10 Absolute Dynamic Topography from Altimetry
173
10.4 Scientific Advances Allowed by Recent Improvements
in MDT Determination
Significant improvements have been made for absolute dynamic topography computation since the launch of the first altimetric satellites more than 20 years ago.
Although most studies based on altimetric data have looked at the variable part
of dynamic topography, the recent improvements have generated an increasing
number of papers looking not anymore at ocean mesoscale variability but at the
interpretation of the full dynamical signal.
By taking a deeper look into their high resolution Mean Dynamic Topography,
Maximenko et al. (2008) reveal the presence of new stationary jet-like striations,
which they validated against historical XBT profiles, highlighting a coherent vertical
structure until at least 700 m depth. Although the dynamics of these structures is not
well understood yet, the emergence of high resolution MDT has made possible the
discovery of such new features.
The good quality of the (Niiler et al., 2003) MDT has allowed (Hughes, 2005) to
compute the near-surface vorticity balance of the Antarctic Circumpolar Current.
They found a compensation of relative and planetary advection at wavelengths
greater than 300–500 km. The resulting total vorticity advection was clearly related
to features in the bottom topography. The strong mean flow in the ACC allowed
the non-linear terms of the vorticity balance to be well resolved. In weaker flow
however, the author insisted on the need of higher resolution MDT as well as error
covariance estimates in order to better quantify the errors.
The improved knowledge of the ocean MDT has also allowed to improve
our understanding of the interaction between the ocean mean circulation
and the mesoscale eddy field. Several studies have recently investigated this
issue. Using AVISO Sea Level Anomalies together with the MDT from
Niiler et al. (2003), Fu, 2006 showed that in areas where the mean flow is
mainly eastward (as the Antarctic Circumpolar Current), the intrinsic westward
eddy propagation can be compensated leading locally to eastward propagating
eddies.
On the other hand, in areas where the mean flow is mainly westward, the intrinsic
westward eddy propagation may be reinforced by the mean flow. Inversely, the role
of mesoscale eddies for reinforcing the time-mean circulation was investigating in
the Kuroshio extension by Qiu and Chen (2010) using AVISO SLA and a mean
filed by Teague et al. (1990). They showed that eddy forcing was sustaining the
time-mean meanders of the Kuroshio Extension against dissipation.
The better estimation of the ocean’s MDT and hence the ocean’s absolute
dynamic topography has also led to the emergence of a number of ocean surface
current’s products as OSCAR (Bonjean and Lagerloef, 2002), SURCOUF (Larnicol
et al., 2006) or the currents computed by Sudre and Morrow (2008). These surface
currents are computed as a sum of the geostrophic component from altimetry and
an estimate of the Ekman component. These currents estimates are commonly used
for a number of applications as ocean model validation, support to offshore activity,
search and rescue, oil spill monitoring. . .
173
10.4 Scientific Advances Allowed by Recent Improvements
in MDT Determination
Significant improvements have been made for absolute dynamic topography computation since the launch of the first altimetric satellites more than 20 years ago.
Although most studies based on altimetric data have looked at the variable part
of dynamic topography, the recent improvements have generated an increasing
number of papers looking not anymore at ocean mesoscale variability but at the
interpretation of the full dynamical signal.
By taking a deeper look into their high resolution Mean Dynamic Topography,
Maximenko et al. (2008) reveal the presence of new stationary jet-like striations,
which they validated against historical XBT profiles, highlighting a coherent vertical
structure until at least 700 m depth. Although the dynamics of these structures is not
well understood yet, the emergence of high resolution MDT has made possible the
discovery of such new features.
The good quality of the (Niiler et al., 2003) MDT has allowed (Hughes, 2005) to
compute the near-surface vorticity balance of the Antarctic Circumpolar Current.
They found a compensation of relative and planetary advection at wavelengths
greater than 300–500 km. The resulting total vorticity advection was clearly related
to features in the bottom topography. The strong mean flow in the ACC allowed
the non-linear terms of the vorticity balance to be well resolved. In weaker flow
however, the author insisted on the need of higher resolution MDT as well as error
covariance estimates in order to better quantify the errors.
The improved knowledge of the ocean MDT has also allowed to improve
our understanding of the interaction between the ocean mean circulation
and the mesoscale eddy field. Several studies have recently investigated this
issue. Using AVISO Sea Level Anomalies together with the MDT from
Niiler et al. (2003), Fu, 2006 showed that in areas where the mean flow is
mainly eastward (as the Antarctic Circumpolar Current), the intrinsic westward
eddy propagation can be compensated leading locally to eastward propagating
eddies.
On the other hand, in areas where the mean flow is mainly westward, the intrinsic
westward eddy propagation may be reinforced by the mean flow. Inversely, the role
of mesoscale eddies for reinforcing the time-mean circulation was investigating in
the Kuroshio extension by Qiu and Chen (2010) using AVISO SLA and a mean
filed by Teague et al. (1990). They showed that eddy forcing was sustaining the
time-mean meanders of the Kuroshio Extension against dissipation.
The better estimation of the ocean’s MDT and hence the ocean’s absolute
dynamic topography has also led to the emergence of a number of ocean surface
current’s products as OSCAR (Bonjean and Lagerloef, 2002), SURCOUF (Larnicol
et al., 2006) or the currents computed by Sudre and Morrow (2008). These surface
currents are computed as a sum of the geostrophic component from altimetry and
an estimate of the Ekman component. These currents estimates are commonly used
for a number of applications as ocean model validation, support to offshore activity,
search and rescue, oil spill monitoring. . .