9 Determining Ocean Circulation and Sea Level from Satellite Altimetry
151
small scales made them survive the substantial instrument noise and orbit errors
of the GEOS-3 altimeter. However, the trade-off between temporal and spatial
resolution of a single altimeter has made it difficult to map the two-dimensional
evolution of mesoscale eddies. Despite this difficulty, many statistical properties
of mesoscale variability were ascertained from altimetry: geographic distribution of energy, spatial and temporal scales, wavenumber-frequency spectrum,
eddy transports, viscosity and diffusivity (see Le Traon and Morrow, 2000 for a
review).
The merging of multiple altimeter data into a gridded data set (Ducet et al., 2000)
created a first opportunity to study the two-dimensional movement of mesoscale
eddies. The intrinsic resolution of the data set is about 150–200 km, allowing to
map eddies of sizes larger than these scales. The data set has created a surge of effort
in tracking eddies, e.g. off the Central America coast (Palacios and Bograd, 2005)
and in the Oyashio (Isoguchi and Kawamura, 2006) and studying their behavior.
Advanced methods exist for tracking certain vorticity properties of eddies and allow
automatic tracking (Isern-Fontanet et al., 2006; Morrow et al., 2004; Chelton et al.,
2007). Figure 9.2 summarizes the properties of large eddies with lifetime grater 12
weeks surveyed by Chelton et al. (2007). A notable feature is that the propagation
of cyclonic eddies has a slight tendency for a poleward deflection from purely westward. Anti-cyclonic eddies have a slight tendency for an equatorward deflection. At
low latitudes, the westward speed of eddy propagation is somewhat less than that
of non-dispersive baroclinic Rossby waves represented by the large-scale variability. At mid and high latitudes, the eddy speed is indistinguishable from the Rossby
wave speed.
Using the maximum correlation method, Fu (2006, 2009) mapped the propagation velocity vector of ocean eddy variability. The method used the spatial and
temporal lags of the maximum correlation of time series of sea surface height
anomalies to compute the velocity of propagation of the dominant variability in the
time series. The results correspond to the energy containing variability mostly associated with mesoscale motions, but cannot distinguish isolated eddies from other
forms of variability like meandering currents and fronts. Displayed in Fig. 9.3 is an
example in the Southern Ocean (from Fu, 2009), showing the vectors of propagation
superimposed on the bottom topography (color shade) along with the tracks of the
Subtropical Front, the Sub-Antarctic Front, and the Polar Front.
Within the Antarctic Circumpolar Current (ACC) which basically flows eastward
parallel to these fronts, eddy propagation is steered from westward to eastward
by the mean currents. A prominent feature is the “U” shaped turn of eddy propagation over a fracture zone centered at 55 ◦ S and 240 ◦ E (the Menard Fracture
Zone). Another notable influence of bottom topography and ACC is the deflection
of eddy paths over the Mid-Atlantic Ridge between 340 ◦ and 360 ◦ E. In the
Argentine Basin, a counter-clockwise gyre-like pattern of eddy propagation is centered over a topographic feature called the Zapiola Rise. The center of the “gyre”
seems to be a region where eddies tend to dissipate. This may be evidence for eddies
being a source of energy driving the large-scale counter-clockwise (anti-cyclonic)
barotropic circulation over the Zapiola Rise (de Miranda et al., 1999).
151
small scales made them survive the substantial instrument noise and orbit errors
of the GEOS-3 altimeter. However, the trade-off between temporal and spatial
resolution of a single altimeter has made it difficult to map the two-dimensional
evolution of mesoscale eddies. Despite this difficulty, many statistical properties
of mesoscale variability were ascertained from altimetry: geographic distribution of energy, spatial and temporal scales, wavenumber-frequency spectrum,
eddy transports, viscosity and diffusivity (see Le Traon and Morrow, 2000 for a
review).
The merging of multiple altimeter data into a gridded data set (Ducet et al., 2000)
created a first opportunity to study the two-dimensional movement of mesoscale
eddies. The intrinsic resolution of the data set is about 150–200 km, allowing to
map eddies of sizes larger than these scales. The data set has created a surge of effort
in tracking eddies, e.g. off the Central America coast (Palacios and Bograd, 2005)
and in the Oyashio (Isoguchi and Kawamura, 2006) and studying their behavior.
Advanced methods exist for tracking certain vorticity properties of eddies and allow
automatic tracking (Isern-Fontanet et al., 2006; Morrow et al., 2004; Chelton et al.,
2007). Figure 9.2 summarizes the properties of large eddies with lifetime grater 12
weeks surveyed by Chelton et al. (2007). A notable feature is that the propagation
of cyclonic eddies has a slight tendency for a poleward deflection from purely westward. Anti-cyclonic eddies have a slight tendency for an equatorward deflection. At
low latitudes, the westward speed of eddy propagation is somewhat less than that
of non-dispersive baroclinic Rossby waves represented by the large-scale variability. At mid and high latitudes, the eddy speed is indistinguishable from the Rossby
wave speed.
Using the maximum correlation method, Fu (2006, 2009) mapped the propagation velocity vector of ocean eddy variability. The method used the spatial and
temporal lags of the maximum correlation of time series of sea surface height
anomalies to compute the velocity of propagation of the dominant variability in the
time series. The results correspond to the energy containing variability mostly associated with mesoscale motions, but cannot distinguish isolated eddies from other
forms of variability like meandering currents and fronts. Displayed in Fig. 9.3 is an
example in the Southern Ocean (from Fu, 2009), showing the vectors of propagation
superimposed on the bottom topography (color shade) along with the tracks of the
Subtropical Front, the Sub-Antarctic Front, and the Polar Front.
Within the Antarctic Circumpolar Current (ACC) which basically flows eastward
parallel to these fronts, eddy propagation is steered from westward to eastward
by the mean currents. A prominent feature is the “U” shaped turn of eddy propagation over a fracture zone centered at 55 ◦ S and 240 ◦ E (the Menard Fracture
Zone). Another notable influence of bottom topography and ACC is the deflection
of eddy paths over the Mid-Atlantic Ridge between 340 ◦ and 360 ◦ E. In the
Argentine Basin, a counter-clockwise gyre-like pattern of eddy propagation is centered over a topographic feature called the Zapiola Rise. The center of the “gyre”
seems to be a region where eddies tend to dissipate. This may be evidence for eddies
being a source of energy driving the large-scale counter-clockwise (anti-cyclonic)
barotropic circulation over the Zapiola Rise (de Miranda et al., 1999).