BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-15-2075-2018Water mass distributions and transports for the 2014 GEOVIDE cruise in the
North AtlanticWater masses in the 2014 GEOVIDE cruiseGarcía-IbáñezMaribel I.maribel.garcia-ibanez@uni.nohttps://orcid.org/0000-0001-5218-0064PérezFiz F.https://orcid.org/0000-0003-4836-8974LherminierPascalehttps://orcid.org/0000-0001-9007-2160ZuninoPatriciaMercierHerléhttps://orcid.org/0000-0002-1940-617XTréguerPaulUni Research Climate, Bjerknes Centre for Climate Research, Bergen
5008, NorwayInstituto de Investigaciones Marinas (IIM, CSIC), Eduardo Cabello 6,
36208 Vigo, SpainIfremer, Univ. Brest, CNRS, IRD, Laboratoire d'Océanographie
Physique et Spatiale (LOPS), IUEM, Plouzané, FranceCentre National de la Recherche Scientifique (CNRS), Ifremer, Institut
de Recherche pour le Développement (IRD), Université de Bretagne
Occidentale (UBO), Laboratoire d'Océanographie Physique et Spatiale
(LOPS), Centre Ifremer de Bretagne, 29280, Plouzané, FranceEnvironmental Sciences Laboratory (LEMAR, UMR 6539) at the European
Institute for Marine Studies (IUEM), Université de Bretagne Occidentale,
CNRS, 29280 Plouzané, FranceMaribel I. García-Ibáñez (maribel.garcia-ibanez@uni.no)9April20181572075209025August201725October201720March201820March2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://bg.copernicus.org/articles/15/2075/2018/bg-15-2075-2018.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/15/2075/2018/bg-15-2075-2018.pdf
We present the distribution of water masses along the GEOTRACES-GA01 section
during the GEOVIDE cruise, which crossed the subpolar North Atlantic Ocean
and the Labrador Sea in the summer of 2014. The water mass structure
resulting from an extended optimum multiparameter (eOMP) analysis provides
the framework for interpreting the observed distributions of trace elements
and their isotopes. Central Waters and Subpolar Mode Waters (SPMW) dominated
the upper part of the GEOTRACES-GA01 section. At intermediate depths, the
dominant water mass was Labrador Sea Water, while the deep parts of the
section were filled by Iceland–Scotland Overflow Water (ISOW) and North-East
Atlantic Deep Water. We also evaluate the water mass volume transports across
the 2014 OVIDE line (Portugal to Greenland section) by combining the water
mass fractions resulting from the eOMP analysis with the absolute geostrophic
velocity field estimated through a box inverse model. This allowed us to
assess the relative contribution of each water mass to the transport across
the section. Finally, we discuss the changes in the distribution and
transport of water masses between the 2014 OVIDE line and the 2002–2010 mean
state. At the upper and intermediate water levels, colder end-members of the
water masses replaced the warmer ones in 2014 with respect to 2002–2010, in
agreement with the long-term cooling of the North Atlantic Subpolar Gyre that
started in the mid-2000s. Below 2000 dbar, ISOW increased its contribution in
2014 with respect to 2002–2010, with the increase being consistent with
other estimates of ISOW transports along 58–59∘ N. We also
observed an increase in SPMW in the East Greenland Irminger Current in 2014
with respect to 2002–2010, which supports the recent deep convection events
in the Irminger Sea. From the assessment of the relative water mass
contribution to the Atlantic Meridional Overturning Circulation (AMOC) across
the OVIDE line, we conclude that the larger AMOC intensity in 2014 compared
to the 2002–2010 mean was related to both the increase in the northward
transport of Central Waters in the AMOC upper limb and to the increase in the
southward flow of Irminger Basin SPMW and ISOW in the AMOC lower limb.
Schematic diagram of the large-scale circulation in the subpolar
North Atlantic adapted from Daniault et al. (2016). Isobaths every 1000 dbar
are represented by black contours. GEOVIDE (GEOTRACES-GA01 section)
hydrographic stations are indicated by yellow dots. The main topographical
features are labelled as follows: Azores–Biscay Rise (ABR), Bight Fracture Zone (BFZ),
Charlie–Gibbs Fracture Zone (CGFZ), Faraday Fracture Zone (FFZ), Iberian Abyssal Plain (IAP), Maxwell Fracture Zone (MFZ) and Mid-Atlantic Ridge (MAR). The main water masses and currents are also
represented: Denmark Strait Overflow Water (DSOW), Deep Western Boundary
Current (DWBC), East Greenland Irminger Current (EGIC), Iceland–Scotland Overflow Water (ISOW), Irminger Current (IC),
Labrador Current (LC), Labrador Sea Water (LSW), Mediterranean Water (MW), North Atlantic Current (NAC) and North-East Atlantic Deep Water lower (NEADWL).
Introduction
The 2014 GEOVIDE cruise consisted of two hydrographic sections: the seventh
repetition of the OVIDE line from Lisbon (Portugal) to Cape Farewell
(Greenland), and a section across the Labrador Sea from Cape Farewell to St.
John's (Canada) (Fig. 1). The GEOVIDE cruise was the major French
contribution to the Global GEOTRACES programme (official GEOTRACES-GA01
section), which aimed to achieve a three-dimensional distribution of the trace
elements and their isotopes (TEIs) in the global ocean (SCOR Working Group,
2007). Obtaining high-quality hydrographic and tracer measurements will
enable us to trace back the origins, pathways and processes governing
the observed TEI distributions.
To provide a framework for interpreting the observed TEI distributions, in
this work, we qualitatively assess the water mass distribution along the
2014 GEOTRACES-GA01 section through an extended optimum multiparameter
(eOMP) analysis (Karstensen and Tomczak, 1998). We extend the study
performed by García-Ibáñez et al. (2015) for the 2002–2010
OVIDE cruises, which described the distributions and temporal variations of
the main water masses along the OVIDE line, and inferred the water mass
transformations within the North Atlantic Subpolar Gyre. As in that work, we
also combine the water mass structure resulting from the eOMP analysis with
the velocity field across the OVIDE line, obtaining water mass volume
transports. The assessment of the water mass volume transports based on
dilutions of “pure” (OMP-based) water masses provides insights into the
circulation features that are particularly useful for areas of complex
currents and water mass transformation, as in the subpolar North Atlantic
(SPNA). Finally, we compare the water mass distribution and transport of the
2014 OVIDE line with the average water mass distribution and transport of
the 2002–2010 OVIDE cruises (García-Ibáñez et al., 2015) and
link the observed changes to major changes in the formation and circulation
of water masses in the SPNA.
(a) Potential temperature (θ, in ∘C), (b) salinity, (c) oxygen (in µmol kg-1),
and (d) silicic acid (in
µmol kg-1) along 2014 GEOVIDE cruise (GEOTRACES-GA01 section,
inset in subplot a), from Portugal (right) to Canada (left). Sample
locations appear as grey dots. The dashed horizontal red line in subplot
(d) represents the isopycnal σ1= 32.15 kg m-3 (where
σ1 is potential density referenced to 1000 dbar), which marks
the limit between the upper and lower limbs of the Atlantic Meridional
Overturning Circulation (AMOC) at the GEOTRACES-GA01 section (Zunino et al.,
2017).
Data and methodsThe hydrographic data
The GEOVIDE cruise (GEOTRACES-GA01 section) was conducted in June–July 2014
and consisted of 78 stations along the eastern SPNA and the Labrador Sea
(Fig. 1). During the first cast in each station, a classical rosette
equipped with 22 Niskin bottles and CTD SBE-911 equipped with a dissolved
oxygen sensor SBE-43 was deployed. This first station cast was used as a
reference for the physical and chemical characterization of water masses.
Discrete sampling for oxygen, nutrients and salinity was performed at all
78 stations. Nutrient concentrations (nitrate and silicic acid) were
measured by the continuous flow analyser, giving the concentration values in
µM (µmol L-1). We transformed measured nutrient
concentrations in µM to µmol kg-1 by dividing by the
density of the sample at 20 ∘C (measurement temperature).
Accuracies were 0.001∘ C for temperature, 0.002 for salinity, 2 µmol kg-1 for oxygen and 0.1 µmol kg-1 for nutrients.
Upper water velocity was continuously measured with two ship-mounted
acoustic Doppler current profilers (ADCPs; Ocean Surveyors).
For further reference, the vertical sections of potential temperature
(θ), salinity, oxygen and silicic acid are shown in Fig. 2.
Hydrographic features and general circulation
The 2014 GEOTRACES-GA01 section crossed regions of water mass transformation
and deep convection leading to water mass formation (e.g. Sarafanov et al.,
2012; García-Ibáñez et al., 2015; Yashayaev and Loder, 2017).
The North Atlantic Current (NAC) carries warm and saline subtropical waters
northwards, towards the north-eastern Atlantic Ocean (Fig. 1). Air–sea
interaction progressively reduces the temperature of the Central Waters
transported by the NAC, being ultimately transformed into Subpolar Mode
Water (SPMW) (McCartney and Talley, 1982; Tsuchiya et al., 1992). The
continuous air–sea interaction along the NAC path leads to a continuous
transition of varieties of Central Waters and SPMWs (McCartney and Talley,
1982; Pollard et al., 1996; Brambilla and Talley, 2008; de Boisséson et
al., 2012). The last stage of the transformation of SPMW is the Labrador Sea
Water (LSW), which is formed in the Labrador and Irminger seas (e.g.
Pickart et al., 2003; de Jong and de Steur, 2016; Fröb et al., 2016;
Piron et al., 2017). After its formation, LSW enters the Deep Western
Boundary Current (DWBC) (Bersch et al., 2007) (Fig. 1), where it joins the
Denmark Strait Overflow Water (DSOW) and the Iceland–Scotland Overflow
Water (ISOW) (Rudels et al., 2002; Tanhua et al., 2008). DSOW forms after
the deep waters of the Nordic Seas overflow the Greenland–Iceland sill and
entrain Atlantic waters (SPMW and LSW) (Read, 2000; Yashayaev and Dickson,
2008). Cascading events of Polar Intermediate Water (PIW) also affect DSOW
in the Irminger Sea (e.g. Pickart et al., 2005; Falina et al., 2012;
Jochumsen et al., 2015). ISOW forms after the Norwegian Sea waters overflow
the Iceland–Scotland sills and entrain SPMW and LSW (van Aken and de Boer,
1995; Dickson et al., 2002; Fogelqvist et al., 2003). Then, ISOW flows
southwards in the Iceland Basin, mainly along the eastern flank of the
Reykjanes Ridge. Through the journey of ISOW in the Iceland Basin,
entrainment events lead to the formation of the North-East Atlantic Deep
Water (NEADW) (van Aken, 2000). NEADW recirculates in the West European
Basin (Fig. 1) and mixes with the surrounding waters, including the
Antarctic Bottom Water (van Aken and Becker, 1996), which results in
thermohaline properties of NEADW that can be approximated as a line
(Saunders, 1986; Mantyla, 1994). The waters of the GEOTRACES-GA01 section
are also influenced by the saline Mediterranean Water (MW), and the
relatively fresh Subarctic Intermediate Water (SAIW). MW enters the North
Atlantic from the Mediterranean Sea after overflowing the Strait of
Gibraltar (Ambar and Howe, 1979; Baringer and Price, 1997). SAIW originates
in the western boundary of the subpolar gyre, i.e. the Labrador Current
(Fig. 1) (Arhan, 1990), by mixing between the warm saline waters of the NAC
with the cold and fresher LSW (Iselin, 1936; Arhan, 1990; Read, 2000). The
thermohaline properties of SAIW vary along its pathway towards the West
European Basin, becoming saltier and warmer (Harvey and Arhan, 1988; Pollard
et al., 2004).
The above-described water mass formation processes occurring in the SPNA
lead to the ventilation and renewal of the intermediate and deep ocean, and
are the start process of the Atlantic Meridional Overturning Circulation
(AMOC) (Kuhlbrodt et al., 2007; Rhein et al., 2011; Sarafanov et al., 2012).
The AMOC consists of two limbs: a warm northward-flowing upper limb, and a
colder southward-flowing lower limb. For the OVIDE line, the upper limb of
the AMOC is constituted by the Central Waters, the SPMW of the Iceland
Basin, SAIW and MW, while the AMOC lower limb is constituted by the SPMW of
the Irminger Sea (IrSPMW), PIW, LSW, ISOW, DSOW and NEADW
(García-Ibáñez et al., 2015).
Extended optimum multiparameter (eOMP) analysis
To solve the complex water mass structure of the SPNA, we used an optimum
multiparameter (OMP) analysis (Tomczak and Large, 1989). This technique has
previously been used to describe in detail the origin, pathways and
transformation of the main water masses in the SPNA (Tanhua et al., 2005;
Álvarez et al., 2004, 2005; García-Ibáñez et al., 2015).
Briefly, OMP analyses consider the properties of a given water sample to be
the result of linear combinations of a finite number of water masses
represented by the so-called source water types (SWTs) (Tomczak, 1999). In
this study, SWTs were characterized by θ, salinity, oxygen, silicic
acid and nitrate (Table 1). Once the SWTs and their physical and chemical
properties are defined, the OMP analysis solves the mixing between SWTs by a
least-square method constrained to be positive definite, giving the
fractions of each SWT (Xi) in each water sample:
d=G∗Xi+r,
where d is the observed property in a water sample, G is the matrix
containing the properties defining the SWTs, Xi is the relative
contributions of each SWT to the sample, and r is the residual. An
additional constraint is mass conservation, which ensures the contributions
of all the SWTs sum to 100 %:
∑Xi=1+r.
In this study, we solved an extended OMP (eOMP) analysis (Karstensen and
Tomczak, 1998), which accounts for the non-conservative behaviour of oxygen
and nutrients by using Redfield-like stoichiometric ratios (R):
d=G∗Xi+ΔO2bio/R+r,
where R is 12 for silicic acid (Perez et al., 1993; Castro et al., 1998),
and 10.5 for nitrate (Takahashi et al., 1985; Anderson and Sarmiento, 1994).
The ΔO2bio term accounts for the changes in oxygen due to
the synthesis and/or remineralisation of the organic matter.
Properties characterizing the source water types (SWTsa) considered in this study with their corresponding standard
deviationsb. The square of correlation coefficients (R2) between
the observed and estimated properties are also given, together with the
standard deviation of the residuals (SDR) and the SDR /ε ratios
from the data below 400 dbar. The ε (standard deviation of the
water sample properties) used to compute the SDR /ε ratios are
listed in Table S1. The last column accounts for the uncertainties in the
SWT contributions.
a ENACW16 and ENACW12= East North Atlantic Central Water of
16 and 12 ∘C, respectively; SPMW8,
SPMW7 and IrSPMW = Subpolar Mode Water of 8 ∘C,
of 7 ∘C and of the Irminger Sea, respectively; LSW = Labrador
Sea Water; SAIW6 and SAIW4= Subarctic Intermediate Water of
6 and 4 ∘C, respectively; MW = Mediterranean Water; ISOW = Iceland–Scotland Overflow
Water; DSOW = Denmark Strait Overflow Water; PIW = Polar Intermediate Water; and
NEADWU and NEADWL= North-East Atlantic Deep Water upper and
lower, respectively.
b The standard deviation of the properties of the SWTs were obtained
following the method described in Text S1 in the Supplement.
c No uncertainty is given for NEADWU since it is was decomposed
between MW, LSW, ISOW and NEADWL (see Sect. 2.3); n/a: not applicable.
(a) Potential temperature (θ)–salinity (S) diagram
including the source water types (SWTs; Table 1) and (b) zoomed for deep
waters. The isopycnal σ1= 32.15 kg m-3 delimiting the
upper and lower limbs of the AMOC at the GEOTRACES-GA01 section is also
plotted. Colour coding represents the mixing groups (legend on c), i.e.
subsets of SWTs susceptible of mixing together. (c) Distribution of the
mixing groups along the GEOTRACES-GA01 section (inset).
To be able to compare the resulting water mass distribution and transport
for OVIDE 2014 with the average of OVIDE 2002–2010
(García-Ibáñez et al., 2015), we used the same eOMP set-up as
in García-Ibáñez et al. (2015), but with readjusted
thermohaline properties for LSW and ISOW to better represent these water
masses in the 2010s (e.g. Hansen et al., 2016; Yashayaev and Loder, 2017).
We used 14 SWTs to solve the water mass structure along the GEOTRACES-GA01
section (Table 1, Fig. 3). The upper waters of the GEOTRACES-GA01 section
were characterized by Central Waters and SPMW. The thermohaline range of the
Central Waters was solved by defining two SWTs that coincide with extremes
of the θ–S line defining the East North Atlantic Central Waters
(ENACW), the predominant variety of the North Atlantic Central Waters to the
east of the Mid-Atlantic Ridge (Iselin, 1936): ENACW of 16 ∘C (ENACW16), whose θ–S characteristics match those from the
warmer central waters of Pollard et al. (1996), and ENACW of
12 ∘C (ENACW12), which represents the upper limit of
ENACW defined by Harvey (1982). The change in temperature of SPMW along the
NAC path cannot be accounted by the OMP analysis, since it is the result of
air–sea interaction (e.g. McCartney and Talley, 1982; Brambilla and Talley,
2008). This problem was solved by defining three SWTs to characterize SPMW:
SPMW of 8 ∘C (SPMW8), SPMW of 7 ∘C
(SPMW7) and SPMW of the Irminger Basin (IrSPMW). SPMW7 and
SPMW8 characterize the thermohaline range of SPMW in the Iceland Basin,
with the θ–S of SPMW8 being representative of that formed
within the Iceland Basin (Brambilla and Talley, 2008), and the θ–S
of SPMW7 to that found over the eastern flank of the Reykjanes Ridge
(Thierry et al., 2008). The θ–S of IrSPMW characterize SPMW found in
the Irminger Sea (Brambilla and Talley, 2008), and are close to those of the
Irminger Sea Water (Krauss, 1995). The intermediate waters of the
GEOTRACES-GA01 section were characterized by LSW, MW and SAIW. The
thermohaline properties of LSW were chosen from the thermohaline properties
of LSW formed in 2008 (LSW2008; Kieke and Yashayaev, 2015; Yashayaev
and Loder, 2009, 2017), which, according to the transit times proposed by
Yashayaev et al. (2007), would have reached the Irminger and Iceland basins
by 2014. The properties of MW were taken from Wüst and Defant (1936)
near Cape St. Vicente, where MW has its θ–S characteristics
established after overflowing the Strait of Gibraltar (Ambar and Howe, 1979;
Baringer and Price, 1997). The thermohaline range of SAIW
(4–7 ∘C and S < 34.9) was represented by two SWTs:
SAIW of 6 ∘C (SAIW6) and SAIW of 4 ∘C
(SAIW4), following the descriptions of Bubnov (1968) and Harvey and
Arhan (1988). Finally, the deep waters of the GEOTRACES-GA01 section were
characterized by DSOW, ISOW and NEADW. The thermohaline properties of ISOW
were defined as the ISOW properties after crossed the Iceland–Scotland sills
defined by van Aken and Becker (1996), and these properties were readjusted by increasing
ISOW temperature and salinity by 0.1 ∘C and 0.01, respectively,
according to the observed changes in the overflow properties since 2002
(Hansen et al., 2016). The thermohaline characteristics chosen for DSOW were
selected from those found by Tanhua et al. (2005) downstream of the
Greenland–Iceland sill. We also included PIW in the analysis to take into
account the dense shelf water intrusions into DSOW. The thermohaline
characteristics selected for PIW are in agreement with those proposed by
Malmberg (1972) and Rudels et al. (2002). NEADW was modelled by the
definition of two SWTs equal to the end-points of the line defining the
thermohaline properties of NEADW in the West European Basin (Saunders, 1986;
Mantyla, 1994; Castro et al., 1998): upper NEADW (NEADWU) and lower
NEADW (NEADWL).
In order to solve an over-determined system of linear mixing equations (Eqs. 2 and 3), a maximum of four SWTs
can be considered simultaneously: one Eq. (3) for each variable defining the SWTs (i.e. 5) and the mass conservation
equation (Eq. 3), and five unknowns – four Xi values and ΔO2bio. This inconvenience was solved by organizing the SWTs into
11 subsets or mixing groups (Fig. 3c), based on the characteristics and
dynamics of the SWTs in the SPNA. The mixing groups are vertically and
horizontally sequenced, and share at least one SWT with the adjacent mixing
groups to ensure water mass continuity (for more details about the eOMP set-up see García-Ibáñez et al., 2015). After obtaining the
Xi values for each water sample, the Xi of NEADWU was decomposed
into 1 % of MW, 13 % of LSW, 36 % of ISOW and 50 % of NEADWL
(van Aken, 2000; Álvarez et al., 2004; Carracedo et al., 2012;
García-Ibáñez et al., 2015).
An important assumption of the methodology is that the physical and chemical
characteristics of the SWTs are considered time-invariant and equally
affected by mixing; hence, changes in the properties of the water masses
over time are reflected through water mass redistributions. The first 75 dbar, where non-conservative behaviour of temperature and salinity is
expected, were excluded from the analysis to avoid changes in water mass
properties due to air–sea interaction. Water samples with salinity lower
than 34.7 were also excluded from the analysis to avoid the influence of
high percentages of fresh water (Daniault et al., 2011).
We tested the robustness of the methodology through a Monte Carlo simulation
(Tanhua et al., 2005), where the physical and chemical properties of both
each SWT and each water sample were randomly perturbed within the standard
deviation of each parameter (Table 1; see also Text S1 and Table S1 in the Supplement). This
allowed an assessment of the sensitivity of the eOMP analysis to potential
measurement errors and temporal variations in the physical and chemical
properties that define the SWTs (Leffanue and Tomczak, 2004). A hundred
Monte Carlo simulations were performed and the eOMP equation system was
solved for each of them. The average standard deviation of the Xi values
(last column in Table 1) is lower than 12 %, which indicates that the
methodology is robust. Additionally, our eOMP analysis is consistent since
its residuals (r in Eq. 3) lack a tendency with depth (Fig. S1 in the Supplement), with the
standard deviations of the residuals being slightly higher than the
measurement errors (Table 1). Besides, the ability of our eOMP analysis to
reproduce the measured values is given as the correlation coefficient
(R2, Table 1) between the measured values (water samples) and the
expected values for the SWT mixing (values of the properties of each water
sample obtained by when substituting Xi values in Eq. 3). The R2
values are higher than 0.993, which again indicates the reliability of our
eOMP analysis.
Water mass distribution resulting from the eOMP analysis for the
2014 GEOVIDE cruise (GEOTRACES-GA01 section, inset in subplot a), from
Portugal (right) to Canada (left). Sample locations appear as grey dots. ABR
refers to Azores–Biscay Rise. Consult Table 1 for water mass acronyms.
Water mass distribution for 2014
The water mass distribution for the 2014 GEOTRACES-GA01 section (Fig. 4) was
obtained through an eOMP analysis (Sect. 2.3).
The Central Waters (ENACW16+ ENACW12) occupy the upper eastern
part of the 2014 GEOTRACES-GA01 section from the Iberian Peninsula until the
Reykjanes Ridge (Fig. 4a, b), with ENACW12 being the dominant one. The
contribution of ENACW12 exceeds 90 % in the upper 500 dbar in the
Iberian Abyssal Plain, following the maximum in θ and the minimum in
silicic acid (Fig. 2a, d). The distribution of the Central Waters is
associated with the NAC, which is associated with the thermohaline front (Fig. 2a, b)
delimiting ENACW12. The westward extension of ENACW12 reflects its
cyclonic circulation in the Iceland Basin and its southward flow over the
eastern flank of the Reykjanes Ridge (Read, 2000; Pollard et al., 2004).
The cooler end-member of the Central Waters, SPMW8, extends below
ENACW12 (Fig. 4c). Air–sea interaction processes along the NAC
transforms both ENACW12 and SPMW8 into SPMW7 (Thierry et al.,
2008; García-Ibáñez et al., 2015), which dominates the upper
1500 dbar of the Iceland Basin and above the Reykjanes Ridge (Fig. 4d). The
distribution of SPMW7 reflects the circulation of the NAC around the
Reykjanes Ridge, from the Iceland Basin to the Irminger Sea (Brambilla and
Talley, 2008). Further transformation of SPMW7 through air–sea
interaction along the path of the NAC leads to the formation of IrSPMW in
the Irminger Sea (Brambilla and Talley, 2008). The main core of IrSPMW is on
the Greenland slope, from where it extends eastwards until the Reykjanes
Ridge (Fig. 4a). This distribution could indicate that the major region of
formation of IrSPMW is the north-west of the Irminger Sea (Brambilla and
Talley, 2008), from where the East Greenland Irminger Current (EGIC) (Fig. 1) would transport it until the GEOTRACES-GA01 section and then to the
Labrador Sea. Once in the Labrador Sea, IrSPMW could act as a precursor for
the upper LSW (Pickart et al., 2003).
SAIW (SAIW6+ SAIW4) extends along the upper western part of the
2014 GEOTRACES-GA01 section, from the Labrador Sea to 20∘ W
(Fig. 4g, h), with SAIW6 being the main end-member and SAIW4 only
found over the Greenland Slope with less than 35 % of contribution
(average of 11 ± 7 %; n= 55). The maximum contributions of SAIW
are in the surface layer of the Labrador Sea, over the Greenland Slope and
in the first 1000 dbar on the eastern side of the Reykjanes Ridge. This
distribution reflects the formation and circulation of SAIW. After its
formation in the Labrador Current (Arhan, 1990), SAIW subducts below the NAC
that transports it to the Iceland Basin (Bubnov, 1968; Arhan, 1990; Read,
2000), where it mixes with SPMWs and Central Waters.
The dominant water mass in the 2014 GEOTRACES-GA01 section is LSW (Fig. 4e),
which extends along the whole section. The highest contribution of LSW is in
the Labrador Sea, with LSW concentrations reaching 100 %. LSW fills the
Labrador Sea from the surface almost to the seafloor, with the higher
concentrations found in the upper 1500 dbar. The distribution of LSW in the
Labrador Sea indicates recent ventilation (Kieke and Yashayaev, 2015), which
is also found in the Irminger Sea (de Jong and de Steur, 2016), where high
concentrations of LSW extend from surface to about 1200 dbar. The high
oxygen concentration found in the upper ∼ 1500 dbar in both
basins (Fig. 2c) corroborate the recent ventilation of the LSW layer in the
Labrador and Irminger Seas. The distribution of LSW in the Labrador Sea
extending deeper than 2000 dbar reflects the diapycnal mixing with ISOW
(Lauderdale et al., 2008) and/or the entrainment of LSW in the ISOW layer
all along the subpolar gyre. The LSW concentration in the Irminger Sea is
lower than in the Labrador Sea, with the upper 1000 dbar of the Irminger Sea
being dominated by a mixture of IrSPMW and LSW. The recent events of deep
convection observed in the Irminger Sea led to the formation of LSW-like
water in the Irminger Sea (de Jong and de Steur, 2016), with characteristics
in between the end-members of IrSPMW and LSW. High LSW concentrations are
also found in the West European Basin between 1000 and 3000 dbar. The
decrease in the contribution of LSW found over the Reykjanes Ridge, where
waters are a mixture between SPMW7, LSW and ISOW, suggests strong
mixing around and over the Reykjanes Ridge (Ferron et al., 2014). This
strong mixing is also observed in the θ and salinity distributions
(Fig. 2a, b) by a deepening of the isotherm of 6 ∘C and the
isohaline of 35. Some authors refer the admixture of Atlantic waters and
ISOW found around and over the Reykjanes Ridge as Icelandic Slope Water
(e.g. Yashayaev et al., 2007), which in our study is represented by mixing
group 4 (SPMW7, LSW, ISOW and NEADWU; Fig. 3c).
The relatively high salinity and low oxygen concentration centred at 1500 dbar
in the West European Basin (Fig. 2b, c) are linked to MW (Fig. 4f). MW
intersects the GEOTRACES-GA01 section over the Iberian shelf, between 500
and 2000 dbar, and spreads westwards until 20∘ W. This
westward extension may result from meddy transport (Arhan and King, 1995;
Mazé et al., 1997) or may be associated with the Azores countercurrent
(Carracedo et al., 2014).
The western part of the 2014 GEOTRACES-GA01 section (west of
20∘ W) below 2000 dbar is dominated by ISOW (Fig. 4b). The
main core of ISOW is located on the eastern flank of the Reykjanes Ridge,
reaching percentages greater than 90 %. From this region, ISOW extends
eastwards into the West European Basin, where it is progressively eroded and
diluted into NEADWL (Fig. 4h). ISOW also fills the deep areas of the
Irminger and Labrador Seas, with concentrations greater than 50 %. This
distribution of ISOW is consistent with its circulation from the
Iceland–Scotland sills, across the Iceland Basin along the eastern flank of
the Reykjanes Ridge, crossing the Charlie–Gibbs Fracture Zone (CGFZ)
(Dickson and Brown, 1994; Saunders, 2001), flowing cyclonically in the
Irminger Sea (Sarafanov et al., 2012) and joining the DWBC (e.g. Price and
Baringer, 1994; Rudels et al., 2002; Tanhua et al., 2008), and then flowing
cyclonically in the Labrador Sea (Xu et al., 2010). The eastward extension
of ISOW into the West European Basin may result from a fraction of ISOW
bypassing the CGFZ (Fig. 1), as reported in previous studies (e.g.
Fleischmann et al., 2001; LeBel et al., 2008; Xu et al., 2010; Zou et al.,
2017).
The bottom areas of the Labrador and Irminger Seas are occupied by DSOW
(Fig. 4g). The DSOW distribution is coincident with a minimum of θ
(< 2 ∘C), a maximum of oxygen and a relative minimum
of silicic acid (Fig. 2a, c, d). The DSOW distribution supports the
circulation scheme for DSOW, which, after its overflow formation on the
Greenland–Iceland sill, joins the DWBC, and then continues flowing within
that current around the Labrador Sea. Percentages of up to 25 % of PIW are
present within the realm of DSOW (average of 11 ± 6 %; n= 91;
Fig. 4f), supporting the existence of entrainment of East Greenland shelf
waters into DSOW (e.g. Pickart et al., 2005; Falina et al., 2012; Jochumsen
et al., 2015). PIW is also found on the continental shelves and slopes of
Greenland and Canada, with the greatest contribution found on the Canadian
shelf. The appearance of PIW on the Canadian shelf is in agreement with the
exchange of Arctic waters occurring via the Canadian Arctic Archipelago,
Baffin Bay and Davis Strait (Curry et al., 2014), which then join the
Labrador Current and intersect the GEOTRACES-GA01 section (Fig. 1).
The dominant deep water in the West European Basin is NEADWL (Fig. 4h),
which extends from 2000 dbar to the bottom. The location of NEADWL
coincides with the higher concentrations of silicic acid measured in section
(> 20 µmol kg-1; Fig. 2d), supporting the influence
of Antarctic Bottom Water in NEADWL (van Aken and Becker, 1996).
Water mass volume transports for 2014
Water mass volume transports across the 2014 OVIDE line (Portugal to
Greenland section) result from combining the water mass fractions from the
eOMP analysis (Xi values) with the absolute geostrophic velocity field. The
absolute geostrophic field orthogonal to the 2014 OVIDE line was estimated
by a box inverse model, using the hydrologic profiles measured at each
station, and constrained by ADCP velocity measurements and by a net volume
transport of 1 ± 3 Sv northwards to ensure mass conservation
(Lherminier et al., 2007, 2010; Zunino et al., 2017).
To allow the combination of the Xi values with the absolute geostrophic
velocity field, the Xi values were linearly interpolated in density
coordinates to match the grid of the absolute geostrophic velocity. Then the
Xi values were multiplied by the absolute geostrophic velocity field, to
estimate the water mass volume transports orthogonal to the section. The
resulting water mass volume transports were then integrated along the
section to obtain the net water mass volume transports (represented in
sverdrups; 1 Sv = 106 m3 s-1) (Fig. 5). Northward
(southward) water mass volume transports are positive (negative). Errors
were computed by propagating both the uncertainty of the Xi values (listed in
Table 1) and the uncertainty of the velocity field.
Net water mass volume transports (in Sv; 1 Sv = 106 m3 s-1)
perpendicular to the OVIDE line for the 2002–2010 period (white
bars; from García-Ibáñez et al., 2015) and for 2014 (grey
bars). Transports are positive (negative) northwards (southwards). Central
Waters refers to the sum of ENACW16, ENACW12 and SPMW8; and
SAIW to the sum of SAIW6 and SAIW4 (consult Table 1 for water mass
acronyms). Error bars represent the error in the net water mass volume
transport for 2014 and the standard deviation from the average net water
mass volume transport for 2002–2010.
We describe the water mass volume transports based on their contribution to
the upper and lower limbs of the AMOC. Across the OVIDE line, the upper and
lower limbs of the AMOC are separated by the isopycnal σ1= 32.15 kg m-3 (Mercier et al., 2015; Zunino et al., 2017), where σ1 refers to potential density referenced to 1000 dbar. The upper limb
of the AMOC for the 2014 OVIDE line is represented by the Central Waters
(ENACW16, ENACW12 and SPMW8; 13.7 ± 1.2 Sv), SPMW7
(3.3 ± 1.9 Sv) and SAIW (SAIW6 and SAIW4; 1.0 ± 0.6
Sv) (Fig. 5). We also included the net northward transport of MW (0.7 ± 0.6 Sv) to the AMOC upper limb, since it contributes to the
formation of intermediate waters in the SPNA (Reid, 1979, 1994). These flows
altogether result in an AMOC upper limb of 18.7 ± 2.4 Sv for OVIDE
2014, which is in good agreement with the 18.7 ± 3.0 Sv for the
intensity of the AMOC reported for OVIDE 2014 by Zunino et al. (2017). The
AMOC intensity is defined as the maximum of the surface-to-bottom integrated
stream function computed in density coordinates (Zunino et al., 2017). The
contributors of the upper limb of the AMOC agree with the subpolar (SAIW and
SPMW7) and subtropical (Central Waters) components of the AMOC at the
OVIDE sections described by Desbruyères et al. (2013).
The observed net volume transport for Central Waters (ENACW16,
ENACW12 and SPMW8) across the 2014 OVIDE line is in agreement with
the average value of 11.6 ± 1.4 Sv for 2002–2010
(García-Ibáñez et al., 2015) but lower than the 19.6 ± 1.7 Sv for 1997 reported as the net transport
of the NAC by Lherminier et al. (2007). The higher volume transport of Central Waters in 1997 is linked
to the higher AMOC intensity reported for that year (23.3 ± 1.2 Sv;
Lherminier et al. 2007) with respect to the AMOC intensity reported for 2014
(18.7 ± 3.0 Sv; Zunino et al., 2017).
Water mass differences between the OVIDE line (inset in subplot
a) in 2014 and the average of 2002–2010, from Portugal (right) to
Greenland (left). Positive (negative) anomalies in the proportion of a water
mass imply a gain (loss) in 2014 compared to 2002–2010. The grey line in
subplots (a) and (b) represents the isopycnal σ3= 41.25 g m-3 discussed in the text. The changes in SAIW4 are not shown due
to its low representation along the section. Sample locations appear as grey
dots. ABR refers to Azores–Biscay Rise. Consult Table 1 for water mass
acronyms.
The AMOC lower limb at the 2014 OVIDE line is, then, constituted by the
remainder water masses, i.e. IrSPMW (-10.2 ± 1.1 Sv), LSW (2.0 ± 1.7 Sv), ISOW (-5.1 ± 1.3 Sv), DSOW (-2.4 ± 0.4 Sv), PIW
(-2.2 ± 0.2 Sv) and NEADWL (0.3 ± 1.7 Sv) (Fig. 5),
resulting in a southward transport of -17.6 ± 3.0 Sv. The net volume
transport of LSW for OVIDE 2014 is in agreement with the 2 ± 1 Sv
reported for OVIDE 2002 by Lherminier et al. (2007). The net southward
transport of ISOW for OVIDE 2014 is significantly higher than previous
estimates of about -3 Sv (e.g. Saunders, 1996; van Aken and Becker, 1996;
Lherminier et al., 2007; Sarafanov et al., 2012). However, it is in
agreement with the -5.8 ± 0.9 Sv observed at the OSNAP array
(58–59∘ N) between July 2014 and July 2016 (Johns et al.,
2017). The transport of PIW was split into that associated with its
distribution in the upper 2000 dbar, and that associated with its
distribution deeper than 2000 dbar (samples assigned to mixing group 3; Fig. 3c), which was added to DSOW to agree with the cascading events occurring
along DSOW pathway (e.g. Pickart et al., 2005; Falina et al., 2012;
Jochumsen et al., 2015). The net volume transport of the shallow core of PIW
across OVIDE 2014 is in agreement with previous estimates of around -2 Sv
entering from the Arctic Ocean (barely -2 Sv reported by Pickart et al., 2005, and an average transport of -2.4 ± 0.3 Sv reported by Falina
et al., 2012, for 2002–2004). The net volume transport of DSOW across OVIDE
2014 (-2.2 ± 0.4 Sv; Fig. 5) is in good agreement with the estimates
of Ross (1984) (from -2 to -3 Sv), Eden and Willebrand (2001) (-2.5 Sv),
Lherminier et al. (2010) (-2 Sv for the OVIDE sections of 2002 and 2004),
and García-Ibáñez et al. (2015) (-2.4 ± 0.3 Sv for OVIDE
2002–2010). However, our estimate is slightly lower than the -3 Sv widely
recognized as the long-term average transport of DSOW (e.g. Dickson and
Brown, 1994; Macrander et al., 2005; Jochumsen et al., 2017). Finally, the
NEADWL net volume transport across OVIDE 2014 is compatible with
previous estimates of about 1 Sv (e.g. van Aken and Becker, 1996).
Differences in the water mass distribution and volume transport between
OVIDE 2014 and the average OVIDE 2002–2010
In this section, we describe and discuss the observed changes in the water
mass distribution along the OVIDE line (Portugal to Greenland) between
2002–2010 and 2014 (Fig. 6). We also describe and discuss how the changes
in water mass distributions are reflected in the water mass volume transport
across the OVIDE line (Fig. 5).
To obtain the differences in water mass proportions between 2002–2010 and
2014, we interpolated the water mass distributions of 2014 from this study
and those of 2002–2010 from García-Ibáñez et al. (2015) to a
common grid using a Delaunay triangulation. The selected grid was the
sampling locations from the OVIDE 2010 cruise. Positive (negative) anomalies in the proportion of a water mass imply a gain
(loss) in 2014 compared to 2002–2010.
The high mesoscale variability of the study area, with changes in the
location of fronts and eddies (Zunino et al., 2017), leads to changes in the
water mass proportions between 2002–2010 and 2014 in alternative patterns
of increases and decreases (Fig. 6). However, some water masses show
persistent and regionally localized changes linked to changes in the
hydrographic properties of the OVIDE section, which are further discussed.
Since our OMP analysis considers time-invariant properties characterizing
the SWTs, the inter-annual variability in the water mass properties at
formation is solved by the OMP analysis through water mass redistributions.
Therefore, the observed changes in the thermohaline properties between
2002–2010 and 2014 are reflected by the redistribution of water masses.
However, note that the properties describing LSW and ISOW were adjusted to
better represent their warming and salinization observed since the 2000s
(see Sect. 2.3) (e.g. Hansen et al., 2016; Yashayaev and Loder, 2017).
For alternative comparative purposes, the water mass distribution using the same SWT properties as
in García-Ibáñez et al. (2015) is given in Fig. S2 in the
Supplement.
The water masses that have experienced the greatest change in their
proportions along the OVIDE line between the 2002–2010 mean and 2014 are
LSW and ISOW. The changes in LSW follow two general patterns separated by
the isopycnal σ3= 41.25 kg m-3: an increase above it
and a decrease below it with respect to 2002–2010 (Fig. 6a). Comparing our
Fig. 6 with Fig. 7 in Zunino et al. (2017), the isopycnal of σ3= 41.25 kg m-3 represents the upper bound of the warming,
salinification and deoxygenation in 2014 with respect to 2002–2012. The
positive LSW anomalies coincide with negative anomalies of the SPMWs and
SAIW6. In the West European Basin, LSW replaced SPMW8 (Fig. 6f);
around and over the Reykjanes Ridge, LSW replaced SPMW7 (Fig. 6c); and
in the Irminger Sea, LSW replaced IrSPMW and SAIW6 (Fig. 6d, e), with
the exception of the EGIC where IrSPMW replaced LSW. The increase in the
relatively cold LSW at the expense of the relatively warm SPMWs is a response to
the cold and fresh anomalies in surface and intermediate waters in the 2014
with respect to 2002–2012 (Zunino et al., 2017), which is in agreement with
the long-term cooling of the North Atlantic Subpolar Gyre that started in
the mid-2000s (e.g. Robson et al., 2016; Piecuch et al., 2017). The
long-term cooling of the subpolar gyre is also observed by a redistribution
in between the different end-members of the Central Waters and SPMW, where
the colder end-members (ENACW12 and IrSPMW; Fig. 6b, d) replaced the
warmer end-members (ENACW16 and SPMW7; Fig. 6d, c) in 2014 with
respect to 2002–2010. The replacement of LSW by IrSPMW and PIW (Fig. 6f) in
the EGIC could be linked to the formation of LSW-like water in the Irminger Sea
(de Jong and de Steur, 2016), in which properties are in between our
end-members for IrSPMW and PIW. The redistribution between LSW and IrSPMW in
the EGIC could have also been caused by the narrowing of the Irminger Gyre in
2014 with respect to 2002–2012 (Zunino et al., 2017).
Below σ3= 41.25 kg m-3, the decrease in the
contribution of LSW in 2014 with respect to 2002–2010 is balanced by an
increase in ISOW (Fig. 6b). This water mass redistribution responds both to
the salinization of LSW (e.g. Yashayaev and Loder, 2017), and to the lower
density of LSW formed in recent years that occupies shallower positions in
the water column. García-Ibáñez et al. (2015) also reported how
the progressive salinization of LSW since the late 1990s resulted in a
progressive decrease in LSW and increase in ISOW. East of
22∘ W, the increase in ISOW compensates for the decrease in
NEADWL, which is linked to a decrease in silicic acid in the range of
20–35 µmol kg-1 (i.e. in the mixing zone between ISOW and
NEADW) in 2014 compared to 2002–2010 (Fig. S3 in the Supplement). The uniform increase in
ISOW is consistent with the increase in volume transport of ISOW observed in
the OSNAP array along 58-59∘ N (Johns et al., 2017; Zou et
al., 2017).
The proportion of DSOW generally decreases with respect to 2002–2010 (Fig. 6e), being partially compensated for by an increase in PIW (Fig. 6f). The
contrasting changes in DSOW corroborates the variability in the composition
of DSOW and in the entrainment events (e.g. Macrander et al., 2005; Tanhua
et al., 2008; Falina et al., 2012; van Aken and de Jong, 2012).
The changes in the remaining water masses are attributable to mesoscale
activity. The changes observed in MW (Fig. 6d) might be caused by meddy
activity (Arhan and King, 1995; Mazé et al., 1997) that, in 2014, mixed
MW with SPMW8 (Fig. 6f). The interchange between SAIW6 (Fig. 6e)
and SPMW7 (Fig. 6c) is explained by the relatively warm and salty
anticyclonic eddy found at the northern limit of the NAC (Zunino et al.,
2017; their Figs. 5 and 7).
The discussed changes in water mass distributions between the 2002–2010
mean and 2014 led to changes in the water mass volume transport across the
OVIDE line (Fig. 5). Most of the changes in the net water mass volume
transports between 2014 and the 2002–2010 mean are within the errors and,
therefore, are not significant, with the exception of SAIW, PIW and ISOW,
which are further discussed.
The observed net volume transport of 1.0 ± 0.6 Sv for SAIW (SAIW6
and SAIW4) across the OVIDE 2014 line is lower than the average value
of 2.2 ± 0.4 Sv for 2002–2010 (García-Ibáñez et al.,
2015), and the 2.9 Sv reported by Álvarez et al. (2004) for 1997. The
lower SAIW transport in 2014 compared to 1997 and 2002–2010 is related to
the generally lower proportions of SAIW in 2014, associated with the cold
and fresh anomalies in surface and intermediate waters in 2014 with respect
to 2002–2012 (Zunino et al., 2017). The net volume transport of the shallow
core of PIW across OVIDE 2014 (-2.2 ± 0.2 Sv) agrees with previous
estimates of around -2 Sv entering from the Arctic Ocean (barely -2 Sv
reported by Pickart et al. (2005), and an average transport of -2.4 ± 0.3 Sv reported by Falina et al. (2012) for 2002–2004). However, the net
volume transport of PIW in 2014 is significantly higher than the average
value of -1.4 ± 0.1 Sv for OVIDE 2002–2010
(García-Ibáñez et al., 2015). The increase in the contribution
of PIW to the EGIC (Fig. 6f) explains the observed increase in the transport
of PIW in 2014.
The net southward transport of ISOW for OVIDE 2014 (-5.1 ± 1.3 Sv) is
significantly higher than previous estimates of about -3 Sv (e.g. Saunders,
1996; van Aken and Becker, 1996; Lherminier et al., 2007; Sarafanov et al.,
2012). The ISOW transport is also significantly higher than the average
value of -2.8 ± 0.8 Sv for OVIDE 2002–2010
(García-Ibáñez et al., 2015). However, the net volume transport
of ISOW found in 2014 is in agreement with the -5.8 ± 0.9 Sv observed
at the OSNAP array (58–59∘ N) between July 2014 and July
2016 (Johns et al., 2017).
The assessment of the relative contribution of each water mass to the AMOC
across the OVIDE line allows the identification of the water masses involved in the
slight increase in the AMOC intensity from the average value of 16.2 ± 2.4 Sv for OVIDE 2002–2010 to 18.7 ± 3.0 Sv for OVIDE 2014 (Zunino et
al., 2017). An increase in the AMOC intensity implies an increase in the net
northward and southward transports of its upper and lower limbs,
respectively. The increase in the AMOC intensity is related to the increase
in the northward transport of the Central Waters in its upper limb, and to
the increase in the southward flow of IrSPMW and ISOW in its lower limb.
Conclusions
We described and discussed the distribution of water masses along the
GEOVIDE cruise (GEOTRACES-GA01 section), crossing the subpolar North
Atlantic Ocean and the Labrador Sea in summer 2014. We also provided the
relative contribution from each water mass to the transports across the
OVIDE line (Portugal to Greenland) of the GEOTRACES-GA01 section by
combining the eOMP results with the velocity field. The water mass structure
along the GEOTRACES-GA01 section, obtained through an eOMP analysis, is
consistent with generally accepted knowledge of the subpolar North Atlantic
circulation, with the exception of higher ISOW presence than the mean values
reported in the literature. Our estimates of water mass transports are in
good agreement with previous studies and match the main features of the
northern North Atlantic Circulation, with the exception of higher-than-expected transport of ISOW linked to the high proportion of this water mass
along the section. However, our net water mass volume transport of ISOW in
2014 is in agreement with the observed increase in the ISOW transport at
58–59∘ N since 2014.
We also assessed the change in the water mass distribution and transport of
the 2014 OVIDE line, by comparing them with the average OVIDE 2002–2010. At the
upper and intermediate water levels, the colder end-members of Central
Waters and SPMW as well as LSW replaced warmer water masses in 2014 with respect
to 2002–2010, in agreement with the observed cooling of the North Atlantic
Subpolar Gyre that started in the mid-2000s. Below 2000 dbar, ISOW presents
greater proportions in 2014 than in 2002–2010, with the increase being
consistent with other estimates of ISOW transports along
58–59∘ N. We also observed an increase in IrSPMW and PIW
within the EGIC in 2014 with respect to 2002–2010, an admixture similar to
LSW, which supports the recent deep convection events in the Irminger Sea.
This comparison of the water mass distribution for OVIDE 2014 and 2002–2010
highlights the utility of the eOMP analysis for identifying temporal
variations in water mass distributions linked to circulation changes and
water mass transformation.
The quantification of the volume transport for each water mass into the two
limbs of the AMOC allowed us to identify the water masses implicated in the
strengthening of the AMOC at the OVIDE line in 2014 in relation to the 2002–2010
average. The increase in the intensity of the AMOC upper limb is
related to the increase in the northward transport of the Central Waters,
which is partially compensated for by the increase in the southward flow of
IrSPMW and ISOW in the AMOC lower limb.
The 2014 GEOVIDE (GEOTRACES-GA01 and GO-SHIP A25) data
from the classical rosette is available at SEANOE:
http://doi.org/10.17882/54653 (Perez et al., 2018). The Greenland–Portugal
ADCP data are available at SEANOE: http://doi.org/10.17882/52153 (Lhermier
and Sharthou, 2017).
The Supplement related to this article is available online at https://doi.org/10.5194/bg-15-2075-2018-supplement.
All authors contributed extensively to the work presented in this paper.
MIGI, FFP and PL designed the research. MIGI, FFP, PL,
PZ, HM and PT analysed the physical and chemical data. MIGI and
FFP developed the code for processing the data. MIGI wrote the
paper and prepared all figures, with contributions from all co-authors.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “GEOVIDE, an international GEOTRACES study along the OVIDE
section in the North Atlantic and in the Labrador Sea (GA01)”. It is not associated with a conference.
Acknowledgements
We are grateful to the captains, staff and researchers who contributed to
the acquisition and processing of hydrographic data. We are especially grateful
to Morgane Gallinari, Emilie Grosstefan and Manon Le Goff, who contributed
to the analysis of nutrients, and to the technical team: Pierre Branellec,
Floriane Desprez de Gésincourt, Michel Hamon, Catherine Kermabon,
Philippe Le Bot, Stéphane Leizour, Olivier Ménage, Fabien
Pérault and Emmanuel de Saint-Léger. For this work Maribel I.
García-Ibáñez and Fiz F. Pérez were supported by the Spanish
Ministry of Economy and Competitiveness through the BOCATS (CTM2013-41048-P)
project, co-funded by the Fondo Europeo de Desarrollo Regional 2014–2020
(FEDER). Maribel I. García-Ibáñez was also supported by the Centre for
Climate Dynamics at the Bjerknes Centre. Pascale Lherminier was supported by
IFREMER. Patricia Zunino was supported by CNRS and IFREMER, within the framework
of the projects AtlantOS (H2020-633211) and GEOVIDE (ANR-13-BS06-0014-02).
Herlé Mercier was supported by CNRS and the AtlantOS H2020 project under grant
number 633211. Paul Tréguer was supported by the LABEX-Mer (French
Government “Investissement d'Avenir” programme, ANR-10-LABX-19-01).
Edited by: Gilles Reverdin
Reviewed by: three anonymous referees
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