Directly-measured mid-depth circulation in the Northeastern North Atlantic Ocean A. S. Bower*, B. Le Cann†, T. Rossby‡, W. Zenk§, J. Gould||, K. Speer¶, P. L. Richardson*, M. D. Prater‡, H-M. Zhang#.
* Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA; † Laboratoire de Physique des Océans, CNRS, UFR Sciences, BP809, 29285 Brest, France ; ‡ Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881, USA; § Institut für Meereskunde, Christian-Albrechts-Universität, D-24105 Kiel, Germany; || Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK; ¶ Department of Oceanography, Florida State University, Tallahassee, FL 32306, USA; # Graduate School of Oceanography, University of Rhode Island. Now at National Climatic Data Center, NOAA, Asheville, NC 28801, USA.
2.Hydrographic Distributions at Two Float Levels 3. Some Individual Float Tracks from Thermocline 4. Mean Streamfunction Fields at Thermocline and 5. Funneling of Floats over Gaps Hydrographic Distributions at Two Float Levels Mean Streamfunction Fields at Thermocline and LSW Levels Summary: and LSW Levelss LSW Levels in the Mid-Atlantic RidgeFunneling of Floats over Gaps in Mid-Atlantic Ridge Pressure on σ = 27.50 Potential Temperature on σ = 27.50 Some Individual Float Tracks from Thermocline and LSW Levels N As part of a large international effort to directly observe the N θ θ o o 65˚N 0 65 o Top panel shows close-up view 56˚N
>10 3000 N 30 65 65 circulation throughout the subpolar North Atlantic Ocean during 3000 IFR 100 9.5 1000 00 2000 200 0 of the southern Reykjanes Ridge PR IrB TH 20 WOCE, several research groups from the U.S., the U.K., Germany RR 200 9 and fracture zone bathymetry IcB 8.5 I IFR 3000 300 8 G 3000 and France collaborated in a major initiative to measure the along with example trajectories 3000 60 o IC 2000 400 7.5 CGFZ RP N RR 00 FI 2 of individual floats on the 27.5σ 7 θ 3000 absolute velocity at two levels in the northeastern North Atlantic 500 IrB N N LS 200 3000 o o RT 6.5 IcB surface that were funneled over 3000 using acoustically-tracked subsurface floats. The northeastern CGFZ 600 6 D 3000 30 55 55 PB LB 52˚N the Charlie-Gibbs and Faraday 0 700 5.5 1000 North Atlantic is important to the thermohaline circulation as this 55 o HB 60˚N FFZ 5 N Fracture Zones as they crossed 800 RB 1000 4.5 00 0 3000 is where warm subtropical water is transported to high latitudes. A 1 900 4 RT the Mid-Atlantic Ridge. Middle FFZ
3000 N N o o BB 1000 3.5 E total of 223 float tracks, representing 328 float-years of data, were MAR panel summarizes these results, 3 CGFZ ABR 2000 45 45 PB 1100 50 o 0 2.5 showing the distribution of A 20 combined to generate maps of mean absolute velocity and eddy N 0 o o o o o 1200 o o o o o 2 latitudes where all upper-level kinetic energy at the thermocline and Labrador Sea Water levels. 50 W 40 W 30 W 20 W 10 W 50 W 40 W 30 W 20 W 10 W 100 48˚N FFZ floats crossed the Mid-Atlantic Salinity on σ = 27.50 B 55˚N Oxygen on σ = 27.50 3000 N We find that most of the mean flow transported northward by the N 3000 o θ o θ Ridge. Floats in the North 36 7.25 45 o G 20 N 30 FC 0
F 0 3
0 65 North Atlantic Current at the thermocline level recirculated within 65 7 NWC Atlantic Current at the upper SA 35.8 OX C the subpolar region, and relatively little entered Rockall Trough or 6.75 level crossed the ridge eastbound 40˚W 36˚W 32˚W 28˚W 24˚W 35.6 the Nordic Seas. Saline Mediterranean Water reached high latitudes 6.5 preferentially over the Charlie- 35.4 6.25 40 o N MAR IbB Gibbs Fracture Zone (CGFZ; not by continuous, broad-scale, mean advection along the eastern 35.2 6 50˚N N N 10 o o 31/61), the Faraday Fracture 5.75 Eastbound boundary as previously described, but by a combination of narrow 35 200 55 55 5.5 Zone (FFZ; 11/61), and to a lesser 50 o o W slope currents and mixing processes. At the Labrador Sea Water 34.8 5.25 W 5 10 NAC extent, the Maxwell Fracture loats 5 o o 1000 34.6 40 W o 20 W Zone (MFZ; 4/61) (red bars; level, a strong, topographically-constrained current associated with 65 o 30 W 0 4.75 20
N # of f N 34.4 N 0 o o 45˚N float depths 200-800 m). The the overflow of dense water from the Norwegian Sea flowed around 4.5 45 34.2 45 0 distribution of float deployment 4.25 Westbound the northwestern Iceland Basin along the continental slope and 00 5 2 o o o o o 34 o o o o o 4 100 latitudes, unfilled bars, is 50 W 40 W 30 W 20 W 10 W 50 W 40 W 30 W 20 W 10 W 60 o Reykjanes Ridge, and closed counterclockwise recirculations existed N significantly different from the adjacent to this boundary current. At both levels, currents crossed distribution of crossing latitudes, 0 50˚W 40˚W 30˚W 20˚W 10˚W 5˚W the Mid-Atlantic Ridge, eastbound and westbound, preferentially B' suggesting that the floats were MFZ FFZ CGFZ BFZ Pressure on σ = 27.77 Potential Temperature on σ = 27.77 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 _> 35 N N 55 o 3 2 1000 o over deep gaps in the ridge. The latter result demonstrates that o θ θ N A' funneled over the various 800 C' (10 m /s) 65 65 fracture zones. North of 53°N, seafloor topography can constrain even upper ocean circulation PR 900 TH > 5 2000 floats crossed the Reykjanes 4.8 1000
patterns, possibly limiting the ocean’s response to climate change. Depth (m) 50 o Ridge from east to west at both 4.6 N 3000 1100 65˚N 0
0 levels (blue for upper-level, green 4.4 2 1200 N N E' for lower level) mainly through 4000 o o 4.2 D' 1 G' 00 200 500 1300 15 46 48 50 52 54 56 58 60 55 55 the Bight Fracture Zone (BFZ) 4 45 1. RAFOS and MARVOR Float Trajectories in the o 1500 1400 N Latitude 3.8 F' and two unnamed gaps near RAFOSSubpolar and MARV NorthOR FloatAtlantic Trajectories in the Subpolar North Atlantic 1500 3.6 53.5°N and 55°N. North of 53°N, the gaps are not aligned east-west and the distribution of float 200 200 N N
N 1600 o o 3.4 crossing latitudes is therefore somewhat wider around these gaps. Bottom panel shows depth along o 40 o 45 45 1700 3.2 N 60˚N the ridge crest.
1800 3 65 I 50oW 40oW 30oW 20oW 10oW 50oW 40oW 30oW 20oW 10oW 0 Eddy Kinetic Energy from Float Observations 20 G o N Salinity on σ = 27.77 Oxygen on σ = 27.77 50 o W o N N 6. Eddy Kinetic W 0 o o θ θ 10 35.5 7 20 65 40 o oW 65 65 W o 20 6.75 Energy from Float 700 35.4 30 W SA OX 400 6.5 200 35.3 55˚N Observations 100 6.25 50 35.2 6 Eddy kinetic energy (EKE; cm2s-2) from 25 35.1 5.75 Representative float trajectories on the 27.5σ surface (top panel) and at the 1500-1750 m level 10 N N θ o o the float data for the 27.5 σ surface 5 35 5.5 (bottom panel) Markers ‘x’ and ‘.’ indicate launch and surface positions, respectively. Floats A, θ 55 55
(top panel) and the 1500-1750 m level N
5.25 o 34.9 B and C in the upper panel, all launched in the North Atlantic Current, indicate the eastward, streamline 1 5 then northeastward flow along the front with average float speed of 10-15 cm-1 s . Floats A and 50˚N spacing (km) (bottom panel). These are the first high- 55 34.8 200 400 4.75 resolution maps of subsurface EKE for N N N B were both funneled across the Mid-Atlantic Ridge at the Charlie-Gibbs Fracture Zone, but at 34.7 o o o 4.5 depths more than 1000 m above the ridge crest (see also Figure 5). In the Iceland Basin, their the subpolar North Atlantic. EKE var- 45 45 34.6 4.25 2 paths diverged: float A looped back westward and then turned northward along the western ies by almost two orders of magnitude 55 o o o o o 34.5 o o o o o 4 4 2 -2 Ir 50 W 40 W 30 W 20 W 10 W 50 W 40 W 30 W 20 W 10 W flank of the Reykjanes Ridge while float B continued northeastward through the Iceland Basin, within the domain, from >400 cm s in 45˚N N 2 -2 o exhibiting significant eddy motions with speed occasionally exceeding 30 cm -1s . Float C was guided 6 the western boundary NAC to ≤10 cm s 45
1 via another deep channel across the ridge, the Faraday Fracture Zone (FFZ), then also turned 50 over the Mid-Atlantic Ridge south of 8 0 20
0 the CGFZ. EKE decreases sharply along
northeastward into the Iceland Basin where it, too, showed relatively strong eddy activity. Float D, velocity scale (cm/s) 10 50oW 40oW 30oW 20oW 10oW which was constrained by strong bathymetric slopes around the Irminger and Labrador Basins, the path of the NAC where it leaves the These maps of mean pressure, potential temperature, salinity and dissolved oxygen (O2) on N o -1 50˚W 40˚W 30˚W 20˚W 10˚W 5˚W western boundary (52°N) and branches two density surfaces corresponding to the two levels of the float data (from HydroBase) set the traces out the continuation of the cyclonic subpolar gyre in the west, with speeds up to 40 cm s 65 FC 1 2 3 4 5 6 7 8 9 10 11 12 into several weaker currents. Distinct hydrographic context for the float observations. Upper panels:σ θ = 27.50, which varies in depth (west of Greenland). Contrary to expectation, floats deployed near the eastern boundary north of 3 2 (10 m /s) local EKE maxima occur in the cen-
N from near the sea surface in the northwestern corner of the domain to ~1000 m near the eastern 50°N typically did not enter Rockall Trough, but drifted into the Iceland Basin along the southern 2 -2 o -1 ter of the Iceland Basin (~200 cm s ), boundary. Lower panels: σθ = 27.77, which ranges in depth from 1100 m in the northwest to 1800 m and western edge of the Rockall Plateau with speeds generally < 15 cm s (float E). Floats launched in the southeast. south of that parallel typically revealed persistent eddy activity and drifted out into the ocean Mean streamfunction for the subpolar North Atlantic from subsurface float data at two levels. a) over the western slope of the Reykjanes 45 Ridge (>200 cm2s-2), and southwest of interior (float F). However, continuous northward flow over and along the steep continental slope upper level, thermocline, density σθ = 27.5; b) lower level, Labrador Sea Water, depth 1500-1750 2 -2 IP At the upper level, the warm subtropical thermocline waters spread northward mainly in was intermittently observed, with speeds generally < 10 cm s-1 (float G). Geographic labels are as in m. See abstract for summary of results. Mapping of gridded float velocity into streamfunction Rockall Plateau (>100 cm s ). These N fields was done using standard objective analysis techniques, with a decorrelation scale of 100 km. maxima also appear in maps of surface o
the northeastern North Atlantic. Two sources for this warm water have been proposed: the Figure 2, with the addition of Hatton Bank (HB) and Faroe Islands (FI). 55 o o o o o Mediterranean Water (MW) in the southeast (θ > 10°C) that enters the North Atlantic through Arrowheads show the direction of flow along the contours. The nomogram in the lower panel gives EKE, with slightly higher magnitude 50 W 40 W 30 W 20 W 10 W -1 2 -2 the Strait of Gibraltar, and the water transported by the Gulf Stream and NAC from the western At the lower level (lower panel), a strong and persistent flow is seen to begin on the western flanks speed in cm s as a function of line separation (solid line for upper panel, dashed for lower panel), (200-400 cm s ), but this subsurface Trajectories of all 223 acoustically-tracked, eddy-resolving subsurface drifting floats launched in basin (5.5°C< θ < 6.5°C). At the lower level, the large-scale temperature contrast from southeast to of Hatton Bank in the northern Iceland Basin and continue southwest of the Faroe Islands towards and the contours give volume transport per unit depth. Note that two higher-resolution contours, map reveals that this variability extends 2 -1 the northern North Atlantic in 1993-1995 and 1996-2001, representing 328 float-years of data. The northwest illustrates the competing influence of Deep Mediterranean Water θ( > 7°C) and LSW the eastern flank of the RR (float A’). This strong flow pattern is accelerated by the Iceland-Scotland 24,000 and 26,000 m s , have been added to the upper panel near the eastern boundary. Labeled through the water column. This is also N floats were deployed at two levels: an upper-ocean thermocline density surface ( = 27.5; red tracks; features are: North Atlantic Current (NAC), Northwest Corner (NWC), Irminger Current (IC). apparent in the lower level EKE pattern, o σθ (θ < 3.4°C). LSW spreads into the eastern North Atlantic across the Mid-Atlantic Ridge at ~50°N. overflow of Nordic waters through the Faroe Bank Channel. Track B’ typifies the organized flow see also Figure 2), and an isobaric layer at 1500-1750 m depth, the core layer of Labrador Sea Water The influence of warmer saltier Iceland Scotland Overflow Water, which enters the northern Iceland of these waters as they cross over the RR into the Irminger Basin. Track C’ traces out the cyclonic which generally tracks that of the upper 45 layer with magnitude about 1⁄2-1/10 that (LSW, blue tracks). The float data were collected as part of the U.S. Atlantic Climate Change Experi- Basin through the Faroe Bank Channel, is apparent along the continental slope south of Iceland circulation at the southern end of the Iceland Basin. It crosses briefly into the Irminger Basin before Acknowledgements: This research was supported by the U.S. National Science Foundation, the Deutsche Forschungsgemeinschaft of the upper level. The maximum in the 50oW 40oW 30oW 20oW 10oW ment (ACCE – a component of the World Ocean Circulation Experiment), the European EURO- and on both flanks of the Reykjanes Ridge θ( > 3.8°C). Geographic abbreviations are: Labrador Sea turning east through the CGFZ into the southern Iceland Basin. Floats D’, and E’ show the spread in Germany, Institut Français de Recherche pour l’Exploitation de la Mer, Service Hydrographique et Océanographique de la FLOAT Project, the German Sonderforschungsbereich (SFB) Subpolar Program, the French Actions de (LS), Irminger Basin (IrB), Reykjanes Ridge (RR), Iceland Basin (IcB), Iceland-Faroes Ridge (IFR), of the LSW into the Iceland Basin and south along the eastern slope of the MAR, respectively. Float Marine, Centre National de la Recherche Scientifique in France, and the European Commission EUROFLOAT Project. We are Iceland Basin at the lower level has a 2 -2 Recherche sur la Circulation dans l’Atlantique Nord-Est (ARCANE) during 1996-2001, and the U.S. North Rockall Plateau (RP), Rockall Trough (RT), Porcupine Bank (PB), Bay of Biscay (BB), Azores-Biscay F’ extends this southward movement much farther south while float G’ indicates the slow drift very grateful for the help and support of all the technicians and research assistants who expertly contributed to this project. magnitude of about 100 cm s . On the other hand, the upper level EKE maximum on the western H. Hunt Furey was instrumental in the preparation of the figures. Atlantic Current Study (NACS) in 1993-1995. The floats were submerged from 1.5-5 years and Rise (ABR), Mid-Atlantic Ridge (MAR), Faraday Fracture Zone (FFZ), Charlie-Gibbs Fracture Zone south in the Iberian Basin. slope of the Reykjanes Ridge is not mirrored at the deeper level, perhaps due to the stronger baro- tracked using an array of 18 moored sound sources that was maintained by the cooperating groups. (CGFZ). For more information, see: Bower, A.S., B. Le Cann, T. Rossby, W. Zenk, J. Gould, K. Speer, P. L. Richardson, M. D. Prater and clinicity of the Irminger Current. Further study is needed to explain the structured distribution of H.-M. Zhang, 2002. Directly-measured, mid-depth circulation of the northeastern North Atlantic Ocean. Nature, 419, 603-607. Labeled landmarks include Greenland (G), Iceland (I), Ireland (Ir) and the Iberian Peninsula (IP). EKE in this region.