flowing Gulf Stream into its continuation as the Azores Current and the northward flowing North Atlantic Current (NAC). The core of the NAC can be traced northward east of Newfoundland (Flemish Cap) into the “northwest corner” where it retroflects in an almost complete circle before separating from the western boundary at about 50–52°N. The core of the NAC continues zonally eastward toward a gap in the Mid-Atlantic Ridge [Carr and Rossby, 2001]. Farther east it splits into two major northeastward flowing branches; one through the Iceland Basin and the other one through the Rockall Trough. These two branches form the major northward pathways of AW in the northern North Atlantic [Fratantoni, 2001].

[6] The swift-flowing branch that continues northeastward through the Rockall Trough, upon encountering the Irish-Scottish shelf, appears to be constrained as a topographically trapped shelf edge current. This current increases its speed along the Scottish slope toward the Faroe-Shetland Channel [Burrows et al., 1999], where it enters the Norwegian Sea. Through the Iceland Basin, the flow appears to be concentrated in a wider, eddy structured western branch (Figure 3a3b), which continues northeastward toward southeastern Iceland. In this area the flow appears to bifurcate into a northward and a southward branch. Its major northward part crosses the Iceland-Faroe Ridge close to Iceland through the “western valley”, and after passing the ridge turns eastward and forms the Iceland-Faroe frontal jet [Perkins et al., 1998].

[7] The bifurcation of the flow southeast of Iceland and a return flow of AW around the Reykjanes Ridge, consists of a strong current southward along the eastern slope, and subsequently northeastward flow along the western slope of the ridge. It then continues westward over the Denmark Strait and bounds the southwest flowing East Greenland Current over the continental shelf, as a concentrated jet on the seaward rise. The observations also show a distinct westward flow across the Reykjanes Ridge close to Iceland, which continues toward the Denmark Strait. Only few drifters released south of Iceland enter the Nordic Seas west of Iceland, with subsequent small velocities west and north of Iceland. These and other data illustrate the sporadic and variable inflow pattern through the Denmark Strait and north of Iceland [Perkins et al., 1998]. Our synthesis of the drifter data in Icelandic waters with emphasis on the circulation east of Iceland and around the Reykjanes Ridge (Figure 1) agrees with the results of Valdimarsson and Malmberg [1999], and Perkins et al. [1998].

3.2. Nordic Seas Pathways Toward the Arctic

[8] According to section 3.1, the AW enters the Nordic Seas through two major pathways: the Faroe-Shetland Channel and over the Iceland-Faroe Ridge. Figure 3a3b shows that the fastest flow in the Iceland-Faroe Front moves eastward with a meandering and unstable structure [Read and Pollard, 1992]. This branch maintains its properties as a frontal jet and continues farther northeastward into the Norwegian Sea as the western branch of the NwAC, after passing to the north of the Faroes. This current then tends to follow the topographic slope of the Vøring plateau [Poulain et al., 1996] toward Jan Mayen. As illustrated schematically in Figure 1, the observations show that the major pathway turns northeastward along the slope of the Mohn Ridge. This pathway then appears to turn northward west of Bear Island and continues along the Knipovich Ridge toward the Fram Strait.

[9] A hydrographic section across the Lofoten basin along 71°N [Mauritzen 1996, Figure 9] shows the AW as a 600 km wide, 800 m deep slab, with a distinct Polar Front over the Mohn Ridge. Along a 73.5°N section [Mauritzen 1996, Figure 11] the AW is shallower and extends about 300 km offshore with a distinct Polar Front over the Knipovich Ridge. The hydrographic sections agree with the SST field in Figure 1, which in combination with drifter observations substantiate this pathway as a topographically steered jet in the Polar Front. The deeper AW in the Lofoten basin coincides with a stronger eddy-like drifter pattern (Figures 3a3b).

[10] North of the Faroes the fastest current bifurcates and partly continues into the Faroe Shetland Channel, along its western slope. Then it retroflects and merges with the Atlantic inflow along the eastern slope of the Faroe-Shetland Channel [Poulain et al., 1996]. This inflow appears as a nearly barotropic current [Burrows et al., 1999]. The flow partly branches into the North Sea and continues northward as the eastern branch of the NwAC along the Norwegian shelf edge toward the Arctic, with a branch bifurcating into the 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 Nordic Seas are identified, using the currents from Lagrangian drifters drogued at 15 m depth, whose speeds exceed 30 cm/s. Combined with hydrography, these observations show the circulation pattern illustrated in Figure 1. The western branch 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, where the AW is subducted [Mauritzen, 1996]. This northward extension of the western branch of the NwAC is in agreement with van Aken et al. [1995], revealing a northward flowing frontal jet over the Knipovich Ridge. Also from a volume flux perspective this understanding of the western branch 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 southern Norwegian Sea 3.4 Sv [Orvik et al., 2001], and over the Knipovich Ridge 3 Sv [van Aken et al., 1995].

[12] In a broad sense, the two-branch structure of the northward flowing AW appears to be established as the zonal, eastward flowing NAC splits into two northward branches (Figure 1). Both branches show topographic steering: the eastern branch flows through the Rockall Trough where a topographic trapped shelf edge current appears to be established upon reaching the Irish-Scottish shelf, and the western branch flows through the Iceland Basin toward Iceland. The observations then show that the AW enters the Nordic Seas mainly through the Iceland-Faroe gap close to Iceland and through the Faroe-Shetland Channel. Thus, the establishment of the two-branch NwAC in the Norwegian Sea-after crossing the Scotland-Greenland Ridge, can be interpreted as an extension of the two major branches in the northern North Atlantic. 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, nearly barotropic shelf edge current and the western branch is a jet in the Polar Front, guided along the dominant topographic features from the Iceland-Faroe Front into the Fram Strait. The extension of AW in the Nordic Seas is then confined to the 200–600 km wide strip between these two major pathways.

[13] In this study, we have shown for the first time the western branch of the NwAC as a continuous jet in the Polar Front, extending from the Iceland-Faroe Ridge to the Fram Strait. The topographic guidance of the frontal jet indicates a deeper current along its pathways [Svendsen et al., 1991], including the slopes of the Vøring Plateau, the Mohn Ridge and the Knipovich Ridge.

Acknowledgments

[14] Thanks are due to Sharon Lukas for performing data processing and providing figures, and to Øystein Skagseth and Alastair Jenkins for constructive criticism of the manuscript.

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