Near-Surface Transport Pathways in the North Atlantic Ocean: Looking for
Throughput from the Subtropical to the Subpolar Gyre
IRINA I. RYPINA AND LAWRENCE J. PRATT
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
M. SUSAN LOZIER
Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, North Carolina
(Manuscript received 10 May 2010, in final form 3 November 2010)
ABSTRACT
Motivated by discrepancies between Eulerian transport estimates and the behavior of Lagrangian surface
drifters, near-surface transport pathways and processes in the North Atlantic are studied using a combination
of data, altimetric surface heights, statistical analysis of trajectories, and dynamical systems techniques.
Particular attention is paid to the issue of the subtropical-to-subpolar intergyre fluid exchange. The velocity
field used in this study is composed of a steady drifter-derived background flow, upon which a time-dependent
altimeter-based perturbation is superimposed. This analysis suggests that most of the fluid entering the
subpolar gyre from the subtropical gyre within two years comes from a narrow region lying inshore of the Gulf
Stream core, whereas fluid on the offshore side of the Gulf Stream is largely prevented from doing so by the
Gulf Stream core, which acts as a strong transport barrier, in agreement with past studies. The transport
barrier near the Gulf Stream core is robust and persistent from 1992 until 2008. The qualitative behavior is
found to be largely independent of the Ekman drift.
1. Introduction
In educating the public on the role of the ocean cir-
culation in climate, the global circulation is often sim-
plified and presented as a branching ‘‘conveyor belt.’’
Transport pathways in this picture are represented in
terms of continuous currents, even though it is known
that the actual movement of water may take place in
eddies and over paths that are intermittent. However,
even in the North Atlantic, the most densely sampled
major basin in the global ocean, it is difficult to move
beyond the conveyor belt model since transport pathways
in the upper and lower limbs of the meridional over-
turning circulation (MOC) are still so poorly understood.
The traditional view (Stommel and Arons 1960) that
recently ventilated waters from the subpolar North At-
lantic flow southward in a deep western boundary cur-
rent (DWBC) has recently been contradicted by a study
of deep floats (Bower et al. 2009), which shows that the
primary southward pathway for these waters lies well
offshore. Similarly, the general expectation that waters
constituting the upper limb of the MOC are carried
continuously into the subpolar gyre via the Gulf Stream/
North Atlantic Current has come under some recent
suspicion. Prior studies have shown that the Gulf Stream
transports approximately 65 Sv (Sv [ 10
6
m
3
s
21
)at
Cape Hatteras (e.g., Johns et al. 1995) and that about
20 Sv of this flow makes its way northward into the
subpolar gyre via the North Atlantic Current. However,
recent studies based on surface drifters show very little
intergyre exchange in the North Atlantic, leaving open
the question of how, where, or when the waters of the
MOC upper limb enter the subpolar gyre to eventually
return to their deep-water formation sites. Clearly, our
understanding of how water parcels move as part of the
North Atlantic MOC remains unclear. The main thrust
of the present paper is to clarify this picture using a
combination of data, models, trajectory statistics, and
methodology from dynamical systems theory.
Observations of surface drifters in the North At-
lantic show surprisingly small connectivity between
Corresponding author address: Irina I. Rypina, Physical Ocean-
ography Department, Woods Hole Oceanographic Institution,
Woods Hole, MA 02543.
E-mail: irypina@whoi.edu
M
AY 2011 R Y P I N A E T A L . 911
DOI: 10.1175/201 JPO4498.1
Ó 2011 American Meteorological Society
1
the subtr opical and subpolar gyres. Of the drifters
deployed south of 458N from 1990 to 2002, only one
reached the subpolar gyre (Brambilla and Talley 2006).
At first glance, these observations suggest that the two
gyres are completely separated from each other at the
surface by a transport barrier. However, because of the
limitations of the drifter dataset, it is also possible that
surface exchange between the gyres is limited to certain
areas or times that were poorly sampled by drifters and/
or occurs on time scales longer than the typical drifter
lifetime.
Brambilla and Talley (2006) investigated the possi-
bility that sampling bias (due to short drifter lifetime) or
Ekman drift could have led to the surprisingly small
number of drifter crossings. They quantified the per-
centage of actual and simulated drifters, deployed in the
rectangular domain (referred to as the Gulf Stream box)
from 358 to 478N, 788 to 488W, that crossed into the sub-
polar gyre. Simulated drifters were advected through the
drifter-derived mean field with and without the Ekman
component and through the mean field with a stochastic
perturbation (representing turbulent eddies) superim-
posed. The authors found that the sampling bias and the
Ekman velocity limit the connectivity between the gyres,
whereas the stochastic perturbation increases the con-
nectivity. Contributions from the Ekman and turbulent
components were estimated to be of the same order and
account for only about 5%–6% decrease or increase,
respectively, in drifter crossings. The authors were also
able to create a set of trajectories with an average life
span of 600 days by compositing shorter trajectories,
which led to a modest 4% increase in drifters moving
from the Gulf Stream box to the subpolar gyre. Note,
however, that these specific numbers apply only to their
rectangular launch site and may not be representative of
other regions. Even with a 10% increase in the number
of drifters that make it to the subpolar gyre, it is difficult
to account for the 20 Sv (of 65 Sv at Hatteras) estimated
to do so.
According to Bingham et al. (2007), another indica-
tion of limited connectivity between the gyres is the loss
of meridional coherence of MOC anomalies at about
408N, which corresponds roughly to the intergyre bound-
ary. In their modeling study, these authors demonstrated
that the loss of coherence could be attributed to the
dominance of decadal variability for the zonally aver-
aged northward transport anomalies north of 408N, in
the subpolar region, compared to the dominance of an-
nual and higher-frequency variability in the subtropical
region.
The impact of interannual variability in the North At-
lantic surface currents on intergyre exchange was further
investigated by Hakkinen and Rhines (2009) using nearly
the same drifter dataset as in Brambilla and Talley (2006).
These autho rs separated the observed drifter tracks into
three intervals (1991–95, 1996–2000, and 2001–05) and
counted the number of drifters moving from the Bram-
billa and Talley Gulf Stream box to the subpolar gyre
during each interval. Their analysis indicated an increase
in the intergyre connectivity after 2001, from which they
inferred a change in surface currents that opened new
pathways from the western subtropical gyre to the sub-
polar latitudes. Although this conclusion is plausible and
supported by observational evidence of an abrupt in-
crease in salinities since 2002 in the Faroe2Shetland
Channel and in the Rockall Trough (Holliday et al.
2008), the increased number of crossings into subpolar
latitudes since 2001 could also be explained by differ-
ences in drifter launch locations before and after 2001.
We will elaborate on this point in the next section after
presenting our analysis.
Though the interpretation of a mean transport across
a fixed section is straightforward, the same is not true for
the passage of drifters across that same section. Thus,
a reconciliation of Eulerian volume transport estimates
with fluid particle trajectories would be required to fully
address the conundrum described above. Here, we focus
primarily on the Lagrangian view of the subtropical to
subpolar exchange in an effort to shed light on these
recent observations and leave such reconciliation to a
later study. In our study of the near-surface subtropical-
to-subpolar fluid exchange in the North Atlantic, a
time-dependent near-surface velocity field has been
constructed by combining a drifter-derived mean field
(similar to that of Brambilla and Talley) with a meso-
scale perturbation based on satellite altimetry. The re-
sulting field, which approximates surface velocities
over 1992–2008, is described in section 2. Also in sec-
tion 2, we show results of several simulated drifter re-
lease experiments that shed light on the near-surface
transport pathways by which subtropical waters flow
into the subpolar region. Recent advances in the theory
and application of dynamical systems (DS) have been
extensively used over the last two decades to clarify the
Lagrangian picture of fluid stirring and exchange pro-
cesses. These methods will be explored in section 3 to
complement more traditional analysis based on tra-
jectory statistics. The results also provide some insights
into the horizontal diffusivity, including the failure of
traditional parameterizations to capture the barrier ef-
fect of the Gulf Stream. Because of the complicated
time dependence of the flow, transport pathways and
processes m ay vary significantly with time. This issue is
addressed in section 4, where we ask whether the nat-
ural variability of surface currents in the North Atlantic
can trigger some qualitative changes in the geometry o f
912 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 41
transport pathways and barriers. Section 5 concen-
trates on the influence of Ekman drift on the exchange
processes. A summary of our studies and conclusions is
given in section 6.
2. Near-surface transp ort pathways in the North
Atlantic: Looking for the subtropical-to-
subpolar gyre throughput
The observation-based model that we have used to
study near-surface fluid exchange in the North Atlantic
consists of a drifter-derived steady background flow
(similar to that of Brambilla and Talley 2006) subject to
an altimetry-based (i.e., surface height anomaly) per-
turbation. This approach was first employed by Rypina
et al. (2009) in the Adriatic Sea. Throughout our work
we have assumed that the flow is two dimensional and
that the streamfunction takes the form
c(u, l, t) 5 c
0
(u, l) 1 c
1
(u, l, t), (1)
where (u, l) are latitude and longitude and the sub-
scripts 0 and 1 denote the steady and the time-dependent
part of the streamfunction, respectively. The fluid par-
ticle trajectories are given by
dl
dt
5
›c/›u
R cosu
;
du
dt
5
›c/›l
R cosu
, (2)
where R is the earth’s radius.
The background streamfunction c
0
(u, l) was con-
structed from time averages of spatially binned (18318
bins) near-surface drifter velocities u
dr
and y
dr
by writing
a finite difference approximation to the equations
›c
0
›u
5 u
dr
;
›c
0
›l
5 y
dr
cosu, (3)
followed by a least squares fitting procedure. The no-
normal flow condition was imposed at the boundary by
setting c
0
5 const everywhere on land. The resulting
streamfunction in Fig. 1 shows two well-defined main
gyres, a smaller subpolar and a larger subtropical gyre.
The two are separated by a dividing streamline (black
curve) that extends from the hyperbolic (saddle type)
stagnation point at about 368N, 768W to the hyperbolic
stagnation point at about 558N, 108W. Because this
separating streamline prevents throughflow from the
subtropical into the subpolar gyre in the mean velocity
field, any throughflow that results in the time-dependent
field is due entirely to the eddy field. An important prop-
erty is that the Gulf Stream lies inside the subtropical gyre
to the southeast of this separating streamline. (The mean
Gulf Stream core was defined as the streamline that has
locally the largest average velocity between 258 and 428N,
808 and 458W.)
A slightly different geometrical configuration (Fig. 2)
arises if one accounts for a small near-surface flow from
the North Atlantic into the Arctic, which brings warm
water poleward where it cools and sinks. However, since
the two geometrical configurations give similar results in
terms of percentage of trajectories that cross from the
subtropical to the subpolar gyre, we focus here only on
the geometry described above.
To construct an unsteady mesoscale surface velocity
field in the North Atlantic, we have superimposed an
altimetry-based perturbation c
1
5 (g/f)h(u, l, t)onc
0
as
described by Eq. (1). Here h is an altimetric sea level
anomaly, g the gravitational acceleration, and f(u) the
Coriolis parameter. The corresponding sea level fields
cover the time interval from 1992 to 2008 with Dt 5 7
days. A plot (Fig. 3) of the 16-yr average kinetic energy
for this field suggests that the most energetic eddies
occur near the Gulf Stream extension. Although our
FIG. 1. The steady drifter-based streamfunction c
0
(l, u) that
describes the mean surface circulation of the North Atlantic. The
magenta curve extending across the North Atlantic is the sepa-
rating streamline.
FIG. 2. The streamfunction c
0
(l, u) field that takes into account
a small near-surface outflow from the North Atlantic into the
Arctic.
M
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observation-based velocity field is entirely based on
data, it has some rather obvious shortcomings as a model
of the North Atlantic near-surface circulation. These
shortcomings include the sparse resolution of both back-
ground flow and perturbation in space and time, neglect of
transport through various straits, errors in u
dr
and y
dr
due
to the limitations of the drifter dataset, and neglect of the
nongeostrophic component in c
1
. Despite these limita-
tions, the observation-based model describes fairly well
the near-surface mesoscale velocity field of the North
Atlantic and its variability. Results of numerical simu-
lations performed using these observation-based veloc-
ities are presented in Figs. 4–8, and 11.
To study intergyre exchange, 90 drifters were released
once per month during the 12 months starting in October
1992 within a zonal band straddling the Gulf Stream near
348N and tracked for two years (Fig. 4). As explained later
in the text we have conducted this analysis for all years in
our temporal domain. Here October 1992–September 1993
is shown as a representative year. Also, we chose two year
integrations since two years corresponds approximately to
the average drifter lifetime and because it matches the in-
tegration time previously used by Brambilla and Talley
(2006). We have shaded black any trajectory that crosses
558N and taken this as a crude criterion for entering the
subpolar gyre. It can be seen that only a few of the trajec-
tories launched in the eastern part of this zonal segment
were able to cross 558N. This suggests that, at least at the
surface, a transport barrier exists that prevents trajectories
originating on its offshore side from entering the subpolar
gyre. We argue that this barrier may be associated with the
instantaneous position of the Gulf Stream core. Because
the Gulf Stream shifts in time, its instantaneous core os-
cillates about its mean position (green curve). Such oscil-
lation may explain the existence of some black launch
locations that lie offshore from the green curve in Fig. 4.
Our inference about the Gulf Stream core as a transport
barrier agrees with the observations of Bower et al. (1985)
showing limited surface exchange across the Gulf Stream
and also with the presence of a strong potential vorticity
gradient across the Gulf Stream.
Figure 5 shows regions of the subtropical gyre that
participate in the exchange with the subpolar gyre. For
a simulated drifter launched at a particular location, this
figure shows the probability that the drifter will cross
a zonal line at 558N within 2 yr. Most of the subtropical
gyre is characterized by virtually zero probabilities (white
area), meaning that drifters launched there do not make it
into the subpolar gyre within 2 yr. Drifters that do reach
the subpolar gyre must be launched inside the gray and
black funnel-shaped region located onshore of the in-
stantaneous position of the Gulf Stream core. Note that
this region is quite narrow to the south of approximately
408N and can easily be missed by a random seeding of
drifters. This result suggests an alternative interpretation
of the main result of Hakkinen and Rhines (2009): the
increased number of drifters crossing from the subtrop-
ical to the subpolar region since 2001 may simply indicate
that more drifters were deployed inside the funnel-
shaped area after 2001 than before 2001. Indeed, a visual
comparison between the distribution of launch locations
before and after 2001 [Fig. 2a in Hakkinen and Rhines
(2009)] and the black and gray funnel-shaped region in
Fig. 5 suggests this may be the case.
FIG. 3. Square root of the time-averaged eddy kinetic energy
computed from altimetric sea surface heights.
FIG. 4. Two-year simulated drifter tracks in the observation-based
system. Drifters were released once per month during the 12 months
starting in October 1992 within a zonal band straddling the Gulf
Stream near 348N. Tracks and release sites are color coded: black for
tracks that crossed 558N and gray for tracks that did not. The blue
curve is the separating streamline; the green curve approximates the
Gulf Stream velocity core of the background flow (dashed where the
core is no longer well defined); and red is a zonal line at 558N. Color-
coded launch locations are also shown in the small subplot below.
914 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 41
In the previous figure, we launched simulated drifters
once per month during the 12 months starting in October
1992. To check that the results would not change for other
years, we have done a more extensive calculation, where
simulated drifters were launched in a subdomain of
the North Atlantic (258–468N, 838–408W) once per month
during 14 years (1992–2006). The resulting probability
map is shown in Fig. 6a. As in Fig. 5, drifters that reach
subpolar latitudes within 2 yr are launched mostly inside
the funnel-shaped region onshore of the Gulf Stream
core. However, some very small but nonzero probabilities
(darkest blue) occur in the interior of the subtropical
gyre. Trajectories of simulated drifters corresponding to
the nonzero probabilities offshore from the Gulf Stream
core in Fig. 6a are illustrated in Fig. 6b. Many of these
trajectories recirculate clockwise, enter the Gulf Stream
near the Straits of Florida, and then flow through the
funnel into the subpolar gyre. Others are entrained into
the Gulf Stream along its seaward extension, a sign of
a vigorous eddy field.
In the previous figures, we have used a rather re-
strictive criterion, a zonal line at 558N, for the intergyre
boundary. An alternative way to define the intergyre
boundary is to use the separating streamline of the steady
flow. This criterion is less restrictive because some tra-
jectories may cross this separatrix briefly at some location
but then cross back into the subtropical gyre at some later
time, staying always to the south from 558N. Note also
that the northernmost point of the separating streamline
reaches 558N—the latitude used as a crude criterion
for entering the subpolar gyre in Figs. 4 and 5. A map
showing probability for a simulated drifter launched at
a particular location to cross the separating streamline
within 2 yr is plotted in Fig. 7. As in Fig. 5, most drifters
that cross the separating streamline are launched in the
narrow dark region located inshore of the Gulf Stream
core. However, more launch locations inside the sub-
tropical gyre are characterized by small but nonzero
probabilities in Fig. 7 than in Fig. 5. These correspond to
drifters that briefly cross the separating streamline but
than cross back and recirculate in the subtropical gyre,
staying always to the south from 558N.
Figures 4–7 show results with 2-yr integration times;
results with longer integration times are shown in Fig. 8.
This figure shows an estimate of the ‘‘cross time’’ T
cross
,
the time needed for a simulated drifter launched at a
particular location to cross a zonal line at 558N. Values
of T
cross
are color coded from blue to red (small to large
T
cross
); black indicates launch locations of drifters that
did not cross 558N by the end of 2008 (i.e., T
cross
.
16 yr). Several interesting conclusions can be drawn
from this figure: first, the blue funnel-shaped region with
cross times T
cross
# 2 yr generally resembles the nonzero-
probability region shown in the top panel of Fig. 5. The
nozzle of the blue funnel in Fig. 8, however, does not
FIG. 5. Probability that a drifter crosses a zonal line at 558Nwithin
2 yr as a function of its launch location in the observationally based
model. Simulated drifters were released once per month during the
12 months starting in October 1992 at each model grid point south of
558N. The red curve indicates the separating streamline of the back-
ground flow; the green curve approximates the Gulf Stream core of
the background flow; and the red line indicates a zonal line at 558N.
FIG. 6. (a) As in Fig. 4 but for simulated trajectories launched
during 14 years (1992–2006). (b) Trajectories of drifters corre-
sponding to the nonzero probabilities offshore from the Gulf
Stream core in Fig. 5a.
M
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