15 MAY 2012
ZHAI AND SHELDON
3619
On the North Atlantic Ocean Heat Content Change between 1955–70 and 1980–95
XIAOMING ZHAI
Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, United Kingdom
LUKE SHELDON
Department of Earth Sciences, University of Oxford, Oxford, United Kingdom
(Manuscript received 4 April 2011, in final form 2 December 2011)
ABSTRACT
The upper-ocean heat content of the North Atlantic has undergone significant changes over the last 50 years
but the underlying physical mechanisms are not yet well understood. In the present study, the authors examine
the North Atlantic ocean heat content change in the upper 700 m between the 1955–70 and 1980–95 periods.
Consistent with previous studies, the large-scale pattern consists of warming of the tropics and subtropics and
cooling of the subpolar ocean. However, this study finds that the most significant heat content change in the
North Atlantic during these two time periods is the warming of the Gulf Stream region. Numerical experiments
strongly suggest that this warming in the Gulf Stream region is largely driven by changes of the large-scale wind
forcing. Furthermore, the increased ocean heat content in the Gulf Stream region appears to feedback on to the
atmosphere, resulting in warmer surface air temperature and enhanced precipitation there.
1. Introduction
Warming of the North Atlantic over the past 50 years
has not been uniform (e.g., Levitus et al. 2000, 2005a;
Lozier et al. 2008). For example, using data from hydrographic stations, Lozier et al. (2008) found in the North
Atlantic that the tropics and subtropics have warmed
but the subpolar ocean has cooled (see also Levitus
et al. 2000). These observations suggest that, instead of
a diffusive process from the surface, ocean heat content
change is largely a consequence of ocean dynamical adjustment to large-scale variability in wind and buoyancy
forcing (e.g., Lozier et al. 2008; Zhai et al. 2011). However, the physical mechanisms behind the observed
ocean heat content change are not yet well understood,
partially due to the lack of subsurface data in the past.
In his seminal paper, Bjerknes (1964) conjectured that
on interannual time scales the ocean reacts passively to the
overlying atmosphere, whereas on decadal/interdecadal
time scales changes in ocean circulation and its associated heat transport become more important—for
Corresponding author address: Xiaoming Zhai, Atmospheric,
Oceanic and Planetary Physics, Department of Physics, University
of Oxford, Oxford OX1 3PU, United Kingdom.
E-mail: [email protected]
DOI: 10.1175/JCLI-D-11-00187.1
Ó 2012 American Meteorological Society
example, in determining ocean heat content and sea
surface temperature (SST) anomalies. Bjerknes’ hypothesis has been supported by both observational and modeling studies. For example, Battisti et al. (1995) showed
that SST variability on interannual time scales over the
Atlantic can be understood in terms of one-dimensional
mixed layer processes, except in regions of the Gulf
Stream and North Atlantic Current where advection by
ocean currents is hypothesized to be important. Modeling studies (e.g., Halliwell 1998; Krahmann et al. 2001;
Dong and Kelly 2004) later demonstrated the importance of geostrophic advection in heat content changes
in the western boundary region, although the exact
driving mechanism remains unclear. In a theoretical
study, Marshall et al. (2001) argued that the meridional
shift in the jet stream associated with the North Atlantic Oscillations (NAO) drives, through anomalous
wind stress curl, an ‘‘intergyre gyre,’’ which alters the
Gulf Stream and North Atlantic Current. The strength
and position of the Gulf Stream have been reported to
respond to the NAO forcing with a delay of 0–3 years
(e.g., Taylor and Stephens 1998; Joyce et al. 2000;
Frankignoul et al. 2001; Eden and Willebrand 2001;
Visbeck et al. 2003), faster than that expected from the
baroclinic response that takes 8–10 years. In the present paper, we examine the upper-ocean heat content
3620
JOURNAL OF CLIMATE
change in the North Atlantic between 1955–70 and
1980–95 using both observations and an ocean general
circulation model.
2. Data analysis
We analyze the ocean heat content/temperature data
from the National Oceanographic Data Center World
Ocean Database. This dataset updates the ocean heat
content estimates made by Levitus et al. (2005a) with
additional historical and modern data including Argo
profiling float data that have been corrected for systematic errors and represents a 3-month average from 1955 to
the present with a horizontal resolution of 18 3 18. Empirically determined corrections for instrumental offsets
of bathythermograph measurements (both expendable
bathythermographs and mechanical bathythermographs)
have been applied to this dataset using data from ocean
station data casts, conductivity–temperature–depth casts,
and moored buoy data (Levitus et al. 2009). We caution
that interpolations are used to fill data gaps, especially in
the early years. The North Atlantic, on the other hand,
has consistently the best observational coverage throughout the historical record. In this study, we focus on the heat
content change in the upper 700 m in the North Atlantic
between two time periods, 1955–70 and 1980–95, which
correspond to persistent negative and positive NAO periods, respectively (Fig. 1a).
Figure 1b shows the ocean heat content change in the
North Atlantic between these two time periods, where
the color red means the ocean is warmer during the
1980–95 time period than the 1955–70 time period and
the color blue means cooler. The large-scale pattern,
that is, warming in the tropics and subtropics and cooling
in the subpolar ocean, is similar to that found by Lozier
et al. (2008). Note that Lozier et al. (2008) used only the
hydrographic station data, and their ocean heat content
is integrated over the whole water column and also over
two slightly longer time periods. However, the most significant heat content change in the North Atlantic we
find is the warming of the Gulf Stream region, where, in
contrast, Lozier et al. (2008) found cooling in their Hydrobase data. This discrepancy appears to be due mostly
to the difference between integrating to 700 m versus
full depth since a similar warming pattern in the Gulf
Stream region is found when the temperature anomaly is
averaged over only the top 750 m using the same HydroBase data (see Fig. 2a in Lozier et al. 2010). We have
also tried different instrument types in the World Ocean
Atlas and got consistent warming in the Gulf Stream
region. We therefore believe the warming in the Gulf
Stream region shown in Fig. 1b is robust, based on analysis using different instrument types and also datasets
VOLUME 25
compiled by averaging on both depth surfaces (Levitus
et al. 2009) and potential density surfaces (Lozier et al.
2010). Note that warming in the Gulf Stream region is
also a prominent feature in EOF analysis (Levitus et al.
2005b). The cooling that occurs over the Gulf Stream
region when integrating the temperature anomaly over
the whole water depth found in Lozier et al. (2008) has
to be caused by a very strong negative temperature
anomaly in the deep ocean. Although interesting, due to
the sparseness of data in the deep ocean, especially before
the Argo era, we focus our study only on the top 700 m.
To understand the mechanisms behind the observed
heat content change, we plot in Fig. 1c the difference of
the net surface heat fluxes between these two time periods using monthly National Centers for Environmental Prediction (NCEP) reanalysis product (Kalnay et al.
1996), where the color red indicates the ocean gaining
more heat through air–sea heat fluxes, and the color blue
indicates the ocean losing more heat during the 1980–95
time period than the 1955–70 time period. The basinwide anomalous heating in the tropics and subtropics is
consistent with the warming there and vice versa for the
subpolar region. However, there is significant cooling
over the Gulf Stream region where the ocean has warmed
the most. Comparison between Figs. 1b and 1c demonstrates that warming in the Gulf Stream region during the
1980–95 time period cannot be a result of enhanced surface heat flux into the ocean but driven instead by changes
in ocean circulation.
At periods longer than a month, one hypothesis (e.g.,
Willebrand et al. 1980) on the oceanic response predicted
by linear theory to large-scale atmospheric disturbances is
a time-dependent Sverdrup balance. Now we estimate
ocean circulation changes in response to changes of the
large-scale wind stress curl using linear flat-bottomed
Sverdrup dynamics. The depth-integrated streamfunction
C is given, at equilibrium, by
C(l, f) 5
1
br
ðl
^ $ 3 t) dl9,
(k
(1)
lE
where f is latitude, r is seawater density, l is longitude,
lE is the longitude of the eastern boundary, b is the
^ is a unit
change of Coriolis parameter with latitude, k
vector pointing in the vertically upward direction, and t
is the wind stress vector. The integral is evaluated from
the eastern boundary to the outside of the western
boundary. Changes of the interior Sverdrup transport
predicted by (1) are expected to be balanced by changes of
the western boundary current transport, forming anomalous horizontal gyre circulations.
Figure 1d shows the difference in the interior Sverdrup
transport between these two time periods (1980–95 minus
15 MAY 2012
3621
ZHAI AND SHELDON
FIG. 1. (a) The winter North Atlantic Oscillation index provided by the Climate Analysis Section, National Center
for Atmospheric Research (NCAR); (b) the ocean heat content change in the upper 700 m estimated from the World
Ocean Database; (c) the difference in the net surface heat flux from NCEP reanalysis data; (d) the difference in the
interior Sverdrup transport estimated from NCEP wind using (1). Here, (b),(c),(d) are the 1980–95 time period minus
the 1955–70 time period. Red indicates ocean warming, the ocean gaining more heat through air–sea heat fluxes and
anomalous northward interior Sverdrup flow, whereas blue indicates ocean cooling, the ocean losing more heat and
anomalous southward interior Sverdrup flow. Changes of the interior Sverdrup transport are expected to be balanced
by changes of the western boundary current transport, forming anomalous horizontal gyre circulations. Here, (b) is in
units of 1020 J, (c) in W m22, and (d) in Sv.
1955–70), where the color red indicates anomalous
northward interior Sverdrup flow, and the color blue
indicates anomalous southward interior Sverdrup flow.
The most prominent feature in Fig. 1d is the anomalous
southward interior Sverdrup transport between 338
and 578N, with a maximum strength of about 6–7 Sv
(1 Sv [ 106 m3 s21) at 458N. This anomalous southward interior transport forced by the large-scale wind
stress curl anomaly is, in turn, balanced by an enhanced
northward Gulf Stream transport, and they together
form an anomalous anticyclonic circulation straddling
the climatological subtropical and subpolar gyres. This
anomalous anticyclonic circulation located between
the two main gyres is dubbed the ‘‘intergyre gyre’’ by
Marshall et al. (2001) (also see Czaja and Marshall
2001; Visbeck et al. 2003). We will show later that the
enhanced western boundary current transport associated with the ‘‘intergyre gyre’’ is largely responsible for
the observed warming in the Gulf Stream region. To
the south of this ‘‘intergyre gyre’’, there are also a few
anomalous recirculations with reduced magnitude and
meridional extent.
The upper-ocean heat content integrated down to a
fixed depth of h is balanced by surface heating/cooling,
the divergence of heat transport and diffusion, and can
be written as
›
›t
ð0
Q
u dz 5 net 2
rcp
2h
ð0
2h
$ (uu) dz 1
ð0
k=2 u dz, (2)
2h
where u is potential temperature, cp is specific heat at
constant pressure, Qnet is the net surface heat flux (positive when the ocean gains heat), k is the thermal diffusion
coefficient, and u is the three-dimensional velocity vector.
When applying (2) to the Gulf Stream region over the
time periods of interest, some terms turn out to be small
and we can simplify the heat budget.
First, consistent with other studies (e.g., Qiu 2000; Dong
and Kelly 2004), contributions of anomalous Ekman
transport/pumping and diffusion to the heat content budget
3622
JOURNAL OF CLIMATE
in the western boundary region are found to be small in
comparison with that of the geostrophic current and are
hence not considered further. Furthermore, the anomalous heat transport in the western boundary region is
found due mostly to advection of the mean state by
anomalous ocean circulation, rather than the transport of
temperature anomalies by the mean flow, supported by
previous studies (e.g., Qiu 2000; Eden and Jung 2001;
Jayne and Marotzke 2001). Eddies have been known to
be important in shaping the large-scale ocean circulation
and transporting heat, especially in the western boundary
regions (e.g., Jayne and Marotzke 2001; Zhai et al. 2004;
Greatbatch et al. 2010). However, it is difficult to use data
to estimate ocean heat content changes associated with
changes of eddy fluxes in the Gulf Stream region, especially in the early years. Previous model–data comparison
studies (e.g., Penduff et al. 2004) suggest only a very weak
link between the NAO-related atmospheric forcing and
eddy kinetic energy variability in the Gulf Stream region.
We therefore assume changes in the Gulf Stream eddy
field between the 1980–95 and 1955–70 periods play only
a secondary role in our heat budget.
Now integrating (2) over the Gulf Stream region, we
get
ð
ð
›
rcp u dV ’ Qnet dS 1 arcp CDu,
›t
(3)
where S is the surface area of the Gulf Stream region,
V 5 S 3 h is the control volume, and Du is the time-mean
horizontal temperature difference between regions where
the Gulf Stream enters the control volume and where
it exits. Note that u, Qnet, and C are anomalies from
the time-mean quantities. The nondimensional coefficient
a in (3) measures the fractional change of the western
boundary current transport that takes place in the upper
700 m. Depending on whether the baroclinic Rossby
wave adjustment processes are complete, changes of the
western boundary current transport can extend to the
whole water depth (a ; 0.2) or be confined to the upper
ocean (a ; 1). On interdecadal time scales, we expect a
to be much larger than 0.2. It is worth noting that the
ocean heat content change due to horizontal gyre circulation changes depends both on the mean temperature gradient and velocity anomaly, both of which are
functions of space. The heat content change is concentrated in the Gulf Stream region (Fig. 1b) because of the
sharp thermal front and large velocity anomalies there.
We now estimate each term in (3) for the control
volume of the upper 700 m between 358 and 498N and
between 75.58 and 34.58W. The ocean heat content
change between these two time periods, 1980–95 minus
1955–70, is estimated to be (6.6 6 1.0) 3 1021 J, and
VOLUME 25
the time-integrated NCEP surface heat flux anomaly is
(21.8 6 0.5) 3 1022 J. There is a large uncertainty associated with the heat transport divergence due to
changes in ocean circulation. For our control volume, we
estimate Du 5 58618C from the World Ocean Database
and C 5 4 6 1 Sv from Fig. 1d. Assuming a 5 0.6, we
estimate the anomalous heat transport into the Gulf
Stream region between the two time periods to be (2.7 6
1.0) 3 1022 J, which is broadly consistent with previous
studies (e.g., Marshall et al. 2001; Visbeck et al. 2003).
For example, Marshall et al. (2001) estimated the heat
transport associated with the ‘‘intergyre gyre’’ to be
;0.07 PW using C 5 3 Sv and Du 5 68C, that is, ;3.4 3
1022 J when integrated for 16 years in our case. Visbeck et al.
(2003) estimated this term to be ;0.05 PW using C 5 6 Sv
and Du 5 28C, or ;2.5 3 1022 J for 16 years. We have,
within error bars, a closed heat budget for the Gulf
Stream region; again we emphasize the large uncertainties
in these estimates. The picture that emerges from the heat
budget analysis is as follows. On decadal/interdecadal
time scales, the meridional shift of the jet stream associated with the positive NAO drives, through an anomalous negative wind stress curl, anomalous southward
Sverdrup transport in the ocean interior at midlatitudes
(between about 308 and 558N; Fig. 1d), which is balanced
by an enhanced western boundary current transport.
This, in turn, transports an anomalous amount of heat
into the Gulf Stream/North Atlantic Current region, the
majority of which is lost to the atmosphere (Fig. 1c). The
residual results in the observed warming in the Gulf Stream
region (Fig. 1b).
There is no good way to estimate from data the change
in ocean circulation due to changes in the buoyancy
forcing and whether it contributes to the observed
warming in the Gulf Stream region. On the other hand,
given that the heat budget for the Gulf Stream region is
closed within error bars without invoking the buoyancy
forcing, it suggests that changes in buoyancy forcing
between the two time periods need play only a secondary
role in determining the observed warming pattern in the
Gulf Stream region. In the next section, we will explore
the mechanisms further using an ocean general circulation model.
3. Numerical experiments
The model used in this section is the Massachusetts
Institute of Technology (MIT) general circulation model
in a global configuration from 788S to 748N. The horizontal resolution is 18 3 18, with 33 geopotential levels
ranging in thickness from 10 m at the surface to 250 m
at the bottom. Exchange with the Nordic Seas, which
lies outside of the model domain, is crudely taken into
15 MAY 2012
ZHAI AND SHELDON
3623
FIG. 2. The changes of the ocean heat content in (a) the upper 700 m, (c) the barotropic streamfunction, and (e) the
Atlantic meridional overturning circulation between the two time periods (1980–95 minus 1955–70) in EXCONTROL
are shown. (b),(d),(f) As in (a),(c),(e), but driven by changes in the surface heat fluxes, that is, EXCONTROL 2
EXWIND. Here, (a),(b) are in units of 1020 J, and (c),(d),(e),(f) are in Sv.
account by restoring the model temperature and salinity fields near the northern boundary toward the
observed, climatological values. In addition, sea surface temperature and salinity are restored to climatological values on a time scale of 30 days to prevent model
drift. Readers are referred to Czeschel et al. (2010) and
Zhai et al. (2011) for more detailed model description.
The model is spunup for 900 years, forced by NCEP
climatological monthly mean forcing. After that, it is
forced by monthly mean NCEP wind stress and heat
fluxes from 1948 to 2000 (the control experiment; hereafter EXCONTROL).
Figures 2a shows the modeled ocean heat content
change in the upper 700 m in the North Atlantic (1980–95
minus 1955–70) in EXCONTROL. Even though there
are some regional differences, the model reproduces
remarkably well the observed pattern of heat content
change in the North Atlantic. In particular, the largest
heat content change in the model between these two time
periods occurs in the Gulf Stream region, just as observed.
3624
JOURNAL OF CLIMATE
There is widespread cooling in the subpolar gyre, with the
maximum cooling being displaced slightly farther to the
east than observed. A cold tongue, southeast of the Gulf
Stream region, extends southwestward to the outside of
the western boundary. There is indication of such a cold
tongue in the data, but it appears less pronounced and
also discontinuous (Fig. 1b). Farther to the south, the
model captures the basinwide warming centered to the
east of the Caribbean Sea.
Figure 2c shows the change in barotropic streamfunction (1980–95 minus 1955–70) in EXCONTROL.
Both the subpolar gyre and the Gulf Stream spin up
during the 1980–95 time period, with a maximum increase of 8 Sv in the Labrador Sea and 7 Sv to the south
of the Grand Banks, respectively. Eden and Jung (2001)
also found that the oceanic response north of 308N to
a persistent positive phase of the NAO is an amplification
of the mean state. Farther to the south, there are three
anomalous recirculation gyres of alternating signs. The
strength of Atlantic meridional overturning circulation
(AMOC) also increases during the 1980–95 time period,
but it is mostly confined to north of 408N with a maximum
increase of 2 Sv at 458N (Fig. 2e).
Given that the model reproduces faithfully the observed heat content change in the North Atlantic, an
additional model run (hereafter EXWIND) is conducted to help to understand the driving mechanism
responsible for the observed pattern (see also Eden
and Willebrand 2001; Lozier et al. 2008). EXWIND is
identical to EXCONTROL, except that in EXWIND
the climatological monthly mean surface heat flux
forcing is used. Since both the heat content and horizontal transport changes in EXWIND are similar to
those in EXCONTROL, we take the difference between them (EXCONTROL 2 EXWIND) to reveal
the role played by the surface heat flux anomalies. The
most significant heat content change in the North Atlantic driven by the surface heat flux anomalies is the
cooling to the east of Newfoundland (Fig. 2b), which
offsets the wind-induced warming there, such that the
net heat content change driven by both the wind stress
and heat fluxes (Fig. 2a) resembles the observations.
Variability of the surface heat flux forcing makes a
relatively small contribution to the modeled warming
in the Gulf Stream region, although it clearly strengthens
the subpolar gyre (Fig. 2d) and the AMOC (Fig. 2f)
during the 1980–95 time period (also see Eden and Jung
2001; Lozier et al. 2010). Our model results confirm the
picture that emerges from data analysis in the previous
section, that is, changes of the large-scale wind forcing
are the main cause of the observed warming in the Gulf
Stream region between the two time periods (1980–95
minus 1955–70).
VOLUME 25
FIG. 3. (a) The change in barotropic streamfunction (1980–95
minus 1955–70) in EXWIND and (b) the negative of Fig. 1d plotted
here for easy comparison with (a). Changes of the interior Sverdrup
transport in (b) are expected to be balanced by changes of the
western boundary current transport, forming anomalous horizontal
gyre circulations similar to that in (a). Both are in Sv.
Figure 3a shows the change in barotropic streamfunction (1980–95 minus 1955–70) in EXWIND, which
captures most of the horizontal gyre circulation change
in EXCONTROL (Fig. 2c), and accounts for, within
observational errors, almost all the observed ocean heat
content change in the Gulf Stream region and south of it.
For easy comparison, we plot in Fig. 3b the interior
Sverdrup transport averaged over the 1955–70 time period minus that over the 1980–95 time period estimated
from NCEP reanalysis data, that is, the negative of Fig. 1d.
Simple Sverdrup theory explains, to leading order,
changes of the depth-integrated ocean circulation in an
ocean general circulation model. For example, Fig. 3b
reproduces the three anomalous recirculation gyres to the
south of the latitude of Cape Hatteras, the anticyclonic
intergyre gyre, and the cyclonic circulation farther to the
north. However, (1) underestimates the strength of the
15 MAY 2012
ZHAI AND SHELDON
3625
intergyre gyre by ;30% to the east of the Grand Bank
and also significantly overestimates the subpolar gyre
circulation, possibly because of nonlinearity and the
presence of bottom topography (e.g., Willebrand et al.
1980; Greatbatch et al. 1991; Zhang and Vallis 2007). A
detailed analysis of the dynamical adjustment processes
involved and their contributions to the observed heat
content change are beyond the scope of this paper and
left for a future study.
4. Influence on the atmosphere
There is a weak, and often inconsistent, response in
atmospheric models to midlatitude SST anomalies (e.g.,
Palmer and Sun 1985; Kushnir and Held 1996). Recent
studies (e.g., Nakamura et al. 2004; Minobe et al. 2008;
Joyce et al. 2009) show that the most likely regions
where the atmosphere responds to the underlying ocean
are the western boundaries, for example, the Gulf Stream.
Since the observed warming in the Gulf Stream region
leads to an enhanced surface heat flux into the atmosphere (Fig. 1c), we now explore the response of the atmosphere to this oceanic heating using NCEP reanalysis
data. Figure 4a shows the surface air temperature difference between the two time periods (1980–95 minus 1955–
70), where there is a band of pronounced warming of the
surface atmosphere over the Gulf Stream region, with the
center of the atmospheric warming slightly downstream
(eastward) of the warming of the Gulf Stream. The amplitude of the surface air temperature change is similar
to that of the SST anomaly (not shown), consistent with
the notion that the atmospheric temperatures and SST
covary at low frequencies. Our study thus supports the
following picture: on decadal/interdecadal time scales,
the large-scale atmospheric variability drives changes of
the upper-ocean heat content in the Gulf Stream region,
which lead to anomalous surface heat fluxes (Fig. 1c).
These anomalous surface heat fluxes act to damp the SST
variability and drive the surface air temperature variability (Fig. 4a), such that the atmospheric temperatures
and SST covary at low frequencies. The upper-ocean
heat content anomaly in the Gulf Stream region continues to imprint itself on SST through the so-called ‘‘reemergence mechanism’’ (Alexander and Deser 1995).
Note that unlike the SST anomalies which decorrelate
seasonally, the upper-ocean heat content anomaly persists
throughout the year, representing an integral measure
of the anomalous heat flux that can potentially influence
the overlying atmosphere (e.g., Alexander and Deser
1995; Deser et al. 2003).
Recently, Minobe et al. (2008) showed that the warm
SST in the Gulf Stream region affects the entire troposphere through surface wind convergence, anchoring a
FIG. 4. Changes of (a) the surface air temperature and (c) precipitation from NCEP reanalysis data (1980–95 minus 1955–70) are
shown. Red indicates warmer surface air temperature and more
precipitation, whereas blue indicates cooler surface air temperature and less precipitation. (b) The annual-mean precipitation is
shown. Here, (a) is in units of 8C, and (b),(c) are in kg m22 s21.
3626
JOURNAL OF CLIMATE
FIG. 5. Time series of (a) the ocean heat content integrated over
the upper 700 m estimated from the World Ocean Database and
(b) precipitation from NCEP reanalysis data in the Gulf Stream
region from year 1955 to year 2009. Both time series are 5-yr
running means and linearly detrended.
narrow band of precipitation along the Gulf Stream with
potentially far-reaching impacts. However, it is not clear
whether decadal SST variability in the Gulf Stream
region can also cause a decadal signal in precipitation
there. The annual-mean precipitation field from NCEP
reanalysis data (Fig. 4b) is very similar to satellite observations and high-resolution model simulations (e.g.,
Minobe et al. 2008; Kuwano-Yoshida et al. 2010), where
we observe a narrow rainfall band confined sharply on
the warmer flank of the SST front associated with the
Gulf Stream. Figure 4c shows the precipitation averaged
over the 1980–95 time period minus that over the 1955–
70 time period. There is a remarkable narrow band of
enhanced precipitation along the Gulf Stream, suggesting the influence of ocean heat content change on local
precipitation over the Gulf Stream region on decadal/
interdecadal time scales, although it is not clear what the
exact mechanisms are. It is possible that the decadal
upper-ocean heat content anomaly in the Gulf Stream
region imprints itself on SST through the ‘‘re-emergence
mechanism’’ (Alexander and Deser 1995), and the resultant SST anomaly influences the troposphere through
anomalous surface wind convergence (e.g., Minobe et al.
2008), causing a decadal rainfall signal there. The enhanced band of precipitation may also be associated
with northward migration of the SST front and its resultant convective precipitation since there appears
to be a band of reduced rainfall immediately to its south.
Figure 5 shows the time series of the low-frequency
upper-ocean heat content from the World Ocean Database and precipitation from NCEP reanalysis data in
VOLUME 25
FIG. 6. The cross correlation between Figs. 5a and 5b is shown.
Negative lags mean the ocean heat content variability in the Gulf
Stream region leads the precipitation variability there.
the Gulf Stream region from year 1955 to year 2009.
Both time series are linearly detrended. The cross-correlation analysis suggests that the upper-ocean heat
content in the Gulf Stream region covaries with the
precipitation there at low frequencies, with the upperocean heat content variability slightly leading (Fig. 6).
We caution that the precipitation field from NCEP reanalysis is still model data. Our results call for further
studies to confirm the link found here between the lowfrequency upper-ocean heat content and precipitation
variability in the Gulf Stream region, and also determine
whether this oceanic influence is confined locally near the
western boundary or it has any far-reaching effects on, for
example, the climate of western Europe.
These observational evidence points toward a twoway coupling between the atmosphere and the ocean at
midlatitudes on decadal/interdecadal time scales, supporting previous studies (e.g., Marshall et al. 2001; Czaja
and Marshall 2001; Kwon et al. 2010), that is, the oceanic
variability forced by the basin-wide large-scale atmospheric forcing feeds back to the atmosphere (mostly) in
the western boundary current regions.
5. Summary and conclusions
The upper-ocean heat content change in the North
Atlantic between two time periods, 1980–95 minus
1955–70, has been investigated using both observations
and an ocean general circulation model. Our main results are as follows.
d
The large-scale heat content change consists of warming in the tropics and subtropics and cooling in the
15 MAY 2012
d
d
d
ZHAI AND SHELDON
subpolar ocean. However, the most significant heat
content change in the North Atlantic is the warming of
the Gulf Stream region.
Warming in the Gulf Stream region is not a result of
enhanced surface heat flux into the ocean but driven
instead by changes in ocean circulation.
Changes in the large-scale wind forcing and associated
changes in the horizontal gyre circulation are found to
be the main cause of the observed warming in the Gulf
Stream region.
The increased ocean heat content in the Gulf Stream
region appears to feedback on to the atmosphere,
resulting in warmer surface air temperature and enhanced precipitation there.
Simple Sverdrup theory captures the leading-order
large-scale changes in the horizontal gyre transport,
which account for most of the observed heat content
change, especially in the tropics and subtropics; however,
nonlinearities and the presence of bottom topography
are clearly important in the Gulf Stream region and the
subpolar gyre (e.g., Greatbatch et al. 1991; Zhang and
Vallis 2007). Future research is clearly needed to improve
the theory. Our study also suggests that the low-frequency
ocean heat content changes in the Gulf Stream region may
feedback on to the atmosphere and cause a low-frequency
rainfall signal along the Gulf Stream. If confirmed, this
may have important implications for decadal/interdecadal
midlatitude air–sea interactions. Further studies are required to show whether this oceanic influence is confined
locally near the western boundary or if it has far-reaching
effects on, for example, the climate of western Europe.
Acknowledgments. We thank David Marshall for constant encouragement, Helen Johnson, Richard Greatbatch,
Ralf Hand, and Noel Keenlyside for helpful discussions,
and three anonymous reviewers for their insightful and
constructive comments that led to a significant improvement in the manuscript. We gratefully acknowledge the
financial support provided by the Department of Earth
Sciences, University of Oxford.
REFERENCES
Alexander, M. A., and C. Deser, 1995: A mechanism for the recurrence of wintertime midlatitude SST anomalies. J. Phys.
Oceanogr., 25, 122–137.
Battisti, D. S., U. S. Bhatt, and M. A. Alexander, 1995: A modeling
study of the interannual variability in the wintertime North
Atlantic Ocean. J. Climate, 8, 3067–3083.
Bjerknes, J., 1964: Atlantic air–sea interactions. Advances in Geophysics, Vol. 10, Academic Press, 1–82.
Czaja, A., and J. Marshall, 2001: Observations of atmosphere–
ocean coupling in the North Atlantic. Quart. J. Roy. Meteor.
Soc., 127, 1893–1916.
3627
Czeschel, L., D. P. Marshall, and H. L. Johnson, 2010: Oscillatory
sensitivity of Atlantic overturning to high-latitude forcing.
Geophys. Res. Lett., 37, L10601, doi:10.1029/2010GL043177.
Deser, C., M. A. Alexander, and M. S. Timlin, 2003: Understanding
the persistence of sea surface temperature anomalies in midlatitudes. J. Climate, 16, 57–72.
Dong, S., and K. Kelly, 2004: Heat budget in the Gulf Stream region: The importance of heat storage and advection. J. Phys.
Oceanogr., 34, 1214–1231.
Eden, C., and T. Jung, 2001: North Atlantic interdecadal variability: Oceanic response to the North Atlantic Oscillation (1865–
1997). J. Climate, 14, 676–691.
——, and J. Willebrand, 2001: Mechanism of interannual to decadal variability of the North Atlantic circulation. J. Climate,
14, 2266–2280.
Frankignoul, C., G. de Cotlogon, T. M. Joyce, and S. Dong, 2001:
Gulf Stream variability and ocean–atmosphere interactions.
J. Phys. Oceanogr., 31, 3516–3529.
Greatbatch, R. J., A. F. Fanning, A. D. Goulding, and S. Levitus,
1991: A diagnosis of interpentadal circulation changes in the
North Atlantic. J. Geophys. Res., 96 (C12), 22 009–22 024.
——, X. Zhai, M. Claus, L. Czeschel, and W. Rath, 2010: Transport
driven by eddy momentum fluxes in the Gulf Stream Extension region. Geophys. Res. Lett., 37, L24401, doi:10.1029/
2010GL045473.
Halliwell, G., 1998: Simulation of North Atlantic decadal/multidecadal
winter SST anomalies driven by basin-scale atmospheric circulation anomalies. J. Phys. Oceanogr., 28, 5–21.
Jayne, S. R., and J. Marotzke, 2001: The dynamics of ocean heat
transport variability. Rev. Geophys., 39, 385–411.
Joyce, T. M., C. Deser, and M. A. Spall, 2000: The relationship
between decadal variability of Subtropical Mode Water
and the North Atlantic Oscillation. J. Climate, 13, 2550–
2569.
——, Y. O. Kwon, and L. Yu, 2009: On the relationship between
synoptic wintertime atmospheric variability and path shifts in
the Gulf Stream and the Kuroshio Extension. J. Climate, 22,
3177–3192.
Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437–471.
Krahmann, G., M. Visbeck, and G. Reverdin, 2001: Formation and
propagation of temperature anomalies along the North Atlantic Current. J. Phys. Oceanogr., 31, 1287–1303.
Kushnir, Y., and I. Held, 1996: Equilibrium atmospheric response
to North Atlantic SST anomalies. J. Climate, 9, 1208–1220.
Kuwano-Yoshida, A., S. Minobe, and S. P. Xie, 2010: Precipitation
response to the Gulf Stream in an atmospheric GCM. J. Climate, 23, 3676–3698.
Kwon, Y. O., M. A. Alexander, N. A. Bond, C. Frankignoul,
H. Nakamura, B. Qiu, and L. Thompson, 2010: Role of the Gulf
Stream and Kuroshio–Oyashio systems in large-scale atmosphere–
ocean interaction: A review. J. Climate, 23, 3249–3281.
Levitus, S., J. I. Antonov, T. P. Boyer, and C. Stephens, 2000:
Warming of the world ocean. Science, 287, 2225–2229.
——, ——, and ——, 2005a: Warming of the world ocean, 1995–2003.
Geophys. Res. Lett., 32, L02604, doi:10.1029/2004GL021592.
——, ——, ——, H. E. Garcia, and R. A. Locamini, 2005b: EOF
analysis of upper ocean heat content, 1956–2003. Geophys.
Res. Lett., 32, L18607, doi:10.1029/2005GL023606.
——, ——, ——, R. A. Locarnini, and H. E. Garcia, 2009: Global
ocean heat content 1955–2008 in light of recently revealed
instrumentation problems. Geophys. Res. Lett., 36, L07608,
doi:10.1029/2008GL037155.
3628
JOURNAL OF CLIMATE
Lozier, M. S., S. Leadbetter, R. G. Williams, V. Roussenov, M. S. C.
Reed, and N. J. Moore, 2008: The spatial pattern and mechanisms of heat-content change in the North Atlantic. Science,
319, 800–803.
——, V. Roussenov, M. S. C. Reed, and R. G. Williams, 2010:
Opposing decadal changes for the North Atlantic meridional
overturning circulation. Nat. Geosci., 3, 728–734.
Marshall, J., H. L. Johnson, and J. Goodman, 2001: A study of the
interaction of the North Atlantic Oscillation with the ocean
circulation. J. Phys. Oceanogr., 14, 1399–1421.
Minobe, S., A. Kuwano-Yoshida, N. Komori, S. P. Xie, and R. J.
Small, 2008: Influence of the Gulf Stream on the troposphere.
Nature, 452, 206–209.
Nakamura, H., A. Shimpo, A. Goto, W. Ohfuchi, and S. P. Xie,
2004: Observed associations among storm tracks, jet stream
and midlatitude oceanic fronts. Earth’s Climate: The Ocean–
Atmosphere Interaction, Geophys. Monogr., Vol. 147, Amer.
Geophys. Union, 1828–1844.
Palmer, T. N., and Z. Sun, 1985: A modeling and observational
study of the relationship between sea-surface temperature
anomalies in the northwest Atlanic and the atmospheric
general circulation. Quart. J. Roy. Meteor. Soc., 111, 947–
975.
Penduff, T., B. Barnier, W. K. Dewar, and J. J. O’Brien, 2004:
Dynamical response of the oceanic eddy field to the North
VOLUME 25
Atlantic Oscillation: A model–data comparison. J. Phys. Oceanogr., 34, 2615–2629.
Qiu, B., 2000: Interannual variability of the Kurishio Extension
system and its impact on the wintertime SST field. J. Phys.
Oceanogr., 30, 1486–1502.
Taylor, A. H., and J. A. Stephens, 1998: The North Atlantic Oscillation and the latitude of the Gulf Stream. Tellus, 50, 134–142.
Visbeck, M., E. P. Chassignet, R. G. Curry, T. L. Delworth, R. R.
Dickson, and G. Krahmann, 2003: The ocean’s response to
North Atlantic Oscillation variability. The North Atlantic Oscillation: Climatic Significance and Environmental Impact, Geophys. Monogr., Vol. 134, Amer. Geophys. Union, 113–146.
Willebrand, J., S. G. H. Philander, and R. C. Pacanowski, 1980: The
oceanic response to large-scale atmospheric disturbances.
J. Phys. Oceanogr., 10, 411–429.
Zhai, X., R. J. Greatbatch, and J. Sheng, 2004: Diagnosing the role of
eddies in driving the circulation of the northwest Atlantic Ocean.
Geophys. Res. Lett., 31, L23304, doi:10.1029/2004GL021146.
——, H. L. Johnson, and D. P. Marshall, 2011: A model of Atlantic
heat content and sea level change in response to thermohaline
forcing. J. Climate, 24, 5619–5632.
Zhang, R., and G. K. Vallis, 2007: The role of bottom vortex
stretching on the path of the North Atlantic Western Boundary
Current and on the northern recirculation gyre. J. Phys. Oceanogr., 37, 2053–2080.