Sujet de thèse 2015-2018

- Titre du sujet de thèse proposé : Caractérisation de la circulation autour, au-dessus et à travers (via des zones de fracture) la dorsale de Reykjanes et des processus dynamiques associés - Directeur de thèse : H. Mercier

- Co-directeur de thèse : V. Thierry

- Laboratoire/unité, département d’accueil : Laboratoire de Physique des Océans

- Ecole doctorale de rattachement (préciser si l’équipe est équipe d’accueil d’une école doctorale, si oui le sujet sera t-il éligible à l’appel d’offre de l’ED ?) : Ecole Doctorale des Sciences de la Mer

- Cofinancement envisagé/obtenu: 50% Ifremer (acquis) ; 50% UBO, Région Bretagne, ou Labex Mer (demandés)

- Employeur envisagé : IFREMER

- Résumé et mots-clés en français: La dorsale de Reykjanes, une structure topographique majeure de l'océan Atlantique Nord située au sud de l'Islande, est localisée dans un endroit stratégique pour la cellule méridienne (MOC) qui est le principal mécanisme pour le transport méridien de chaleur, le sel et le carbone dans cet océan. Elle influence en effet les principaux chemins suivis par les deux branches de la MOC et est la porte vers les zones de convection profonde pour les eaux chaudes de sa branche supérieure. Cependant, la structure de l'écoulement le long et au-dessus de la dorsale et les régimes dynamiques dominants sont encore mal caractérisés et compris. En outre, les modèles numériques utilisés à la fois pour les scénarios climatiques (1 ° à 1/4 °) et pour les prévisions opérationnelles à court terme (1/12 °) présentes des déficiences dans le gyre subpolaire. Ceci est probablement dû à une mauvaise représentation des interactions courant topographie autour de la dorsale de Reykjanes, qui tend à apporter trop d'eau chaude et salée vers le bassin d’Irminger et la mer du Labrador. Cependant, la connaissance de la structure des écoulements et des processus à l’œuvre à partir d'observation in situ, qui servirait de référence pour les modèles de circulation générale, est encore limitée dans cette région.

Dans le cadre du projet RREX et en utilisant des données existantes (Argo, altimétrie, projet OVIDE) et de nouvelles données (hydrographie, ADCP) acquises au cours d'une campagne hydrographique prévue en Juin 2015, l'objectif de cette thèse est (1) de documenter la circulation autour et au-dessus de la ride de Reykjanes et (2) d'identifier les processus dynamiques contrôlant la connexion entre les deux côtés de la ride.

Mots clés : interaction courant-bathymétrie, analyse de données, hydrologie, processus dynamiques, Océan Atlantique Nord

- Titre, résumé et mots-clés en anglais Characterization of the circulation around, above and across (through fracture zones) the Reykjanes Ridge and of the associated dynamical processes The Reykjanes Ridge, a major topographic feature of the North-Atlantic Ocean lying south of Iceland, is localized in a strategic place for the Meridional Overturning Cell (MOC) that is the primary mechanism for the meridional transport of heat, salt and carbon in this ocean. It influences the main paths followed by the two limbs of the MOC and is the gate towards the deep convection areas for the warm water masses of the upper limb. However, the structure of the flow along and over the ridge and

the prevailing dynamical regimes are still poorly characterized and understood. In addition, numerical models used both for climate scenarios (1° to 1/4°) and operational short-term forecasts (1/12°) present deficiencies in the subpolar gyre. This is possibly due to a wrong representation of flow-topography interactions around the Reykjanes Ridge, which tends to bring too much warm and salty water to the Irminger and Labrador Seas. However, knowledge from in situ observation of flow pathways and processes at work that would serve as benchmark for general circulation models are still missing in this region.

As part of the RREX project and using existing dataset (Argo, altimetry, OVIDE project) and new dataset (hydrography, ADCP) acquired during an hydrographic survey planned in June 2015, the objective of this PhD is (1) to document the circulation around and over the ridge and (2) to identify the processes controlling the dynamical connections between the two sides of the ridge.

Keywords: current-topography interactions, data analysis, hydrography, dynamical processes, North-Atlantic Ocean

- Profil de candidature souhaitée Master 2 en océanographie physique ou météorologie dynamique. Master degree in physical oceanography or dynamical meteorology. Projet scientifique Scientific context Topographic features such as ocean basin geometry and ocean ridges have a major influence on the oceanic circulation. Such complex current-topography interactions prevail in the subpolar gyre of the North-Atlantic Ocean (Figure 1) and particularly near the Reykjanes Ridge, a major topographic feature, which lies in a central position in the subpolar gyre and greatly influences the exchanges between the eastern and western parts of the gyre (Figure 1). The Reykjanes Ridge is localized in a strategic place for the Meridional Overturning Cell (MOC), the primary mechanism for the spreading of heat, salt and carbon and other biogeochemical properties (Mercier et al, 2013; Perez et al, 2013). Indeed, the Reykjanes Ridge lies at the crossroads of the warm and salty subtropical water masses transported by the upper limb of the MOC and the cold and less salty subpolar water masses transported by the lower limb of the MOC. It is also the gateway to the deep convection areas for the warm and salty water masses of the upper limb.

Figure 1: Schematic of the horizontal circulation in the North-Atlantic Ocean. The surface/subsurface circulation is in red/orange. IC and NAC: Irminger Current and North-Atlantic Current. The deep currents are in blue. ISOW and DWBC: Iceland Scotland Overflow Water and Deep Western Boundary Current. The green arrows represent cold and fresh surface coastal currents: the Labrador, East and West Greenland Currents (LC, EGC, WGC). The. Labrador Sea Water (LSW) spreading is represented in purple. The northward Irminger Current (IC) flows on the western side of the Reykjanes Ridge. Position of the OVIDE (black dots), AR7E (yellow dots), and 60°N (green dots) sections are displayed.

LSW

LSW

RRMW

ISOW

ISOW

Reykjanes Ridge

ISW

The Reykjanes Ridge clearly imposes a constraint on the spatial pattern of the subpolar gyre circulation (Bower et al, 2002) and water masses (Thierry et al, 2008). From bottom to top, a general anticyclonic circulation is observed around the ridge (Figs. 1, 3). At depth, the Iceland-Scotland Overflow Water (ISOW) is carried southwestward on the eastern flank of the ridge and most of it is exported into the Irminger Sea where it flows northeastward below the Irminger Current. At shallower level, the surface-intensified Irminger Current flows northeastward on the western side of the ridge. On the eastern side of the ridge, the southwestward East Reykjanes Ridge Current is more barotropic (Figs. 1, 3). The Icelandic Slope Water (ISW), a warm and salty water mass lying above the ridge (van Aken and de Boer, 1995; Read, 2001), separates two pools of Labrador Sea Water that are observed at intermediate depth on each side of the Reykjanes Ridge. The Reykjanes Ridge Mode Water (RRMW), a Subpolar Mode Water variety, is observed on top of those water masses (Brambilla et al 2008, Thierry et al 2008) (Figure 2).

Figure 2: Mean 2002-2010 salinity along the Ovide line near the Reykjanes Ridge. The main water masses observed there are the Iceland Scotland Overflow Water (ISOW), Reykjanes Ridge Mode Water (RRMW), Labrador Sea Water (LSW), and Icelandic Slope Water (ISW).

Figure 3: Mean 2002-2010 circulation along the Ovide near the Reykjanes Ridge. Northeastward (southwestward) flow is in red (blue). The red and blue lines indicate the region where the flow was northeastward (southwestward) at each Ovide cruise. The gray lines represent contours of constant density. IC and ERRC stands for Irminger Current and East Reykjanes Ridge Current, respectively.

As shown by Ovide data (Fig. 1), the currents form a relatively large baroclinic flow on the western side of the ridge (the Irminger Current), but they are narrow and barotropic on the eastern side where they form a banded structure (Fig. 3). The narrow southwestward East Reykjanes Ridge Current flows within 100km from the top of the ridge. In Fig. 3, the currents on the two sides of the ridge have similar intensities, which is in contradiction with Reverdin et al. (2003) and Flatau et al. (2003) who observed, in analyzing surface drifter data, a weak southwestward flow on the eastern flank of the ridge and an intense Irminger Current flowing northeastward west of the ridge. Interestingly, the summer mean velocity field over 2002-2010 in Fig. 3 shows that the narrow southwestward current in the upper layers of the water column is bounded to the east by a narrow (100km wide) northeastward flow. Other evidence of such northeastward flow is found in the literature (Langseth and Boyer, 1972, Kraus, 1995, van Aken 1995, Knutsen et al, 2005).

ERRC IC

The connection between the ridge current system and the export of warm and salty water masses into the Irminger Basin and Labrador Sea is not fully understood, although this connection is thought to ultimately influence the formation of intermediate and deep waters in those basins (Cuny et al, 2002, Myers et al, 2007). While Lherminier et al (2010) have estimated that the transport over the Reykjanes Ridge between the Ovide line and Iceland ranged between 9.6 ± 2.1 Sv in 2002 and 13.8 ± 2.1 Sv in 2004, Chafik et al (2014), considering roughly the same latitude band, have recently suggested from indirect measurements that the mean flow across the Reykjanes Ridge is very weak. Further investigations are required to determine whether the cross ridge transfer occurs as a large scale flow, whether it is channeled by the fracture zones as suggested by isopycnal floats (Bower et al, 2002) or whether other processes, like eddies as suggested by Krauss (1995) and Argo float displacements (not shown), are involved. At deeper levels, the ISOW is known to escape the Iceland Basin toward the Irminger Sea through the Charlie Gibbs Fracture Zone. Based on direct measurements, Saunders (1994) estimated this transport to 2.4 Sv. However, it is likely that a westward flow of ISOW occurs north of the Charlie Gibbs Fracture Zone (CGFZ). Lagrangian floats (Lankhorst and Zenk, 2006) and model outputs (Xu et al 2010) have revealed that westward escapes of ISOW through multiple gaps in the ridge are possible, especially at the Bight Fracture Zones (herafter BFZ). Further investigations of the structure of the flow around the ridge, for quantification of the flow over the ridge with respect to the flow around it as well as the flow of ISOW through the Bight Fracture Zone are required. Further investigations of the dynamical connections between the two sides of the ridge and of the prevailing dynamical balances are also required. Among the various possible mechanisms at play, one can cite the large-scale theory that would lead to a continuous flow along f/h contours (f being the Coriolis parameter and h the layer thickness), baroclinic and barotropic instabilities associated with the vertical and horizontal shears of the currents, eddies produced in the wake of topographic features, detachments of the boundary current that can potentially create unstable sharp fronts (Flor et al., 2011). The structure of the currents with respect to the ridge may also be dependent on the large scale structure of the subpolar gyre and for instance of the varying intensity and properties of the NAC branches that enter the Iceland Basin (de Boisséson et al, 2012; Desbruyères et al., 2013), or may be driven by deep mixing, either enhanced along the topography (Ferron et al, 2014) or within fracture zones (Ferron et al, 1998). Objective of the PhD and methodology As part of the Reykjanes Ridge Experiment (RREX) project, in which we propose a thorough investigation of flow-topography interactions in the key region surrounding Reykjanes Ridge trough an integrated approach combining in situ data analyses, theoretical approach and realistic modeling, the main objective of the PhD thesis is to (1) document the circulation around and over the ridge and (2) identify the processes controlling the dynamical connections between the two sides of the ridge from observations. Using Argo data and hydrographic and ADCP data from the RREX hydrographic survey conducted in June 2015 (see Fig. 4), the PhD student will provide a 3D estimate of the geostrophic currents, and water masses distribution around the ridge. Then, the PhD student will be in charge of the quantification of the southward and northward transports along the Reykjanes Ridge at three different locations (the three sections perpendicular to the ridge) as well as the quantification of the transport across the ridge and particularly of the transport of ISOW through the BFZ (at 57°N) and CGFZ (at 52°N) and maybe also through other gaps in the ridge. The high spatial resolution data set acquired in the BFZ and CGFZ will allow a dedicated study of the dynamics of the flow and mixing of water masses. A snapshot of the vertical structure of the currents and transports will then be estimated as well as their evolution along the axis of the ridge. In combining transports value with tracer analysis (T, S, O2), the PhD student will provide estimation on the way the Irminger Current is fed by a cross-ridge flow during its northward journey along the ridge and help determine the main paths followed by the RRMW and ISOW at the time of the cruise. Specific attention will be given to the role of eddies

and flow instabilities that are predicted to occur in the wake of the Reykjanes Ridge. For this, the in situ analysis will be complemented by an analysis of altimetry data and Argo float displacements. As a result of this PhD thesis, we expect a better understanding of the respective role of the large-scale flow, eddies and fracture zones in the dynamical connections between the two sides of the ridge.

Figure 4: Position of the CTDO2 casts (red dots). Mixing measurements (VMP) will be done every other cTD casts. The yellow dots on section BC and DE indicate the position of the two ASFAR devices that will release 1 Argo float every 3 months. The black points indicate the moorings location (not discussed in text). The BFZ and CGFZ are located near 57°N (close to point H) and 52°N, respectively.

Workplan Month 1 to 3: Bibliography, database generation (Argo, hydrography, ADCP, altimetry) Month 4 to 18: 3D estimate of the currents and water masses distribution around the ridge; estimation of the transports along, above and across the ridge; estimation of the importance of fracture zones in the croos-ridge transports; writing of a first publication on the fine description of the circulation around and over the ridge. Month 19 to 33: Combination of transports estimates with tracers and mixing measurements to understand water-mass (RRMW, ISOW) evolution during their journey above and across the ridge with a specific focus on mixing in the fracture zones and eddies; writing of a second publication;. Month 34 to 36: Writing of the PhD thesis Collaborations RREX project: The PhD student will work in close collaboration with other partners of the Reykjanes Ridge Experiment (RREX) project (PI: V. Thierry). In this project, we propose a thorough investigation of flow-topography interactions in the key region surrounding Reykjanes Ridge trough an integrated approach combining in situ data analyses, theoretical approach and realistic modeling in order to address the three following objectives: (1) Document the circulation around and over the ridge and identify the processes controlling the dynamical connections between the two sides of the ridge; (2) Quantify and understand the water mass transformation in the vicinity of the Reykjanes Ridge; (3) Identify key parameters in general circulation models that are critical for an adequate representation of the circulation and water mass transformation at the Reykjanes Ridge and provide recommendations for the improvement of the ocean component of the next generation of climate models The PhD work will mainly be a contribution to the first objective, but it will also contribute to the two other objectives 2 and 3 as part of the synthesis done at the end of the project. Results of the thesis will be useful for model evaluation, in particular the specific high-resolution regional model configuration (1/36°, about 100 vertical levels) that will be developed by C. Talandier (LPO) in 2015/2016, in close collaboration with the Drakkar consortium, to assess the influence of vertical resolution and

bathymetry representation (e.g. fracture zones) for the representation of the flow dynamics along and over the ridge in ocean models. Other collaborations: Fiz Perez and Aida Rios (BOCATS projects, continuation of OVIDE project), Artem Sarafanov, Anastasia Falina and Alexey Sokov from the Shirshov Institute of Oceanology (SIO), DRAKKAR consortium, Associated projects: EQUIPEX NAOS, projet H2020 AtlantOS, Euro-Argo, Argo, OSNAP project. References van Aken H. M., 1995: Mean currents and current variability in the Iceland Basin, Netherlands Journal of Sea

Research, 33, 2, 135-145. van Aken,H.M. and de Boer, C.J., 1995: On the synoptic hydrography of intermediate and deep water masses in

the Iceland Basin. Deep-Sea Research, 42, 165–189. Bower A., B. Le Cann, T. Rossby, W. Zenk, J. Gould, K. Speer, P. L. Richardson, M.D. Prater and H.-M. Zhang,

2002: Directly measured mid-depth circulation in the northeastern North Atlantic Ocean. Nature, 419, 603-607. doi: 10.1038/nature01078.

Brambilla, E., and L. D. Talley, 2008: Subpolar Mode Water in the northeastern Atlantic: 1. Averaged properties and mean circulation, Journal of Geophysical Research,113 (C04025), doi:10.1029/2006JC004062.

de Boisséson, E., V. Thierry, H. Mercier, G. Caniaux, and D. Desbruyères, 2012: Origin, formation and variability of the Subpolar Mode Water located over the Reykjanes Ridge, J. Geophys. Res., 117, C12005, doi:10.1029/2011JC007519

Chafik L. T. Rossby, and C. Schrum, 2014: On the spatial structure and temporal variability of poleward transport between Scotland and Greenland, Journal of Geophysical Research, doi:10.1002/2013JC009287

Cuny J., P. B. Rhines, P. P. Niiler and S. Bacon, 2002: Labrador Sea Boundary Currents and the Fate of the Iminger Sea Water, J. Phys. Oceanogr., 32, 627-647.

Desbruyères, D., V. Thierry and H. Mercier, 2013: Simulated decadal variability of the meridional overturning circulation across the A25-Ovide section. Journal of Geophysical Research Oceans, 118 (1), 462-475, doi:10.1029/2012JC008342

Flór J.-B. , H. Scolan and J. Gula, 2011: Frontal instabilities and waves in a differentially rotating fluid. Journal of Fluid Mechanics, 685, pp 532-542 doi:10.1017/jfm.2011.338

Flór J.-B. and I. Eames, 2002 : Dynamics of monopolar vortices on a topographic beta-plane. Journal of Fluid Mechanics, 456, pp 353-376 doi:10.1017/S0022112001007728

Ferron, Bruno, Herlé Mercier, Kevin Speer, Ann Gargett, Kurt Polzin, 1998: Mixing in the Romanche Fracture Zone. J. Phys. Oceanogr., 28, 1929–1945.

Ferron B., F. Kokoszka, H. Mercier, and P. Lherminier, 2014 : Dissipation rate estimates from microstructure and finescale internal wave observations along the A25 Greenland-Portugal OVIDE line. Submitted to J. Atmos. Ocean. Tech.

Krauss W., 1995 : Currents and mixing in the Irminger Sea and in the Iceland Basin, J. Geophys. Res.,100, C6, 10,851-10,871.

Knutsen Ø, H. Svendsen, S. Østerhus, T. Rossby and B. Hansen, 2005: Direct measurements of the mean flow and eddy kinetic energy structure of the upper ocean circulation in the NE Atlantic, Geophys. Res. Lett., 32, L14604, doi:10.1029/2005GL023615.

Lankhorst M. and W. Zenk, 2006: Lagrangian Observations of the Midepth and Deep Velocity Fields of the Northeastern Atlantic Ocean, 36, 43-63.

Lherminier, P., H. Mercier, T. Huck, C. Gourcuff, F. F. Perez, P. Morin, A. Sarafanov, A. Falina, 2010: The Atlantic Meridional Overturning Circulation and the Subpolar Gyre observed at the A25-OVIDE section in June 2002 and 2004. Deep Sea Research Part I, 56 (11), 1374-1391, doi:10.1016/j.dsr.2010.07.009.

Mercier, H., P. Lherminer, A. Sarafanov, F. Gaillard, N. Daniault, D. Desbruyères, A. Falina, B. Ferron, T. Huck, V. Thierry, 2013. Variability of the meridional overturning circulation at the Greenland–Portugal OVIDE section from 1993 to 2010. Prog. Oceanogr. (2013), http://dx.doi.org/10.1016/j.pocean.2013.11.001

Myers P.G., N. Kulan and M. H. Ribergaard, 2007: Iminger Water variability in the West Greenland Current, Geophys., Res. Lett., 34, L17601, doi:10.1029/2007/GL030419

Perez, F. F., H. Mercier, M. Vazquez-Rodriguez, P. Lherminier, A. Velo, P. Pardo, G. Roson, A. Rios, 2013: Reconciling air-sea CO2 fluxes and anthropogenic CO2 budgets in a changing North Atlantic. Nature Geosciences, 6, 146-152, doi:10.1038/ngeo1680. (Cover highlight)

Rattan, S., P. Myers, A.M. Treguier, S. Theetten, A. Biastoch, C. Boening, 2010: Towards an Understanding of Labrador Sea Salinity Drift in Eddy-Permitting Simulations. Ocean Modelling, 35, 1-2, 77-88, doi:10.1016/j.ocemod.2010.06.007

Read, J.F., 2001: CONVEX-91: water masses and circulation of the Northeast Atlantic subpolar gyre. Progress in Oceanography, 48, 461–510.

Saunders P. M., 1994: The flux of overflow water through the Chralie-Gibbs Fracture Zone, J. Geophys. Res., 99, C6, 12343-12355.

Thierry, V., E. de Boisséson, and H. Mercier (2008), Interannual variability of the Subpolar Mode Water properties over the Reykjanes Ridge during 1990–2006, J. Geophys. Res., 113, C04016, doi:10.1029/2007JC004443.

Treguier A.M., S. Theetten, E.P. Chassignet, T. Penduff, R. Smith, L. Talley, J.O. Beismann and C. Böning, 2005 : The North Atlantic Subpolar Gyre in Four High-Resolution Models, J. Phys. Oceanogr., 35, 757-774.

Våge K., R. S. Pickart, A. Sarafanov, Ø. Knutsen, H. Mercier, P. Lherminier, H. M. van Aken, J. Meincke, D. Quadfasel, and S. Bacon, 2011: The Irminger Gyre: Circulation, convection and interannual variability, Deep-Sea Res. I, 58, 590-614, doi:10.1016/j.dsr.2011.03.001

Xu X. W. J. Schmitz Jr, H. E. Hurlburt, P. J. Hogan, and E. P. Chassignet, 2010: Transport of Nordic Seas overflow water into and within the Irminger Sea: An eddy-resolving simulation and observations, 115, C12048, doi:10.1029/2010JC006351.