been computed using satellite altimetry data
(e.g. Johnson et al., 1992; Morrow et al., 1992).
An example of current ellipses estimated from the
T/P data in the California Current region is shown
in Fig. 3.3.15 (after Strub et al., 1997), along with
the uncertainty in the estimates. Notable features
in the core of the current are the high eddy kinetic
energy and the tendency of along-shore orientation
of the velocity variability. The error estimates
might be too large for the region north of 45°N,
where the size of the ellipses is smaller than that of
the errors.
3.3.4.3 Estimation of current transport and
variability
The temporal variability of the transport of surface
currents can be readily determined from the difference in altimetric sea-surface-height anomalies
(with temporal mean removed) across the domain
of the current (Zlotnicki, 1991). Determination of
the absolute velocity and transport of currents and
eddies is problematic because the knowledge of the
geoid at the mesoscale is not sufficiently accurate
for oceanographic applications. Significant effort
has been made to construct local geoid models
based on oceanographic and geodetic data for altimetric studies of ocean currents. The accuracy of
these geoids is highly inhomogeneous, varying from
5 to 40 cm (e.g. Rapp and Wang, 1994; Hwang,
1996). Their utility is thus mixed. However, oceanographic measurements made simultaneously with
altimetry can be used to estimate the local geoid
without the use of any geodetic data.
Using in-situ hydrographic observations made
by AXBTs along GEOSAT ground tracks at times
close to a satellite overpass, Mitchell et al. (1990)
estimated the along-track geoid in the Gulf Stream
region by subtracting the dynamic height computed from the in-situ data from the altimetric sea
surface height. Such a ‘synthetic geoid’ was then
used for estimating dynamic heights and hence
absolute cross-track geostrophic velocities from
other altimeter measurements along the same track.
Among many sources of errors, the unknown contribution of barotropic current to the sea surface
height is an outstanding one. Kelly et al. (1991)
used direct measurements of ocean current velocity
made by ADCP under a GEOSAT track from
Bermuda to Cape Cod to estimate the sea surface
dynamic topography for determining the geoid.
However, the ageostrophic component of velocity
becomes a source of error, among others. Nevertheless, these synthetic geoid models are more
accurate than most of the gravimetric geoids along
the specific satellite tracks where the synthetic
geoids are derived.
Howden et al. (2000) used the gravimetric
geoid of Rapp and Wang (1994) with T/P data to
study the transport of the Gulf Stream. They
derived a relation between sea surface dynamic
height and baroclinic transport at various depths.
This relation was then used to estimate the baroclinic transport from the height difference across
the Gulf Stream measured by altimetry. The crossstream sea surface height profile was derived in a
stream coordinate (relative to the moving stream
axis) rather than an Eulerian coordinate. The
results based on the stream coordinate were much
more stable than those based on an Eulerian coordinate. The results, showing very little interannual
variability over the 5-year study period, were compared favourably with estimates based on XBT
and ADCP observations made by a ship periodically sailing between New Jersey and Bermuda.
The relationship between the volume transport
of the Kuroshio and ocean topography was investigated by an effort of repeated hydrographic surveys and deployment of moored instruments along
a line underneath a T/P ground track south of
Japan (Imawaki et al., 1997). Absolute ocean
topography was estimated from hydrographic and
current meter data during a 2-year period. The difference in the topography across the Kuroshio was
found to be highly correlated with the volume
transport of the upper 1000 m. The standard deviation of a linear fit between the directly measured
transport and the transport estimated from the
ocean topography is only 4.3 Sv over a range of
34–91 Sv. Using the mean ocean topography thus
obtained as a reference for the T/P sea surface
height anomalies (relative to the mean calculated
over the same period as the in-situ data), Imawaki
et al. (1997) estimated the variability of the
Kuroshio volume transport from the T/P data over
longer time spans than the in-situ measurement
(Fig. 3.3.16). There is no apparent annual cycle in
the variability, which is dominated by interannual
and intraseasonal time scales. The variability of
the Kuroshio transport is greater than that of the
Gulf Stream by about a factor of two in terms
of the ratio of standard deviation to the mean
(Howden et al., 2000).
3.3 Ocean Circulation and Variability from Satellite Altimetry
165
Fu
(e.g. Johnson et al., 1992; Morrow et al., 1992).
An example of current ellipses estimated from the
T/P data in the California Current region is shown
in Fig. 3.3.15 (after Strub et al., 1997), along with
the uncertainty in the estimates. Notable features
in the core of the current are the high eddy kinetic
energy and the tendency of along-shore orientation
of the velocity variability. The error estimates
might be too large for the region north of 45°N,
where the size of the ellipses is smaller than that of
the errors.
3.3.4.3 Estimation of current transport and
variability
The temporal variability of the transport of surface
currents can be readily determined from the difference in altimetric sea-surface-height anomalies
(with temporal mean removed) across the domain
of the current (Zlotnicki, 1991). Determination of
the absolute velocity and transport of currents and
eddies is problematic because the knowledge of the
geoid at the mesoscale is not sufficiently accurate
for oceanographic applications. Significant effort
has been made to construct local geoid models
based on oceanographic and geodetic data for altimetric studies of ocean currents. The accuracy of
these geoids is highly inhomogeneous, varying from
5 to 40 cm (e.g. Rapp and Wang, 1994; Hwang,
1996). Their utility is thus mixed. However, oceanographic measurements made simultaneously with
altimetry can be used to estimate the local geoid
without the use of any geodetic data.
Using in-situ hydrographic observations made
by AXBTs along GEOSAT ground tracks at times
close to a satellite overpass, Mitchell et al. (1990)
estimated the along-track geoid in the Gulf Stream
region by subtracting the dynamic height computed from the in-situ data from the altimetric sea
surface height. Such a ‘synthetic geoid’ was then
used for estimating dynamic heights and hence
absolute cross-track geostrophic velocities from
other altimeter measurements along the same track.
Among many sources of errors, the unknown contribution of barotropic current to the sea surface
height is an outstanding one. Kelly et al. (1991)
used direct measurements of ocean current velocity
made by ADCP under a GEOSAT track from
Bermuda to Cape Cod to estimate the sea surface
dynamic topography for determining the geoid.
However, the ageostrophic component of velocity
becomes a source of error, among others. Nevertheless, these synthetic geoid models are more
accurate than most of the gravimetric geoids along
the specific satellite tracks where the synthetic
geoids are derived.
Howden et al. (2000) used the gravimetric
geoid of Rapp and Wang (1994) with T/P data to
study the transport of the Gulf Stream. They
derived a relation between sea surface dynamic
height and baroclinic transport at various depths.
This relation was then used to estimate the baroclinic transport from the height difference across
the Gulf Stream measured by altimetry. The crossstream sea surface height profile was derived in a
stream coordinate (relative to the moving stream
axis) rather than an Eulerian coordinate. The
results based on the stream coordinate were much
more stable than those based on an Eulerian coordinate. The results, showing very little interannual
variability over the 5-year study period, were compared favourably with estimates based on XBT
and ADCP observations made by a ship periodically sailing between New Jersey and Bermuda.
The relationship between the volume transport
of the Kuroshio and ocean topography was investigated by an effort of repeated hydrographic surveys and deployment of moored instruments along
a line underneath a T/P ground track south of
Japan (Imawaki et al., 1997). Absolute ocean
topography was estimated from hydrographic and
current meter data during a 2-year period. The difference in the topography across the Kuroshio was
found to be highly correlated with the volume
transport of the upper 1000 m. The standard deviation of a linear fit between the directly measured
transport and the transport estimated from the
ocean topography is only 4.3 Sv over a range of
34–91 Sv. Using the mean ocean topography thus
obtained as a reference for the T/P sea surface
height anomalies (relative to the mean calculated
over the same period as the in-situ data), Imawaki
et al. (1997) estimated the variability of the
Kuroshio volume transport from the T/P data over
longer time spans than the in-situ measurement
(Fig. 3.3.16). There is no apparent annual cycle in
the variability, which is dominated by interannual
and intraseasonal time scales. The variability of
the Kuroshio transport is greater than that of the
Gulf Stream by about a factor of two in terms
of the ratio of standard deviation to the mean
(Howden et al., 2000).
3.3 Ocean Circulation and Variability from Satellite Altimetry
165
Fu