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Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light of international partnership and collaboration

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Labrador Sea Water (LSW), the lightest contribution to North Atlantic Deep Water (NADW) and one of most prominent water masses of the subpolar North Atlantic, has seen remarkable changes over the past century. LSW originates in the Labrador Sea, where it is formed through wintertime ocean convection of varying intensity, depth and spatial extent. Formation of LSW, followed by its respective injection into the mid-depth circulation system, is mandatory for ventilating and renewing water layers of the interior ocean. Indispensably important for unraveling the history of variability in formation and properties of LSW as well as for mapping its large-scale spreading and export are sustained physical and chemical observations from the deep ocean. These observations started at the beginning of the 20th century from occasional mostly national surveys and today constitute large-scale multi-national collaborative efforts including a vast arsenal of sophisticated instrumentation. In a historical context, we revisit major milestones over the past 100 years which have established and are constantly adding to shaping today’s knowledge on LSW, and present first details on the latest vintage of LSW generated during the strong winter of 2013/2014. Respective Argo data reveal mixed-layer depths greater than 1700 m marking formation of a new cold and fresh anomaly that has spread since then over the subpolar North Atlantic. We further summarize the on-going observational efforts in the subpolar North Atlantic and present a compilation of hydrographic standard lines that serve to provide top-to-bottom information on NADW components.
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Studies of Labrador Sea Water formation and variability in the subpolar
North Atlantic in the light of international partnership and collaboration
Dagmar Kieke
a,
, Igor Yashayaev
b
a
Institut für Umweltphysik, AG Ozeanographie, Universität Bremen, Otto-Hahn-Allee, 28359 Bremen, Germany
b
Ocean Sciences Division, Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada
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Article history:
Available online xxxx
abstract
Labrador Sea Water (LSW), the lightest contribution to North Atlantic Deep Water (NADW) and one of the
most prominent water masses of the subpolar North Atlantic, has seen remarkable changes over the past
century. LSW originates in the Labrador Sea, where it is formed through wintertime ocean convection of
varying intensity, depth and spatial extent. Formation of LSW, followed by its respective injection into the
mid-depth circulation system, is mandatory for ventilating and renewing water layers of the interior
ocean. Indispensably important for unraveling the history of variability in formation and properties of
LSW as well as for mapping its large-scale spreading and export are sustained physical and chemical
observations from the deep ocean. These observations started at the beginning of the 20th century from
occasional mostly national surveys and today constitute large-scale multi-national collaborative efforts
including a vast arsenal of sophisticated instrumentation. In a historical context, we revisit major mile-
stones over the past 100 years which have established and are constantly adding to shaping today’s
knowledge on LSW, and present first details on the latest vintage of LSW generated during the strong
winter of 2013/2014. Respective Argo data reveal mixed-layer depths greater than 1700 m marking for-
mation of a new cold and fresh anomaly that has spread since then over the subpolar North Atlantic. We
further summarize the on-going observational efforts in the subpolar North Atlantic and present a com-
pilation of hydrographic standard lines that serve to provide top-to-bottom information on NADW
components.
Ó2014 Elsevier Ltd. All rights reserved.
Introduction
Over the past century, the western subpolar North Atlantic
attracted a close attention concerning different kinds of ocean
observations, making it the best-studied and monitored basin of
the world ocean. The initial efforts that started the century of
intense international and multidisciplinary oceanographic field
research in the subpolar North Atlantic have their roots in naviga-
tion, including insuring save and secure waterways, and explora-
tion. Identifying ice extents and iceberg dangers are of critical
importance as they pose a threat to commercial shipping, fishing,
oil exploration and other public, industrial, and economical needs
involving the ocean. The fishing industry itself has seen remarkable
changes through this period. The most dramatic impact of econ-
omy and even demography had the shutdown of fish catches e.g.
in Canadian waters in the 1980s to 1990s due to overfishing and
spatial shifts of fish populations prior to and following the collapse
(Rose et al., 2000). Knowledge on environmental and climate
conditions and associated variability were proved crucial to under-
stand changes in the ecosystem and recovery of fish stocks
(Lehodey et al., 2006).
Most notably, however, the processes relevant for (both regio-
nal and planetary) climate variability originate or are modulated
in the North Atlantic and have an impact on the regional and with-
out overstatement global climate (e.g., Marshall et al., 2001;
Vellinga and Wood, 2002). The subpolar North Atlantic, in particu-
lar, hosts a source of formation of its characteristic intermediate
and deep water masses (e.g. Dickson et al., 2008). They constitute
the northern loop of the Atlantic meridional overturning circula-
tion (AMOC) and connect the warm and saline near-surface waters
imported from the subtropics via the Gulf Stream and North Atlan-
tic Current (NAC) to the cold and deep returning flow, known as
the deep and abyssal limb of the AMOC. Water of subtropical origin
advects heat and subsequently releases it to the atmosphere along
its path toward the Polar Seas. This branch is often identified as
Atlantic Water that crosses the Greenland-Scotland Ridge and
enters the Nordic Seas (Greenland, Icelandic and Norwegian Seas)
and eventually remote polar basins (e.g., Hansen et al., 2003;
Yashayaev and Seidov, 2015). There (in the Nordic Seas), the
http://dx.doi.org/10.1016/j.pocean.2014.12.010
0079-6611/Ó2014 Elsevier Ltd. All rights reserved.
Corresponding author.
E-mail address: dkieke@uni-bremen.de (D. Kieke).
Progress in Oceanography xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Progress in Oceanography
journal homepage: www.elsevier.com/locate/pocean
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
Atlantic Water feeds Nordic water mass formation regions (e.g.,
Mauritzen, 1996; Isachsen et al., 2007). Another branch originating
from the NAC follows the given basin geometry of the subpolar
North Atlantic and via the Irminger Sea enters the Labrador Sea,
where it jointly with Arctic outflow and other local waters feeds
the source of water mass formation in the Labrador and Irminger
Seas. High oceanic heat losses between Labrador and Greenland,
strongest during the winter period, reduce the density stratifica-
tion and make the affected waters sink to greater depths (e.g.,
McCartney and Talley, 1982; McCartney, 1992), with the mean
buoyancy-forced downwelling happening near the boundaries of
the Labrador Sea (e.g. Pickart and Spall, 2007; Spall, 2010). Local-
ized water mass formation regions act as windows to the deep
ocean, as surface waters are transformed to intermediate and deep
waters by a complex chain of physical processes, thus introducing
characteristics previously imprinted at the surface into the deep
ocean (like contents of high dissolved oxygen, transient tracers,
and anthropogenic carbon). These deep waters are carried far into
the southern hemisphere, ventilate the intermediate to deep ocean,
and feed among others the formation of Circumpolar Deep Water,
thus demonstrating the importance of North Atlantic water masses
for the global ocean (e.g. Schmitz, 1996). Changes in the deep
water formation have direct consequences for the ventilation of
the deep ocean (e.g., Körtzinger et al., 2004; Stendardo and
Gruber, 2012), the ocean’s potential to store anthropogenic carbon
(e.g., Sabine et al., 2004; Steinfeldt et al., 2009), and the strength of
the AMOC and the associated heat transport (Srokosz et al., 2012).
The research efforts initiated and maintained in the North
Atlantic for more than a century delivered many insights into the
ocean circulation and both structure and variability of water
masses and helped to identify processes that modulate ocean and
climate variability in the western subpolar North Atlantic on a
large range of time scales of ocean variability. While scientific
motivation differed from cruise to cruise and project to project,
data of the past 100 years of oceanographic observations in the
subpolar North Atlantic constitutes an important legacy to today’s
society, as it is necessary and mandatory to describe observable
changes in a world facing global warming and expected conse-
quences (IPCC, 2013). Furthermore, available data is incorporated
into numerical ocean and climate models that serve to better
understand and improve prediction of ocean and climate variabil-
ity and to assess past and future states of the ocean circulation.
Here, we review important milestones of the past 100 years of
North Atlantic observational hydrography and focus on the historic
developments that have shaped our knowledge on the cold and
dense water masses forming the deep return branch of the AMOC,
collectively known as the North Atlantic Deep Water (NADW).
NADW consists of an upper and lighter and a lower and denser
component. The upper component originates in the Labrador Sea
located between Canada and Greenland, as reflected in its name
– Labrador Sea Water (LSW). It is produced by deep wintertime
ocean convection and is injected into the large-scale mid-depth
to deep circulation system via various pathways as, for example,
those revealed in parameter maps and float trajectories (e.g.
Talley and McCartney, 1982; Sy et al., 1997;Fischer and Schott,
2002;Yashayaev et al., 2007; Bower et al., 2009; Kieke et al.,
2009), removing it from the formation region. The lower compo-
nent of NADW originates from the Nordic Seas source waters spill-
ing over the Denmark Strait (Denmark Strait Overflow Water,
DSOW) and the Iceland-Scotland Ridge (Iceland-Scotland Overflow
Fig. 1. Overview on major oceanographic observational programs conducted in the subpolar North Atlantic over the past 100 years, with relevance for the formation of
Labrador Sea Water.
2D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
Water, ISOW; also often referred to as Northeast Atlantic Deep
Water, NEADW). All three water masses showed strong pentadal
(DSOW, e.g., Yashayaev and Dickson, 2008) to interdecadal (NEA-
DW, the same source) variability in all measured properties and
volume. The transport change was also found to be significant
though with no clearly discernible trend (Schott et al., 2004;
Kieke and Rhein, 2006; Jochumsen et al., 2012; Myers and Kulan,
2012; Mertens et al., 2014). Nonetheless, the most remarkable
changes ever seen in the intermediate and deep waters were
observed in the layer occupied by LSW. The physical and chemical
properties of this water mass as well as formation rate (reflected in
the corresponding volume changes observed both locally and
remotely) changed strongly throughout the past decades. What
played an important role in discovering these changes were the
national (mostly Canadian and German) and progressively broad-
ening and deepening and becoming more and more systematic
international efforts. These activities collectively deliver multi-
platform data at high quality and often in real-time (thus) allowing
the detection of important signals. Considering the importance of
LSW for rejuvenating and ventilating the intermediate to deep
ocean, the topic of revealing and monitoring its variability and
dynamics presents a very good example of international research
cooperation, fruitful collaboration, and synergetic efforts in a
region critical for ocean’s water mass renewal, ventilation and cli-
mate change that were historically put together by different
groups, e.g., research institutes, agencies and other public organi-
zations, and international bodies (Fig. 1).
A prehistory of deep ocean monitoring and process studies in
the Labrador Sea
The tragic sinking of RMS Titanic in April of 1912 brought
awareness and raised attention to processes on both shallow and
deep waters even not directly affected by or, in their (own) turn,
directly affecting seasonal ice cover, which led to the initiation of
the International Ice Patrol (IPP) survey in 1914 (Ricketts and
Trask, 1932;Fig. 1). This survey was the first precedent of
annual-to-seasonal monitoring of the northwest Atlantic, including
the Labrador Sea, and spanning more than half a century. While the
respective oceanographic unit was disestablished in 1982
(Capelotti, 1996), the IIP continued to deliver important informa-
tion on local sea ice conditions. Thus, today’s knowledge on water
mass formation in the Labrador Sea and dynamics of the boundary
currents dates back to the beginning of the 20th century. Smith
et al. (1937) reviewed scientific efforts conducted during the late
19th–early 20th century, often motivated by fishery research. In
their famous data compilation they systematically synthesized
oceanographic observations derived from expeditions conducted
in the western subpolar North Atlantic during 1928 to 1935 and
put them into relation to older observations available at that time.
Respective section data already revealed the nowadays well-
known doming of isotherms and a comparatively homogeneous
distribution of temperature and salinity at mid-depths in the
Labrador Sea, indicative of convective processes and water mass
transformation happening there. Smith et al. (1937) introduced
the conditions for the formation of deep water in the western
North Atlantic that were further on adopted by Sverdrup et al.
(1942) and represented established knowledge that remained
hardly changed until the 1950s – water gaining high salinity by
excess evaporation at lower latitudes is imported into the North
Atlantic; in the Irminger and Labrador Seas it is mixed with low
salinity water of Arctic origin, but still preserves a high salinity;
winter-time cooling initiates the onset of convective currents
reaching from the surface to the bottom which consequently leads
to the formation and renewal of deep and bottom waters of high
salinities and temperatures close to the freezing point – a concept
that was considerably modified and refined later on.
From the IGY surveys of 1957 and 1958 to the winter cruises of
the mid-to-late 1970s – First dedicated efforts to investigate
water mass formation in the Labrador Sea
Since the above-mentioned early efforts a lot of progress has
been made giving us know a corrected and much more detailed
picture of water mass formation in the Labrador Sea. Systematic
multi-national surveys conducted as part of the International Geo-
physical Year (IGY) 1957/58 program and the interlinked Polar Front
Survey marked an unprecedented milestone in international col-
laboration and coordinated research in the subpolar North Atlantic
(Fig. 1). Among the various IGY objectives, the analysis of the deep
circulation was one of the central ones. When motivating the Ger-
man contribution to IGY, Dietrich (1957) pointed to the annual
cycle in the deep water renewal of the Labrador and Irminger Seas
as a crucial process for the ventilation of the deep North Atlantic in
general. From observations of the mid-1930s (partly dating back to
the early Meteor cruises) and mid-1950s he postulated the deep
water renewal down to a depth of about 2500 m in localized
regions of the Labrador and Irminger Seas.
Using hydrographic data retrieved from Nansen bottle casts with
attached reversing thermometers, Fuglister (1960) compiled a first
IGY atlas that focused on vertical sections from the period 1954 to
1959, but excluded the western subpolar North Atlantic. Dietrich
(1969) further extended the IGY data set and presented quasi-synoptic
large-scales maps of temperature and salinity data, displayed at
different depth levels from the surface down to the bottom and for
the late winter and the late summer season of 1957/1958. Mapping
the winter distribution for the Labrador Sea was, however, excluded
due to the absence of respective data (Dietrich, 1969).
Lazier (1973) summarized the knowledge about the renewal of
LSW existing until then and elaborated on the renewal of LSW
based on data stemming from the large-scale cruise conducted in
the Labrador Sea and southern Irminger Sea between March and
May 1966 with the Canadian Scientific Ship (CSS)Hudson. Maps of
water mass properties on isopycnal surfaces served to localize
regions in the Labrador Sea that are likely to succumb to water
mass renewal, though respective observations actually did not
reveal direct evidence of deep convective mixing happening at that
time.
The CSS Hudson cruises of the years 1965–1967 (Grant, 1968)
and the no less famous Erika Dan cruise from early 1962
(Worthington and Wright, 1970) marked a period of intensive win-
tertime research carried out in the Labrador Sea. Unfortunately for
the scientific community, this period was characterized by low
atmospheric forcing being expressed by a sequence of moderate
winters and a prolonged phase of negative North Atlantic Oscilla-
tion (NAO), resulting in reduced LSW renewal (Lazier, 1973;
Clarke and Gascard, 1983). Changes in the NAO index represent
increases or decreases in the sea level pressure differences between
Iceland and the Azores (e.g., Hurrell, 1995), characterizing variabil-
ity of dominant states of the atmospheric circulation over the North
Atlantic (Hauser et al., 2015; Yashayaev and Seidov, 2015). A phase
of positive NAO is characterized by increased westerly winds at
mid-latitudes. Consequently, winters over Europe are milder and
those over the western North Atlantic are colder and represent a
larger surface forcing that has an impact on water mass formation
in the western subpolar North Atlantic. At times of negative NAO
phases, this relationship reverses (The Lab Sea Group, 1998).
Time series data retrieved from the Ocean Weather Ship (OWS)
Station Bravo located in the central Labrador Sea at 56°30
0
N, 51°W,
turned out to be inevitable valuable to gain insight into water mass
D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx 3
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
variability on shorter and longer time scales (e.g. Lazier, 1973,
1980; Straneo, 2006). The OWS program, with ten sites occupied
in the North Atlantic over varying periods, was primarily initiated
for the needs of aviation and commercial shipping and had its roots
dating back to the early 1920s, with OWS Bravo being maintained
from 1945 to 1974 (Lazier, 1980). Starting in January 1964 and
running for more than a full decade ending in January 1974 a par-
ticular hydrographic sampling program was conducted that
included daily Nansen bottle casts down to a water depth of
1500 m (Shuhy, 1969). By evaluating parts of the OWS Bravo data,
Lazier (1973) identified traces of deep convection in temperature
profiles of January 1967 and suggested, based on this evidence,
that ‘‘oceanographic surveys to examine the formation processes
in detail’’ may be planned. In further evaluating the OWS Bravo
data Lazier (1980) constructed monthly time series of temperature,
salinity, and density at 11 individual depths located between 10 m
and 1500 m, thus covering that part of the water column that is
typically affected by oceanic convection. Apart from being able to
describe the annual cycle of temperature and salinity in the upper
water column over this 10 year period, Lazier (1980) noticed a con-
siderable decrease in salinity in the upper layers between 1967 and
1971. Water mass properties recorded throughout this period
pointed to a decrease in water mass formation resulting from
low wintertime heat losses over the Labrador Sea (Lazier, 1980;
Smith and Dobson, 1984). Convection depths reduced to the upper
100–200 m range of the water column, but increased again in the
winter of 1972 to depths of 1500 m. These observations marked
again a milestone, as the need for time series observations suitable
to describe the history of formation and change and to identify
responsible physical processes became evident. The freshening of
the Labrador Sea and the temporary cessation of deep water
renewal in the late 1960s to early 1970s were later on attributed
to the passage of the so-called Great Salinity Anomaly (GSA)
(Dickson et al., 1988), a freshening event that originated from
anomalously high Arctic freshwater exports caused by anomalous
atmospheric conditions prevailing at that time and subsequently
advected throughout the entire subpolar North Atlantic. Remark-
able salinity anomalies, yet not as magnificent as the GSA (which
passage through the North, Norwegian and Barents Sea is presently
recapped by Yashayaev and Seidov, 2015), have been observed to
occur in the Labrador Sea in the 1980s and 1990s, though with dif-
ferent processes involved in their formation (e.g., Häkkinen, 2002;
Karcher et al., 2005).
Relevant physical processes and different parameters involved
in the renewal of LSW were further worked out in a series of papers
by Clarke and Gascard (1983), Gascard and Clarke (1983), and
Clarke and Coote (1988). Respective field data were obtained from
cruises conducted in the late winters of 1976 and 1978 by the same
CSS Hudson. Hydrographic observations now being carried out with
ship-based and Batfish-operated CTD systems were further comple-
mented by moored current meters. This series of dedicated obser-
vational efforts and a thorough analysis that followed had
comprised the first systematic effort directed to map and conse-
quently understand a convective process in the Labrador Sea.
Clarke and Gascard (1983) furthermore compared results from
the Labrador Sea to the observations of deep water renewal in the
western Mediterranean Sea that may easily reach the depths of
2000 m, while being intermittent in time and involving a hierarchy
of spatial scales from the order of 100 km to 1 km (see review by
Marshall and Schott, 1999). This led to the notion that the LSW
renewal processes in the two regions are comparable to a certain
extent and, as had already been done for the Mediterranean Sea,
can be divided into three phases: (1) a preconditioning phase asso-
ciated with a reduction in the stratification enabled by the cyclonic
circulation combined with significant surface buoyancy loss, (2) a
violent mixing phase associated with an increase in density through
convective mixing and respective plume formation, and (3) a phase
of spreading and lateral exchange between the convection site and
the ambient fluid involving a suite of advective processes and baro-
clinic instabilities (Clarke and Gascard, 1983; Marshall and Schott,
1999). While the general concepts applied to the Labrador Sea were
widely accepted (Marshall and Schott, 1999), there are particular
differences in the two regions e.g. regarding specific scales
involved, the different regional settings, the linkage of LSW forma-
tion history to the NAO, different vulnerability to freshwater sur-
face buoyancy forcing (Lilly et al., 1999).
With more and more data available and increasing length of
sustained time series, the changes of LSW on interannual to
multi-decadal time scales are becoming more and more evident.
An earlier long-term and broad-scale summary by Talley and
McCartney (1982) who exploited the extended OWS Bravo data
set for the period 1948–1974, revealed decadal changes in temper-
ature, salinity, density, and formation of LSW.
The legacy of WOCE – Toward multi-national interdisciplinary
coordinated efforts
Over the past decades, LSW showed significant variability in its
properties that is associated with variations in the strength and
persistency of open-ocean convection. In record years more than
two-thirds of the water column succumbed to convective over-
turning which contributes to the renewal of this water mass
(e.g., Lazier et al., 2002). Modern efforts to estimate long-term var-
iability in the water mass properties of the NADW components
have established intra-annual to multi-decadal time series of tem-
perature and salinity (e.g., Curry et al., 1998; Stramma et al., 2004;
Yashayaev, 2007; Jochumsen et al., 2012), contents of dissolved
oxygen (e.g., van Aken et al., 2011; Stendardo and Gruber, 2012),
anthropogenic carbon (e.g., Steinfeldt et al., 2009; Racapé et al.,
2013), trace gases (e.g., Azetsu-Scott et al., 2003; Kieke et al.,
2007), and even overturning and respective variability (Pickart
and Spall, 2007; Mercier et al., 2015). Respective data are collected
by repeated ship surveys using water sampling systems with
attached high-quality conductivity–temperature–depth/oxygen
(CTD/O) sensors operated at high vertical resolution and calibrated
against standards, by long-term moorings installed at key locations
in the Labrador and Irminger Seas, the Newfoundland Basin, the
overflow regions and deep basins of the eastern subpolar North
Atlantic, as well as by autonomously drifting profiling Argo floats
measuring temperature and salinity over the upper 2000 m of
the water column, while (also) providing Lagrangian
measurements.
The beginning of the internationally coordinated World Ocean
Circulation Experiment (WOCE,Fig. 1) that aimed at observing the
three-dimensional structure of the global ocean, a prerequisite
for improving predictions from ocean and climate models
(Siedler et al., 2001), was crucial for the understanding of LSW var-
iability. WOCE started its field phase in 1990 and followed after the
prototype-like large-scale field program Sections of the former
Soviet Union that started in 1981 and lasted until the collapse of
the Soviet Union (Lappo et al., 1995). Several hydrographic one-
time and repeat lines were established in the world ocean
(Siedler et al., 2001), among those the famous AR7W line running
from the Canadian continental slope across the central Labrador
Sea towards Greenland and occupied for the first time in 1990.
The WOCE Hydrographic Program (WHP) did not only focus on con-
ventional hydrographic observations with first and second order
quality control, but provided for the first time extensive chemical
data like nutrients, transient tracers (e.g. chlorofluorocarbons
(CFC), helium, tritium), and further parameters of the carbon sys-
tem from basin scales to global coverage. Many of these parame-
ters are now assembled into quality-controlled and synthesized
4D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
large-scale collections of hydrographic and chemical data, like
GLODAP, CARINA and expected follow-up efforts (Key et al.,
2004, 2010), compiled within the European CarboOcean and Carbo-
Change programs (Fig. 1).
AR7W was sampled by the Canadian partners from the Bedford
Institute of Oceanography (BIO) of the Department of Fisheries and
Oceans (DFO) operating from the same vessel (CSS, later Canadian
Coast Guard Ship,CCGS,Hudson)
1
once or on two occasions twice a
year, first as part of WOCE, then CLIVAR and, presently, AZOMP
(Fig. 1). In the period 1996–1998 repeats of up to three times
a year were conducted by international collaborators as further
programs like the Labrador Sea Convection Experiment (The Lab
Sea Group, 1998) or the German Sonderforschungsbereich (SFB)
460 (Schott et al., 2004; Stramma et al., 2004) were initiated in
the mid-1990s (Fig. 1). Strengthening the WOCE efforts, these
programs significantly contributed to a deepened understanding
of the physical processes and temporal and spatial scales
involved in the water mass transformation of the Labrador Sea
(e.g., The Lab Sea Group, 1998; Lilly et al., 1999; Marshall and
Schott, 1999; Lazier et al., 2002; Pickart et al., 2002; Rhein
et al., 2002; Stramma et al., 2004). As of today, on-going scien-
tific collaborative efforts as the German cooperative research pro-
ject (VV) RACE and its predecessor VV North Atlantic, the Canadian
VITALS and the multi-national European Union (EU) programs
THOR followed nowadays by NACLIM (Fig. 1) join AZOMP in data
collection along the AR7W line as part of large-scale tracer sur-
veys and activities related to deployments of moorings, floats,
and gliders, important ingredients in process-oriented and
export-related coordinated monitoring strategies comprising all
NADW components (e.g. Dengler et al., 2006; Fischer et al.,
2015).
Data obtained during the early WOCE period revealed a deep
dense and extremely voluminous class of LSW extensively doc-
umented in scientific literature (e.g., Curry et al., 1998; Lazier
et al., 2002; Yashayev et al., 2003; Kieke et al., 2006;
Yashayaev, 2007). However, the mode of LSW that was formed
through the early 1990s is quite exceptional in the ocean in
general and in the North Atlantic and is far from being seen
as a typical, but rather extraordinarily and unique one.
Yashayaev et al. (2008) and Kieke et al. (2009) show and dis-
cuss this mode based on different complementary analyses. At
that time, a series of harsh winters related to pronounced posi-
tive states of the NAO (Curry et al., 1998; Lazier et al., 2002)
preconditioned the ocean and reduced the density stratification.
A strong wintertime surface forcing with high heat losses to the
atmosphere resulted in convection depths exceeding 2000 m
(Lazier et al., 2002). Consequently, a huge newly formed volume
of LSW built up that was never observed before and since then
as being as homogeneous, thick, fresh, and cool. Different den-
sity based LSW definitions (Curry et al., 1998; Kieke et al.,
2006; Rhein et al., 2011) and direct volumetric definitions
(Yashayaev, 2007; Yashayaev et al., 2007) revealed layer thick-
nesses at the time of the surveys that were about 1800 m to
more than 2000 m (Fig. 2).
The Argo period – Developing capabilities for real-time
monitoring of variability and water mass formation throughout
ocean basins
The past and present ship-based data collection was and still is
mostly centered around warm months with few exceptions like the
CCGS Hudson cruises in fall 1996 and 2002 and the two Knorr
cruises of the winters 1997 and 1998 (e.g., The Lab Sea Group,
1998; Pickart et al., 2002). The international Argo program with
nowadays more than 3500 individual floats drifting freely in the
world ocean delivers hydrographic data for the upper 2000 m since
about the year 2000. The Argo data set (e.g. Yashayaev and Loder,
2009) and previous floats studies in the mid to late 1990s (e.g.,
Lavender et al., 2000, 2002; Steffen and D’Asaro, 2002; Avsic
et al., 2006), therefore, are immeasurably valuable for measuring
and monitoring winter convection as it delivers data throughout
the entire year, allowing to better determine typical summer and
winter conditions for each year.
Overall, milder winter conditions established after 1996 and
accompanied by a general shift to lower NAO resulted in a reduc-
tion of convectively generated mixed layer depths to about
1000 m with a general range of 700–1400 m (Fig. 3). A direct con-
sequence of this weakened forcing was the formation of a lighter
mode or year class of LSW following the winter of 1999–2000,
termed LSW
2000
in some studies (Yashayaev, 2007; Yashayaev
et al., 2007) or upper LSW in others studies (Stramma et al.,
Fig. 2. Temporal evolution of the layer thickness [m] of two modes of LSW derived from CTD data of the central Labrador Sea in the period 1988–2014. Upper LSW (uLSW) is
shown in red, dense or deep LSW (dLSW) is highlighted in blue. Superimposed is a respective uLSW estimate derived from the Argo program (black). A 3-yr filter is applied to
smooth the time series (solid lines). Uncertainties are presented as vertical bars. Update from Rhein et al. (2011). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
1
The AR7W line was occupied by a different Canadian ship only twice – in 1990
and 2012, by CSS Dawson and CCGS Martha L. Black, respectively.
D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx 5
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of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
2004; Kieke et al., 2006, 2007). Stratification in the upper Labrador
Sea increased (Lazier et al., 2002), and layer thickness estimates
revealed larger volumes of the lighter LSW mode formed since
the late 1990s at the expense of the denser mode formed in the
early 1990s (Kieke et al., 2006). An updated time series of layer
thickness estimates of both modes defined by the isopycnal ranges
is presented in Fig. 2. Respective time series show an out-of-phase
evolution, with one mode increasing in thickness resulting from
increased formation at the expense of the other mode. Estimates
mostly rely on available ship-based data stemming from annual
repeats. Apart from the dramatic changes noticed and discussed
since the early 1990s available Argo data reveals the seasonal cycle
in the layer thickness evolution of the lighter or upper LSW mode,
with layer thickness increasing over the winter period and subse-
quently decreasing again. This seasonal signal is dominated by
the seasonal vertical displacement of the isopycnal defining the
upper limit of uLSW (
r
h
= 27.68 kg/m
3
) with local wintertime out-
cropping and cannot be captured by the ship surveys. Whether it is
fully resolved by the Argo data set depends on the number, actual
location, and lateral distribution of the available floats.
Gelderloos et al. (2013) summarize the estimates of maximum
mixed-layer depths derived from ship-based measurements,
moorings, and floats for the period 1993–2009, which demon-
strates the strong multi-platform-based scientific effort to estab-
lish such metrics. These estimates are revisited here, further
expanded and updated until winter 2014 (Fig. 3). The time series
clearly reveals the changing maximum mixed-layer depth in the
past 25 years with considerable reduction between the mid-
1990s and mid-2000s, with this depth becoming more variable
and even more sporadic in the subsequent years. It cannot really
be said that intermediate-depth convection has ever left or, even
more, returned to the Labrador Sea. The recent pattern reminds a
random behavior of a typical climatic variable, suggesting that in
a certain year convection can reach quite deep. Indeed, the dec-
ade’s greatest mixed-layer depths were observed in 2008 and
2014 (Fig. 3). The latter event also caused the largest layer thick-
ness of uLSW observed in the past 25 years (Fig. 2).
Several studies revealed a warming of the dense LSW mode
since the mid-1990s coinciding with an increase in salinity, a
decrease in its thickness, and thus a deepening of its core (e.g.,
Azetsu-Scott et al., 2003; Rhein et al., 2011; Yashayaev, 2007;
Yashayaev et al., 2007; Yashayaev and Loder, 2009; Fischer et al.,
2010). The loss of LSW from the respective depth and density
was compensated by the Icelandic Slope Water arriving to the Lab-
rador Sea from the northeastern subpolar North Atlantic
(Yashayaev et al., 2007, 2008). It was also supplying heat and salt
mixing into the aging LSW class.
Winter of 2007/2008 showed a pronounced atmospheric cool-
ing over the Labrador Sea resulting in deep convection events hav-
ing extended as deep as 1600 m and possibly deeper, approaching
if not coming in contact with the density range of the old dense
LSW mode after its long isolation. This event was analyzed by using
the hydrographic measurements collected in the 2008 May CCGS
Hudson cruise (Yashayaev and Loder, 2009) and Argo float profiles
(Våge et al., 2008; Yashayaev and Loder, 2009), with the respective
estimates included in Fig. 3. After a relative increase of maximum
Fig. 3. (a) Winter NAO-Index (December to March), following Jones et al. (1997), with pink highlighting positive NAO and blue negative NAO phases. Note the reversed y-axis.
(b) Estimates of the maximum mixed-layer depth [m] from the central Labrador Sea and derived from various sources as summarized by Gelderloos et al. (2013) and updated
for the period 2009–2014 from Argo float data collected in the central Labrador Sea. A06: Avsic et al. (2006); GVK13: Gelderloos et al. (2013); L02: Lazier et al. (2002); L03:
Lilly et al. (2003); P02: Pickart et al. (2002); V09: Våge et al. (2008); YL09: Yashayaev and Loder (2009). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
6D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx
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of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
convection depth in the winter of 2007/2008, we observed convec-
tive mixing reaching below 1000 m in at least four years, with
probably one of the deepest happened just recently in the winter
of 2013–2014 resulting in mixed-layers exceeding 1700 m and
forming another fairly fresh and cold LSW vintage (Fig. 4). Argo-
derived temperature and salinity evolutions for the years 2002–
2014 averaged over 500–1000 dbar and 1000–1500 dbar (Fig. 5)
in the central Labrador Sea highlight those periods when convec-
tion was intensive enough to penetrate to depths of at least
1500 m. General tendencies towards an increasing warming of
the interior Labrador Sea over the Argo period (Figs. 4 and 5) were
abruptly interrupted at times of convection extending to greater
depths (2008, 2012, and 2014; compare Fig. 3) and picked up again
afterwards. A general increase of salinity at mid-depths persisted
until 2011/2012, and then salinities started to decrease again.
The future continuation of the Argo time series will show whether
this freshening persists over a longer period, possibly either shap-
ing into a new longer-term oscillation adding to those observed in
the Labrador Sea over the past years (van Aken et al., 2011), or con-
tributing to shorter cycles variability. The upper part of the water
column shows a pronounced seasonal cycle in temperature and
salinity (Fig. 5a and c). Temperature differences between summer
2007 and winter 2008 were similar to those of the period summer
2013 to winter 2014. The surface layer was, however, more saline
in summer 2007 compared to summer 2013, which points to lower
amount of buoyancy that must have been removed during water
mass transformation in 2007/2008 compared to 2013/2014. Ice
coverage data from the Canadian Ice Service (http://www.ec.gc.
ca/glaces-ice) indicate a higher weekly ice coverage for the season
2013/2014 compared to 2007/2008. The net annual surface heat
losses method computed using the method by Yashayaev and
Loder (2009) show increased heat losses in the years when convec-
tion exceeded 1300 m in depth. A more detailed (and in-depth)
analysis of the impact of atmospheric forcing and oceanic advec-
tion of heat on convection is presently underway.
Winter forcing of the season 2013/2104 thus led to a new fresh
and cold LSW anomaly (Fig. 4) that is thought to already spread
along the propagation pathways into the interior subpolar North
Atlantic. In addition to providing basin-wide all-season measure-
ments of physical and bio-chemical characteristics in both upper
and intermediate-depth layers (Figs. 2–5), the drifting Argo floats,
usually reporting their position every ten days, provide a unique
means for measuring and mapping ocean circulation. Fig. 6 shows
the most detailed view of mid-depth circulation in the Labrador
Sea based on observations. Displacements between consecutive
positions of individual Argo floats from a 10-year period, between
2002 and 2012, were summed up in 0.5°geographic coordinate
cells. Each cell accounts crossings by floats even if data transmis-
sion occurred outside that cell. The resulting velocity field reveals
an improved presentation of the known features of the Labrador
Sea circulation, such as the intensified boundary current along
the rim of the western basin, the flow towards the Mid-Atlantic
Ridge (Talley and McCartney, 1982; Fischer and Schott, 2002) pre-
sumably following the general pathway of the North Atlantic Cur-
rent (Kieke et al., 2009), pronounced recirculation in the central
Labrador Sea and off the boundary current south of 55°N(Fischer
Fig. 4. Argo derived temporal evolution of potential temperature (a) and salinity (b) in the western to central Labrador Sea for the period 2002–2014. Figure partly updated
from Yashayaev and Loder (2009).
D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx 7
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
and Schott, 2002), and export into the Irminger Sea (Lavender et al.,
2000; Yashayaev et al., 2008). Furthermore, some new features,
like the eastward flow exiting the Labrador Sea in its middle sec-
tion, can be easily identified in the maps of the average float dis-
placements. Note the weakening of the mean currents in the top
part of Fig. 6 that is caused by variations in current direction, which
also produce eddies.
Present observational challenges and opportunities
Oceanic deep convection is difficult to observe directly due to
the intermittent character and small spatial scales of convective
events and the necessity to have wintertime observations. Deep-
sea moorings from the central Labrador Sea deliver high resolution
time series that are able to capture short-term events ranging on
time scales from hours to days provided that they are installed at
the right spot at the right time (e.g., Lilly et al., 1999; Avsic et al.,
2006). Mooring activities in the Labrador Sea actually intensified
in the mid-1990s when the NAO started to change its phase thus
pointing to an expected weakening in the surface forcing (The
Lab Sea Group, 1998;Fig. 3a). Present extensive mooring efforts
concentrate on the NADW export within the boundary current
monitored since the mid-1990s at 53°N as part of SFB 460 (e.g.
Dengler et al., 2006; Fischer et al., 2010) and today continued as
part of the RACE and NACLIM efforts and further complemented
by a mooring array deployed recently at the southwestern tip of
Greenland as part of the US contribution to the multi-national
OSNAP initiative. Changes in the formation rate of LSW are there-
fore often inferred from indirect large-scale methods integrating
data over certain periods using, for example, water mass diagnostic
Fig. 5. Temperature and salinity derived from Argo profiles of the western to central Labrador Sea and averaged over the upper 100 dbar range (a and c) and 500–1000 dbar
was well as 1000–1500 dbar range (b and d). Available data from the central Labrador Sea were bin-averaged to 14 days, and respective time series subsequently smoothed by
applying a low-pass filter of 60 days.
8D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx
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of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
approaches and surface buoyancy forcing fluxes or changes in tran-
sient tracer inventories. Haine et al. (2008, also see references
therein) summarized and compared available evidence from a
large number of different approaches and methods to estimate
LSW formation rates and put them into relation to respective
results from ocean model simulations. While tendencies for the
1990s were in general agreement, particular observational results
differed by a factor of four to six, probably indicating more varia-
tion in the methods than in the variability of LSW production itself.
As pointed out in this study, the need to quantify uncertainties and
identify a consistent definition of water mass formation still pre-
vails today.
Tracer observations with regard to chlorofluorocarbon (CFC)
were first conducted in the Labrador Sea in 1986 (Wallace and
Lazier, 1988) and revealed deep convection happening on much
shorter time scales than air–sea interchange, as newly ventilated
water showed CFC undersaturations as low as 60%. Since then,
CFC measurements were included into the WOCE program and
were continued since the early 2000s as part of several national
and multi-national efforts (Fig. 1). The year 1997 marked the best
spatial resolution of CFC data ever obtained as more than six
different tracer groups participated in the field program (e.g.,
Rhein et al., 2002; Lebel et al., 2008). Insight from tracer
inventories changing in time have significantly contributed to
our understanding regarding LSW formation changes (e.g.
Smethie et al., 2000; Smethie and Fine, 2001; Rhein et al., 2002,
2011; Azetsu-Scott et al., 2003; Kieke et al., 2006, 2007; Lebel
et al., 2008). Since the mid-2000s this number reduced to typically
two to three, and particularly the eastern subpolar North Atlantic
nowadays faces a lower number of tracer observations (now
including sulphurhexafluoride, SF
6
) and coarser spatial coverage
compared to the late 1990s/early 2000s. This makes establishing
LSW formation rates based on changing tracer inventories of cer-
tain water masses more challenging and requires new approaches.
Rhein et al. (2011), for example, derived correlations between CFC
content of the LSW layer and its respective salinity, thus making
use of the larger amount of hydrographic data to close data gaps
in large-scale tracer inventories.
Formation of intermediate water masses in the Irminger Sea,
maybe not as deep, dense and voluminous as LSW, but yet contrib-
uting to overall ocean ventilation and water renewal was another
topic that came into focus in the past decade. Early indications that
winter convection can penetrate quite deep were already presented
by Nansen (1912). Considerable reassessment was done since the
early 2000s through the analysis of repeat hydrography data
stemming from the WOCE-line A1E/AR7E that crosses the southern
Irminger Sea (e.g. Pickart et al., 2003; Sarafanov et al., 2007), hydro-
graphic float profiles and tracer data (e.g. Bacon et al., 2003;
Fig. 6. Mid-depth circulation at around 1000 m derived from Argo float data of the period 2002–2012. Displacements between consecutive float positions were summed in
0.5°geographic coordinate cells. Different current regimes are highlighted by different colors and lengths of arrows, see legend in upper right corner of the map. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx 9
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
Centurioni and Gould, 2004; Kieke et al., 2006; Yashayaev and
Loder, 2009), and more recently mooring efforts (de Jong et al.,
2012). Respective results point to local convection occasionally cre-
ating mixed layers down to about 800 m or deeper that lead to for-
mation of mode waters suggesting to have a density comparable to
the ‘original’ LSW but being warmer and more saline (Pickart et al.,
2003). In contrast, Yashayaev and Loder (2009) compared Argo data
from the Labrador and Irminger Sea showing LSW spread into the
Irminger Sea underneath the locally formed water. Respective anal-
ysis of CTD sections from the pre-Argo period showed thinner deep
mixed layers overlain by a local mixed layer, and a general tendency
toward warmer and more saline properties, thus pointing to a
locally formed mode originating in the Irminger Sea and showing
features distinguishable from the LSW. Estimates of the strength
of local water mass formation in the Irminger Sea not to say vari-
ability of the formation rate are still missing.
Observations and findings discussed so far became possible
with the developing broad national and international collabora-
tions concerning fieldwork and data exchange (Fig. 1). These efforts
started to emerge in a systematic way in the mid-20th century,
intensified during the WOCE period and were brought to a new
unprecedented level through developments of the global network
of Argo floats and satellite altimetry (e.g. Gelderloos et al., 2013).
Fig. 7 summarizes past (since the 1990s) and present oceanographic
section lines with repeated hydrographic measurements. These lines
were and still are occupied as part of ship-based surveys conducted
with varying frequency and persistency, e.g., seasonally, annually,
biennially or more episodically. Most of this ship-based sampling
takes place between spring and fall. Though some of these, hereafter
standard, lines are not critical for monitoring properties and subse-
quent spreading of LSW, as they are too shallow or restricted to
somewhat narrower zones, like the ridges, channels and shelves sur-
rounding the subpolar North Atlantic, shown hydrographic lines are
respected as part or important components of one effective and effi-
cient ocean’s hydrographic observational network. All efforts associ-
ated with the standard lines in the subpolar North Atlantic are
important to studying of deeper NADW components and/or
exchanges between the basin margins and the interior. They fur-
thermore provide the basis for systematically conducting sophisti-
cated localized process studies involving Eulerian and Lagrangian
flow analyses by means of deployed moorings, gliders, floats of var-
ious types and bottom-mounted inverted echo sounders.
Some of the former WOCE repeat sections, like AR7W (during
WOCE addressed as A1W/AR7W), still persist today in their
original form, although the station grid on some of these lines
is now more detailed. The eastern extension of AR7W, the
WOCE line A1E/AR7E running from Greenland towards Europe,
formerly occupied mostly annually by German and Dutch insti-
Fig. 7. Observational hydrographic standard lines with focus on physical oceanographic observations and relevance for assessing variations in the strength of the AMOC and
contributing NADW components. Gray lines denote repeated sections of the 1990s and early 2000s (WOCE and VEINS/ASOF) that are presently inactive. Colored lines
highlight present hydrographic standard lines occupied in the framework of various scientific programs and repeated at different schedules. Text labels refer to either
established names of the respective standard line, scientific projects or responsible institutions running the repeat surveys along these lines. Abbreviations not already
introduced in Fig. 1 are: DMI/GINR, sections of the Danish Meteorological Institute/Greenland Institute of Natural Resources; EEL, Extended Ellett Line; MAR, Mid-Atlantic
Ridge section; OB, Orphan Basin section; XHL, eXtended Halifax Line. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
10 D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
tutions, was discontinued in its original form. However, some
parts of A1E/AR7E or lines close to those are now co-occupied
by at least three national and international programs, among
such are the French OVIDE (e.g. Mercier et al., 2015), the Rus-
sian 59°30
0
N sections (commonly referred to as 60°N), and the
US, UK, and Dutch efforts that merged into the recently estab-
lished multi-national OSNAP program, which together with the
Canadian VITALS are the latest additions to the on-going activi-
ties in the subpolar North Atlantic (Figs. 1 and 7). The former
WOCE-line A2/AR19 section running from the Grand Banks
towards the European Shelf along nominally 43°Nto48°Nis
close to the 47°N line presently occupied as part of the German
RACE initiative.
Both, the European Union program VEINS conducted during
1990–1997 and followed by the multi-national ASOF study initi-
ated in 2000, address(ed) the exchange between the Arctic Ocean
and the subpolar North Atlantic (Dickson et al., 2008), which
implemented repeated observational lines crossing the Irminger
Sea at several latitudes. Parts of these repeat sections in the north-
ern and central Irminger Sea have been transitioned into occa-
sional occupations, mostly in the framework of localized process
studies, but are no longer active as frequently repeated standard
lines. The Denmark Strait is the northern limit of the subpolar
North Atlantic with surveys on seasonal to annual time scales for
the purpose of recording and investigating the import of Denmark
Strait Overflow Water, the densest NADW component, into the
subpolar basin.
Deep ocean observations in the eastern subpolar North Atlantic
are presently provided through biennial to occasional surveys
along major lines crossing the eastern basins that aim at measuring
the latitudinal exchange of water masses (e.g. the OVIDE line from
Greenland to Portugal, the CLIVAR section along 20°W, the OSNAP-
east line, and the eastern extension of the 47°N section occupied as
part of RACE and approximately following the eastern part of the
former WOCE-line A2/AR19). The 47°N section, further strength-
ened by deployments of moorings and bottom-mounted echo
sounders along its western part (Mertens et al., 2014), is presently
the southernmost cross-subpolar North Atlantic standard line
addressing southward deep water export out of the northern
basins and northward near-surface import of subtropical origin.
In contrast, the eXtended Halifax Line (XHL) centered at 60°Wis
annually occupied by the BIO as a part of the Canadian AZOMP pro-
gram to investigate import and export conditions at the southern
boundary of the subpolar gyre. The overall impression here is that
key sites concerning the formation of lighter and dense NADW
components or the export of the Atlantic Water towards the Nordic
Seas are presently and over the next few years will be well covered
by the scientific community. The interior subpolar gyre, however,
shows larger regions not well covered by deep ship-based observa-
tions. At present only the MAR-line, which is covering the latitudi-
nal range from the Charlie-Gibbs Facture Zone located at 52°30
0
N
south to 47°N, is sustained and annually visited in the framework
of RACE to investigate cross-basin exchange and associated vari-
ability. To the north of 53°N, there is a larger region bracketed by
the AR7W and the OSNAP-west lines from the west and by the
OVIDE line from the east without any regularly repeated hydro-
graphic standard line in between. However, this is the region
where the NADW components of the Labrador and Irminger Seas
are heavily exchanged. Thus, the deep part of the water column
carrying the overflow components is presently not covered sys-
tematically, as present-day Argo floats do not penetrate into the
overflow layers. In addition, the northwestern Labrador Sea is not
regularly visited, but convection may become more limited over
years shifting to the north. Therefore, the band where all those dif-
ferent lines meet is well sampled – but partly at the expense of
southerly and northerly regions. The on-going studies will show
whether there will be growing need to re-establish and occupy
again standard sections like the former WOCE-lines A20 and A25
crossing the subpolar basin from Greenland to the Azores.
Conclusions
In this paper, we revisit major milestones of ocean observations
that contributed to our today’s knowledge on deep ocean variabil-
ity and water mass formation in the subpolar North Atlantic, which
is done through relating all key phases of observations to the his-
tory and research of one of the most important water masses of
the North Atlantic – the LSW. The LSW has seen remarkable
changes throughout the past century. The latest distinct year class
of LSW (LSW
2014
) was formed in the winter of 2013–2014. It
appeared very strongly in both physical and chemical properties
measured as part of Argo and of DFO’s AZOMP May occupation of
AR7W (a paper is currently in preparation), and can be seen in
hydrographic data as a water that is colder and fresher than in sev-
eral preceding years. In this paper, we limit a discussion of this
event, important to the subpolar North Atlantic, to showing
time-depth evolution of temperature and salinity based on Argo
float profiles. The strong winter of 2013/2014 produced mixed-
layer depths of about 1700 m that were comparable to those
observed in winter 2008. The winter cooling in 2011–2012 created
the third deepest mixed layers (about 1500 m) within the past five
years. As such, deep convection creating mixed layer depths of
more than 1500 m was more often present in the past five than
in the mid-1990s to mid-2000s (Fig. 3). The fresh and cold signal
of the LSW
2014
is thought to propagate along known spreading
pathways. Observing its arrival in different parts of the subpolar
North Atlantic with the help of collective complementing observa-
tional efforts (summarized in Fig. 7) will be instrumental to re-
evaluating our existing knowledge of LSW production and export
rates, and its spreading and arrival times.
Unraveling physical processes that contribute to or dominate
this variability and assessing its impact on AMOC variations
requires sustained integrated and broader cross-disciplinary inter-
nationals efforts that would provide the necessary observations
from the deep ocean. This would facilitate the arrival at more accu-
rate estimates of LSW production, its variation/variability and
impact on both horizontal and overturning circulation, renewal
of intermediate and even deeper waters across the subpolar North
Atlantic. As long as the Argo program continues and expiring floats
are replaced regularly, it is possible to monitor the seasonal to
interannual evolution of hydrographic changes in the Labrador
Sea and on larger spatial scales linked to changing environmental
conditions. As the profiling range of the floats is limited to
2000 m respective variations in the denser LSW mode cannot be
resolved which emphasizes the need to sustain existing ship-based
hydrographic standard lines and to push towards efforts extending
the vertical range of the existent Argo array. First pilot deploy-
ments of Deep Argo devices delivering profiles down to 4000 m
are presently carried out. Once the technology is mature, sampling
strategies are defined and Deep Argo is implemented, it may
become feasible to analyze the structure and variability of the dee-
per part of the water column (Johnson and Lyman, 2014).
Acknowledgements
This work would not have been possible without the dedicated
work of all scientists and crew working for a century at sea under
often unfavourable conditions and contributing to unraveling the
secrets of one of the most prominent water masses of the Atlantic
Ocean. We thank two anonymous reviewers whose comments
helped to improve this manuscript. We further thank a large num-
D. Kieke, I. Yashayaev / Progress in Oceanography xxx (2015) xxx–xxx 11
Please cite this article in press as: Kieke, D., Yashayaev, I. Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light
of international partnership and collaboration. Prog. Oceanogr. (2015), http://dx.doi.org/10.1016/j.pocean.2014.12.010
ber of scientists who provided information on their national pro-
grams that greatly helped to compile information shown in
Fig. 7. Funding for DK comes from BMBF-RACE (Grant 03F0605C)
and Cluster of Excellence – Marum, OC-1. IY is supported by the
Bedford Institute of Oceanography, Department of Fisheries and
Oceans (DFO), Canada. DFO provides ongoing support to the Atlan-
tic Zone Off-Shelf Monitoring Program (AZOMP), comprising
annual occupations of the AR7W and XHL lines and the Atlantic
Canadian contribution to the International Argo program. Argo data
were collected and made freely available by the International Argo
Program and the national programs that contribute to it (http://
www.argo.ucsd.edu,http://argo.jcommops.org). The Argo Program
is part of the Global Ocean Observing System. We thank B. Klein,
Bundesamt für Seeschifffahrt und Hydrographie, Hamburg, Ger-
many, for kindly providing additional quality control on a North
Atlantic subset of the Argo data. This subset was downloaded from
the Coriolis data repository on June 25, 2014.
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... While basin-scale and multidecadal-scale fluctuations in the properties of the upper ∼700 m of the suboplar gyre are well described by the Atlantic Multidecadal Variability (AMV; Zhang, 2008;Polyakov et al., 2010;McCarthy et al., 2015;Frajka-Williams et al., 2017;Desbruyères et al., 2020) and its associated mechanisms , the origin of deeper subpolar variability is not yet clear, and various theories exist on the mechanisms of deep ventilation. Reconciling these perspectives is important for improving model representation and predictability of the deep ocean (Kieke & Yashayaev, 2015), and for our understanding of how biogeochemical variables such as CO 2 and nutrients are sequestered (Fröb et al., 2016;Hill et al., 2004). ...
... The central Labrador Sea is considered a primary location of deep ventilation. This has commonly been thought to occur through variability in open-ocean deep convection strength, particularly in terms of how it enables time-varying changes in ocean column-integrated buoyancy exchange with the atmosphere (Kieke & Yashayaev, 2015;van Aken et al., 2011;Yashayaev & Loder, 2016). Yet, observational analyses show that the strength of Labrador Sea deep convective ventilation varies at decadal timescales, whereas deep temperature and salinity Abstract The ventilation of the central Labrador Sea is important for the uptake of ocean tracers and carbon. ...
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Plain Language Summary The central Labrador Sea is a globally‐important region for the modification of deep temperature and salinity. However, the nature of the ventilation mechanisms of the central Labrador Sea is not well understood. The conventional perspective is that time‐varying convective mixing in the central Labrador Sea actively modifies deep properties by mediating exchange with the atmosphere. Here using historical ocean observations, we show that in addition to this mechanism the Labrador Sea open‐ocean deep convection also acts to vertically redistribute large‐scale upper ocean anomalies that can originate outside of the central Labrador Sea and that are associated to variations in the Atlantic Multidecadal Variability, the dominant mode of basin‐wide low frequency temperature and salinity fluctuations. Using a simple multiple linear regression model, which combines predictors for the two mechanisms that affect deep central Labrador Sea properties, we demonstrate that we can closely reconstruct the observed decadal variability of the upper ∼2,000 m of the central Labrador Sea temperature, salinity and density. The results will help with the development of decadal prediction systems by improving our understanding of the mechanisms leading to decadal variability in the deep ocean.
... For a long time, it has been the prevailing view that deep convection in the subpolar North Atlantic predominantly occurs in the western SPG in a relatively confined region in the Labrador Sea, hereafter referred to as the primary deep convection region (Yashayaev, 2007). However, there were early reports (e.g., Nansen, 1912) and a recently increasing number of publications suggesting at least sporadic deep convection also in the eastern SPG south of Cape Farewell and in the Irminger Sea, hereafter referred to as the secondary deep convection regions (Bacon et al., 2003;de Jong et al., 2012de Jong et al., , 2018Falina et al., 2007;Fröb et al., 2016;Kieke & Yashayaev, 2015;Martin & Moore, 2007;Paquin et al., 2016;Pickart, Spall, et al., 2003;Pickart, Straneo, et al., 2003;Pickart et al., 2008;Piron et al., 2016Piron et al., , 2017Sarafanov, 2009;Sarafanov et al., 2018;Sproson et al., 2008;Våge et al., 2008Våge et al., , 2009Zunino et al., 2020). In particular, analyses based on Argo data indicate that after a period with mainly low deep convection activity in the 2000s and early 2010s, deep convection resumed in 2015-2018 at the gyre-scale (Piron et al., 2017;Zunino et al., 2020). ...
... In short, a complex interplay of a multitude of factors in the ocean-atmosphere-cryosphere system determine when and where deep convection occurs in the subpolar North Atlantic. Due to its complexity, and despite intense research in the field (Kieke & Yashayaev, 2015), the nature of deep convection variability in the subpolar North Atlantic is still not fully understood and hard to predict (e.g., Våge et al., 2009). In particular, it is not clear how often and to what spatial extent deep convection and associated deep water formation occur outside the primary deep convection region in the central Labrador Sea, that is, in the secondary deep convection regions south of Cape Farewell and in the Irminger Sea. ...
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Deep convection and associated deep water formation are key processes for climate variability, since they impact the oceanic uptake of heat and trace gases and alter the structure and strength of the global overturning circulation. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea in the western subpolar gyre (SPG). However, there is increasing observational evidence that deep convection also has occurred in the eastern SPG south of Cape Farewell and in the Irminger Sea, in particular, in 2015–2018. Here we assess this recent event in the context of the temporal evolution of spatial deep convection patterns in the SPG since the mid-twentieth century, using realistic eddy-rich ocean model simulations. These reveal a large interannual variability with changing contributions of the eastern SPG to the total deep convection volume. Notably, in the late 1980s to early 1990s, the period with highest deep convection intensity in the Labrador Sea related to a persistent positive phase of the North Atlantic Oscillation, the relative contribution of the eastern SPG was small. In contrast, in 2015–2018, deep convection occurred with an unprecedented large relative contribution of the eastern SPG. This is partly linked to a smaller north-westward extent of deep convection in the Labrador Sea compared to previous periods of intensified deep convection, and may be a first fingerprint of freshening trends in the Labrador Sea potentially associated with enhanced Greenland melting and the oceanic advection of the 2012–2016 eastern North Atlantic fresh anomaly.
... Deep and surface circulation in the SPG and Labrador Sea are very important elements of the Atlantic Meridional Overturning Circulation (AMOC) cell and consequently global ocean circulation. Formation of Labrador Sea Water (LSW) contributing to (upper) NADW has, however, been found to show large variability throughout the past decades 9,[19][20][21] . During the LGM, the Labrador Sea is believed to have been characterised by cold sea-surface temperatures and perennial sea-ice cover along the Canadian and Greenland margins, while the central North Atlantic and Norwegian Sea may have been seasonally ice-free 2 . ...
... Part of the warmer and more saline WGIW water masses reaches the interior of the Labrador Sea basin south of the Davis Strait as a weak, eddy-dominated flow 22 . This flow of saline water to the central Labrador Sea is crucial for the formation of Labrador Sea Water (LSW), a deep to intermediate water mass contributing to upper NADW 9,19 . However, a part of the relatively warm and saline Atlanticsourced subsurface waters (WGIW) continues with the WGC northward along the shelf and upper slope of West Greenland across Davis Strait and into the Baffin Bay 27,28 . ...
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The Last Glacial Maximum (LGM, 23–19,000 year BP) designates a period of extensive glacial extent and very cold conditions on the Northern Hemisphere. The strength of ocean circulation during this period has been highly debated. Based on investigations of two marine sediment cores from the Davis Strait (1033 m water depth) and the northern Labrador Sea (2381 m), we demonstrate a significant influx of Atlantic-sourced water at both subsurface and intermediate depths during the LGM. Although surface-water conditions were cold and sea-ice loaded, the lower strata of the (proto) West Greenland Current carried a significant Atlantic (Irminger Sea-derived) Water signal, while at the deeper site the sea floor was swept by a water mass comparable with present Northeast Atlantic Deep Water. The persistent influx of these Atlantic-sourced waters entrained by boundary currents off SW Greenland demonstrates an active Atlantic Meridional Overturning Circulation during the LGM. Immediately after the LGM, deglaciation was characterized by a prominent deep-water ventilation event and potentially Labrador Sea Water formation, presumably related to brine formation and/or hyperpycnal meltwater flows. This was followed by a major re-arrangement of deep-water masses most likely linked to increased overflow at the Greenland-Scotland Ridge after ca 15 kyr BP.
... Due to our experimental setup and as expected from literature (Kieke and Yashayaev, 2015;Palter et al., 2016;Fischer et al., 2018), NADW is majorly advected within the boundary currents close to the continental slope or the shelf break. Already west of the Eirik Ridge we noticed an enhanced divergence of the particle pathways, which coincides with trajectories from RAFOS floats of the OSNAP float program (Zou et al., 2021). ...
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This is a preprint (before peer review) of our paper "Major sources of North Atlantic Deep Water in the subpolar North Atlantic from Lagrangian analyses in a high–resolution ocean model". In this work we use Lagrangian analysis to find the sources of NADW within the subpolar Gyre in the ocean model VIKING20X-JRA- OMIP. In this study we show the multiple sources of NADW passing 53°N contributing to similar density regimes. The classical view of density classes being directly linked to a common formation region holds only partly within our experiments. We rather find that different origins in combination with transformation processes such as diapycnal mixing along the pathways lead to water mass properties that can be very similar at the southern exit of the Labrador Sea.
... The time evolution of the MLD in the Labrador Sea, shown in Figure 5, allows to see that the convection in the Labrador Sea has a strong interannual variability and can be compared with 2004-2014 observation-based estimate of convective activity from Figure 3 of Kieke and Yashayaev (2015), with some years with almost no convection activity (e.g., 2010) while others exhibit strong convective events (e.g., 2008). The strong convective event found in 2015 is also in agreement with observations (Yashayaev and Loder, 2017). ...
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The Atlantic Meridional Overturning Circulation (AMOC) is a crucial element of the Earth climate. It is a complex circulation system difficult to monitor and to model. There is considerable debate regarding its evolution over the last century as well as large uncertainty about its fate at the end of this century. We depict here the progress since the IPCC SROCC report, offering an update of its chapter 6.7. We also show new results from a high-resolution ocean model and a CMIP6 model to investigate the impact of Greenland Ice Sheet (GrIS) melting, a key uncertainty for past and future AMOC changes. The ocean-only simulation at 1/24° resolution in the Arctic-North Atlantic Ocean performed over the period 2004–2016 indicates that the spread of the Greenland freshwater runoff toward the center of the Labrador Sea, where oceanic convection occurs, seems larger in this model than in a CMIP6 model. Potential explanations are related to the model spatial resolution and the representation of mesoscale processes, which more realistically transport the freshwater released around the shelves and, through eddies, provides strong lateral exchanges between the fine-scale boundary current and the convective basin in the Labrador Sea. The larger freshening of the Labrador Sea in the high-resolution model then strongly affects deep convection activity. In the simulation including GrIS melting, the AMOC weakens by about 2 Sv after only 13 years, far more strongly than what is found in the CMIP6 model. This difference raises serious concerns on the ability of CMIP6 models to correctly assess the potential impact of GrIS melting on the AMOC changes over the last few decades as well as on its future fate. To gain confidence in the GrIS freshwater impacts on climate simulations and therefore in AMOC projections, urgent progress should be made on the parameterization of mesoscale processes in ocean models.
... The lower isopycnal for UNADW at both sections is 27.80 kg m −3 , chosen to exclude the lower overflow component of NADW (LNADW). We refer to UNADW and LNADW as the water masses contained in the lower limb of the MOC (e.g., 3,24 ). For the thickness calculation in the basin interior, we add a planetary potential vorticity constraint (<4 × 10 −12 m −1 s −1 ) to identify newly-formed deep waters 25 . ...
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Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region. Western boundary current variability in the subpolar North Atlantic is thought to reflect interior convection changes and determine Atlantic Meridional Overturning Circulation variability. Here, the authors show with an extended OSNAP time series that neither linkage is robust due to the complex dynamics in the region.
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Recent studies using data from the OSNAP observational campaign and from numerical ocean models suggest that the Iceland Basin and the Irminger Sea may be more significant for formation of upper North Atlantic Deep Water than the Labrador Sea. Here, we present a set of hindcast integrations of a global 1/4° NEMO simulation from 1958 until nearly the present day, forced with three standard forcing data sets. We use the surface‐forced stream function, estimated from surface buoyancy fluxes, along with the overturning stream function, similarly defined in potential density space, to investigate the causal link between surface forcing and decadal variability in the strength of the Atlantic meridional overturning circulation (AMOC). We use the stream functions to demonstrate that watermasses in the simulations are transformed to higher densities as they propagate around the subpolar gyre from their formation locations in the north‐east Atlantic and the Irminger Sea, consistent with the picture emerging from observations. The surface heat loss from the Irminger Sea is confirmed to be the dominant mechanism for decadal AMOC variability, with the heat loss anomaly from the Labrador Sea having about half the magnitude. A scalar metric based on the surface‐forced stream function, accumulated in time, is found to be a good predictor of changes in the overturning strength. The AMOC variability is shown to be related to that of the North Atlantic Oscillation (NAO), primarily through the surface heat flux, itself dominated by the air‐sea temperature difference, but also with some local feedback from the SST to the surface fluxes.
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Since 2014, an array of current meters deployed in the Iceland Basin as part of the Overturning in the Subpolar North Atlantic Program has provided new measurements of the southward flow of Iceland‐Scotland Overflow Water (ISOW) along the eastern flank of the Reykjanes Ridge. The location of the array, near 58–59°N, captures the ISOW plume at the farthest downstream location in the Iceland Basin before significant amounts of ISOW can flow into the Irminger Basin through deep fractures in the Reykjanes Ridge. The net transport of the ISOW plume at this location—approximately 5.3 Sv based on the first 4 years of observations—is significantly larger than previous values obtained farther north in the Iceland Basin, suggesting that either previous measurements did not fully capture the plume transport or that additional entrainment into the ISOW plume occurs as it approaches the southern tip of the Reykjanes Ridge. A detailed water mass analysis of the plume from continuous temperature/salinity observations shows that about 50% of the plume transport (2.6 Sv) is derived from dense waters flowing over the Nordic Sea sills into the Iceland Basin, while the remainder is made up of nearly equal parts of entrained Atlantic thermocline water and modified Labrador Sea Water. The overall results from this study suggest that the ISOW plume approximately doubles its transport through entrainment, similar to that of the Denmark Strait overflow plume in the Irminger Sea that forms the other major overflow source of North Atlantic Deep Water.
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The rapid warming of our planet has resulted in accelerated melting of ice in polar regions. Currently we have limited knowledge on how, where and when the surface meltwater layer is mixed with the underlying ocean due to lack of observations in these remote areas. We present a lightweight (17 kg) and low-cost (6000€) instrument for autonomous profiling across the strongly stratified upper layer in Arctic coastal waters, freshened by the riverine input and meltwater from glaciers, icebergs, and sea ice. The profiler uses a specially designed plunger buoyancy engine to displace up to 700 cm³ of water and allows for autonomous dives to 200 m depth. It can carry different sensor packages and convey its location by satellite communication. Two modes are available: (a) a free-floating mode and (b) a moored mode, where the instrument is anchored to the seafloor. In both modes, the profiler controls its velocity of 12 ± 0.3 cm/s resulting in 510 ± 22 data points per 100 m depth. Equipped with several sensors, e.g. conductivity, temperature, oxygen, and pressure, the autonomous profiler was successfully tested in a remote Northeast Greenlandic fjord. Data has been compared to traditional CTD instrument casts performed nearby.
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Published in Progress in Oceanography, 2015, Volume 132, March 2015, Pages 68–127. http://www.sciencedirect.com/science/journal/00796611/132/supp/C The focus of this work is on the temporal and spatial variability of the Atlantic Water (AW). We analyze the existing historic hydrographic data from the World Ocean Database to document the long-term variability of the AW throughflow across the Norwegian Sea to the western Barents Sea. Interannual-to-multidecadal variability of water temperature, salinity and density are analyzed along six composite sections crossing the AW flow and coastal currents at six selected locations. The stations are lined up from southwest to northeast – from the northern North Sea (69°N) throughout the Norwegian Sea to the Kola Section in the Barents Sea (33°30′E). The changing volume and characteristics of the AW throughflow dominate the hydrographic variability on decadal and longer time scales in the studied area. We examine the role of fluctuations of the volume of inflow versus the variable local factors, such as the air-sea interaction and mixing with the fresh coastal and cold Arctic waters, in controlling the long-term regional variability. It is shown that the volume of the AW, passing through the area and affecting the position of the outer edge of the warm and saline core, correlates well with temperature and salinity averaged over the central portions of the studied sections. The coastal flow (mostly associated with the Norwegian Coastal Current flowing over the continental shelf) is largely controlled by seasonal local heat and freshwater impacts. Temperature records at all six lines show a warming trend superimposed on a series of relatively warm and cold periods, which in most cases follow, with a delay of four to five years, the periods of relatively low and high North Atlantic Oscillation (NAO), and the periods of relatively high and low Atlantic Multidecadal Oscillation (AMO), respectively. In general, there is a relatively high correlation between the year-to-year changes of the NAO and AMO indices, which is to some extent reflected in the (delayed) AW temperature fluctuations. It takes about two years for freshening and salinification events and a much shorter time (of about a year or less) for cooling and warming episodes to propagate or spread across the region. This significant difference in the propagation rates of salinity and temperature anomalies is explained by the leading role of horizontal advection in the propagation of salinity anomalies, whereas temperature is also controlled by the competing air-sea interaction along the AW throughflow. Therefore, although a water parcel moves within the flow as a whole, the temperature, salinity and density anomalies split and propagate separately, with the temperature and density signals leading the salinity signal. A new hydrographic index, coastal-to-offshore density step, is introduced to capture variability in the strength of the AW volume transport. This index shows the same cycles of variability as observed in temperature, NAO and AMO but without an obvious trend.KeywordsNordic SeasBarents SeaNorwegian SeaSubpolar GyreNorth AtlanticClimate changeAtlantic WaterOcean CirculationTemperatureSalinityDensityWarmingAnomalyFreshwaterNorth Atlantic OscillationAtlantic Multidecadal Oscillation
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The atmosphere and ocean of the North Atlantic have undergone significant changes in the past century. To understand these changes, their mechanisms, and their regional implications requires a quantitative understanding of processes in the coupled ocean and atmosphere system. Central to this understanding is the role played by the dominant patterns of ocean and atmospheric variability which define coherent variations in physical characteristics over large areas.Cluster analysis is used in this article to identify the patterns of the North Atlantic atmospheric variability in the subseasonal and interannual spectral intervals. Four dominant subseasonal weather regimes are defined using Bayesian Gaussian mixture models. All correlation patterns of the Sea Level Pressure (SLP) anomalies with the membership probability time series for the weather regimes show similarities with the dipole structure typical for the North Atlantic Oscillation (NAO). The SLP patterns of two of the regimes represent the opposite phases NAO+ and NAO-. The two other weather regimes, the Atlantic Ridge (AR) and Scandinavian-Greenland dipole (SG), have dipole spatial structures with the northern and southern centers of action shifted with respect to the NAO pattern. These two patterns define blocking structures over Scandinavia and near the southern tip of Greenland, respectively. The storm tracks typical for the four regimes resemble the well known paths for positive/negative phases of NAO for the NAO+/NAO- weather regimes, and paths influenced by blocking off the south Greenland tip for AR and over Scandinavia for SG. The correlation patterns of momentum and heat fluxes to the ocean for the four regimes have tripole structures with positive (warm) downward heat flux anomalies over the Subpolar North Atlantic (SPNA) for the NAO- and the AR and negative heat flux anomalies over the SPNA for the NAO+. The downward heat flux anomalies associated with the SG are negative over the Labrador Sea and positive over the eastern SPNA.The long term impact of the weather regimes on the regional climate is characterized by their distribution; i.e. the frequency of occurrence and persistence in time of each of them. Four typical distributions of the weather regimes are identified in this study which are associated with four dominant spatial interannual patterns representing the phases of two asymmetrical “modes”. The first two patterns have the spatial structures of positive and negative phases of the North Atlantic Oscillation (NAO). The third and fourth patterns, here referred to as G+ and G-, define the opposite phases of a mode, that has a spatial structure defined by three centers found over Florida, south of Greenland and over Scandinavia. The NAO+ interannual patterns are associated with negative anomalies of the surface downward heat flux and ocean heat content over the SPNA. The NAO- and G+ are associated with positive anomalies of heat flux and ocean heat content.In the 1960s the dominant NAO- and G+ interannual patterns favored warmer than normal atmospheric and ocean temperatures over the SPNA. The winters in the late 1980s and early 1990s over the SPNA were colder than normal. This decadal shift in the atmospheric state between 1970s and 1980s was associated with a change in the dominant interannual patterns towards NAO+ and G- in the late 1980s and early 1990s. The recent warming of the SPNA since the mid-1990s was related to dominance of the G+/G- interannual patterns in the distribution of interannual patterns probability membership.Our analysis suggests that this decadal variability was associated with long term shifts in atmospheric behavior over the SPNA that can be described by a change in the 1980s of the distribution of membership probabilities for the the interannual patterns. Within the interannual pattern phase space, this change is characterized with a shift from the NAO-/G+/G- subspace in the 1950 and 1960s, towards NAO+/G+/G- since the mid 1980s.
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