oceanographic data assimilation has been motivated to a large extent by the advent of high-quality
altimeter data, whose large-scale regular sampling
has made data assimilation a fruitful approach to
exploiting the utility of the data. A large body of
literature exists on model assimilation of altimeter
data (see Fukumori, 2000, for a review). Most of
the progress to date has been focused on methodologies and their demonstration. The challenge is
the quantification of the skill of a numerical model
for optimizing the prediction of subsurface fields
from altimeter data. Ultimately the framework
provided by altimetry with an assimilating model
will be used to incorporate other WOCE observations in a dynamically and statistically consistent
manner to create a description of the entire state
of the ocean during the WOCE period. Some preliminary progress has been reported in Stammer et
al. (1997) and Fukumori et al. (1999). Preliminary
results showing the utility of assimilating altimeter
data in improving the prediction of ENSO have
been reported by Ji et al. (2000).
The utility of altimetry for global ocean synthesis underscores the importance of keeping precision satellite altimetry missions continuous in time
for climate studies. Strategies for such sustained
observing systems were discussed in Nowlin
(1999). In the near term, T/P will be succeeded
in the year 2001 by its follow-on mission called
Jason-1, which will make observations in the same
orbit as T/P with expected performance exceeding
that of T/P. Jason-1 is a continuing collaboration
between the USA and France. While Jason-1
will demonstrate the operational applications of
altimetry, strategies for making altimetry become
part of a long-term ocean observing system are a
pressing issue. Also of importance is the deployment of in-situ observation network complementary to altimetry observations such as the array of
Argo floats (Wilson, 2000).
A Jason series has been proposed to maintain
long-term precision altimetric observations of the
global ocean topography. The concept is to forge a
long-term collaboration between the USA and
France in ocean altimetry and its operational implementation and application. The performance goal is
to progress from the T/P standard systematically to
an accuracy of 1 cm through dedicated effort and
new technologies. While other ocean topography
observing systems will continue or emerge (tide
gauge network, other altimetry missions, new
remote sensing techniques), the Jason series will
provide a benchmark for calibrating and integrating other measurements to achieve a globally
consistent data set.
As noted in Section 3.3.1 and several other
sections, a single altimeter suffers from a compromise in sampling the spatial and temporal scales
of ocean variability. Multiple conventional nadirlooking altimeters are needed to map the ocean
mesoscale eddies (Greenslade et al., 1997; Le
Traon and Dibarboure, 1999). During the Jason-1
time frame, the European ENVISAT will make altimetric measurement in the ERS orbit, providing a
dual-altimeter coverage of the global ocean. New
technologies have provided a promising approach
to low-cost altimeters for multiple deployments in
the future (e.g. Raney, 1998). In the mean time,
new techniques are being developed for mapping
ocean topography with two-dimensional spatial
resolutions. Interferometric techniques are a
promising approach to wide-swathe altimetry by
measuring the interference between signals received
by two altimeters flown on the same spacecraft
with a slight off-nadir pointing (Rodriguez et al.,
2000). The reflections of GPS signals off the ocean
surface are being analysed for detecting ocean
topographic features (LaBrecque et al., 1998). Such
developments of new techniques and technologies
should be an integral part of the strategy for a
long-term observing system.
A substantial improvement of the knowledge of
the geoid is expected in the next 5–10 years.
GRACE, a USA/Germany joint mission, will
utilize a pair of low-earth-orbiting satellites whose
relative distance is precisely determined (Wahr
et al., 1998; Davis et al., 1999). The measurement
of the distance plus onboard accelerometer measurements will be analysed to improve the geoid
accuracy to less than 1 cm at spatial scales greater
than 200 km (half wavelength). GRACE is planned
for launch in 2001. The GOCE mission, currently
planned by the European Space Agency, will fly
a system based on gradiometry and GPS precision tracking in a low orbit. GOCE is expected
to extend the 1 cm geoid accuracy to a scale of
100 km. With such accurate knowledge of the
geoid, absolute surface geostrophic current velocity and its variability will be determined from
altimeter data with details including the swift
boundary currents and mesoscale eddies (LeGrand
and Minster, 1999). Such information will provide
3.3 Ocean Circulation and Variability from Satellite Altimetry
171
Fu
altimeter data, whose large-scale regular sampling
has made data assimilation a fruitful approach to
exploiting the utility of the data. A large body of
literature exists on model assimilation of altimeter
data (see Fukumori, 2000, for a review). Most of
the progress to date has been focused on methodologies and their demonstration. The challenge is
the quantification of the skill of a numerical model
for optimizing the prediction of subsurface fields
from altimeter data. Ultimately the framework
provided by altimetry with an assimilating model
will be used to incorporate other WOCE observations in a dynamically and statistically consistent
manner to create a description of the entire state
of the ocean during the WOCE period. Some preliminary progress has been reported in Stammer et
al. (1997) and Fukumori et al. (1999). Preliminary
results showing the utility of assimilating altimeter
data in improving the prediction of ENSO have
been reported by Ji et al. (2000).
The utility of altimetry for global ocean synthesis underscores the importance of keeping precision satellite altimetry missions continuous in time
for climate studies. Strategies for such sustained
observing systems were discussed in Nowlin
(1999). In the near term, T/P will be succeeded
in the year 2001 by its follow-on mission called
Jason-1, which will make observations in the same
orbit as T/P with expected performance exceeding
that of T/P. Jason-1 is a continuing collaboration
between the USA and France. While Jason-1
will demonstrate the operational applications of
altimetry, strategies for making altimetry become
part of a long-term ocean observing system are a
pressing issue. Also of importance is the deployment of in-situ observation network complementary to altimetry observations such as the array of
Argo floats (Wilson, 2000).
A Jason series has been proposed to maintain
long-term precision altimetric observations of the
global ocean topography. The concept is to forge a
long-term collaboration between the USA and
France in ocean altimetry and its operational implementation and application. The performance goal is
to progress from the T/P standard systematically to
an accuracy of 1 cm through dedicated effort and
new technologies. While other ocean topography
observing systems will continue or emerge (tide
gauge network, other altimetry missions, new
remote sensing techniques), the Jason series will
provide a benchmark for calibrating and integrating other measurements to achieve a globally
consistent data set.
As noted in Section 3.3.1 and several other
sections, a single altimeter suffers from a compromise in sampling the spatial and temporal scales
of ocean variability. Multiple conventional nadirlooking altimeters are needed to map the ocean
mesoscale eddies (Greenslade et al., 1997; Le
Traon and Dibarboure, 1999). During the Jason-1
time frame, the European ENVISAT will make altimetric measurement in the ERS orbit, providing a
dual-altimeter coverage of the global ocean. New
technologies have provided a promising approach
to low-cost altimeters for multiple deployments in
the future (e.g. Raney, 1998). In the mean time,
new techniques are being developed for mapping
ocean topography with two-dimensional spatial
resolutions. Interferometric techniques are a
promising approach to wide-swathe altimetry by
measuring the interference between signals received
by two altimeters flown on the same spacecraft
with a slight off-nadir pointing (Rodriguez et al.,
2000). The reflections of GPS signals off the ocean
surface are being analysed for detecting ocean
topographic features (LaBrecque et al., 1998). Such
developments of new techniques and technologies
should be an integral part of the strategy for a
long-term observing system.
A substantial improvement of the knowledge of
the geoid is expected in the next 5–10 years.
GRACE, a USA/Germany joint mission, will
utilize a pair of low-earth-orbiting satellites whose
relative distance is precisely determined (Wahr
et al., 1998; Davis et al., 1999). The measurement
of the distance plus onboard accelerometer measurements will be analysed to improve the geoid
accuracy to less than 1 cm at spatial scales greater
than 200 km (half wavelength). GRACE is planned
for launch in 2001. The GOCE mission, currently
planned by the European Space Agency, will fly
a system based on gradiometry and GPS precision tracking in a low orbit. GOCE is expected
to extend the 1 cm geoid accuracy to a scale of
100 km. With such accurate knowledge of the
geoid, absolute surface geostrophic current velocity and its variability will be determined from
altimeter data with details including the swift
boundary currents and mesoscale eddies (LeGrand
and Minster, 1999). Such information will provide
3.3 Ocean Circulation and Variability from Satellite Altimetry
171
Fu