The
IAG/IAPSO Joint Working Group (JWG) on Geodetic Effects of Nontidal Oceanic
Processes was formed at the XXII General Assembly of the IUGG that was held in
Birmingham during July, 1999 for the purpose of: (1) promoting investigations
of the effects of nontidal oceanic processes on the Earth’s rotation,
deformation, gravitational field, and geocenter; and (2) fostering interactions
between the geodetic and oceanographic communities in order to gain greater
understanding of these effects. Since it was formed, five
meetings-of-opportunity of the JWG have been held: (1) on December 15, 1999 in
conjunction with the 1999 Fall Meeting of the AGU that was held in San
Francisco, California; (2) on April 27, 2000 in conjunction with the XXV
General Assembly of the EGS that was held in Nice, France; (3) on March 29,
2001 in conjunction with the XXVI General Assembly of the EGS that was held in
Nice, France; (4) on April 25, 2002 in conjunction with the XXVII General
Assembly of the EGS that was held in Nice, France; and (5) on April 7, 2003 in
conjunction with the 2003 Joint Assembly of the EGS-AGU-EUG that was held in
Nice, France. Summaries of the latter four meetings have been or will soon be
published in the IAG Newsletter that appears in the Journal of Geodesy.
In the
last few years a number of exciting developments have occurred in the area of
ocean/solid Earth interactions. The importance of oceanic processes to exciting
polar motion in general (Furuya & Hamano 1998; Ponte
et al. 1998; Johnson et al. 1999; Nastula
& Ponte 1999; Chen et al. 2000;
Nastula et al. 2000, 2002; Wünsch
2000, 2002a, 2002b; Ponte et al.
2001, 2002; Thomas et al. 2001;
Leuliette et al. 2002a; Ponte and Ali
2002) and the Chandler wobble in particular (Celaya et al. 1999; Ponte & Stammer 1999; Gross 2000; Brzezinski &
Nastula 2002; Brzezinski et al.,
2002b; Gross et al. 2003; Liao et al. 2003) has been demonstrated by
applying ocean models to Earth rotation studies. In addition, ocean models have
enabled the relatively smaller, but still detectable, influence of oceanic
processes on the length-of-day (lod) to be studied (Marcus et al. 1998; Johnson et al.
1999; Chen et al. 2000; Kakuta et al. 2000; Ponte & Stammer 2000;
Höpfner 2001; Ponte and Ali 2002; Ponte et
al. 2001, 2002). The oceanic torques acting on the solid Earth are also
being studied (de Viron et al. 2002;
Fujita et al. 2002; Hughes 2002). As
global ocean general circulation models continue to improve, and ocean data
assimilation systems develop (Ponte et
al. 2001), more progress can be expected.
Since the
formation of the Joint Working Group, the effect of oceanic mass redistribution
on the orbits of satellites (Chen et al.
1999a; Johnson et al. 2001a), the
Earth’s gravitational field (Cazenave et
al. 1999; Cheng & Tapley 1999; Gruber et al. 2000; Foldvary and Fukuda 2001a, 2001b, 2002; Johnson et al. 2001b; Leuliette et al. 2002b; Wünsch et al. 2002; Reigber et al. 2003), geocenter (Cazenave et al. 1999; Chen et al. 1999b; Bouillé et al.
2000; Johnson et al. 2001b; Crétaux et al. 2002), surface gravity
measurements (Sato et al. 2001), and
station positions (Mangiarotti et al.
2001) have also been studied. The launch of CHAMP and GRACE will enable even
more detailed studies of the influence of the oceans on the Earth’s
gravitational field (Wahr et al.
1998). Furthermore, CHAMP and GRACE will directly measure the mass term of the
Earth rotation excitation functions (Gross 2001, 2003) as well as fluctuations
in ocean-bottom pressure (Ponte 1999). Thus, the next few years should prove as
exciting as have the last few years in studying the geodetic effects of
nontidal oceanic processes.
As
can be seen in the studies of, for example, the EOPs (Earth Orientation
Parameters) and the change in the J2 term of the global gravity field, the
oceans play an important role in Earth system dynamics (ESD). Measurements of
sea surface height (SSH) variability obtained from satellite altimetry have
revolutionized our knowledge of the role of the oceans. For the ESD, the
important physical quantity directly affecting its change is the mass
redistribution in the oceans, not the change in the SSH itself, because, as is
well known, the observed SSH variations from satellite altimeters include as a
major part steric changes due to thermal and salinity changes in the oceans
which do not produce any gravity effects. By combining the altimeter data with
the data obtained from satellite gravity missions such as CHAMP, GRACE and GOCE,
we can expect to separate the steric changes and to reveal the true mass
transport in the oceans. On the one hand, the validation and calibration of
satellite data based on observations on the ground, in the ocean and on the sea
floor are important for interpreting the satellite data because the observation
data by satellites are a result of integrating over many related phenomena.
More over, for the data obtained from the gravity satellites, the effect of air
pressure changes on the ocean surface is contaminated by an effect of aliasing.
In order to study the mass transport in the oceans and the aliasing effect in
the satellite gravity data due to air mass changes on the sea surface, it is
recommended to develop network campaign observations with ocean bottom pressure
gauges at selected locations in the world oceans. An ocean region where the El
Niño effect appears is a candidate for this observation purpose and the network
should be designed so that we can detect the mass transport in not only the east-west
direction but also the north-south direction. Reports
from individual JWG members on their activities are given below:
Brzezinski.
We studied the nontidal oceanic influences on Earth rotation
by using the methods which had been earlier developed and applied for
estimating atmospheric effects (Brzezinski et
al. 2002a). We focused our attention on the excitation of the 14-month free
Chandler wobble. By using an 11-year time series of the ocean angular momentum
(OAM) we concluded (Brzezinski and Nastula 2002) that within the limits of
accuracy the coupled system atmosphere/ocean fully explains the observed
Chandler wobble during the period 1985-1996. Similar study using a 50-year OAM
series (Brzezinski et al. 2002b)
yielded less promising results, which could be attributed to the differences in
the underlying ocean circulation models. Our first attempt to estimate the
nontidal oceanic contribution to nutation (Petrov et al. 1999) showed that the OAM series produced so far were still
not adequate for studying the diurnal and subdiurnal effects. The most
important tasks for the future within the subject of the oceanic excitation of
Earth rotation were pointed out in a review paper (Brzezinski 2003).
B. Chao.
The
Space Geodesy research group at the NASA Goddard Space Flight Center made
numerous studies on oceans' roles in changing the Earth's rotation and
low-degree gravity field. In addition to the global effects, three specific
ocean basins have been targeted: North Atlantic (in association with the North
Atlantic Oscillation), Mediterranean Sea (a joint project with scientists from
the University of Alicante, Spain), and the extratropical Pacific (in searching
for the causes of the 1998 anomaly in the Earth's J2 series found in
satellite-laser-ranging data). Three main types of data are examined: (1)
TOPEX/Poseidon altimetry data, (2) sea-surface temperature data, (3) ocean
general circulation model output. Extensive use of the Empirical Orthogonal
Function / Principal Component numerical technique has been made. Progress has
been reported at various international meetings, including the AGU, EGS, and
T/P SWT. Publications include Chao and Zhou (1999), Johnson et al. (1999, 2001b), Chen et al.
(2000), O’Connor et al. (2000), Chao et al. (2001), and Fujita et al. (2002).
T. Johnson.
The
U.S. Naval Observatory in an effort to improve its predictions of UT1-UTC
examined the usefulness of introducing atmospheric angular momentum data into
the EOP combination and prediction process. The study showed that the
atmosphere could not account for all of the variability on time scales ranging
from six days to 15 days. Using the Parallel Ocean Climate Model (POCM), we
determined that this variability was the result of non-tidal ocean variability
(Johnson et al. 2003). These results
along with the results of earlier studies (Ponte et al. 1998; Johnson 1998; Johnson et al. 1999; Ponte et al.
2002) indicate that the ocean appears to excite variations in Earth's rotation
on time scales ranging from a few days to a few years. In another study,
Johnson et al. (2001b) indicated that
non-tidal oceanic variability could account for some of the perturbations in
the orbits of the Lageos I and Lageos II satellites on timescales of several
days to a few years that result from temporal variations in the Earth's
gravitational field but that are unexplained by the atmosphere. Furthermore, it
was shown by Johnson et al. (2001a)
that the effects of oceanic variability could be observed in the orbit
perturbations of GPS satellites. We have also shown that the intermediate and
lower layers of global ocean circulation models appear to require much more
than 30 years to spin up (Johnson 1999). However, the application of sea level
adjustments, also known as the Greatbatch correction, effectively removes most
of this effect as well as the effects of the Boussinesq approximation. These
results indicate that the examination of trends in momentum would be a better
test of model spin-up than the use of kinetic energy.
J. Nastula.
It was shown that oceanic excitation, when added to atmospheric
excitation, leads to substantial improvements in the agreement with observed
polar motion excitation at seasonal and intraseasonal periods (Nastula and
Ponte 1999). It was also shown by Brzezinski and Nastula (2002) that variations
of the angular momentum of the coupled atmosphere/ocean system (with a little
bit higher contribution from the ocean source) can explain within the error
limits the observed Chandler motion during the 1985-1996 interval. In more recent
studies, Brzezinski et al. (2002b)
extended the analysis of Brzezinski and Nastula (2002) by using a 50-year time
series of OAM estimated by Ponte (2000, personal communication). The obtained
estimate of the oceanic excitation power, 18.1 mas2/ cpy, is in good
agreement with the residual excitation derived from a simultaneous use of polar
motion and atmospheric angular momentum data, 20.3 mas2/ cpy. Regional patterns of atmospheric and oceanic excitation
were analysed separately and compared with each other (Nastula et al. 2000, 2002). The results confirm
recent findings that oceans supplement the atmosphere as an important source of
polar motion excitation. Analysis of regional AAM and OAM signals were
performed for monthly and longer periods (Nastula et al. 2000) and for periods shorter than 10 days (Nastula et al. 2002). The influence of specific
geographic areas on polar motion excitation was discovered. Regional
characteristics of short period excitation of polar motion are generally in
agreement with those obtained from analyses performed for signals at monthly
and longer periods. The AAM and OAM signals associated with pressure terms were
found to be of the same order of magnitude, while signals associated with winds
were substantially larger then those associated with ocean currents. The
strongest polar motion excitation due to variability of atmospheric pressure,
oceanic pressure and wind terms is connected with some specific areas over
northern and southern mid-latitudes. The spatial pattern of pressure plus
inverted barometer (IB) term is dominated, however, by maxima over land areas
with Eurasia being especially important. Oceanic excitation due to currents is
strong in the North Pacific and the Southern Oceans. Variability in oceanic
bottom pressure tends to be large in mid and high latitude regions.
R. Ponte.
Ponte
et al. (2002)
addressed the influence of climate variability on ocean angular momentum (OAM).
Possible anthropogenically induced signals included trends and changes in the
seasonal cycle of OAM, but their effects on Earth rotation were relatively
weak. In contrast, OAM signals related to natural climate variability were
found to be important sources of excitation, particularly for the annual,
Chandler, and Markowitz wobbles. Ponte and Ali (2002) demonstrated the role of
OAM signals for excitation of sub-weekly polar motion and length-of-day
variations and the importance of deviations from an inverted barometer response
to atmospheric pressure at these rapid time scales. Uncertainties in the
atmospheric pressure fields remain a problem in determining those signals
(Ponte and Ray 2002; Ponte and Dorandeu 2003). Similar uncertainties in
seasonal wind stress torques over the ocean, which affect OAM and the planetary
angular momentum budget, were discussed in Ponte et al. (2003).
T. Sato.
Differential
GPS observations were conducted on the fast ice in Lutzow-Holm Bay, Antarctica
(Aoki et al. 2000). The vertical
displacement, was clearly detected. Tidal variation derived from GPS showed
good agreement with those from pressure gauges. GPS measurements of the vertical
displacement of fast ice near Syowa Station, Antarctica, were conducted between
April and December 1998 (Aoki et al.
2002). The GPS derived sea level, combined with observed sea ice thickness,
supported a conventional bottom pressure gauge result with an RMS error of
0.007 m. Coherent sea level variations were clearly detected for five coastal
tide gauge data around Antarctica on intraseasonal time scales (Aoki 2002). The
coherent variations had significant negative correlations with an index of the
atmospheric annular mode variation (Antarctic Oscillation). Important new
findings from 14 months of observations of ocean bottom pressure variations in
the southeastern Pacific are reported by Fujimoto et al. (2003). One is a pressure increase starting in December 1997
at almost the same time as the termination of the 1997-98 El Niño. It is also
coincident with a remarkable change in the J2 term of the Earth's gravity
field. These results suggest that the El Niño might have brought about mass
redistribution in the eastern Pacific Ocean. The other feature in the
observations is a local pressure variation across the spreading axis of the
ultra-fast spreading southern East Pacific Rise. It is estimated that the
seafloor near the spreading axis was depressed at a rate of about 20 mm/month.
The gravity effects of sea surface height (SSH) variations were studied by
Fukuda et al. (1999). They applied
the EOF (Empirical Orthogonal Function) analysis to both SSH data and induced
gravity changes and showed that one of the EOF components was strongly
correlated with ENSO (El Niño/Southern Oscillation) like SSH variations. Based
on the POCM (Parallel Ocean Climate Model, Stammer et al. 1996) and the TOPEX/POSEIDON (T/P) altimeter, the effects of
SSH variations on gravity observations were estimated (Sato et al. 2001). The thermal steric
component of SSH variations was estimated by assuming a simple linear
relationship between the time variations in the SSH and SST fields. The
predicted gravity changes at the three observation sites (i.e. Esashi in Japan,
Canberra in Australia and Syowa Station in Antarctica) were compared with the
actual data obtained from the superconducting gravimeters installed at these
three sites. We have also tried to investigate the effects of SSH on the
gravity observations in other frequency bands. Our computations suggest that
ENSO-like ocean oscillations contribute 2 to 3 microgals peak-to-peak amplitude
to gravity variations in the equatorial Pacific at the maximum.
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S., Coherent sealevel response to the Antarctic Oscillation, Geophys. Res. Lett., 29(12),
doi:10.1029/2002GL015733, 2002.
Aoki,
S., T. Ozawa, K. Doi, and K. Shibuya, GPS observation of the sea level variation in Lutzow-Holm Bay, Antarctica, Geophys. Res. Lett., 27, 2285-2288,
2000.
Aoki,
S., K. Shibuya, A. Masuyama, T. Ozawa, and K. Doi, Evaluation of seasonal sea
level variation at Syowa Station, Antarctica, using GPS observations, J. Oceanogr., 58, 519-523, 2002.
Bouillé,
F, A. Cazenave, J. M. Lemoine, and J. F. Crétaux, Geocentre motion from the
DORIS space system and laser data to the Lageos satellites: Comparison with
surface loading data, Geophys. J. Int.,
143, 71-82, 2000.
Brzezinski,
A., Oceanic excitation of polar motion and nutation: An overview, in IERS Technical Note 30: Proc. IERS Workshop
on Combination Research and Global Geophysical Fluids, edited by B.
Richter, in press, Bundesamts für Kartographie und Geodäsie, Frankfurt am Main,
Germany, 2003.
Brzezinski,
A., and J. Nastula, Oceanic excitation of the Chandle wobble, Adv. Space Res., 30, 195-200, 2002.
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A., Ch. Bizouard, and S. Petrov, Influence of the atmosphere on Earth rotation:
What new can be learned from the recent atmospheric angular momentum
estimates?, Surv. Geophysics, 23,
33-69, 2002a.
Brzezinski,
A., J. Nastula, and R. M. Ponte, Oceanic excitation of the Chandler wobble
using a 50-year time series of ocean angular momentum, in Vistas for Geodesy in the New Millennium, edited by J. Adám and
K.-P. Schwarz, pp. 434-439, IAG Symposia vol. 125, Springer-Verlag, New York,
2002b.
Cazenave,
A., F. Mercier, F. Bouille, and J. M. Lemoine, Global-scale interactions
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Celaya,
M. A., J. M. Wahr, and F. O. Bryan, Climate-driven polar motion, J. Geophys. Res., 104, 12813-12829,
1999.
Chao, B.
F., and Y.-H. Zhou, Meteorological excitation of interannual polar motion by
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B. F., W. O’Connor, D. Zheng, and A. Y. Au, Reply to Wunsch, J. Geophys. Int., 146, 266, 2001.
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J. L., C. R. Wilson, R. J. Eanes, and B. D. Tapley, Geophysical contributions
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J. L., C. R. Wilson, R. J. Eanes, and R. S. Nerem, Geophysical interpretation
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Chen,
J. L., C. R. Wilson, B. F. Chao, C. K. Shum, and B. D. Tapley, Hydrological and
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Cheng,
M., and B. D. Tapley, Seasonal variations in low degree zonal harmonics of the
Earth’s gravity field from satellite laser ranging observations, J. Geophys. Res., 104, 2667-2681, 1999.
Crétaux,
J.-F., L. Soudarin, F. J. M. Davidson, M.-C. Gennero, M. Bergé-Nguyen, and A.
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Viron, O., V. Dehant, and H. Goosse, The “hidden torque”: The art, for a
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L., and Y. Fukuda, Evaluation of temporal variations on the gravity field
caused by geophysical fluids and their possible detection by GRACE, in Gravity, Geoid, and Geodynamics 2000,
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L., and Y. Fukuda, IB and NIB hypotheses and their possible discrimination by
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L., and Y. Fukuda, Effects of atmospheric variations on the marine geoid
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R. S., CHAMP, mass displacements, and the Earth’s rotation, in First CHAMP Mission Results for Gravity,
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J., Atmospheric, oceanic, and hydrological contributions to seasonal variations
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C. W., Torques exerted by a shallow fluid on a non-spherical, rotating planet, Tellus, 54A, 56-62, 2002.
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T. J., The role of the ocean in the planetary angular momentum budget, Ph.D.
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T. J., The effects of the correction for mass non-conservation in global ocean
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T. J., C. R. Wilson, and B. F. Chao, Oceanic angular momentum variability
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T. J., P. Kammeyer, and J. Ray, The effects of geophysical fluids on motions of
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T. J., C. R. Wilson, and B. F. Chao, Nontidal oceanic contributions to
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T. J., B. J. Luzum, and J. Ray, Improved near-term UT1R predictions using
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S., A. Cazenave, L. Soudarin, and J. F. Crétaux, Annual vertical crustal
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W., B. F. Chao, D. Zheng, and A. Y. Au, Wind stress forcing of the North Sea
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R. M., A preliminary model study of the large-scale seasonal cycle in bottom
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R. M., and J. Dorandeu, Uncertainties in ECMWF surface pressure fields over the
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R. M., and R. D. Ray, Atmospheric pressure corrections in geodesy and
oceanography: A strategy for handling air tides, Geophys. Res. Lett., 29, doi:10.1029/2002GL016340, 2002.
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R. M., and D. Stammer, Role of ocean currents and bottom pressure variability
on seasonal polar motion, J. Geophys.
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R. M., and D. Stammer, Global and regional axial ocean angular momentum signals
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R. M., D. Stammer, and C. Wunsch, Improving ocean angular momentum estimates
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R. M., J. Rajamony, and J. M. Gregory, Ocean angular momentum signals in a
climate model and implications for Earth rotation, Clim. Dyn., 19, 181-190, 2002.
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R. M., A. Mahadevan, J. Rajamony, and R. D. Rosen, Uncertainties in seasonal
wind torques over the ocean, J. Climate,
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H. Lühr, and P. Schwintzer, pp. 128-133, Springer-Verlag, New York, 2003.
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Y. Fukuda, Y. Aoyama, H. McQueen, K. Shibuya, K. Asari, and M. Ooe, On the
observed annual gravity variation and the effect of sea surface height
variations, Phys. Earth Planet. Inter.,
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