INTERNATIONAL ASSOCIATION FOR THE PHYSICAL SCIENCES OF THE OCEAN
of the International Union of Geodesy and Geophysics
COMMISSION ON MEAN SEA LEVEL AND TIDES
Report of the CMSLT 1995-1999
and
proposals for recommendation to the IAPSO executives
As indicated in its title, the domains of
concern of the Commission are “Mean Sea
Level” and “Tides”. The role of the
Commission is to provide a mechanism for
scientists worldwide to interact on on-going
or new needed studies on these fields. Its also
acts as a source of scientific advice for
organizations like the International Oceanographic Commission on the development of
systems aiming at the improvement of
understanding or monitoring of the sea level
variations on the short and long term.
Recent renewed interest on Tides
Precise knowledge of tides is required in
many fields of Geodesy and Geophysics.
Over the last decade, in response to the
particularly demanding requests of the
satellite altimetry community, an intense
activity has led to very significant progresses.
Our ability to very precisely predict deep
ocean tides has been tremendously improved,
to the accuracy of 2 to 3 cm worldwide (Shum
et al., 1997). Our understanding of the
dynamics the tides has led to new insights on
their energetics. And the impact of the ocean
tides in the many fields of geophysics has
been revisited. A particularly fruitful meeting
has been organized in London in October
1996, co-sponsored by IAPSO-CMSLT, USNASA, UK Joint Association for Geophysics
and Proudman Oceanographic Institute.
Entitled “Tidal Science 1996”, it was held on
the Seventieth Birthday of David E.
Cartwright. The major presentations have
been published as a special issue by Progress
in Oceanography under the editorial board of
R. Ray and P. Woodworth.
Another domain of renewed interest for ocean
tides is related to the understanding of the
ocean mixing. Maintenance of the large scale
thermohaline circulation has long presented a
problem to oceanographers. Recent observations and theoretical works suggest that
much of the mixing may in spots near areas of
rough topography, and that barotropic tidal
currents provide a plausible source of energy
to maintain these mixing processes, through
internal tides (Munk and Wunsch, 1998).
Investigations are on-going, based on the new
tidal models issued from the analysis of the
altimetric data from TOPEX /POSEIDON and
ERS1-2 on one side, and on the WOCE in situ
measurements on the other side. New
important conclusions are expected on this
topic, and recommendations have already
been proposed by the oceanographic
community to include tides in Ocean General
Circulation Models (Stammer et al., 1998).
Geodetic Fixing of Tide Gauge Bench
Marks
Following the conclusions of the workshops
held under the initiative of the Commission at
the Woods Hole Oceanographic Institution in
November 1988 (Carter et al, 1988) and at
the Institute of Oceanographic Sciences in
December 1993 (Carter, 1994), a joint IGS
and PSMSL workshop has been held at JPL,
Pasadena, in March 1997, to urge the
implementation of GPS stations for
continuous positioning of the tide gauge
benchmarks (Neilan et al, 1998). A
preliminary report has been circulated (Bewis,
1998) and a follow-up meeting was held in
Toulouse in May 1999. The workshop
recommendations included the establishment
of a joint IAG-IAPSO working group for
station selection, writing of standards for
installations, data processing and archiving.
This working group has still to be established.
The Commission requests that IAPSO and
IAG urge the establishment of a joint
working group for scientific advice and
promotion for geodetic fixing of the tide
gauge benchmarks of the Global Sea Level
Observing System and regional networks
(station selection, writing of standards for
installations, data processing and archiving).
Permanent Service for Mean Sea Level
One of the responsabilities of the CMSLT is
to counsel the PSMSL, one of the FAGS data
services within the ICSU family. An Advisory
Panel for direct interaction with users of the
service has been established with the
agreement of the President of the Commission
for day to day advice of the PSMSL Director
Philip Woodworth (see separate report).
The Commission has been asked to consider
the recent efforts going on at Bidston to
assemble a global ocean bottom pressure
database, known as GLOUP (Global
Undersea Pressure). Its purpose is to provide
easy access to the historical database of insitu ocean bottom pressure measurements.
One of the reasons for compiling these data in
an accessible database is the approval for
launch of a satellite mission (GRACE) which
will be able to measure the fluctuations in
ocean bottom pressure by the gravitational
influence of the associated changes in
distribution of atmospheric and ocean mass.
Interpretation of GRACE results will be
strongly dependent on the information from
in-situ bottom pressure measurements.
The Commission recommends the endorsement by IAPSO of the GLOUP activity
managed within the PSMSL.
Sea level and Geoid
The need for a more precise geoid at high
resolution has long been recognized,
especially since the availability of highly
precise altimetry. Indeed, altimeter data are
used up to now in a repeat track mode only,
and for the ocean current variability
investigations. The determination of the
permanent component of the ocean circulation
from satellite altimetry requires a high
accurate and high resolution definition of the
geoid over the ocean. NASA has approved the
launch of GRACE, which (as said above) will
concentrate on the determination of the
temporal variations of the earth’s gravity
field. ESA is considering the GOCE mission
which goal is to achieve the determination of
a highly precise geoid at high resolution (up
to 100 km half wavelength). The two types of
missions are thus of major interest for
oceanography and mean sea level studies.
The Commission recommends that IAPSO, in
association with IAG, through an IUGG
resolution, calls upon the space agencies to
pursue these activities energetically.
Global Sea Level Observing System
The Commission continues to advise the
GLOSS network. In association with the last
meeting of the GLOSS Group of Experts in
Toulouse, a joint session with the IAPSO
Commission has been organized in May 1999
on the use of sea level for Ocean Circulation
studies over the Arctic and Atlantic Ocean.
The papers presented to this workshop will be
edited by G. Mitchum, the convenor, and will
be published as part of the IOC technical
series.
Individuals from the Commission have
actively contributed to the writing of the new
GLOSS implementation plan which has been
finally approved at the June 1997 IOC
Assembly (GLOSS Implementation Plan,
1997).
This plan has been further reviewed during an
International Sea Level Workshop for climate
applications and research held with the
sponsorship and support of the US National
Oceanic and Atmospheric Administration
(NOAA). Among the conclusions of the
workshop, it was suggested that a Science
Steering Group for sea level will be formed
for scientific advice for climate (see
International Sea Level Workshop Report,
Honolulu, Hawaii, June 1997, OOPC report).
Following this recommendation, the GLOSS
Group of Experts 6th Meeting 1999 has
proposed that this steering group should be a
joint committee with OOPC, CLIVAR-UOP
and IAPSO-CMSLT.
The Commission recommends to IAPSO to
endorse the establishment of a Scientific
Steering Group for Sea Level related to
Climate, as a joint sub-committee of the
GLOSS Group of Experts and related bodies
(at present OOPC, CLIVAR/UOP, IAPSO/
CMSLT).
European Sea-Level Observing System
(EOSS) initiative.
The status of the EOSS initiative was
presented at the Commission meeting held in
Birmingham during the UGGI XXII General
Assembly. The main objective of EOSS is to
establish a European Sea Level Service that
guarantees and coordinates the long term
monitoring activities and data exchange along
the European coastline (Plag, 1999). The
Commission welcomed this initiative and
noted its interest to follow this experience
which could serve as a pilot experiment for
regional implementation of GLOOS-like
activities. EOSS initiative was encourage to
be pursued, and it was suggested that status
reports be presented on regular basis to the
Commission.
The Future
The Commission should continue to consider
and encourage the above domains of research
related to sea level.
Tidal dissipation, internal wave and tidal
mixing are an area of renewed interest, which
needs further consideration. Of major interest
for applications are the investigations aiming
to improve the systematic exploitation of
satellite altimetric data for tidal studies and
prediction model developments over continental shelves and shallow seas. The GLOSS
requirement for the national agencies to
supply hourly data in place of monthly means
has to be strongly supported.
The importance of preserving the quality of
our coastal environment implies to increase
the scientific interest on local sea level
measurements, analysis and prediction.
With the new coming missions JASON1 and
ENVISAT, satellite altimetry will continue to
offer a major way of observing sea level at the
quasi global scale for the many domains in
geophysics related to sea level. Among the
new problems recently identified in the
processing of satellite altimetry for oceanic
studies is the question on how well the ocean
react to atmospheric pressure (the so-called
inverse barometer response hypothesis). Also
of major concern for studies of the sea level
variations at the interannual and longer time
scales is the dual strategy combining altimetry
and tide gauge for long term drift control.
The complementary observing system
proposed in the GLOSS Implementation Plan
for sea level monitoring has to be supported,
including an extended network of sea level
(GLOSS-LTT), a program for geodetic
positioning, and an effort for data archeology.
The close relations with the geodetic
community, especially in the IAG, should be
continued and enhanced, in order to
consolidate the monitoring systems under
development (through GPS, Doris, and other
techniques).
The new IPCC report put some emphasis on
the analysis of the outputs of the predictions
coming from coupled Ocean Atmosphere
coupled models. Considering the important
progress recently observed in numerical
modeling, the Commission must encourage
studies based on this approach to reduce the
uncertainties in predictions of sea level rise.
On the same question of sea level rise and
climate, the Commission should think about
developing a dialogue with the glaciologists
investigating the evolution of the ice caps and
glaciers.
Members
A. Cazenave (France)
Satellite Altimetry
M. Bevis (USA)
GPS
C. Boucher (France)
DORIS
References
A. de Mesquita (Brazil)
Extremes
Blewis M., 1998, Continuous GPS
positioning of tide gauges: some preliminary
consideration. Internal report.
J. Gregory (UK)
Climate modelling
B. Mitchell (Australia)
Tidal gauges
GLOSS Implementation Plan, 1997, IOC
Technical Series N°50, UNESCO 1997.
G. Mitchum (USA)
Satellite and gauges
D. Pugh (UK)
Former President
R. Ray (US)
Tides and satellites
S. Shetye (India)
Tide gauges
Munk, W. and Wunsch, C. (1997) The Moon,
of Course… Oceanography, 10, 3, 132-134.
Ray R. D. and P. L. Woodworth, 1997, Tidal
Sciences 1996, Prog. Oceanogr., 40.
Neilan R. and P. L. Woodworth, 1997,
Workshop on methods for monitoring sea
level: GPS and Tide gauge benckmark
monitoring, GPS calibration. GS/PSMSL
Report.
Plag H. P., 1999, European sea Level
Observing System (EOSS): Status and Future
developments, in preparation.
Stammer D., I. Fukumori and C. Wunsch,
Large scale Ocean State Estimation, WOCE
Report N°161/98, GODAE Report N°2.
Shum C.K. et al., 1997, Accuracy Assessment
of Recent Ocean Tide Models, J. Geophys.
Res., 102 (C11), 25173-25194.
Christian Le Provost,
President
July 1999.