Nathalie Daniault

Circulation and mixing of Mediterranean water west of the Iberian Peninsula

The spreading of water of Mediterranean origin west of the Iberian Peninsula was studied with hydrographic data from several recent cruises and current measurements from the BORD-EST programme. The vertical breakdown of the “Mediterranean salt” content reveals the dominant contribution of the so-called lower core of the outflow (60%), and the significant fraction (22%) brought downward to levels below 1500 m by diffusion. Intense salinity maxima in the upper core (18%) are only encountered south of 38°N in the vein flowing northward along the continental slope, and at a few stations in the deep ocean. Apart from the coastally trapped vein, other preferred paths of the water mass are revealed by the horizontal distributions of salinity maximum and Mediterranean Water percentage. One is southward, west of the Gorringe Bank, and two northwestward ones lie around 40°N and west of the Galicia Bank. Year-long velocity measurements in the Tagus Basin show westward mean values of 7 × 10−2 m s−1 at 1000 m associated with a very intense mesoscale variability. This variability is related to the pronounced dynamical signature of the outflow which favours instability in any branch having detached from the slope current. From a mixing point of view, the strong interleaving activity occurring near Cape St-Vincent is illustrated, but its contribution to the downstream salinity decrease in the coastally trapped vein is weak. Current and meddy detachment play the dominant role, with a scaling estimate of their associated lateral diffusivity of order 500 m2 s−1. The statistical distribution of the density ratio parameter, which governs double-diffusion at the base of the Mediterranean Water, was found to be very tight around Rπ = 1.3 in the temperature range of 5°C< φ < 8°C. North of 40°N, the presence of a fraction of Labrador Sea Water in the underlying water is shown to decrease that parameter and should favour the formation of salt fingers.

Mean full-depth summer circulation and transports at the northern periphery of the Atlantic Ocean in the 2000s

A mean state of the full-depth summer circulation in the Atlantic Ocean in the region in between Cape Farewell (Greenland), Scotland and the Greenland-Scotland Ridge (GSR) is assessed by combining 2002–2008 yearly hydrographic measurements at 59.5°N, mean dynamic topography, satellite altimetry data and available estimates of the Atlantic–Nordic Seas exchange. The mean absolute transports by the upper-ocean, mid-depth and deep currents and the Meridional Overturning Circulation (MOCσ = 16.5 ± 2.2 Sv, at σ0 = 27.55) at 59.5°N are quantified in the density space. Inter-basin and diapycnal volume fluxes in between the 59.5°N section and the GSR are then estimated from a box model. The dominant components of the meridional exchange across 59.5°N are the North Atlantic Current (NAC, 15.5 ± 0.8 Sv, σ0 27.55) east of the Reykjanes Ridge, the northward Irminger Current (IC, 12.0 ± 3.0 Sv) and southward Western Boundary Current (WBC, 32.1 ± 5.9 Sv) in the Irminger Sea and the deep water export from the northern Iceland Basin (3.7 ± 0.8 Sv, σ0 27.80). About 60% (12.7 ± 1.4 Sv) of waters carried in the MOCσ upper limb (σ0 27.55) by the NAC/IC across 59.5°N (21.1 ± 1.0 Sv) recirculates westward south of the GSR and feeds the WBC. 80% (10.2 ± 1.7 Sv) of the recirculating NAC/IC-derived upper-ocean waters gains density of σ0 27.55 and contributes to the MOCσ lower limb. Accordingly, the contribution of light-to-dense water conversion south of the GSR (∼10 Sv) to the MOCσ lower limb at 59.5°N is one and a half times larger than the contribution of dense water production in the Nordic Seas (∼6 Sv).

The 1992–2009 transport variability of the East Greenland‐Irminger Current at 60°N

The East Greenland Irminger Current (EGIC) decadal transport variability likely influences deep convection intensity in the Labrador and Irminger Seas but is poorly known yet. The EGIC transport west of the 2000 m isobath was estimated, for the first time, between 1992 and 2009 by combining surface geostrophic velocities derived from altimetry with an estimate of the vertical structure of the transport variability statistically determined from a moored array deployed in 2004-2006. The reconstructed 17-year time series of the EGIC transport was then validated against independent estimates confirming that, indeed, the vertical distribution of the EGIC variability has not changed significantly over the last two decades. The 1992-2009 mean transport is 19.5 Sv with a standard error of 0.3 Sv (1 Sv = 10(6) m(3) s(-1)). In 1992-1996, the EGIC transport was close to the average. Over the following decade (1997-2005), the EGIC transport declined by 3 Sv (15%) so that the 2004-2006 mean transport inferred from the moored array is 2.2 Sv (10%) less than the 1992-2009 mean. It was followed by a period of higher transport. The seasonal to interannual transport variability is coherent with the variability of the windstress curl at the center of the Irminger Sea.

On the Cascading of Dense Shelf Waters in the Irminger Sea

AbstractHydrographic data collected in the Irminger Sea in the 1990s–2000s indicate that dense shelf waters carried by the East Greenland Current south of the Denmark Strait intermittently descend (cascade) down the continental slope and merge with the deep waters originating from the Nordic Seas overflows. Repeat measurements on the East Greenland shelf at ~200 km south of the Denmark Strait (65°–66°N) reveal that East Greenland shelf waters in the Irminger Sea are occasionally as dense (σ0 > 27.80) as the overflow-derived deep waters carried by the Deep Western Boundary Current (DWBC). Clear hydrographic traces of upstream cascading of dense shelf waters are found over the continental slope at 64.3°N, where the densest plumes (σ0 > 27.80) originating from the shelf are identified as distinct low-salinity anomalies in the DWBC. Downstream observations suggest that dense fresh waters descending from the shelf in the northern Irminger Sea can be distinguished in the DWBC up to the latitude of Cape Farewell...

Circulation and Transport at the Southeast Tip of Greenland

Abstract The circulation and related transports at the southeast tip of Greenland are determined from direct current observations of a moored current meter array. The measurements cover a time span from June 2004 to June 2006. The net mean total southwestward transport of the East Greenland–Irminger Current from the midshelf (20 km off the coast at 60°N) to the 2070-m isobath (about 100 km offshore) was estimated as 17.3 Sv (Sv ≡ 106 m3 s−1) with an uncertainty of 1 Sv. The transport variability is characterized by a standard deviation of 3.8 Sv with a peak-to-peak amplitude up to 30 Sv. The seasonal variability has an amplitude of 1.5 Sv. Frequencies around 0.1 day−1 dominate the signal, although a variability at lower frequency (∼1 month−1) also appears in winter. The coherence between the observed transport variability and the wind stress curl variability over the Irminger Sea differs significantly from 0 at the 95% confidence level for periods greater than 5 days.

Evidence of Mediterranean Water dipole collision in the Gulf of Cadiz

A collision of Mediterranean Water dipoles in the Gulf of Cadiz is studied here, using data from the MedTop and Semane experiments. First, a Mediterranean Water eddy (meddy) was surveyed hydrologically in November 2000 southwest of Cape Saint Vincent. Then, this meddy drifted northeastward from this position, accompanied by a cyclone (detected only via altimetry), thus forming a first dipole. In February 2001, a dipole of Mediterranean Water was measured hydrologically just after its formation near Portimao Canyon. This second dipole drifted southwestward. The western and eastern meddies had hydrological radii of about 22 and 25 km, respectively, with corresponding temperature and salinity maxima of (13.45°C, 36.78) and (11.40°C, 36.40). Rafos float trajectories and satellite altimetry indicate that these two dipoles collided early April 2001, south of Cape Saint Vincent, near 35°30′N, 10°15′W. More precisely, the eastern meddy wrapped around the western one. This merger resulted in an anticyclone (a meddy) which drifted southeastward, coupled with the eastern cyclone. Hydrological sections across this final third resulting dipole, performed in July 2001 in the southern Gulf of Cadiz, confirm this interaction: the thermohaline characteristics of the final meddy can be tracked back to the original structures. The subsequent evolution of this dipole was analyzed with Rafos float trajectories. A numerical simulation of the interaction between the two earlier dipoles is also presented. We suggest that these dipole collisions at the Mediterranean Water level may represent a mechanism of generation of the larger meddies that finally leave the Gulf of Cadiz.

The use of free-drifting meteorological buoys to study winds and surface currents

Abstract Free-drifting meteorological buoys have been developed with a capability to estimate the magnitude and direction of surface winds. It is shown, using wind data and assuming the buoys are Langrangian tracers of surface currents, that short-term variations of both quantities are correlated in agreement with Ekmans theory.

Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special_issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue. 1 Scientific context and objectives Understanding the distribution, sources, and sinks of trace elements and isotopes (TEIs) will improve our ability to understand past and present marine environments. Some TEIs are toxic (e.g. Hg), while others are essential micronutrients involved in many metabolic processes of marine organisms (e.g. Fe, Mn). The availability of TEIs therefore constrains the ocean carbon cycle and affects a range of other biogeochemical processes in the Earth system, whilst responding to and influencing global change (de Baar et al., 2005; Blain et al., 2007; Boyd et al., 2007; Pollard et al., 2007). Moreover, TEI interactions with the marine food web strongly depend on their physical (particulate/dissolved/colloidal/soluble) and chemical (organic and redox) forms. In addition, some TEIs are diagnostic in allowing the quantification of specific mechanisms in the marine environment that are challenging to measure directly. A few examples include (i) atmospheric deposition (e.g. 210Pb, Al, Mn, Th isotopes, 7Be; Baker et al., 2016; Hsieh et al., 2011; Measures and Brown, 1996); (ii) mixing rates of deep waters or shelf-to-open ocean (e.g. 231Pa/230Th,114C, Ra isotopes, 129I, 236U; van Beek et al., 2008; Casacuberta et al., 2016; Key et al., 2004); (iii) boundary exchange processes (e.g. εNd, Jeandel et al., 2011; Lacan and Jeandel, 2001, 2005); and (iv) downward flux of organic carbon and/or remineralization in deep waters (e.g. 234Th/238U, 210Pb/210Po, Baxs; Buesseler et al., 2004; Dehairs et al., 1997; Roca-Martí et al., 2016). In such settings, TEIs provide chemical constraints and allow the estimation of fluxes which was not possible before the development of their analyses. Finally, paleoceanographers are wholly dependent on the development of tracers, many of which are based on TEIs used as proxies, in order to reconstruct past environmental conditions (e.g. ocean productivity, patterns and rates of ocean circulation, ecosystem structures, ocean anoxia; Henderson, 2002). Such reconstruction efforts are essential to assess the processes involved in regulating the global climate system, and possible future climate change variability. Despite all these major implications, the distribution, sources, sinks, and internal cycling of TEIs in the oceans are still largely unknown due to the lack of appropriate clean sampling approaches and insufficient sensitivity and selectivity of the analytical measurement techniques until recently. This last point has improved very quickly as significant improvements in the instrumental techniques now allow the measurements of concentrations, speciation (physical and chemical forms), and isotopic compositions for most of the elements of the periodic table which have been identified either as relevant tracers or key nutrients in the marine environment. These recent advances provide the marine geochemistry community with a significant opportunity to make subBiogeosciences, 15, 7097–7109, 2018 www.biogeosciences.net/15/7097/2018/ G. Sarthou et al.: French GEOTRACES North Atlantic Transect (GA01) 7099 Figure 1. Schematic diagram of the mean large-scale circulation adapted from Daniault et al. (2016) and Zunino et al. (2017). Bathymetry is plotted in color with color changes at 100 and 1000 m and every 1000 m below 1000 m. Black dots represent the Short station, yellow stars the Large ones, orange stars the XLarge ones, and red stars the Super ones. The main water masses are indicated: Denmark Strait Overflow Water (DSOW), Iceland–Scotland Overflow Water (ISOW), Labrador Sea Water (LSW), Mediterranean Water (MW), and lower North East Atlantic Deep Water (LNEADW). stantial contributions to a better understanding of the marine environment. In this general context, the aim of the international GEOTRACES programme is to characterize TEI distributions on a global scale, consisting of ocean sections, and regional process studies, using a multi-proxy approach. The GEOVIDE section is the French contribution to this global survey in the North Atlantic Ocean along the OVIDE section and in the Labrador Sea (Fig. 1) and complements a range of other international cruises in the North Atlantic. GEOVIDE leans on the knowledge gained by the OVIDE project during which the Portugal–Greenland section has been carried out biennially since 2002, gathering physical and biogeochemical data from the surface to the bottom (Mercier et al., 2015; Pérez et al., 2018). Rationale for the GEOVIDE section i. The North Atlantic Ocean plays a key role in mediating the climate of the Earth. It represents a key region of the Meridional Overturning Circulation (MOC) and a major sink of anthropogenic carbon (Cant) (Pérez et al., 2013; Sabine et al., 2004; Seager et al., 2002). Since 2002, the OVIDE project has contributed to the observation of both the circulation and water mass properties of the North Atlantic Ocean. Despite the importance of the MOC on global climate, it is still challenging to assess its strength within a reasonable uncertainty (Kanzow et al., 2010; Lherminier et al., 2010). The MOC strength estimated from in situ measurements on OVIDE cruises has thus helped to validate a time series for the amplitude of the MOC (based on altimetry and ARGO float array data) that exhibits a drop of 2.5± 1.4 Sv (95 % confidence interval) between 1993 and 2010 (Mercier et al., 2015), consistent with other modelling studies (Xu et al., 2013). This time series, along with the in situ data, shows a recovery of the MOC amplitude in 2014 at a value similar to those of the mid1990s, confirming the importance of the decadal variability in the subpolar gyre. During OVIDE, the contributions of the most relevant currents, water masses, and biogeochemical provinces were localized and quantified. This knowledge was crucial for the establishment of the best strategy to sample TEIs in this specific region. In addition to the OVIDE section, the Labrador Sea section offered a unique opportunity to complement the MOC estimate, to analyse the propagation of anomalies in temperature and salinity (Reverdin et al., 1994), and to study the distribution of TEIs along the boundary current of the subpolar gyre, coupling both observations and modelling. Moreover, recent results provided evidence that CO2 uptake in the North Atlantic was reduced by the weakening of the MOC (Pérez et al., 2013). The most significant finding of this study was that the uptake of Cant occurred almost exclusively in the subtropical gyre, while natural CO2 uptake dominated in the subpolar gyre. In light of these new results, one issue to be addressed was the coupling between the Cant and the transport of water, with the aim to understand how the changes in the ventilation and in the circulation of water masses affect the Cant uptake and its storage capacity in the various identified provinces (Fröb et al., 2018). Finally, as the subpolar North Atlantic forms the starting point for the global ocean conveyor belt, it is of particular interest to investigate how TEIs are transferred to the deep ocean through both ventilation and particle sinking, and how deep convection processes impact the TEI distributions in this key region. ii. A better assessment of the factors that control organic production and export of carbon in the productive North Atlantic Ocean together with a better understanding of the role played by TEIs in these processes is research priorities. Pronounced phytoplankton blooms occur in the North Atlantic in spring in response to upwelling and water column destratification (Bury et al., 2001; Henson et al., 2009; Savidge et al., 1995). Such www.biogeosciences.net/15/7097/2018/ Biogeosciences, 15, 7097–7109, 2018 7100 G. Sarthou et al.: French GEOTRACES North Atlantic Transect (GA01) blooms are known to trigger substantial export of fastsinking particles (Lampitt, 1985), and can represent a major removal mechanism for particulate organic carbon, macronutrients, and TEIs to the deep ocean. iii. In the North Atlantic, TEI distributions are influenced by a variety of sources including, most importantly, the atmosphere and the margins (Iberian, Greenland, and Labrador margins). 1. Atmosphere. Atmospheric inputs (e.g. mineral dust, anthropogenic emission aerosols) are an important source of TEIs to the North Atlantic Ocean due to the combined effects of anthropogenic emissions from industrial/agricultural sources and mineral dust mobilized from the arid regions of North Africa (Duce et al., 2008; Jickells et al., 2005). Model and satellite data for the GEOVIDE section suggested that an approximately 10fold decrease in the atmospheric concentrations of mineral dust was expected from south to north (Mahowald et al., 2005). As there had been relatively few aerosol TEI studies in the northern North Atlantic compared to the tropical and subtropical North Atlantic prior to GEOVIDE, constraining atmospheric deposition fluxes to this region had been identified as a research priority (de Leeuw et al., 2014). During the GEOVIDE campaign, a multi-proxy approach (e.g. aerosol trace element concentrations, dissolved and particulate Al and Mn, seawater 210Pb, Fe, Nd, and Th isotopes, 7Be) was taken to achieve the objective of better constraining the atmospheric deposition fluxes of key trace elements. 2. Margins. The continental shelves can act