3.3.1 Altimeter observations
Space-age technologies have made satellite remote
sensing a powerful new tool with which to study
the earth on a global scale. The only truly global
observations made with regular sampling in space
and time during WOCE (the World Ocean Circulation Experiment) were obtained from spaceborne sensors. However, the opacity of the ocean
to electromagnetic sensing has limited most spaceborne measurements to the properties of the very
top surface layer of the ocean (such as sea surface
temperature and colour). The promise of measuring the height of sea surface using radar altimetry
(Chelton et al., 2000) emerged in the 1970s and
provided a tantalizing opportunity to oceanographers for obtaining information about the dynamics of the ocean at greater depths. This is because
the sea surface height relative to the geoid is the
dynamic topography of the ocean surface (referred
to as the ocean topography hereafter), which is
related to the circulation and density field of
the entire water column. Without being able to
measure the ocean topography directly, oceanographers have constructed it using density measurement
to compute the dynamic height of the sea surface
in the past. Despite the great potential value of
satellite altimetry, the challenge has been the
required measurement accuracy. A simple analysis
(e.g. Wunsch and Stammer, 1998) shows that a
1 cm tilt in the ocean topography is associated
with a mass transport of 7 Sv (1 Sv:10
6 m
3 s
91 ) in
an ocean of 4000 m depth at 24° latitude, if the
entire water column moves at the same velocity.
The actual transport varies with latitude and the
vertical distribution of current velocity, but this
value provides a rough estimate. Such a magnitude
is an appreciable fraction of the transport of the
Florida Current (ϳ30 Sv), for example. Therefore
the tall order for satellite altimetry is to make
measurement of the sea surface height from space
with an accuracy on the order of 1 cm.
Shown in Fig. 3.3.1 is the geometry of satellite
altimetry. There are three components in the determination of ocean topography, : the altimeter
range measurement, h; the radial height of the
orbit, H; and the geoid height, N. They are related
as follows:
:H9h9N
(3.3.1)
Determination of all three with an accuracy of
1 cm is extremely challenging (see Chelton et al.,
2000 for a review). The measurement of h is
affected by uncertainties from sources including
imperfect knowledge of the interaction of electromagnetic waves with the rough seas, the delay in the
propagation of electromagnetic waves through the
atmosphere and the ionosphere, as well as the calibration of the instrument’s electronics. The determination of H is made by computations that rely on a
precise knowledge of the earth’s gravity field, the
ability to model the non-gravitational forces such as
radiation pressure and atmospheric drag, and the
ability to track the location of the spacecraft precisely. The knowledge of N has to be obtained from
3.3
Ocean Circulation and Variability from
Satellite Altimetry
Lee-Lueng Fu
141
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
Space-age technologies have made satellite remote
sensing a powerful new tool with which to study
the earth on a global scale. The only truly global
observations made with regular sampling in space
and time during WOCE (the World Ocean Circulation Experiment) were obtained from spaceborne sensors. However, the opacity of the ocean
to electromagnetic sensing has limited most spaceborne measurements to the properties of the very
top surface layer of the ocean (such as sea surface
temperature and colour). The promise of measuring the height of sea surface using radar altimetry
(Chelton et al., 2000) emerged in the 1970s and
provided a tantalizing opportunity to oceanographers for obtaining information about the dynamics of the ocean at greater depths. This is because
the sea surface height relative to the geoid is the
dynamic topography of the ocean surface (referred
to as the ocean topography hereafter), which is
related to the circulation and density field of
the entire water column. Without being able to
measure the ocean topography directly, oceanographers have constructed it using density measurement
to compute the dynamic height of the sea surface
in the past. Despite the great potential value of
satellite altimetry, the challenge has been the
required measurement accuracy. A simple analysis
(e.g. Wunsch and Stammer, 1998) shows that a
1 cm tilt in the ocean topography is associated
with a mass transport of 7 Sv (1 Sv:10
6 m
3 s
91 ) in
an ocean of 4000 m depth at 24° latitude, if the
entire water column moves at the same velocity.
The actual transport varies with latitude and the
vertical distribution of current velocity, but this
value provides a rough estimate. Such a magnitude
is an appreciable fraction of the transport of the
Florida Current (ϳ30 Sv), for example. Therefore
the tall order for satellite altimetry is to make
measurement of the sea surface height from space
with an accuracy on the order of 1 cm.
Shown in Fig. 3.3.1 is the geometry of satellite
altimetry. There are three components in the determination of ocean topography, : the altimeter
range measurement, h; the radial height of the
orbit, H; and the geoid height, N. They are related
as follows:
:H9h9N
(3.3.1)
Determination of all three with an accuracy of
1 cm is extremely challenging (see Chelton et al.,
2000 for a review). The measurement of h is
affected by uncertainties from sources including
imperfect knowledge of the interaction of electromagnetic waves with the rough seas, the delay in the
propagation of electromagnetic waves through the
atmosphere and the ionosphere, as well as the calibration of the instrument’s electronics. The determination of H is made by computations that rely on a
precise knowledge of the earth’s gravity field, the
ability to model the non-gravitational forces such as
radiation pressure and atmospheric drag, and the
ability to track the location of the spacecraft precisely. The knowledge of N has to be obtained from
3.3
Ocean Circulation and Variability from
Satellite Altimetry
Lee-Lueng Fu
141
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER