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


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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.
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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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... The subpolar North Atlantic (SPNA) is a key region for the Atlantic Ocean meridional circulation (AMOC) that plays important role in shaping regional and global climate (Stouffer et al. 2006;Trenberth and Fasullo 2017;Lozier et al. 2019). Therefore, the impact of accelerating Greenland Ice Sheet melting on the SPNA and AMOC has gained considerable attention (e.g., Bakker et al. 2016;Böning et al. 2016;Castelao et al. 2019). However, the extent and time scales of the SPNA response to the Greenland freshwater flux anomalies as well as the residence time of the Greenland freshwater are still unclear. ...
... where t is time (years since 1993), F 0 5 21.8 km 3 yr 21 , and the rate of change (increase) of the Greenland discharge p 5 15.9 km 3 yr 22 (the 95% CI for p is [9.1, 21.3]; CI is confidence interval), which is similar to the estimate (16.9 km 3 yr 22 ) used by Böning et al. (2016). The GFWA is defined as the time-integrated Greenland discharge anomaly from time t 0 to t (Fig. 2c) ...
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The impact of increasing Greenland freshwater discharge on the subpolar North Atlantic (SPNA) remains unknown as there are uncertainties associated with the time scales of the Greenland freshwater anomaly (GFWA) in the SPNA. Results from numerical simulations tracking GFWA and an analytical approach are employed to estimate the response time suggesting a decadal time scale (13 years) required for the SPNA to adjust for increasing GFWA. Analytical solutions obtained for a long-lasting increase of freshwater discharge show a non-steady state response of the SPNA with increasing content of the GFWA. In contrast, solutions for a short-lived pulse of freshwater demonstrate different responses of the SPNA with a rapid increase of freshwater in the domain followed by an exponential decay after the pulse has passed. Derived theoretical relation between time scales show that residence time scales are time-dependent for a non-steady state case and asymptote the response time scale with time. Residence time of the GFWA deduced from Lagrangian experiments is close to and smaller than the response time, in agreement with the theory. The Lagrangian analysis shows dependence of the residence time on the entrance route of the GFWA and on the depth. The fraction of the GFWA exported through Davis Strait has limited impact on the interior basins, whereas the fraction entering the SPNA from the southwest Greenland shelf spreads into the interior regions. In both cases, the residence time of the GFWA increases with depth demonstrating long persistence of the freshwater anomaly in the subsurface layers.
... On inter-annual to decadal timescales, AMOC variability has been associated with buoyancy anomalies in the subpolar region, particularly in the Labrador Sea (Delworth et al., 1993;Medhaug et al., 2012). This mechanism is linked to variability in mixed layer depth and the occurrence of deep convection over the same region, particularly in NH winter (Böning et al., 2006;Biastoch et al., 2008;Robson et al., 2012;Wang et al., 2015). Mixed-layer anomalies in the Labrador Sea are an indication of the strength of deep convection in this region, which has been shown to be associated with AMOC variations in modelling studies (Eden and Willebrand, 2001;Eden and Jung, 2001) and observations (Latif and Keenlyside, 2011). ...
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Variations in the strength of the Northern Hemisphere winter polar stratospheric vortex can influence surface variability in the Atlantic sector. Disruptions of the vortex, known as sudden stratospheric warmings (SSWs), are associated with an equatorward shift and deceleration of the North Atlantic jet stream, negative phases of the North Atlantic Oscillation, and cold snaps over Eurasia and North America. Despite clear influences at the surface on sub-seasonal timescales, how stratospheric vortex variability interacts with ocean circulation on decadal to multi-decadal timescales is less well understood. In this study, we use a 1000 year preindustrial control simulation of the UK Earth System Model to study such interactions, using a wavelet analysis technique to examine non-stationary periodic signals in the vortex and ocean. We find that intervals which exhibit persistent anomalous vortex behaviour lead to oscillatory responses in the Atlantic Meridional Overturning Circulation (AMOC). The origin of these responses appears to be highly non-stationary, with spectral power in vortex variability at periods of 30 and 50 years. In contrast, AMOC variations on longer timescales (near 90-year periods) are found to lead to a vortex response through a pathway involving the equatorial Pacific and quasi-biennial oscillation. Using the relationship between persistent vortex behaviour and the AMOC response established in the model, we use regression analysis to estimate the potential contribution of the 8-year SSW hiatus interval in the 1990s to the recent negative trend in AMOC observations. The result suggests that approximately 30 % of the trend may have been caused by the SSW hiatus.
... The Subpolar Gyre (SPG) of the North Atlantic is a cyclonically circulating oceanic gyre that exhibits pronounced decadal-to-multidecadal variability in its properties (Delworth et al., 1993;Marshall et al., 2001;Böning et al., 2006;Born and Mignot, 2012). Over the last sixty years, periods of strong decadal trends in surface temperature of the SPG (-60:10 • E, 50:62 • N) have been observed-cooling in the 1960s, the warming in the 1990s and the recent cooling in the 2000s (Dickson et al., 1988;Bersch et al., 2007;Reverdin, 2010;Robson et al., 2016;Piecuch et al., 2017;Holliday et al., 2020). ...
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The Barents Sea is a key region in the Earth System and is home to highly productive marine resources. An integrated approach for strategic sustainable management of marine resources in such shelf-sea marine ecosystems requires, among many other aspects, a robust understanding of the impact of climate on local oceanic conditions. Here, using a combined observational and modelling approach, we show that decadal climatic trends associated with the North Atlantic Subpolar Gyre (SPG), within the period 1960–2019, have an impact on oceanic conditions in the Barents Sea. We relate hydrographic conditions in the Barents Sea to the decadal variability of the SPG through its impact on the Atlantic Inflow via the Faroe-Shetland Channel and the Barents Sea Opening. When the SPG warms, an increase in the throughput of subtropical waters across the Greenland-Scotland Ridge is followed by an increase in the volume of Atlantic Water entering the Barents Sea. These changes are reflected in pronounced decadal trends in the sea-ice concentration and primary production in the Barents Sea, which follow the SPG after an advective delay of 4–5 years. This impact of the SPG on sea-ice and primary production provides a dynamical explanation of the recently reported 7-year lagged statistical relationship between SPG and cod ( Gadus morhua ) biomass in the Barents Sea. Overall, these results highlight a potential for decadal ecosystem predictions in the Barents Sea.
... Despite showing similar decadal-scale variability, assim-i1 and assim-i2 have different means and long-term trends. The stronger SPG circulation of assim-i2 goes in tandem with a stronger AMOC, and it is likely that these two are related (Eden and Willebrand, 2001;Eden and Jung, 2001;Böning et al., 2006). ...
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The Norwegian Climate Prediction Model version 1 (NorCPM1) is a new research tool for performing climate reanalyses and seasonal-to-decadal climate predictions. It combines the Norwegian Earth System Model version 1 (NorESM1) – which features interactive aerosol–cloud schemes and an isopycnic-coordinate ocean component with biogeochemistry – with anomaly assimilation of sea surface temperature (SST) and T/S-profile observations using the ensemble Kalman filter (EnKF). We describe the Earth system component and the data assimilation (DA) scheme, highlighting implementation of new forcings, bug fixes, retuning and DA innovations. Notably, NorCPM1 uses two anomaly assimilation variants to assess the impact of sea ice initialization and climatological reference period: the first (i1) uses a 1980–2010 reference climatology for computing anomalies and the DA only updates the physical ocean state; the second (i2) uses a 1950–2010 reference climatology and additionally updates the sea ice state via strongly coupled DA of ocean observations. We assess the baseline, reanalysis and prediction performance with output contributed to the Decadal Climate Prediction Project (DCPP) as part of the sixth Coupled Model Intercomparison Project (CMIP6). The NorESM1 simulations exhibit a moderate historical global surface temperature evolution and tropical climate variability characteristics that compare favourably with observations. The climate biases of NorESM1 using CMIP6 external forcings are comparable to, or slightly larger than those of, the original NorESM1 CMIP5 model, with positive biases in Atlantic meridional overturning circulation (AMOC) strength and Arctic sea ice thickness, too-cold subtropical oceans and northern continents, and a too-warm North Atlantic and Southern Ocean. The biases in the assimilation experiments are mostly unchanged, except for a reduced sea ice thickness bias in i2 caused by the assimilation update of sea ice, generally confirming that the anomaly assimilation synchronizes variability without changing the climatology. The i1 and i2 reanalysis/hindcast products overall show comparable performance. The benefits of DA-assisted initialization are seen globally in the first year of the prediction over a range of variables, also in the atmosphere and over land. External forcings are the primary source of multiyear skills, while added benefit from initialization is demonstrated for the subpolar North Atlantic (SPNA) and its extension to the Arctic, and also for temperature over land if the forced signal is removed. Both products show limited success in constraining and predicting unforced surface ocean biogeochemistry variability. However, observational uncertainties and short temporal coverage make biogeochemistry evaluation uncertain, and potential predictability is found to be high. For physical climate prediction, i2 performs marginally better than i1 for a range of variables, especially in the SPNA and in the vicinity of sea ice, with notably improved sea level variability of the Southern Ocean. Despite similar skills, i1 and i2 feature very different drift behaviours, mainly due to their use of different climatologies in DA; i2 exhibits an anomalously strong AMOC that leads to forecast drift with unrealistic warming in the SPNA, whereas i1 exhibits a weaker AMOC that leads to unrealistic cooling. In polar regions, the reduction in climatological ice thickness in i2 causes additional forecast drift as the ice grows back. Posteriori lead-dependent drift correction removes most hindcast differences; applications should therefore benefit from combining the two products. The results confirm that the large-scale ocean circulation exerts strong control on North Atlantic temperature variability, implying predictive potential from better synchronization of circulation variability. Future development will therefore focus on improving the representation of mean state and variability of AMOC and its initialization, in addition to upgrades of the atmospheric component. Other efforts will be directed to refining the anomaly assimilation scheme – to better separate internal and forced signals, to include land and atmosphere initialization and new observational types – and improving biogeochemistry prediction capability. Combined with other systems, NorCPM1 may already contribute to skilful multiyear climate prediction that benefits society.
... The rate and direction of northward transport of warm waters and the density and depth of the southward return flow comprise the AMOC. The formation of North Atlantic Deep Water (NADW) from intense surface cooling returns dense watermasses south (Böning et al., 2006;Lohmann et al., 2014;Marshall & Schott, 1999). The Gulf Stream and the North Atlantic Current (also referred to as the North Atlantic Drift or Trans Atlantic Current) are major sources of warm surface waters through the horizontal gyre circulation. ...
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The North Atlantic ocean is key to climate through its role in heat transport and storage. Climate models suggest that the circulation is weakening but the physical drivers of this change are poorly constrained. Here, the root mechanisms are revealed with the explicitly transparent machine learning (ML) method Tracking global Heating with Ocean Regimes (THOR). Addressing the fundamental question of the existence of dynamical coherent regions, THOR identifies these and their link to distinct currents and mechanisms such as the formation regions of deep water masses, and the location of the Gulf Stream and North Atlantic Current. Beyond a black box approach, THOR is engineered to elucidate its source of predictive skill rooted in physical understanding. A labeled data set is engineered using an explicitly interpretable equation transform and k‐means application to model data, allowing theoretical inference. A multilayer perceptron is then trained, explaining its skill using a combination of layerwise relevance propagation and theory. With abrupt CO2 quadrupling, the circulation weakens due to a shift in deep water formation regions, a northward shift of the Gulf Stream and an eastward shift in the North Atlantic Current. If CO2 is increased 1% yearly, similar but weaker patterns emerge influenced by natural variability. THOR is scalable and applicable to a range of models using only the ocean depth, dynamic sea level and wind stress, and could accelerate the analysis and dissemination of climate model data. THOR constitutes a step toward trustworthy ML called for within oceanography and beyond, as its predictions are physically tractable.
The interannual variability of the Mediterranean overturning circulation is investigated using a high-resolution (1/36°) ocean model. As the overturning circulation regulates the replenishment and ventilation of the deep layers, we study the spatiotemporal scales of the maximum value of the overturning streamfunction over three main sub-basins of dense water formation (Aegean Sea, Adriatic, and the northwestern Mediterranean). The variability of the zonal overturning is also discussed. The spectrum analysis shows that the overturning variability has its largest signal on annual timescales in all sub-basins, explained by perpetual winter formation. On shorter frequencies (decadal) there are marked differences observed, due to regional processes of the overturning cells, led by buoyancy flux long-term variability in each sub-basin. The decomposition of the total overturning circulation into barotropic, geostrophic shear, and Ekman components revealed weakening and strengthening for the Aegean and Adriatic Sea total overturning, respectively, with opposite trends for the barotropic and geostrophic shear components. The simultaneous contribution of the Ekman and geostrophic component to the total overturning differentiates the variability of zonal overturning circulation from the local meridional overturning circulation of the three sub-basins. The cross spectra between the maximum overturning value and the buoyancy fluxes also revealed that the system keeps the “memory” of this forcing and shows annual variability.
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate through its transport of heat in the North Atlantic Ocean. Decadal changes in the AMOC, whether through internal variability or anthropogenically forced weakening, therefore have wide-ranging impacts. In this Review, we synthesize the understanding of contemporary decadal variability in the AMOC, bringing together evidence from observations, ocean reanalyses, forced models and AMOC proxies. Since 1980, there is evidence for periods of strengthening and weakening, although the magnitudes of change (5–25%) are uncertain. In the subpolar North Atlantic, the AMOC strengthened until the mid-1990s and then weakened until the early 2010s, with some evidence of a strengthening thereafter; these changes are probably linked to buoyancy forcing related to the North Atlantic Oscillation. In the subtropics, there is some evidence of the AMOC strengthening from 2001 to 2005 and strong evidence of a weakening from 2005 to 2014. Such large interannual and decadal variability complicates the detection of ongoing long-term trends, but does not preclude a weakening associated with anthropogenic warming. Research priorities include developing robust and sustainable solutions for the long-term monitoring of the AMOC, observation–modelling collaborations to improve the representation of processes in the North Atlantic and better ways to distinguish anthropogenic weakening from internal variability. The Atlantic Meridional Overturning Circulation (AMOC) has a key role in the climate system. This Review documents AMOC variability since 1980, revealing periods of decadal-scale weakening and strengthening that differ between the subpolar and subtropical regions.
A hierarchy of global 1=4° (ORCA025) and Atlantic Ocean 1=20° nested (VIKING20X) ocean-sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50-60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation of the Atlantic Meridional Overturning Circulation (AMOC), and in particular the long-term temporal evolution, strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5° N. This points to a dominant role of the wind forcing. The ability of the model to represent regional observations in western boundary current (WBC) systems at 53° N, 26.5° N and 11° S is explored. The question is investigated of whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to that of the zonally integrated AMOC. Apart from the basin-scale measurements at 26.5° N, it is shown that in particular the outflow of North Atlantic Deepwater at 53° N is a good indicator of the subpolar AMOC trend during the recent decades, once provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddyrich VIKING20X is capable of representing realistic forcingrelated and ocean-intrinsic trends.
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Variations in the strength of the Northern Hemisphere winter polar stratospheric vortex can influence surface variability in the Atlantic sector. Disruptions of the vortex, known as sudden stratospheric warmings (SSW), are associated with an equatorward shift and deceleration of the North Atlantic jet stream, negative phases of the North Atlantic Oscillation as well as cold snaps over Eurasia and North America. Despite clear influences at the surface on sub-seasonal timescales, how stratospheric vortex variability interacts with ocean circulation on decadal to multi-decadal timescales is less well understood. In this study, we use a 1000-year pre-industrial control simulation of the UK Earth System Model to study such interactions using a wavelet analysis technique to examine non-stationary periodic signals in the vortex and ocean. We find that intervals which exhibit persistent anomalous vortex behaviour lead to oscillatory responses in the Atlantic Meridional Overturning Circulation (AMOC). The origin of these responses appears to be highly non-stationary with spectral power in vortex variability and the AMOC at periods of 30 and 50 years. In contrast, AMOC variations on longer timescales (near 90-year periods) are found to lead to a vortex response, through a pathway involving the equatorial Pacific and Quasi-biennial Oscillation. Using the relationship between persistent vortex behaviour and the AMOC response established in the model, we use a regression analysis to estimate the potential contribution of the 8 year SSW hiatus interval in the 1990s to the recent negative trend in AMOC observations. The result suggests that approximately 30 % of the trend may have been caused by the SSW hiatus.
The Labrador Current (LC) provides freshwater from the Arctic to the North Atlantic, modulating the Atlantic Meridional Overturning Circulation (AMOC), and therefore affecting the broader North Atlantic climate. The Holocene alongflow variability of the LC vigor, and the associated forcing mechanisms, are poorly understood due to the limited data near the southern limit of the LC. Here we present a new 9.4 ka record of distal LC vigor over the Scotian Shelf using the sortable-silt proxy, which allows for the first time an assessment of the alongflow changes in Holocene LC vigor and hence its forcing mechanisms. LC speed on the Scotian Shelf decreased slightly from 9.4 to 8.0 ka, during which the 8.1 ka meltwater event had a strong influence. The LC progressively intensified from 8.0 to 5.0 ka, weakened between 5.0 and 1.8 ka and gradually intensified from 1.8 to 0.5 ka. Our synthesis reveals that the Holocene flow history of the LC appears geographically variable due to the interaction of the inner and outer LC. The mean size of the sortable silt data on the Scotian Shelf involve inner or outer LC signals in different periods of the Holocene. The LC vigor on the Scotian Shelf between 9.4e8.0 ka and 1.8e0.5 ka represent the outer LC, which is consistent with the stronger West Greenland Current and increased influx of Atlantic-sourced water to the outer LC. We find a broad agreement between inner LC vigor and AMOC-related sea surface temperature (SST) of the subpolar North Atlantic and the North Atlantic Oscillation (NAO), which suggests that a strong (weak) inner LC is generally associated with regional warm (cold) climate and negative (positive) NAO. The outer LC vigor is dominated by the NAO during the Holocene and partly controlled by freshwater supply between 10.0 and 5.0 ka.We also demonstrate the negative/positive link between the inner/outer LC vigor and the NAO on a millennium time scale. This study improves our understanding of LC variability and sensitivity to anthropogenic warming, and suggest that inner (outer) LC vigor may experience not only a decreasing (increasing) trend in a future warmer climate, with additional effects resulting from enhanced melting of the Greenland ice sheet.
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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.
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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.
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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.
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.
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.
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.
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.
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.