Decadal variability of subpolar gyre transport and its reverberation in the North Atlantic overturning

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DOI: 10.1029/2006GL026906
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1] Analyses of sea surface height (SSH) records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the North Atlantic subpolar gyre during the 1990s. Here we report hindcast simulations with high-resolution ocean circulation models that demonstrate a close correspondence of the SSH changes with the volume transport of the boundary current system in the Labrador Sea. The 1990s-decline, of about 15% of the long-term mean, appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the North Atlantic Oscillation (NAO). The changes in the subpolar gyre, as manifested in the deep western boundary current off Labrador, reverberate in the strength of the meridional overturning circulation (MOC) in the subtropical North Atlantic, suggesting the potential of a subpolar transport index as an element of a MOC monitoring system.
Decadal variability of subpolar gyre transport and its reverberation in
the North Atlantic overturning
C. W. Bo¨ning,
M. Scheinert,
J. Dengg,
A. Biastoch,
and A. Funk
Received 15 May 2006; revised 22 August 2006; accepted 5 September 2006; published 29 September 2006.
[1] Analyses of sea surface height (SSH) records based on
satellite altimeter data and hydrographic properties have
suggested a considerable weakening of the North Atlantic
subpolar gyre during the 1990s. Here we report hindcast
simulations with high-resolution ocean circulation models that
demonstrate a close correspondence of the SSH changes with
the volume transport of the boundary current system in the
Labrador Sea. The 1990s-decline, of about 15% of the long-
term mean, appears as part of a decadal variability of the gyre
transport driven by changes in both heat flux and wind stress
associated with the North Atlantic Oscillation (NAO). The
changes in the subpolar gyre, as manifested in the deep western
boundary current off Labrador, reverberate in the strength of
the meridional overturning circulation (MOC) in the
subtropical North Atlantic, suggesting the potential of a
subpolar transport index as an element of a MOC monitoring
Citation: Bo¨ning, C. W., M. Scheinert, J. Dengg,
A. Biastoch, and A. Funk (2006), Decadal variability of subpolar
gyre transport and its reverberation in the North Atlantic overturning,
Geophys. Res. Lett., 33, L21S01, doi:10.1029/2006GL026906.
1. Introduction
[2] The cyclonic circulation of the subpolar gyre in the
North Atlantic (Figure 1) represents an important part of the
global thermohaline circulation (THC). The eastern, north-
ward flowing portion of the gyre includes various, vigorously
eddying branches of the North Atlanti c Current (NAC),
supplying the northeastern Atlantic with warm, saline waters
of subtropical origin. Subsequently cooled through surface
heat loss in winter, the different return flows merge along the
southeastern continental slope off Greenland, to form an
intense boundary current system, with a volume transport
of 40 to 50 Sv (1 Sv = 10
), around the Labrador Sea
[Pickart et al., 2002; Fischer et al., 2004] (Figure 2). About a
third of this transport comprises the different constituents of
the North Atlantic Deep Water (NADW) that feed the deep
limb of the meridional overturning circulation (MOC) of the
mid-latitude North Atlantic [Lumpkin and Speer, 2003]. The
potential threat to the MOC and its related northward trans-
port of heat due to anthropogenic climate change [Gregory et
al., 2005] has led to major efforts in designing a monitoring
system able to detect changes in the MOC transport in the
subtropical North Atlantic [Hirschi et al., 2003] where ship-
based transoceanic sections have provided repeated snap-
shots of the MOC [Bryden et al., 2005]. The detectabilit y of a
potential (multi-)decadal MOC signal related to changes in
subarctic deep water formation is impeded, however, due to
contaminations by high-frequency fluctuations, particularly
due to local wind forcing [Baehr et al., 2006]. Here we
demonstrate that decadal MOC signals of subarctic origin can
be traced back to pronounced changes in the strength of the
deep western boundary current of the subpolar gyre.
3] It is well established that the hydrographic properties
in the subpolar North Atlantic undergo pronounced varia-
tions on decadal-to-centennial time scales, primarily as a
consequence of changes in the local atmospheric conditions
associated with the NAO [Curry et al., 1998]. These are
particularly manifested in the properties of the Labrador Sea
Water (LSW), the upper constituent of the deep MOC limb,
generated by enhanced air-sea heat fluxes and subsequent,
deep convective mixing events during wintertime storms. A
prominent, well documented period of c hange occurred
during the 1990s, where a phase of exceptionally intense
convection related to the high NAO-index years of the early
1990s, was followed by a period of weak convection after
1994 [Lazier et al., 2002].
4] Recent studies based on satellite-tracked drifting
buoys and satellite-altimeter data have added to this picture
by also documenting evidence for changes in the gyre
circulation during the 1990s. Ha¨kkinen and Rhines [2004,
hereinafter referred to as HR] used sea surface height (SSH)
data based on altimeter records for 1992 to 2002 to propose a
‘gyre index’ for the strength of the cyclonic circulation in
the subpolar North Atlantic, and reported a substantial
decline in this index after 1994. Their analysis suggested a
link between the gyre strength and the cessation of deep
convection in the Labrador Sea associated with the trend in
the net heat fluxes, indicating a possible importance of the
dynamical variability in the subpolar gyre for the evolution of
the thermohaline circulation in the Atlantic. The evolution of
the SSH pattern and the concomitant changes in the eastward
extension of the gyre were reproduced with an Ocean General
Circulation Model (OGCM) forced by atmospheric reanal-
yses [Ha´tu´n et al., 2005]. In this study we examine the nature
and role of the subpolar gyre variability by assessing the
correspondence of a SSH-based ‘gyre index’ with the actual
volume transport of the gyre, by elucidating the forcing
mechanisms contributing to this variability during the last
decades, and by demonstrating its reperc ussion for the basin-
scale MOC.
2. Model Experiments
[5] We use a sequence of OGCM simulations, with hori-
zontal resolutions (longitude by latitude) of 1/12° by 1/12°
cos () and 1/3° by 1/3° cos ()( being latitude), forced by
monthly flux anomalies derived from the NCEP/NCAR
reanalysis data [Kalnay et al., 1996], utilizing a primitive
equation model that has been developed for studying the
GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L21S01, doi:10.1029/2006GL026906, 2006
IFM-GEOMAR Leibniz-Institut fu¨r Meereswissenschaften, Kiel,
Copyright 2006 by the American Geophysical Union.
L21S01 1of5
wind- driven and thermohaline circulation in the Atlantic
Ocean (Family of Linked Atlantic Model Experiments,
FLAME). The z-coordinate model is based on a modified
version of the Modular Ocean Model (MOM2) [Pacanowski,
1995]. All model cases cover the Atlantic Ocean up to 70°N,
using 45 levels in the vertical, isopycnal mixing schemes, and
a bottom boundary layer formulation following Beckmann
and Do¨scher [1997]. Our study builds on previous applica-
tions of the FLAME hierarchy (with 4/3° , 1/3°, and 1/12°
resolutions) to issues of deep water formation [Bo¨ning et al.,
2003] and eddy variability [Eden and Bo¨ning, 2002] in the
Labrador Sea, the mechanisms of decadal MOC variability
[Eden and Willebrand, 2001, hereinafter referred to as EW],
and the propagation of subarctic MOC anomaly signals to the
tropics [Getzlaff et al., 2005]. The model experiments sim-
ulate the ocean’s response for 1958 2001 (in the 1/3°-case;
following a 50-year climatological spin-up) and 1987 2004
(1/12°-case; 10-year spin-up) to interannual variations in
wind stress and heat flux. The heat fluxes are computed with
the linearized bulk formulation of EW. Sea surface salinity as
well as the hydrographic conditions near the northern bound-
ary are restored to climatological conditions on a time scale of
15 days, effectively eliminating possible effects of changing
outflow conditions from the Nordic Seas.
3. Changes in the Subpolar Gyre Transport
[6] In the altimeter data analysis of HR, the interannual
variation in the gyre circulation during 1992 to 2002 was
depicted by the principal component of an empirical orthog-
onal function (EOF) analysis, as well as by the actual SSH
anomalies in the central Labrador Sea; both indices showing
a similar rise of about 8 cm after 1994. The 1/12°-model
simulation is assessed in Figure 3a by inspecting the central-
gyre SSH evolution in comparison to altimeter products. The
monthly values are governed by strong intra-seasonal fluc-
tuations and obviously lack correlation between model and
data. (These fluctuations are much weaker in the 1/3-model
and may thus partly reflect stochastic, eddy-related ‘noise’’.)
The 2-year low-pass filtered SSH anomalies indicate a
comparable behavior as observed, with a rise between 1994
and 1998 of about 10 cm.
7] The relation of this SSH-based index to the actual
volume transport of the subpolar gyre is inspected in
Figure 3b. In the model the SSH-index covaries (r = 0.74)
with the maximum cyclonic gyre transport (as defined by the
minimum of the transport streamfunction at 57° –58°N): it
declines by 7 8 Sv between 1994 and 1999, but rises again
by 4 5 Sv until 2003. The latter feature is consistent with
measurements in the deep Labrador Current which suggested
an increased LSW transport between the late 90s and 2001
2005 (M. Dengler et al., The Deep Labrador Current and
its variability in 1996 2005, submitted to Geophysical
Research Letters, 2006, hereinafter referred to as Dengler et
al., submitted manuscript, 2006). Comparison of the 1/12°-
and 1/3°-simulations shows that the higher resolution, al-
though essential for a realistic representation of the narrow
boundary flows and the mesoscale eddies which govern the
intra-seasonal variability in the subpolar gyre [Eden and
Bo¨ning, 2002], is of little consequence for capturing the
low-frequency variability of the gyre transport. The 1/3°-
hindcast can hence usefully be applied to obtain a longer term
perspective of the subpolar gyre intensity: it depicts the mid-
Figure 1. Schematic of the topography (water depths
smaller than 1500 m are shaded grey) and circulation of the
western subpolar North Atlantic, illustrating the import of
warm water by the NAC (red), its recirculation and
successive cooling in the Irminger Basin (red dashed), and
the deep western boundary current (blue), by which the cold
deep waters outflowing from the Nordic Seas (light blue) and
formed by deep winter conv ection (shaded blue) are exported
from the Labrador Sea. Indicated (black lines) are the sites
(near 53°N and 56°N) of multi-year current measurements off
the Labrador continental slope [Fischer et al., 2004], and the
area (black box) taken for the computation of the SSH
variability in the central Labrador Sea (57°N, 52°W)
following the definition of Ha¨kkinen and Rhines [2004].
Figure 2. Cross-section of mean meridional velocity and
(southward) volume transports (heavy black; in Sv) between
potential density (
) surfaces in the western boundary
current of the Labrador Sea at 53°N in the 1/12°-model. The
transports for the LSW (
= 27.7427.80) and overflow
water layers (
> 27.8) compare to measurements by Fischer
et al. [2004] near 53°N of 11.4 Sv (
= 27.74 27.80) and
13.8 Sv (
> 27.8).
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1990s’ decline as part of a decadal variability pattern, super-
imposed on a longer-term trend from a phase of minimum
transports in the late 1960s, to a maximum during the early
1990s. This behavior appears consistent with the (fragmen-
tary) observa tional ev idence (as discussed, e.g., by HR)
which suggested the early-to-mid 1990s to be un usually
8] Insight into the causes of the transport variability is
provided by a comparison of the 1/3°- and 1/12°-model
hindcasts with a companion experiment (1/3°-HEAT) in
which the interannual forcing variability is artificially re-
stricted to the heat flux, while the wind stress is kept a
climatological, repeating annual cycle (Figure 3c). In addi-
tion to the effect of the changing heat flux emphasized by HR
(associated mainly with changes in wintertime wind intensity
related to the NAO), and consistent with idealized response
experiments by Esselborn and Eden [2001], the model results
point to a substantial, additional effect of wind stress on the
gyre variability; the early 1990s appear exceptional in that
both forcing factors acted in concert to produce the prominent
transport maximum in 1994.
4. Relation to MOC Changes
[9] Is there a repercussion of changes in the subpolar gyre
for the large-scale meridional transports of mass and heat in
the North Atlantic further south? A host of model studies
has established the response of the mid- to low-latitude
MOC to variations in subarctic deep water formation [e.g.,
EW; Ha¨kkinen, 1999; Bentsen et al., 2004; Gulev et al.,
2003; Bailey et al., 2005]. Whereas models suggest
longer-term MOC tr ends to be governed mainly by the
Nordic Sea outflow conditions, changes in Labrador Sea
convection appear as the prime cause of MOC variability
on interannual-to-decadal time scales [Schweckendiek and
Willebrand, 2006]. Since year-to-year changes in deep
water formation rates are difficult to quantify observationally,
it appears of interest to examine here whether an indirect, but
potentially more accessible account of convection changes
can be based on the transport changes in the subpolar gyre.
Specifically, we seek a volume transport metric primarily
governed by changes in air-sea heat flux, i.e., the primary
control of convection variability (as discussed in the model
studies cited above).
10] Since the maximum gyre transport discussed above
(Figure 3c), and thus the SSH-index (Figure 3b), involve a
significant contribution from wind stress changes, we turn
attention on the transport manifested at the western bound-
ary of the southwestern Labrador Sea, a site (see Figures 1
and 2) well-accessible to long-term current measurements as
demonstrated by Fischer et al. [2004] and Dengler et al.
(submitted manuscript, 2006). The t otal, top-to-bottom
WBC transport variability in the model basically parallels
the maximum gyre transport examined in Figure 3 (not
shown). The effect of the variable wind stress appears
diminished, however, in the lower portion of the WBC, i.e.,
in the components eventually feeding the lower limb of the
MOC: as demonstrated in Figure 4a, the deep WBC transport
corresponds to the variability in convection intensity (r = 0.78
(0.73) for a lag of 1 year (2 years), convection leading), used
here as a simple measure of Labrador Sea Water formation:
stronger southward transport episodes are notably associated
with the convection events in the mid-70s, mid-80s and, most
prominently, early 90s. (Alternative use of a ‘formation rate’
based on the wintertime increase in LSW volume leads to
similar results; this rate varies between 0 2 Sv in weak and
68 Sv in s trong convection years, with no significant
differences between the 1/3°- and 1/3°-HEAT cases.)
11] How is the decadal variability in the subarctic deep
water formation reflected in the basin-scale MOC? As dis-
cussed by Eden and Greatbatch [2003], the dynamical signal
at the exit of the subpolar basin near 45°N lags convection
changes by about 2 years associated with the advective
spreading of the newly formed water mass; south of that
there is a rapid southward propagation of MOC signals via
fast boundary wave processes [e.g., Getzlaff et al., 2005].
This behavior can be seen in 1/3°-HEAT: Figure 4b depicts a
Figure 3. (a) SSH anomaly in the central Labrador Sea
(57°N, 52°W) in the 1/12 °-model simulation (in red; thin
curve: monthly values, heavy curve: 2-yr. low-pass filtered)
and altimeter data (black), based on merged products from
TOPEX/POSEIDON, ERS-2, Geosat Follow-on, Jason-1
and Envisat. (b) Comparison of the central gyre SSH-index
(red) with the gyre transport in the 1/12°-model (blue), and
the 1/3°-hindcast (black dotted); negative transport and
SSH anomalies indicating a stronger cyclonic circulation.
(c) Subpolar gyre transport in the 1/3°-model forced by
interannually-varying heat fluxes and wind stresses (as in
Figure 3b), compared to 1/3°-HEAT forced by interannual
heat fluxes and climatological (repeated annual cycle) wind
stresses (heavy black curve). All time-series in Figures 3b
and 3c are 2-year filtered.
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MOC variability clearly following convection events and
rapidly communicated toward the tropical Atlantic. Note,
however, that the MOC amplitude associated with the vig-
orous convection changes is only O(12 Sv) in mid-
latitudes, and becomes even weaker in the subtropics. The
possibility of detecting such thermohaline signals of subarc-
tic origin is exacerbated further due to their superposition by
vigorous wind-driven transport fluctuations [EW; Jayne and
Marotzke, 2001; Shaffray and Sutton, 2004]. In Figure 4c this
is exemplified for an arbitrary latitude (36°N): in the 1/3°-
and 1/12°-hindcasts the decadal MOC-anomalies related to
the subarctic buoyancy forcing (as given by 1/3°-HEAT) are
effectively masked by a wind-driven signal of about twice the
amplitude. Concerning the detectability of MOC changes of
subarctic (thermohaline) origin, it is thus important to find
this signal linked to the (rather pronounced) variability of the
subpolar gyre: Figure 4c shows the contribution to the total
MOC variability due to changes in the subarctic deep water
formation (as seen in 1/3°-HEAT) co-varying with the dense
fraction of the WBC transport off Labrador (as simulated in
the 1/3°-hindcast which includes the wind-driven variability)
(r = 0.71 (0.67) for 36°N, and 0.58 (0.56) at 26°N for lags of
0 (1) years).
5. Conclusions
[12] Ha¨kkinen and Rhines [2004] noted a substantial
decline in their SSH-based ‘gyre index’ for the strength of
the subpolar gyre after 1994, suggesting a link to the
cessation of deep convection in the Labrador Sea and
implications for the thermohaline circulation in the Atlantic.
The model simulations discussed here put these observations
into a multi-decadal context of transpor t changes in the
subpolar gyre: while, as in the previous study using a
different model (MICOM) by Ha´tu´n et al. [2005], the major
decline in the SSH-index during the 1990s is reproduced in
the present hindcast, the concomitant drop in t he gyre
transport (of 7 8 Sv, or about 15% of the long-term mean)
appears as part of a decadal variability: it follows a strength-
ening trend of the gyre intensity since about 1970, and is
followed by an increase during 19992003. Whereas the
total, top-to-bottom transport variability appears forced by
both heat flux and wind stress anomalies, the model results
indicate that the deep fraction of the boundary current in the
southwestern Labrador Sea represents a signal of primarily
thermohaline origin: the decadal variability of the deep WBC
basically follows, with a lag of 1 2 years, that of the intensity
of deep winter convection. An important consequence of this
behavior is that a transp ort anomaly in the deep W BC
represents a harbinger of the basin-scale MOC response to
changes in the subarctic thermohaline forcing.
13] It has to be noted that the present model configuration,
by imposing climatological conditions at the northern bound-
ary (70°N), excludes changes in the outflows from the Nordic
Seas and thereby a potential source of gyre transport and
MOC variability. Whereas, according to analyses by Latif et
al. [2006], variations in the outflows may have been of
secondary importance compared to the effect of Labrador
Sea convection variability during the past decades, the long-
term evolution of the MOC, such as the possible weakening
during the 21st c entury as projected in various climate
models [Gregory et al., 2005], is found to be governed
primarily by changes i n the density of the outflows
[Schweckendiek and Willebrand, 2006]. It needs to be inves-
tigated whether a similar link between subpolar gyre and
MOC transports, as in the case of the convection-related var-
iability discussed here, also holds in an overflow-dominated
climate change scenario: if established for that case, transport
measurements in the Labrador Sea boundary current system
could provide an important, complementary contribution to a
monitoring system aiming at an early detection of potential
anthropogenic changes in the Atlantic MOC.
14] Acknowledgments. This work was supported by the Deutsche
Forschungsgemeinschaft in the framework of Sonderforschungsbereich 460
Dynamik thermohaliner Zirkulationsschwankungen, and by the DEKLIM
program of BMBF. We gratefully acknowledge the contributions of C. Eden,
L. Czeschel and J.-O. Beismann to the FLAME developments and integra-
tions. The ocean model integrations were performed at DKRZ, Hamburg and
HLRS, Stuttgart.
Figure 4. (a) Variability of the dense portion (LSW and
deeper) of WBC transport at 53°N in the 1/3°- (dashed black)
and 1/1 2° -models (blue), in relation to the convection
intensity as given by the anomalies in the mean depth of
the mixed layer in winter (in green; the anomalies represent
the deviations from a time-mean depth of 1030 m, taken
over the area of 56.5° –58.5°N, 55° –53°W; the green
shading highlights strong convection episodes); all time
series 2-year filtered. (b) Anomalies in the MOC transport (in
Sv) as a function of latitude and time in 1/3°-HEAT, showing
the rapid southward spreading of the dynamic response to the
variation in LSW formation. (MOC anomalies are defined as
the deviations from the time-mean MOC at each latitude; the
maximum mean MOC transport is 18 Sv at about 40°N.)
(c) Variability of the MOC transport at 36°N in the 1/3°- and
1/12°-models (thin purple and blue curves, respectively), and
in 1/3°-HEAT (red), in relation to the deep WBC at 53°Nin
the 1/3°-model (dashed black; as in Figure 4a).
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    To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.
  • Article
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    Despite a strong focus on latitudinal continuity of the Atlantic Meridional Overturning Circulation (AMOC) variability, transport continuity in different layers that constitute the AMOC lower limb has received considerably less attention. In this study, we investigate the transport connectivity of Upper North Atlantic Deep Water (UNADW) and Lower NADW (LNADW), with both defined by density. Using two ocean circulation models and an ocean reanalysis, we find that subpolar-originated transport anomalies, particularly for UNADW, do not propagate to the subtropics over a period of five decades. We also find that transports in both layers are linked to AMOC at subpolar latitudes, yet only LNADW transport shows linkage to AMOC in the subtropical gyre. Thus, latitudinal AMOC continuity is likely unrelated to transport continuity in any single layer, but rather a result of connection between subpolar-AMOC and subtropical-LNADW transport. An exception to this generalization is possible with strong LNADW transport events.
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    The Reykjanes Ridge is a major topographic feature located south of Iceland in the North-Atlantic Ocean that strongly influences the subpolar gyre (SPG) circulation. Based on velocity and hydrographic measurements carried out along the crest of the Reykjanes Ridge from the Icelandic continental shelf to 50°N during the RREX cruise in June – July 2015, we derived the first direct estimates of volume and water mass transports over the Reykjanes Ridge. North of 53.15°N, circulation was mainly westward; south of this latitude it was mainly eastward. The westward transport was estimated at 21.9 ± 2.5 Sv (Sv = 106 m3 s-1) and represents the SPG intensity. The westward flows followed two main pathways at 57°N near the Bight Fracture Zone and at 59 – 62°N. We argue that those pathways were connected to the northern branch of the North Atlantic Current and to the Sub-Arctic Front respectively, which were both intersected by the southern part of the section. In addition to this horizontal circulation, mixing and bathymetry shaped the water mass distribution. Water mass transformations in the Iceland Basin lead to the formation of weakly stratified SubPolar Mode Water (SPMW). We explain why SPMW, the main water mass contributing to the westward flow, was denser at 57°N than at 59 – 62°N. At higher densities, both Intermediate Water and Icelandic Slope Water contributed more to the westward transport across the Reykjanes Ridge than the sum of Labrador Sea Water and Iceland-Scotland Overflow Water.
  • Article
    While the influence of the subpolar gyre (SPG) on thermohaline variability in the eastern North Atlantic is well documented, the extent and timescale of the influence of the SPG on North Sea is not well understood. This is primarily because earlier investigations on the causes of variability in the North Sea water properties mostly focused on the role of atmosphere and deployed regional models. Here using a historical simulation with the Max Planck Institute Earth System Model (MPI-ESM), we investigate circulation and water mass variability in key regions, namely, the Rockall Trough and the Faroe-Scotland Channel, which link the North Atlantic to the North Sea. We find that salinity covaries with advective lags in these three regions and that the northern North Sea salinity follows the Rockall Trough with a lag of 1 year. We show that recurring and persistent excursions of salinity anomalies into the northern North Sea are related to the SPG strength and not to the local acceleration of the inflow. Furthermore, we illustrate that the SPG signal is more pronounced in salinity than in temperature and that this simulated SPG signal has a period of 30–40 years. Overall, our study suggests that, at low frequency, water mass variability originating in the North Atlantic dominates changes in the North Sea water properties over those due to local wind-driven volume transport.
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    A new high resolution deglacial and Holocene Sea Surface Temperature (SST) reconstruction is presented for the Alboran Sea (western Mediterranean), based on Mg/Ca ratios measured in the planktonic foraminifera Globigerina bulloides. This new record is evaluated by comparison with other Mg/Ca–SST and previously published alkenone–SST reconstructions from the same region for both Holocene and glacial period. In all cases there is a high degree of coherence between the different Mg/Ca–SST records but strong discrepancies when compared to the alkenone–SST records. We argue that these discrepancies are due to differences in the proxy-response during deglaciation which we hypothesize to reflect a resilience strategy of G. bulloides changing its main growth season. In contrast, short-term Holocene SST variability is larger in the Mg/Ca–SST than in the alkenone–SST records. It is proposed that larger Mg/Ca–SST variability to be the result of spring season variability, while the smoothed alkenone–SST variability represents average annual temperatures. Mg/Ca–SST record differentiates the Holocene in three periods (1) The warmest SST values occurred during the Early Holocene (11.7–9kyrBP); (2) During the middle Holocene occurred a continuous cooling trend that culminated with the coldest Holocene SST in a double peak structure centred at around 4.2kyrBP; (3) The Late Holocene (4.2kyrBP to the present) did not follow any clear cooling/warming trend but millennial-scale oscillations were enhanced. This SST evolution is discussed in the context of changing properties in the Atlantic inflow associated to North Atlantic circulation conditions and also to local hydrographical and atmospheric changes. To conclude, we propose a tight link between North Atlantic circulation patterns and inflow of surface waters into the Mediterranean playing a major role in the controls of Holocene climatic variability of this region.
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    The Atlantic meridional overturning circulation (AMOC) plays a fundamental role in the climate system, and long-term climate simulations are used to understand the AMOC variability and to assess its impact. This study examines the basic characteristics of the AMOC variability in 44 CMIP5 (Phase 5 of the Coupled Model Inter-comparison Project) simulations, using the 18 atmospherically-forced CORE-II (Phase 2 of the Coordinated Ocean-ice Reference Experiment) simulations as a reference. The analysis shows that on interannual and decadal timescales, the AMOC variability in the CMIP5 exhibits a similar magnitude and meridional coherence as in the CORE-II simulations, indicating that the modeled atmospheric variability responsible for AMOC variability in the CMIP5 is in reasonable agreement with the CORE-II forcing. On multidecadal timescales, however, the AMOC variability is weaker by a factor of more than 2 and meridionally less coherent in the CMIP5 than in the CORE-II simulations. The CMIP5 simulations also exhibit a weaker long-term atmospheric variability in the North Atlantic Oscillation (NAO). However, one cannot fully attribute the weaker AMOC variability to the weaker variability in NAO because, unlike the CORE-II simulations, the CMIP5 simulations do not exhibit a robust NAO-AMOC linkage. While the variability of the wintertime heat flux and mixed layer depth in the western subpolar North Atlantic is strongly linked to the AMOC variability, the NAO variability is not.
  • Article
    We analyze sources of ocean heat content (OHC) variability in the eastern North Atlantic subpolar gyre from both Eulerian and Lagrangian perspectives within two ocean simulations from 1990 to 2015. Heat budgets reveal that while the OHC seasonal cycle is driven by air-sea fluxes, interannual OHC variability is driven by both air-sea fluxes and the divergence of ocean heat transport, the latter of which is dominated by the oceanic flux through the southern face of the study area. Lagrangian trajectories initialized along the southern face and run backward in time indicate that interannual variability in the subtropical-origin volume flux (i.e., the upper limb of the overturning circulation) drives variability in the temperature flux through the southern face. As such, the heat carried by the imported subtropical waters is an important component of the eastern subpolar gyre heat budget on interannual time scales.
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    Ocean uptake of anthropogenic heat over the past 15 years has mostly occurred in the Southern Ocean, based on Argo float observations. This agrees with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), where the Southern Ocean (south of 30°S) accounts for 72% ± 28% of global heat uptake, while the contribution from the North Atlantic north of 30°N is only 6%. Aerosols preferentially cool the Northern Hemisphere, and the effect on surface heat flux over the subpolar North Atlantic opposes the greenhouse gas (GHG) effect in nearly equal magnitude. This heat uptake compensation is associated with weakening (strengthening) of the Atlantic meridional overturning circulation (AMOC) in response to GHG (aerosol) radiative forcing. Aerosols are projected to decline in the near future, reinforcing the greenhouse effect on the North Atlantic heat uptake. As a result, the Southern Ocean, which will continue to take up anthropogenic heat largely through the mean upwelling of water from depth, will be joined by increased relative contribution from the North Atlantic because of substantial AMOC slowdown in the twenty-first century. In the RCP8.5 scenario, the percentage contribution to global uptake is projected to decrease to 48% ± 8% in the Southern Ocean and increase to 26% ± 6% in the northern North Atlantic. Despite the large uncertainty in the magnitude of projected aerosol forcing, our results suggest that anthropogenic aerosols, given their geographic distributions and temporal trajectories, strongly influence the high-latitude ocean heat uptake and interhemispheric asymmetry through AMOC change.
  • Article
    In multimillennial global warming simulations with the GISS-E2-R climate model, we observe multicentennial shutdowns with restoration and fast overshooting in North Atlantic Deep Water production despite the absence of exogenous freshwater input. AMOC (Atlantic Meridional Overturning Circulation) cessation is associated with a sea surface salinity reduction, initiated by increases in precipitation over evaporation as the climate warms. These multicentury shutdowns are the direct result of cooling in the North Atlantic associated with an aerosol indirect effect on cloud cover. The local cooling reduces evaporation within the North Atlantic, while warming elsewhere provides moisture to maintain nearly unperturbed precipitation in this region. As global warming continues, warm temperature (low density) anomalies spread northward at depth in the North Atlantic eventually destabilizing the water column, even though precipitation input at the surface is initially unchanged. Internal ocean freshwater transports do not play an important role in initiating this behavior, as assumed by some standard metrics of AMOC stability. The importance of the aerosol indirect effect in these runs is due to its role in strengthening the sea surface temperature-evaporation feedback; this suggests a renewed focus on surface flux observations to help assess overturning stability. The length of the AMOC reduction, and its rapid recovery, may be relevant to the onset and end of the Younger Dryas, which occurred within a warming climate during the last deglaciation. ©2018. American Geophysical Union. All Rights Reserved. This article has been contributed to by US Government employees and their work is in the public domain in the USA.
  • Article
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    Changes of the meridional overturning circulation (MOC) due to surface heat flux variability related to the North Atlantic Oscillation (NAO) are analyzed in various ocean models, i.e., eddying and non-eddying cases. A prime signature of the forcing is variability of the winter-time convection in the Labrador Sea. The associated changes in the strength of the MOC near the subpolar front (45°N) are closely related to the NAO-index, leading MOC anomalies by about 2-3 years in both the eddying and non-eddying simulation. Further south the speed of the meridional signal propagation depends on model resolution. With lower resolution (non-eddying case, 4/3° resolution) the MOC signal propagates equatorward with a mean speed of about 0.6 cm/s, similar as spreading rates of passive tracer anomalies. Eddy-permitting experiments (1/3°) show a significantly faster propagation, with speeds corresponding to boundary waves, thus leading to an almost in-phase variation of the MOC transport over the subtropical to subpolar North Atlantic.
  • Article
    Interannual sea surface height (SSH) variability as measured by the Topex/Poseidon satellite altimeters is investigated for the North Atlantic Ocean between 1992 and 1998. The SSH variability exhibits a basin-wide coherent dipole structure between the subtropical and the subpolar North Atlantic. The SSH dipole pattern changed sign between 1995 and 1996, coinciding with a change of sign of the North Atlantic Oscillation (NAO). The large-scale SSH pattern is reproduced with an ocean general circulation model, and can be traced back to changes in the atmospheric forcing related to the NAO. The model reveals that the interannual SSH anomalies are mainly caused by changes in the oceanic heat transport which are connected with the response of the large-scale ocean circulation to changes in the wind stress curl. Variations in the local heat flux reinforce these SSH anomalies but are of minor importance.
  • Article
    A fundamental feature of the climate system is that heat is transported poleward by both the atmosphere and the oceans. The atmospheric and Atlantic Ocean energy transports in HadCM3, the Met Office Climate Model, have been investigated to gain a new perspective on the mechanisms that govern air-sea interaction over the Atlantic. The time-series of the annual mean atmospheric and Atlantic Ocean energy transports are anticorrelated at high latitudes and correlated in the subtropics. The extratropical relationship between the energy transports in the atmosphere and the North Atlantic Ocean arises from the anticorrelation between the subtropical and high latitude zonal windstress over the North Atlantic Ocean. In the tropics the atmospheric and Atlantic Ocean energy transports are only weakly correlated but even on interannual time-scales the Atlantic Ocean energy transports are coherent in the Northern and Southern Tropical Atlantic Oceans. Coastal upwelling induced by changes in the trades winds over the Caribbean is communicated to the rest of the Tropical Atlantic Ocean via boundary and equatorial Kelvin waves and Rossby waves in the ocean interior. The adjustment of the Tropical Atlantic Ocean to this upwelling leads to the spatial coherence in the energy transports.
  • Article
    The hydrographic structure of the Labrador Sea during wintertime convection is described. The cruise, part of the Deep Convection Experiment, took place in February-March 1997 amidst an extended period of strong forcing in an otherwise moderate winter. Because the water column was preconditioned by previous strong winters, the limited forcing was enough to cause convection to approximately 1500 m. The change in heat storage along a transbasin section, relative to an occupation done the previous October, gives an average heat loss that is consistent with calibrated National Centers for Environmental Prediction surface heat fluxes over that time period (~200 W m -2). Deep overturning was observed both seaward of the western continental slope (which was expected), as well as within the western boundary current itself-something that had not been directly observed previously. These two geographical regions, separated by roughly the 3000-m isobath, produce separate water mass products. The offshore water mass is the familiar cold/fresh/dense classical Labrador Sea Water (LSW). The boundary current water mass is a somewhat warmer, saltier, lighter vintage of classical LSW (though in the far field it would be difficult to distinguish these products). The offshore product was formed within the cyclonic recirculating gyre measured by Lavender et al. in a region that is limited to the north, most likely by an eddy flux of buoyant water from the eastern boundary current. The velocity measurements taken during the cruise provide a transport estimate of the boundary current "throughput" and offshore "recirculation." Finally, the overall trends in stratification of the observed mixed layers are described.
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    Analyses of ocean observations and model simulations suggest that there have been considerable changes in the thermohaline circulation (THC) during the last century. These changes are likely to be the result of natural multidecadal climate variability and are driven by low-frequency variations of the North Atlantic Oscillation (NAO) through changes in Labrador Sea convection. Indications of a sustained THC weakening are not seen during the last few decades. Instead, a strengthening since the 1980s is observed. The combined assessment of ocean hydrography data and model results indicates that the expected anthropogenic weak- ening of the THC will remain within the range of natural variability during the next several decades.
  • Article
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    Current hydrographic data can provide snapshots but no continuous timeseries of the meridional overturning circulation (MOC). Using output from two eddy-permitting numerical ocean models we test the feasibility of a monitoring system for the MOC in the North Atlantic. The results suggest that a relatively simple arrangement, using moorings placed across a longitude-depth section and the zonal wind stress, is able to capture most of the MOC strength and vertical structure as a function of time. Being closely related to the transport of energy to the North Atlantic, measuring the MOC would open the prospect of having continuous information about a key element of northern hemisphere climate.
  • Article
    A high-resolution model of the North Atlantic Ocean is used to examine the potential of chlorofluorocarbon (CFC) inventories for calculating the rate of Labrador Sea Water (LSW) formation. While the simulated CFC-11 inventory and its geographical distribution in 1997 is fairly similar to observations, the model indicates pronounced variations in the history of CFC uptake, reflecting pulsations in LSW renewal in response to changes in wintertime atmospheric conditions. The LSW formation rate based on the volume of newly homogenized water during a winter season varies between 0 Sv and 11 Sv, and it is correlated (with a lag of 1 year) with the North Atlantic Oscillation (NAO) Index. The CFC-based estimate of the mean LSW formation rate is 3.5–4.4 Sv, approximately representing the mean volumetric formation rate (4.3 Sv) for the period 1970–1997.
  • Article
    Climate models used to produce global warming scenarios exhibit widely diverging responses of the thermohaline circulation (THC). To investigate the mechanisms responsible for this variability, a regional Atlantic Ocean model driven with forcing diagnosed from two coupled greenhouse gas simulations has been employed. One of the coupled models (MPI) shows an almost constant THC, the other (GFDL) shows a declining THC in the twenty-first century.The THC evolution in the regional model corresponds rather closely to that of the respective coupled simulation, that is, it remains constant when driven with the forcing from the MPI model, and declines when driven with the GFDL forcing. These findings indicate that a detailed representation of ocean processes in the region covered by the Atlantic model may not be critical for the simulation of the overall THC changes in a global warming scenario, and specifically that the coupled model's rather coarse representation of water mass formation processes in the subpolar North Atlantic is unlikely to be the primary cause for the large differences in the THC evolution.Sensitivity experiments have confirmed that a main parameter governing the THC response to global warming is the density of the intermediate waters in the Greenland Iceland Norwegian Seas, which in turn influences the density of the North Atlantic Deep Water, whereas changes in the air sea heat and freshwater fluxes over the subpolar North Atlantic are only of moderate importance, and mainly influence the interannual decadal variability of THC.Finally, as a consequence of changing surface fluxes, the Labrador Sea convection ceases by about 2030 under both forcings (i.e., even in a situation where the overall THC is stable) indicating that the eventual breakdown of the convection is likely but need not coincide with substantial THC changes.
  • Article
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    Decadal-scale climate variability in the North Atlantic thermohaline circulation is simulated using a sigma-coordinate primitive equation model, forced by NCEP NCAR reanalysis surface forcing fields for the period from 1958 to 1997. Surface heat and freshwater flux are expressed in terms of surface thermal and haline density inputs, diagnosed by the model. Variability in surface density fluxes is closely correlated with the North Atlantic Oscillation and demonstrates differences with the original surface heat and freshwater fluxes. Leading modes of surface water mass transformation are considered in the T S plane. They identify decadal-scale variability associated with the transformation of the Labrador Sea Waters and Subtropical Mode Waters. Analysis of the model responses to the surface forcing shows an immediate reaction of meridional heat transport to the wind stress curl, resulting in a decrease of meridional heat transport at 48°N and an increase in the subtropics. Delayed baroclinic responses to the surface heat forcing are identified at time lags of 3 and 7 yr. The 3-yr response is represented by an increase in the total meridional heat transport in subpolar latitudes and its simultaneous increase in the Tropics and midlatitudes. The 7-yr delayed response to the surface heat forcing is associated with the strengthening of meridional heat transport at all latitudes. However, 7-yr responses may be influenced by the self-correlation in the meridional heat transport and forcing function. Meridional overturning is largely responsible for the variability observed, demonstrating high correlation with meridional heat transport.