First direct estimates of volume and water mass transports across the Reykjanes Ridge

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S-ADCP referenced geostrophic velocities

Dynamic height referenced to the surface was computed from temperature, salinity and pressure at all CTDO2 stations. Their horizontal gradients were then computed between two adjacent stations and used in the thermal wind equation to compute geostrophic velocities referenced to the surface. In presence of a sloping topography, the geostrophic velocity between two CTDO2 stations cannot be computed in the bottom triangle, which is to say below the deepest common level (DCL) of a pair of stations. To estimate the geostrophic flow in the bottom triangle, Ganachaud (2003) recommended the interpolation of hydrographic properties at the shallowest station from adjacent data. We used a second-order polynomial fit suggested by Ganachaud (2003) and computed hydrographic properties below the DCL at the shallowest station by second-order interpolation of the hydrographic properties at the DCL and below. Geostrophic velocities were computed in the bottom triangle of each pair of stations based on the interpolated dynamic heights. Because the interpolation led to unrealistic velocity at the station pair 114 – 115, the geostrophic velocity in the bottom triangle was computed as decreasing to zero at the bottom.
An absolute geostrophic field was estimated by adjusting the geostrophic field referenced to the surface to S-ADCP absolute velocity measurements following Lherminier et al. (2007) and Gourcuff et al. (2011). The absolute geostrophic profile was computed by adding a constant velocity correction to the geostrophic velocity profile referenced to the surface. The correction is the difference in a reference layer Lref between the S-ADCP velocities horizontally averaged between the two stations of the pair and the geostrophic velocities referenced to the surface. To do this, it is best that the physical contents of the geostrophic and S-ADCP velocities in layer Lref be as similar as possible. This means that we should avoid depths where ageostrophic motions, caused by bottom and wind frictions, interactions of tides with bathymetry, inertial oscillations, or cyclogeostrophic terms are the most intense (Ganachaud, 1999). Selecting Lref = 600 – 1000 m led to the best agreement between the geostrophic and S-ADCP profiles. In order to remove “small scale side effects” when averaging S-ADCP data over limited horizontal and vertical distances, we filtered the original 2-km × 16-m gridded S-ADCP velocities horizontally and vertically using Lanczos filters with respective cutoff wave numbers of 1/8 km-1 and 1/400 m-1 applied consecutively. The cutoff frequencies are of the order of magnitude of the Rossby radius horizontally and of Lref thickness vertically. The filtering decreased significantly the root-mean-square (RMS) difference between the geostrophic velocities and the S-ADCP velocities. Above the BFZ and CGFZ, strong ageostrophic motions prevented robust determination of the reference velocity at the 2-km resolution allowed by the CTD sampling. A single reference velocity was thus estimated over the BFZ and CGFZ (stations 96 – 101 and 119 – 122 respectively). Figure 3.5 shows the resulting absolute geostrophic velocity section along the Reykjanes Ridge.

Transport estimates and errors

Transport across the Ridge Section is the sum of geostrophic and Ekman transports. To compute the geostrophic transport, each geostrophic velocity was assigned to a surface equal to the distance between the stations of each pair multiplied by the vertical resolution of the geostrophic velocity profile. Then, the transport for a region limited by the hydrographic stations in the horizontal, and constant depths, isopycnals, or bathymetry in the vertical, was computed as the sum of the products of the geostrophic velocities by the associated surfaces over the region considered. Figure 3.6 shows the resulting top-to-bottom integrated transport along the Ridge Section cumulated from Iceland to 50°N.
Errors for the total transports were computed as follows. The main source of error in the geostrophic transports comes from errors in the determination of the reference velocity, which are the sum of errors due to S-ADCP instrumental noise Einst_noise, S-ADCP calibration error Einst_bias and errors due to the presence of ageostrophic motions in the S-ADCP velocities Eageo. An additional error Ebott comes from the extrapolation of the hydrographic properties in the bottom triangles (Ganachaud, 2003). The Ekman transport error EEkman mainly comes from the uncertainty in the wind stress data. Accounting for all contributions, the error in the total transports for a given region Etransport can be written as: Etransport = ([ δlayersurface EEkman2 + S.Eageo2 + S.Einst_noise 2 + δlayerbottom Ebott2 ]) + S.Einst_bias (3.1) Where S is the surface of the area over which the transport is computed. δlayersurface and δlayerbottom are Kronecker deltas, indicating that those errors are taken into account only when the surface or bottom layers are included in the region. The errors are considered as random, except for the S-ADCP calibration error, which is a systematic error.
The instrumental error was estimated as the mono-ping standard deviation (0.23 m s-1), given by the manufacturer, divided by the number of S-ADCP measurements used to calculate the referenced velocity. To obtain an error for the transport, Einst_noise was multiplied by the surface S of the considered region. Over a horizontal distance of 30 km and a layer thickness of 1500 m, Einst_noise is equal to 0.01 Sv. Its contribution to the error on the top-to-bottom integrated transport along the Ridge Section was estimated at 0.7 Sv.
To estimate the error for the absolute geostrophic velocity due to ageostrophic motions, we followed Lherminier et al. (2007) and considered the length scale Lg, set at half the Rossby radius, below which ageostrophic motions dominate. About half of the variance associated with these scales was removed by filtering the S-ADCP data as described above. We computed the variance of the S-ADCP signal in the layer Lref for each Lg segment along the Ridge Section. Then, assuming a decorrelation between ageostrophic signals for one Lg segment to the other, we computed an averaged ageostrophic variance for the region considered. At the first order, this variance is due to the sum of the ageostrophic signal and instrumental noise. We thus removed the instrument noise variance from the small-scale variance to obtain an estimate of the variance of the ageostrophic motions. This variance was then divided by N, with N the number of segments Lg in the horizontal for the area considered. The square root of this value, multiplied by the surface of the considered region, gives the ageostrophic transport error S.Eageo. Typically, for a 30-km distance and a layer
thickness of 1500 m, the error was ~ 0.06 Sv. On the top-to-bottom integrated transport along the Ridge Section, this error was estimated at 1 Sv.

Water mass characterization

By referring to the literature and to properties observed along the Ridge Section (Figure 3.3, Figure 3.4), we identified four layers delimited by isopycnals that encompass nine main water masses. Layer 1, defined by σ0 < 27.52 kg m-3, contains North Atlantic Central Water (NACW) of subtropical origin (Iselin, 1936) and Sub-Arctic Water (SAW) of subpolar origin (Dickson et al., 1988). These water masses are separated by a sharp salinity front defined by approximately 34.94 at station 130 and separating NACW to the south from SAW to the north.
Further north, a weaker salinity front near station 91 separates SAW to the south from NACW to the north. There, NACW properties differ from those observed south of station 130 due to air-sea heat loss in the Iceland Basin (Figure 3.4). Layer 2, defined by 27.52 < σ0 < 27.71 kg m-3, contains Sub-Arctic Intermediate Water (SAIW) of subpolar origin and is characterized by salinity below 34.94 (Arhan, 1990), and Subpolar Mode Water (SPMW) with salinity above 34.94 and relatively low potential vorticity (q < 6.10-11 m-1 s-1). The potential vorticity was computed as q = ! ! !!, where f is the Coriolis parameter, ρ0 is the reference density and ρ is the potential density referenced to the mid-depth interval over which the vertical gradient of density is computed. The homogeneous SPMW is formed in the winter mixed layer and may also be fed by underlying intermediate waters when the winter mixed layer is deep enough (de Boisséson et al., 2012; Brambilla & Talley, 2008; Thierry et al., 2008). Layer 2 also contains Intermediate Water (IW) associated with patches of low oxygen concentration (O2 < 272 µmol kg-1) and high salinity (S > 34.94), and lies just above the isopycnal 27.71 in the northern part of the Reykjanes Ridge and in the NAC. Carried by the Gulf Stream and subsequently by the NAC, the aged IW is biogeochemically defined by minima of O2 and maxima of NO3 (Van Aken & De Boer, 1995).

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The top-to-bottom cross-ridge flow

We computed the top-to-bottom vertically integrated transports, cumulated southward from Iceland (Figure 3.6), from the absolute geostrophic velocities (Figure 3.5). Positive values correspond to eastward velocities. Starting from Iceland, the cumulative transport decreases until 56.1°N, indicating a westward flow. This flow is intensified between 62 and 59°N and above the BFZ, between 57.3 and 56.1°N, revealing two main cross-ridge flows. Between 56.1 and 53.15°N, the cumulative transport reaches a plateau, indicating weak flows of opposite direction. South of 53.15°N, the transport sharply increases corresponding to an eastward flow.
A first quantification of the cross-ridge transports was obtained by considering the cumulative transports in four regions (Figure 3.6). Region 1 (south of 53.15°N) delimits the eastward flowing NAC, but also encompasses some of the westward flow of ISOW at the CGFZ. In region 1, the top-to-bottom integrated transport was estimated at 40.2 ± 2.3 Sv. As revealed by the absolute geostrophic velocity section (Figure 3.5), the NAC divides in two branches that are respectively aligned with the FFZ, centered at 50.5°N, and the CGFZ, centered at 52.5°N. The top-to-bottom transports of the two NAC branches over the FFZ and CGFZ were estimated at 22.8 ± 1.1 Sv and 17.4 ± 1.7 Sv, respectively. From the northern boundary of region 1 to the Icelandic slope, relatively intense westward flows alternate with relatively weak eastward flows (Figure 3.5). Region 2 (between 53.15 and 56.1°N) is located south of the BFZ and is characterized by no net flow (0 ± 1.4 Sv, Figure 3.6). The top-to-bottom transports in the two main pathways at the BFZ (region 3 between 56.1 and 57.3°N) and at 59 – 62°N were estimated at -8.0 ± 0.5 Sv and -13.6 ± 0.8 Sv, respectively. The overall transport in regions 2 – 4, which corresponds to the intensity of the subpolar gyre, was estimated at – 21.9 ± 2.5 Sv.

Water mass transports across the Reykjanes Ridge

To quantify the contributions of the water masses to the cross-ridge flow, we computed their transport according to the water mass definition (Table 3.1) in the density layers they belong to. We then cumulated these transports from Iceland to 50°N (Figure 3.7) and in the four regions (Figure 3.8). Layer 1 thickness varies strongly with latitude, from ~ 600 m at 50°N to ~ 200 m north of 53°N (Figure 3.3). Accordingly, the bulk of the transport in this layer occurs to the south of the section in the NAC (region 1). Here, the eastward transports of SAW and NACW were estimated at 8.0 ± 0.2 Sv and 4.1 ± 0.2 Sv, respectively (Figure 3.7, Figure 3.8). No NACW was transported in region 2, but a westward flow of NACW was also observed in regions 3 and 4 (-2.0 ± 0.1 Sv).

Table of contents :

Contents
1 Introduction
1.1 Role of the North-Atlantic Ocean on the climate system
1.2 Mean circulation in the northern North-Atlantic Ocean
1.3 State of the art of North-Atlantic water masses
1.3.1 SubPolar Mode Water
1.3.2 Intermediate Water
1.3.3 Labrador Sea Water
1.3.4 Icelandic Slope Water
1.3.5 Iceland-Scotland Overflow Water
1.4 Impact of the topography on the North-Atlantic SubPolar Gyre: some key elements
1.4.1 Impact of topographic features on the flow
1.4.2 The Reykjanes Ridge
1.4.3 Cross-ridge flow
1.4.4 Along-ridge flow
1.5 The Reykjanes Ridge Experiment Project
1.6 Aims of the PhD thesis
2 Data and methods
2.1 Data
2.1.1 CTDO2 data
2.1.2 Lowered-ADCP data
2.1.3 Shipboard-ADCP data
2.1.3.1 S-ADCPs configuration during RREX2015 cruise
2.1.3.2 S-ADCP data processing
2.1.3.3 Instrumental errors
2.1.3.4 Conclusion
2.1.4 The AVISO data set
2.1.5 Atmospheric reanalysis
2.2 Computation of geostrophic transports
2.2.1 General Principle
2.2.2 Bottom triangles
2.2.3 Computation of the absolute reference velocities
2.2.4 Determination of the absolute reference layer
2.2.5 Conclusion
3 First direct estimates of volume and water mass transports across the Reykjanes Ridge
3.1 Introduction
3.2 Data and Methods
3.2.1 Description of the cruise
3.2.2 Data sets
3.2.3 S-ADCP referenced geostrophic velocities
3.2.4 Transport estimates and errors
3.2.5 Water mass characterization
3.3 Results: transports across the Reykjanes Ridge
3.3.1 The top-to-bottom cross-ridge flow
3.3.2 Water mass transports across the Reykjanes Ridge
3.4 Discussion
3.4.1 Circulation across the Reykjanes Ridge
3.4.2 NAC water masses
3.4.3 Subpolar Mode Water and Intermediate Water
3.4.4 Iceland-Scotland Overflow Water
3.4.5 Water mass transformations
3.5 Conclusion
4 Formation and evolution of the East Reykjanes Ridge Current and Irminger Current
4.1 Introduction
4.2 Data and Methods
4.2.1 Data sets
4.2.2 S-ADCP referenced geostrophic velocities and transport estimates
4.2.3 Water mass characterization
4.3 Results: Connections between the Iceland Basin and the Irminger Sea
4.3.1 Horizontal and vertical structures of the along-ridge currents
4.3.2 Hydrography of the eastern flank of the Reykjanes Ridge
4.3.3 Hydrography of the western flank of the Reykjanes Ridge
4.3.4 Circulation in density layers
4.4 Discussion
4.4.1 Large-scale circulation of the ERRC
4.4.2 Large-scale circulation of the IC
4.5 Conclusion
5 Deep through-flow in the Bight Fracture Zone
5.1 Introduction
5.2 Data and Methods
5.2.1 Bathymetry of the Bight Fracture Zone
5.2.2 Hydrographic sections
5.2.3 Deep-Arvor floats
5.3 Results: Through-flow in the Bight-Fracture Zone
5.3.1 The eastern sill of the Bight Fracture Zone
5.3.2 The rift valley of the Reykjanes Ridge
5.3.3 Exit of ISOW toward the Irminger Sea
5.3.4 Circulation of ISOW through the BFZ
5.3.5 Deep-Arvor float trajectories in the BFZ
5.4 Discussion
5.5 Conclusion
6 Conclusions and perspectives
6.1 Estimation of geostrophic transports
6.2 Intensity and structure of the subpolar gyre across the Reykjanes Ridge
6.3 Link between distribution of the cross-ridge flow and large-scale circulation of the subpolar gyre
6.4 Circulation and evolution of Iceland-Scotland Overflow Water across the Reykjanes Ridge
6.5 Formation, connection and evolution of the East Reykjanes Ridge Current
6.6 Connections between Irminger Current and cross-ridge flow
A Sequence of operations during the RREX2015 cruise
B Résumé en Français
B.1 Objectifs de la thèse
B.2 Données et méthodes
B.3 Première estimation directe de transports de volume et de masse d’eau à travers la dorsale de Reykjanes
B.4 Formation et évolution du ERRC et IC
B.5 Circulation profonde dans la zone de fracture Bight
Bibliography

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