GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L23303, doi:10.1029/2004GL021200, 2004
The interannual variability of the North Atlantic Ocean revealed by
combined data from TOPEX//Poseidon and Jason altimetric
measurements
Lee-Lueng Fu
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
Received 4 August 2004; accepted 19 October 2004; published 8 December 2004.
[1] A decade-long record of sea surface height from
combined altimeter data taken by the TOPEX/Poseidon
and Jason satellites was analyzed for studying the
interannual variability of the North Atlantic Ocean. On
time scales of 5– 6 years, variations of sea surface height
have maximum amplitudes in three areas: the subpolar
gyre, the Gulf Stream gyre (the Gulf Stream and its
recirculation), and the subtropical region. The variation of
the subpolar gyre is 180 degrees out of phase with that of the
Gulf Stream gyre at its eastern end. The variation of the Gulf
Stream gyre at its western end is connected to that of
the subtropical region, exhibiting phase propagation from
the subtropics all the way to the eastern end of the Gulf
Stream gyre. The patterns of phase change suggest possible
roles of Rossby waves in the dynamics of the basin-wide
INDEX TERMS: 4556 Oceanography: Physical: Sea
variability.
[3] In a recent study, Häkkinen and Rhines [2004]
reported a decadal trend of slowing down of the subpolar
gyre of the North Atlantic. Their findings were primarily
based on the altimetry data from the TOPEX/Poseidon
satellite. From analysis of empirical orthogonal functions
(EOF), they identified a leading EOF characterized by a
basin-wide pattern of rising sea level in the subpolar gyre
and falling sea level in the Gulf Stream gyre (defined here
as the Gulf Stream and its recirculation system). Interesting
interannual variability is also revealed in the EOF. This
leading mode accounts for only 11% of the total variance,
however.
level variations; 4215 Oceanography: General: Climate and
interannual variability (3309); 4576 Oceanography: Physical:
Western boundary currents. Citation: Fu, L.-L. (2004), The
interannual variability of the North Atlantic Ocean revealed by
combined data from TOPEX/Poseidon and Jason altimetric
measurements, Geophys. Res. Lett., 31, L23303, doi:10.1029/
2004GL021200.
1. Introduction
[2] The circulation of the North Atlantic Ocean is of
critical importance to long-term climate change because
of the perceived roles played by the formation of the
North Atlantic deep water [Broecker, 1997]. It is therefore
important to monitor and understand the variability of the
North Atlantic. However, basin-wide observations of the
variability of the North Atlantic are limited in both spatial
and temporal coverage. Our understanding of the variability
of the North Atlantic circulation has largely been based on
sparsely located in-situ data and numerical models [e.g.,
Joyce and Robbins,1996; Ezer et al., 1995]. Some important
basin-wide changes have been discovered from sea surface
temperature (SST) observations [Hansen and Bezdek, 1996;
Sutton and Allen, 1997], but it is difficult to determine the
underlying dynamic processes from SST. The advent of
satellite altimetry observations of sea surface height (SSH)
has offered a new opportunity to study the circulation and
dynamics of oceanic variability. For example, Häkkinen
[2001] used altimetry data and numerical models to study
the interannual variability of the circulation and meridional
heat transport of the North Atlantic Ocean.
Copyright 2004 by the American Geophysical Union.
0094-8276/04/2004GL021200$05.00
Figure 1. SSH time series at (a) 63°N, 330°E, (b) 43°N,
330°E, and (c) 20°N, 330°E. The thick solid lines show the
low-passed SSH with variance at periods shorter than
18 months removed.
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Figure 2. The spatial pattern of the leading CEOF: (a) amplitude in arbitrary units; (b) phase in degrees. The asterisks in
(b) are the locations where the phase is plotted in Figure 3b.
[4] The objective of the present study is the interannual
variability of the North Atlantic derived from analyzing a
combined record of 11.7 years’ worth of SSH observations
from TOPEX/Poseidon and Jason satellites. The technique
of complex-valued empirical orthogonal function (CEOF)
[see Horel, 1984] was used to extract coherent large-scale
variability from the data record. The advantage of CEOF
over regular EOF is the ability to take into account continuous phase variations in the separation of modal structures.
Regular EOF can only account for 180 degree phase
difference in a mode. The resulting leading CEOF turns
out to be more energetic and informative than the regular
EOF. The data and analysis methods are described in the
next section followed by discussions of the results.
2. Data Analysis and Descriptions
[5] Altimetric observations of SSH from the TOPEX/
Poseidon (T/P) satellite and its successor Jason satellite
were used in the study. The T/P data cover a period from
October 2, 1992 – August 10, 2002. The Jason data became
available along the same ground tracks of T/P after January,
2002. A relative bias of 13 cm between the two altimeter
measurements was established from analysis of the overlapping data records [Vincent et al., 2003]. After subtracting
the relative bias from the Jason data, the two data records
were combined to create global SSH time series with a
record length of 11.7 years.
[6] Both T/P and Jason altimeters measure the height of
sea surface above a reference ellipsoid every 6.2 km along
repeat ground tracks every 10 days. The longitudinal
distance between ground tracks is approximately 200 km
at mid latitudes. The procedures of data processing are
described by Fu [2003] with the standard corrections
applied, including the tidal and inverted barometer corrections. The mean sea surface model supplied in the Geophysical Data Records was first removed from the SSH
observations and the residuals were interpolated to a set of
normal points (6.2 km apart) for each repeat track. A record
time mean from the combined T/P and Jason time series was
removed from the residual SSH at each normal point. The
resultant SSH ‘‘anomalies’’ were used for the study.
[7] In order to focus on the large spatial scales of the
interannual variability, we need to filter out the mesoscale
variability. A Gaussian-weighted smoothing scheme was
applied to the data to create smoothed SSH maps on
uniform 1° 1° grids every 3 days. The smoothing scale
was chosen to create a cutoff wavelength of 500 km.
The transfer function of the smoothing was discussed by
Fu [2003]. The mesoscale variability with wavelengths
shorter than 500 km was essentially removed by the
smoothing.
Figure 3. (a) SSH time series of the leading CEOF at three locations: Solid line (320°E 43°N), dashed-dotted (293°E,
35°N), and thin dashed (330° E, 63° N). (b) The phase of the leading CEOF at the locations marked by asterisks along the
line shown in Figure 2b. The abscissa corresponds to numbered locations beginning at the eastern tropical region.
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Figure 4. Maps of SSH (in cm) from the leading CEOF sampled every 15 months beginning in August, 1993 as shown in
(a) and ending in October, 1999 as shown in (f).
[8] Shown in Figure 1 are three sample SSH records from
the subpolar region (at 63°N), the North Atlantic Current
region (at 43°N), and the subtropical region (at 20°N) along
the longitude of 330°E in the eastern North Atlantic. The
annual cycles and other high-frequency variations are
quite prominent. To focus on the interannual time scales,
variability with periods shorter than 18 months was
removed from the data. The low-frequency residuals are
also shown in Figure 1. These signals reveal some similarity
between the subpolar region and the subtropical region, but
exhibit quite different characteristics in the North Atlantic
Current region.
[9] To illustrate the basin-wide spatial and temporal
structures of the low-frequency variability, the technique
of CEOF was applied to the low-passed SSH data. Shown in
Figure 2 is the spatial variability of the leading mode, which
accounts for 34% of the variance. A regular EOF analysis
was also performed to the data. The resulting leading mode
accounts for only 17% of the variance. The significant
difference in the efficiency of the two approaches indicates
the importance of the phase information of the variability
captured by the CEOF mode to be discussed below.
[10] The spatial pattern of the amplitude of the leading
mode (Figure 2a) is characterized by three branches located
in the subtropics at 15°N –25°N, the Gulf Stream gyre at
35°N–45°N, and the subpolar gyre at 55°N – 65°N. The
maximum amplitude is located in the Gulf Stream gyre with
a peak-to-trough variation of 15 cm (Figure 3a, solid line).
The temporal variability of the mode reveals a time scale of
5 – 6 years. The time series exhibits the completion of
2 cycles of the mode within the 11.7 year record.
[11 ] The spatial pattern of the phase of the mode
(Figure 2b) reveals that the Gulf Stream gyre is generally
180 degrees out of phase with the other two branches. This
is basically consistent with the characteristics of the leading
regular EOF reported in Häkkinen and Rhines [2004].
However, the CEOF is able to delineate the details of the
phase relations among different regions of the mode and
thus extract a more complete modal structure of the
variability. A notable feature is the progression of phase
changes along the line marked with asterisks in Figure 2b.
This line goes through the local amplitude maxima in the
three branches of the mode. In fact, the subtropical branch is
somewhat connected to the Gulf Stream branch at the
western end of the basin. There is a gradual progression
of phase in this connecting region. A plot of the phase along
the line is shown in Figure 3b. The eastern end of the Gulf
Stream gyre (also called the North Atlantic Current) is in
opposite phase with the subpolar gyre with a sharp gradient
of phase between them. This 180 degree phase difference is
illustrated in Figure 3a (solid line vs thin dashed line). There
is a region of more gradual change of phase between the
subtropical branch and the Gulf Stream gyre at the western
end of the basin.
[12] Displayed in Figure 4 are six SSH maps from the
leading CEOF sampled every 15 months to illustrate a cycle
of the interannual variability. The phase propagation from
the subtropics to the Gulf Stream gyre can be visualized,
especially in Figures 4b and 4e. Note the onset of phase
propagation from southwest to northeast in the Gulf Stream
gyre following the phase change in the subtropical region.
The phase shift between the west end and the east end of the
Gulf Stream gyre is also illustrated in Figure 3a (solid line
vs dashed-dotted line).
3. Discussions
[13] The pattern of the leading CEOF (Figure 2a) is
quite similar to that of the North Atlantic SST ‘‘tripole’’
[Marshall et al., 2001], which is characterized by three
distinct regional maxima of low-frequency SST variability
in the subpolar gyre, the Gulf Stream gyre, and the Canary
Basin of the subtropical eastern North Atlantic. It is generally believed that the SST tripole is the ocean’s response to
the forcing of the atmosphere-ocean heat flux [Cayan,
1992]. The frequency spectrum of the SST tripole does
reveal enhanced variance at periods of 5 – 6 years [Marshall
et al., 2001]. It is therefore plausible that the SSH variability
is also the ocean’s response to the heat flux forcing, as
suggested by the modeling study of Häkkinen [2001]. The
SSH variability has revealed the ocean’s dynamic response
to the atmospheric forcing.
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[14] The spatial variability of the phase of the interannual
SSH variability has provided new information on the
connection between the Gulf Stream gyre and the subtropical region, where the SSH changes lead those in the Gulf
Stream region. A recent modeling study by Köhl [2004]
indicates that the meridional overturning circulation at 30°N
in the Atlantic Ocean is sensitive to changes in the temperature and salinity of the Canary Basin of the eastern
subtropical North Atlantic. This sensitivity is achieved
through Rossby waves of advective type similar to those
operating in the subtropical Pacific described by Galanti
and Tziperman [2003]. These Rossby waves, amplified
through the baroclinic instability of the Atlantic North
Equatorial Current [Keffer, 1983], transmit signals across
the basin to affect the Gulf Stream and thus the overturning
circulation across 30°N. This underlying dynamic process is
indication that the SSH variability is not just a steric
response to the heat flux forcing, but also involves a
dynamic response which presumably plays a role in affecting the atmosphere-ocean heat flux [Czaja et al., 2003].
[15] Acknowledgments. The research presented in the paper was
carried out at the Jet Propulsion Laboratory, California Institute of
Technology, under contract with the National Aeronautic and Space
Administration. Support from the TOPEX/Poseidon and Jason Projects is
acknowledged.
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L.-L. Fu, Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109-8099, USA. ([email protected])
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