![Cross-section of mean meridional velocity and (southward) volume transports (heavy black; in Sv) between potential density ( 0 ) surfaces in the western boundary current of the Labrador Sea at 53°N in the 1/12°-model. The transports for the LSW ( 0 = 27.74 –27.80) and overflow water layers ( 0 > 27.8) compare to measurements by Fischer et al. [2004] near 53°N of 11.4 Sv ( 0 = 27.74 – 27.80) and 13.8 Sv ( 0 > 27.8).](https://www.researchgate.net/profile/C_Boening/publication/228621472/figure/fig1/AS:393788192247808@1470897829293/Cross-section-of-mean-meridional-velocity-and-southward-volume-transports-heavy-black_Q320.jpg)
Decadal variability of subpolar gyre transport and its reverberation in
the North Atlantic overturning
C. W. Bo¨ning,
1
M. Scheinert,
1
J. Dengg,
1
A. Biastoch,
1
and A. Funk
1
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
system.
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
6
m
3
s
1
), 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
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IFM-GEOMAR Leibniz-Institut fu¨r Meereswissenschaften, Kiel,
Germany.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL026906$05.00
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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 (
0
) surfaces in the western boundary
current of the Labrador Sea at 53°N in the 1/12°-model. The
transports for the LSW (
0
= 27.74–27.80) and overflow
water layers (
0
> 27.8) compare to measurements by Fischer
et al. [2004] near 53°N of 11.4 Sv (
0
= 27.74– 27.80) and
13.8 Sv (
0
> 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
energetic.
[
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
6–8 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(1–2 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 1999–2003. 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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