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Major Pathways of Atlantic Water in the Northern North Atlantic and Nordic GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 19, 1896, doi:10.1029/2002GL015002, 2002 Major pathways of Atlantic water in the northern North Atlantic and Nordic Seas toward Arctic Kjell Arild Orvik Geophysical Institute, University of Bergen, Bergen, Norway Peter Niiler Scripps 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 in Norwegian Sea. Orvik et al. [2001] identified these two the northern North Atlantic and Nordic Seas are identified branches as an eastern branch which acts as a nearly as current speeds above 30 cm/s, using 1014 Lagrangian barotropic shelf edge current, and a western branch which drifters combined with previously published hydrography. is a topographically steered jet in the Polar Front (the The inflow over the Scotland-Greenland ridge and transition zone between Atlantic and Arctic water in the establishment of the two-branch Norwegian Atlantic Nordic Seas). Current (NwAC) are described in light of the circulation [3] This study is based on about twice the number of in the northern North Atlantic. The western branch of the observations available to earlier studies, and will represent a NwAC appears as a jet in the Polar Front, topographically synthesis and extension of previous and recent findings in guided from the Iceland-Faroe Front, through the Nordic the northern North Atlantic, and the Nordic Seas. To the Seas toward Fram Strait. The eastern branch starts as a drifter data we add hydrography, and this leads to some shelf edge current above the Irish-Scottish continental revision of previous conclusions, particularly in the Nordic shelf, and after passing through the Faroe-Shetland Seas; [e.g. Poulain et al., 1996]. A primary purpose of this Channel, it continues northward along the Norwegian paper is to substantiate the overall circulation pattern in shelf edge toward the Arctic, with a branch bifurcating into Figure 1, where schematics of major pathways of near- the Barents Sea. The NwAC appears to maintain its two- surface AW are shown superimposed on the sea surface branch structure throughout the Nordic Seas, with the temperature (SST) obtained from an AVHHR image. Atlantic water confined to a 200–600 km wide Repeated references will be made to this figure, where the wedge. INDEX TERMS: 4512 Oceanography: Physical: establishment and pathways of the two-branch NwAC is Currents; 4532 Oceanography: Physical: General circulation; 4528 emphasized in light of the inflow pattern and connection Oceanography: Physical: Fronts and jets; 4536 Oceanography: with the northern North Atlantic. Only the overall circu- Physical: Hydrography. Citation: Orvik, K. A., and P. Niiler, lation pattern obtain during the 1989–2001 observation Major pathways of Atlantic water in the northern North Atlantic period is described here. Variability on seasonal and annual and Nordic Seas toward Arctic, Geophys. Res. Lett., 29(19), 1896, timescales is not taken into account. doi:10.1029/2002GL015002, 2002. 1. Introduction 2. Data and Data Processing [4] The bulk of the data set was compiled using the [2] In a global warming perspective, the inflow of warm and saline water from the northern North Atlantic into the SVP Lagrangian drifters, drogued at 15 m depth. Their Nordic Seas (Norwegian, Greenland and Iceland Seas), and data collection, transmission and water following charac- its extension and flow northward toward higher latitudes, is teristics are described by Niiler [2001]. The complete data of great importance. This study emphasizes the near-surface set is described by Reverdin et al. [2002]. A composite pathways of Atlantic Water (AW) in the northern North plot of the drifter data density and deployment sites is Atlantic and Nordic Seas (Figure 1), and is based on 1014 shown in Figure 2. The figure shows a high data density Lagrangian drifters released during 1989–2001. From a all over the northern North Atlantic, in particular near the subset of the data used here, previous studies have been Gulf Stream and its eastern and northern extensions. For published for the near-surface circulation in the following the Nordic Seas, the data density is high around Iceland, areas: the Nordic Seas [Poulain et al., 1996], Icelandic the Scotland-Greenland inflow area and in the eastern waters [Valdimarsson and Malmberg, 1999], and south of Norwegian Sea, covering the AW. The major northward the Iceland Faroe Ridge [Fratantoni, 2001]. Poulain et al. pathways of AW are identified by selecting the fastest [1996] revealed for the first time a two-branch structure of moving drifters, with 24-hour average speed greater than the Norwegian Atlantic Current (NwAC) in the southern 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 Copyright 2002 by the American Geophysical Union. fields. The strong currents are defined by many different 0094-8276/02/2002GL015002$05.00 drifters that have entered regions of strong flow, and are 2 - 1 2 - 2 ORVIK AND NIILER: MAJOR PATHWAYS OF ATLANTIC WATER 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. not necessarily continuous pathways in which any partic- Trough. These two branches form the major northward ular drifter remains over a great distance. pathways of AW in the northern North Atlantic [Fratantoni, 2001]. [6] The swift-flowing branch that continues northeast- 3. Results ward through the Rockall Trough, upon encountering the 3.1. Northern North Atlantic Pathways Toward the Irish-Scottish shelf, appears to be constrained as a topo- Nordic Seas graphically trapped shelf edge current. This current increases [5] The strong currents shown in Figure 3 clearly illus- its speed along the Scottish slope toward the Faroe-Shetland trate the well-known flow fields related to the western Channel [Burrows et al., 1999], where it enters the Norwe- boundary current system of the Gulf Stream with a bifurca- gian Sea. Through the Iceland Basin, the flow appears to be tion of the eastward flowing Gulf Stream into its continu- concentrated in a wider, eddy structured western branch ation as the Azores Current and the northward flowing (Figures 3a–3b), which continues northeastward toward North Atlantic Current (NAC). The core of the NAC can be southeastern Iceland. In this area the flow appears to traced northward east of Newfoundland (Flemish Cap) into bifurcate into a northward and a southward branch. Its major the ‘‘northwest corner’’ where it retroflects in an almost northward part crosses the Iceland-Faroe Ridge close to complete circle before separating from the western boun- Iceland through the ‘‘western valley’’, and after passing dary at about 50–52°N. The core of the NAC continues the ridge turns eastward and forms the Iceland-Faroe frontal zonally eastward toward a gap in the Mid-Atlantic Ridge jet [Perkins et al., 1998]. [Carr and Rossby, 2001]. Farther east it splits into two [7] The bifurcation of the flow southeast of Iceland and a major northeastward flowing branches; one through the return flow of AW around the Reykjanes Ridge, consists of Iceland Basin and the other one through the Rockall a strong current southward along the eastern slope, and ORVIK AND NIILER: MAJOR PATHWAYS OF ATLANTIC WATER 2 - 3 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. Figures 3a–3b 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 Figure 2. Data density as 24-hour average observations in follow the topographic slope of the Vøring plateau [Poulain 2° latitude by 6° longitude bins computed from 1014 et al., 1996] toward Jan Mayen. As illustrated schematically Lagrangian drifters. Release locations are indicated as black in Figure 1, the observations show that the major pathway dots for the 966 SVP-drifters drogued at 15 m depth, and turns northeastward along the slope of the Mohn Ridge. blue dots for the 48 Meldrum drifters at 50 m. This pathway then appears to turn northward west of Bear Island and continues along the Knipovich Ridge toward the subsequently northeastward flow along the western slope of Fram Strait. the ridge. It then continues westward over the Denmark [9] A hydrographic section across the Lofoten basin Strait and bounds the southwest flowing East Greenland along 71°N[Mauritzen 1996, Figure 9] shows the AW as Current over the continental shelf, as a concentrated jet on a 600 km wide, 800 m deep slab, with a distinct Polar Front Figure 3a.
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