Chapter 10
Absolute Dynamic Topography from Altimetry:
Status and Prospects in the Upcoming
GOCE Era
Marie-Helene Rio
10.1 Introduction
In the oceans, all major current systems, which transport heat and mass all around
the globe, regulating our climate, can be considered, in a first approximation, to be
in geostrophic equilibrium, meaning that they can be simply derived by the knowledge of the ocean absolute dynamic topography which is the sea level above an
hypothetical ocean at rest (the geoid). Up to a certain point, the ocean dynamic
topography can be measured from space by satellite altimetry. The limitation stands
in the fact that the actual quantity η measured by an altimeter is the sea level above
the ellipsoid, which differs from the absolute dynamic topography h by the value of
the geoid height N:
h = η − N
(10.1)
While the computation and exploitation of the full ocean dynamical signal h has
been for long hampered by the lack of an accurate geoid, the successful launch,
on March 2009, of the GOCE (Gravity Field and Steady-State Ocean Circulation)
satellite has opened a new prospect for the use of altimetric data. The exploitation
of GOCE data will allow to resolve for the first time the spatial scales of the geoid
down to around 100 km with centimetric accuracy. Together with GRACE data,
this will lead to the estimation of an accurate geoid at all spatial scales greater than
100 km.
The objective of this paper is, shortly before actual GOCE data become available,
to draw an overview of what has been achieved in the last 20 years for absolute
dynamic topography estimation and exploitation, as well as to discuss the benefits
and limits of future GOCE data for further improving our knowledge of the ocean
absolute sea level.
Since the very beginning of altimetry, the so-called “repeat-track” method
(Cheney et al., 1983) has allowed to cope with the uncertainty on the geoid: Sea
Level Anomalies are derived along the altimetric satellite tracks subtracting from
M.-H. Rio (B)
CLS-DOS, Parc Technologique du Canal, Ramonville Saint Agne 31526, France
e-mail: mrio@cls.fr
165
V. Barale et al. (eds.), Oceanography from Space,
DOI 10.1007/978-90-481-8681-5_10, C
Springer Science+Business Media B.V. 2010
Absolute Dynamic Topography from Altimetry:
Status and Prospects in the Upcoming
GOCE Era
Marie-Helene Rio
10.1 Introduction
In the oceans, all major current systems, which transport heat and mass all around
the globe, regulating our climate, can be considered, in a first approximation, to be
in geostrophic equilibrium, meaning that they can be simply derived by the knowledge of the ocean absolute dynamic topography which is the sea level above an
hypothetical ocean at rest (the geoid). Up to a certain point, the ocean dynamic
topography can be measured from space by satellite altimetry. The limitation stands
in the fact that the actual quantity η measured by an altimeter is the sea level above
the ellipsoid, which differs from the absolute dynamic topography h by the value of
the geoid height N:
h = η − N
(10.1)
While the computation and exploitation of the full ocean dynamical signal h has
been for long hampered by the lack of an accurate geoid, the successful launch,
on March 2009, of the GOCE (Gravity Field and Steady-State Ocean Circulation)
satellite has opened a new prospect for the use of altimetric data. The exploitation
of GOCE data will allow to resolve for the first time the spatial scales of the geoid
down to around 100 km with centimetric accuracy. Together with GRACE data,
this will lead to the estimation of an accurate geoid at all spatial scales greater than
100 km.
The objective of this paper is, shortly before actual GOCE data become available,
to draw an overview of what has been achieved in the last 20 years for absolute
dynamic topography estimation and exploitation, as well as to discuss the benefits
and limits of future GOCE data for further improving our knowledge of the ocean
absolute sea level.
Since the very beginning of altimetry, the so-called “repeat-track” method
(Cheney et al., 1983) has allowed to cope with the uncertainty on the geoid: Sea
Level Anomalies are derived along the altimetric satellite tracks subtracting from
M.-H. Rio (B)
CLS-DOS, Parc Technologique du Canal, Ramonville Saint Agne 31526, France
e-mail: mrio@cls.fr
165
V. Barale et al. (eds.), Oceanography from Space,
DOI 10.1007/978-90-481-8681-5_10, C
Springer Science+Business Media B.V. 2010