206
W. Alpers et al.
Eddies strongly affect the dynamics of the ocean and can transfer heat, salt, trace
gases, nutrients, and CHL across frontal zones (see e.g., Olson 1991; Morrow et al.
2003). In particular, they also can transport nutrients from upwelling regions into
oligotrophic ocean regions causing enhanced CHL concentration there (Falkowski
et al. 1991; Levy et al. 2001; Lin et al. 2010). Although already more than 30 years
ago, Gower et al. (1980) have pointed out that phytoplankton patchiness is linked
to mesoscale eddies, it has been realized only recently that eddies play a key role
in the variability of the CHL distribution in the World’s ocean (Williams 2011).
Chelton et al. (2011) state that most of the variability of CHL distribution results
from redistribution of CHL caused by advection with the mesoscale flow field and
not from changes in the local phytoplankton growth.
Often oceanic eddies are divided into mesoscale and submesoscale eddies with
horizontal scales above and below the first baroclinic Rossby radius of deformation,
respectively. The Rossby radius is larger than 100 km in the inner tropics, around
50 km in the upwelling region off West Africa and becomes smaller towards north
reaching about 10 km in the subpolar gyre (Chelton et al. 1998). Mesoscale eddies
having spatial scales above 100 km have been studied intensively in the last two
decades by using radar altimeter (RA) data and by assimilating them in global ocean
circulation models. They are measured by altimeters via sea surface height anomalies
from which current velocities are retrieved by using the geostrophic approximation
(see e.g., Fu and Holt 2008; Chelton et al. 2011). Present-day global ocean circulation models have horizontal resolutions of the order of 10 km and thus are suited
for modeling mesoscale oceanic eddies, but not submesoscale ones (Maltrud and
McClean 2005; Le Galloudec et al. 2008; Capet et al. 2008).
Mesoscale eddies can be detected from satellites also by infrared and optical sensors, like the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard
the Terra and Aqua satellites, and by Synthetic Aperture Radars (SARs). However,
these sensors are not well suited to study the dynamics of mesoscale eddies systematically. Optical/infrared sensors provide useful data only when there are no or only
few clouds in the imaged scene. On the other hand, SAR data are not dependent on
cloud coverage and the time of the day, but their application is limited by the fact
that a given ocean area is imaged very infrequently by a SAR on a single satellite and
that it is often difficult to identify unambiguously features visible on SAR images as
radar signatures of oceanic eddies.
Eddies with horizontal scales below 100 km, which we shall call in the following
small-scale eddies, are not detectable by present-day space-borne RAs, but they can
be detected from satellites by high-resolution optical/infrared sensors and by SARs.
These small-scale eddies have been observed from space first on sunglint images
(Soules 1970; Scully-Power 1986) and later also on synthetic aperture radar (SAR)
images (Fu and Holt 1983; Johannessen et al. 1993, 1996; Munk et al. 2000; DiGiacomo and Holt 2001; Ivanov and Ginzburg 2002; Yamaguchi and Kawamura 2009;
Karimova 2012). Unlike mesoscale eddies, small-scale eddies cannot be modeled by
using traditional quasi-geostrophic theory (Thomas et al. 2008). Simulations carried
out with models with horizontal resolution of the order of 1 km show that small-scale
eddies are generated by instabilities of geostrophic surface flows and that they are
W. Alpers et al.
Eddies strongly affect the dynamics of the ocean and can transfer heat, salt, trace
gases, nutrients, and CHL across frontal zones (see e.g., Olson 1991; Morrow et al.
2003). In particular, they also can transport nutrients from upwelling regions into
oligotrophic ocean regions causing enhanced CHL concentration there (Falkowski
et al. 1991; Levy et al. 2001; Lin et al. 2010). Although already more than 30 years
ago, Gower et al. (1980) have pointed out that phytoplankton patchiness is linked
to mesoscale eddies, it has been realized only recently that eddies play a key role
in the variability of the CHL distribution in the World’s ocean (Williams 2011).
Chelton et al. (2011) state that most of the variability of CHL distribution results
from redistribution of CHL caused by advection with the mesoscale flow field and
not from changes in the local phytoplankton growth.
Often oceanic eddies are divided into mesoscale and submesoscale eddies with
horizontal scales above and below the first baroclinic Rossby radius of deformation,
respectively. The Rossby radius is larger than 100 km in the inner tropics, around
50 km in the upwelling region off West Africa and becomes smaller towards north
reaching about 10 km in the subpolar gyre (Chelton et al. 1998). Mesoscale eddies
having spatial scales above 100 km have been studied intensively in the last two
decades by using radar altimeter (RA) data and by assimilating them in global ocean
circulation models. They are measured by altimeters via sea surface height anomalies
from which current velocities are retrieved by using the geostrophic approximation
(see e.g., Fu and Holt 2008; Chelton et al. 2011). Present-day global ocean circulation models have horizontal resolutions of the order of 10 km and thus are suited
for modeling mesoscale oceanic eddies, but not submesoscale ones (Maltrud and
McClean 2005; Le Galloudec et al. 2008; Capet et al. 2008).
Mesoscale eddies can be detected from satellites also by infrared and optical sensors, like the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard
the Terra and Aqua satellites, and by Synthetic Aperture Radars (SARs). However,
these sensors are not well suited to study the dynamics of mesoscale eddies systematically. Optical/infrared sensors provide useful data only when there are no or only
few clouds in the imaged scene. On the other hand, SAR data are not dependent on
cloud coverage and the time of the day, but their application is limited by the fact
that a given ocean area is imaged very infrequently by a SAR on a single satellite and
that it is often difficult to identify unambiguously features visible on SAR images as
radar signatures of oceanic eddies.
Eddies with horizontal scales below 100 km, which we shall call in the following
small-scale eddies, are not detectable by present-day space-borne RAs, but they can
be detected from satellites by high-resolution optical/infrared sensors and by SARs.
These small-scale eddies have been observed from space first on sunglint images
(Soules 1970; Scully-Power 1986) and later also on synthetic aperture radar (SAR)
images (Fu and Holt 1983; Johannessen et al. 1993, 1996; Munk et al. 2000; DiGiacomo and Holt 2001; Ivanov and Ginzburg 2002; Yamaguchi and Kawamura 2009;
Karimova 2012). Unlike mesoscale eddies, small-scale eddies cannot be modeled by
using traditional quasi-geostrophic theory (Thomas et al. 2008). Simulations carried
out with models with horizontal resolution of the order of 1 km show that small-scale
eddies are generated by instabilities of geostrophic surface flows and that they are