From ten years of observations of the Atlantic meridional
overturning circulation (MOC) at 26

The Atlantic meridional overturning circulation (MOC) is a key part of
the global ocean circulation, redistributing heat and properties around
the globe. The continuous daily time-series observations at

From the first year of observations,

Numerical investigations into the sources of variability to the Atlantic
MOC interannual variability suggest that much of the variability may be
attributable to winds

Map of the study area. The RAPID array is shown with dashed lines
crossing the Atlantic around

In this paper, we introduce the 10-year record of the MOC at

The international

Here, we focus on the transbasin or mid-ocean (MO) transport, from
which the UMO is derived. The MO transport is constructed from three
parts:

The external flow,

The MO transport can be further divided into its contributions to the
upper and lower branches of the overturning circulation. The UMO is
defined as the depth integral of MO transport between the surface and
the time-varying depth of maximum overturning, roughly 1100

For the analysis presented here, we start with the RAPID data as processed
for the publicly available data set. This processing involves filtering
individual instrument records with a 2-day low-pass filter to remove the
tides, and subsampling onto 12-hourly intervals. From this subsampled
data set, transport components are computed, then further 10-day low-pass
filtered with a fifth-order Butterworth filter before the compensation
transport is calculated

For the purpose of calculating isopycnal displacements

All of the 10-year time series of transport components at

Transport-per-unit-depth anomalies of the mid-ocean transport at

Time series of transport anomaly for MOC and components as in
Fig. 2, deseasonalized and low-pass filtered with 1.5

The MOC exhibits substantial variability at timescales longer than
annual (Fig. 4). Interannual variability of the MOC derives primarily
from the UMO component, with a negative trend in both over the full
10-year record. The mean and standard deviation of the MOC for the first
5 years of observations is

Mean

Transport-per-unit depth anomaly profiles show the depth structure of
mid-ocean transport variations. In the top 1100

During the first 3 years of observations (2004–2007), the
components of the MOC (FC, UMO, and Ekman) showed little co-variability,
leading to the conclusion that components contribute their variability
independently to the MOC

The FC carries most of the waters of the Gulf Stream across 26

Transport anomaly time series (left column) for the

Here we consider compensation between FC and UMO, the transbasin transport
east of the Bahamas. This compensation can be clearly seen by plotting their
detrended anomaly time series (Fig. 5a). Certain events stand out,
demonstrating almost perfect correspondence between the two time series, with
examples including February–May 2007, September 2008–June 2009, August
2010–January 2011 and August 2012–March 2013 (highlighted in the figure).
Notably, these episodes of correlation are absent in the first
3

Fluctuations in UMO compensate fluctuations in the FC by similar magnitudes
(

Using the low-pass filtered time series, this high degree of compensation is
absent (Fig. 5c and d). Instead, strong interannual variability in the UMO
remains (Fig. 5c). By comparison, the low-pass filtered FC shows little
interannual variability, consistent with previous work that indicated that
the interannual and longer period FC variability is of much smaller amplitude
than the sub-annual variability (e.g.

Another – and perhaps more remarkable – correlation that emerges from this
analysis is between the deepest limb of the southward mid-ocean
transports (LNADW) and the surface meridional Ekman transport (Fig. 6a).
Using the detrended anomaly time series, the typically southward
LNADW transport can be seen to reduce or even temporarily reverse to
northward during strong Ekman transport reversals. (See, e.g., events in
December 2009–April 2010, November 2010–January 2011, February–March 2013.) The correlation is
statistically significant (

As Fig. 5 but for the Ekman and

As with the FC and UMO compensation, magnitudes of fluctuations between Ekman
and LNADW match (

To identify possible lags between the UMO and FC or Ekman and LNADW, we use
the 10-day filtered time series. For both correlations, between the
UMO and FC and between the Ekman and LNADW, the timescale of the response is
fast (Fig. 7). For the LNADW and Ekman correlation, a maximum correlation of

10-day filtered transport anomaly time series of UMO and FC

The hypsometric compensation term (

The deep maximum in transport per unit depth implies that there are
considerable changes in the deep shear integrated across the width of the
basin, which are reflected in the

While the stratification at

Comparing the UMO transport with both eastern and western boundary isopycnal
displacement time series, we find strong correlations (Fig. 8a). In the west,
displacements between 300 and 1200

Correlation between isopycnal displacements at each depth and
transport time series, where time series are deseasonalized and detrended.

The FC is also highly correlated with the western boundary thermocline
displacement (Fig. 8a). The sign of the correlation has flipped, consistent
with the anti-correlation noted between the FC and UMO. This relationship is
statistically significant, even though the isotherms covarying with the FC
are 150

Time series of deseasonalized, detrended transports and isopycnal
depths.

The correlation between LNADW transport and isopycnal displacements is
significant at depth (1500

Isopycnal displacements in a reduced region (2700–3300

One of the key results presented here is that the UMO and FC transports often compensate each other – i.e. their signs differ but anomalies match – resulting in greatly reduced impact of their individual fluctuations on the total MOC variability. However, this compensation is dependent on timescales. At low-frequencies, the compensation does not dominate (Fig. 5c), and the large interannual variability and trend in the UMO transport has a strong projection onto the interannual variability and trend of the MOC.

To investigate the co-variability for different timescales, we evaluate the
coherence calculated using a multitaper spectrum following

Coherence between MOC components: UMO and FC (magenta), LNADW and Ekman (green) and FC and Ekman (grey), where time series are the original 10-day filtered (seasonal variations retained). The top panel shows coherence, where significance is delimited by the black horizontal line. The lower panel shows the phase relationship at each period in degrees.

These results at first appear to contradict

The compensation between the FC and UMO is non-stationary. A windowed
correlation between the FC and UMO prior to 2007 shows no significant
correlation, either between the full time series or the high-pass filtered
time series (not shown). From

The MOC at

In the top 1000

Trend in isopycnal displacements and transports.

The trend in

A southward intensification of

The trend in

Here we have identified significant compensation, dependent on timescale,
between components of the MOC: for example, on sub-annual timescales when
the FC is stronger northward in the western boundary, the UMO compensates
with stronger southward flow between the Bahamas and Canary islands. While
these components are largely independent, they are weakly coupled due to the
construction of the

However, by construction the

The compensation between the UMO and FC has the appearance of
a time-varying horizontal or gyre circulation, but is limited to
sub-annual timescales similar to those of eddies identified in

The relationship between surface winds and deep isopycnal displacements
is harder to understand, and we presently do not have a dynamical
explanation for this behaviour. Nevertheless, the observations are quite
clear: when there is anomalous southward Ekman transport (resulting from
westerly winds), isopycnals at

The observed high-degree of variability on sub-annual timescales of the
UMO and LNADW transports may contribute to the apparent absence of
meridional coherence between observations at

The record of basin-wide MOC transport variability at

The main result of this paper has been detailing newly identified
compensations between MOC components (UMO and FC, and LNADW and Ekman).
Using the 10-year record, we now find that on shorter timescales (periods
shorter than 1

There is a key difference between these two compensating transport pairs.
Between the FC and UMO, compensated high-frequency transport anomalies
results in a horizontal circulation anomaly: that is, northward flow in the
FC (in the top 700

Finally, investigating longer-term variations of the MOC, we can localize
the origin of the intensifying trend in the MOC to isopycnal displacements on
the western boundary. Observed transport fluctuations on interannual and
longer timescales present a different story. From the 10-year record, the
mid-ocean transports (rather than FC and Ekman) are primarily responsible for
the low-frequency variability and trend of the overturning. Furthermore, the
trend in transport variability is associated with the persistent deepening of
isopycnals below the thermocline at the western boundary. These displacements
are greatest in the abyss (

Data from the RAPID Climate Change
(RAPID)/Meridional overturning circulation and heat flux array (MOCHA), Western
Boundary Time Series (WBTS) projects are funded by the Natural
Environment Research Council (NERC), National Science Foundation (NSF,
OCE1332978) and National Oceanic and Atmospheric
Administration (NOAA), the Climate Program Office – Climate Observation Division. Data are freely available from

Florida Current transports are funded by the
NOAA and are available
from

Wavelet code provided by A. Grinsted, J. Moore and S. Jevrejeva. Special
thanks to the captains, crews, and technicians, who have been invaluable in the
measurement of the MOC at