54
Chapter 4: Physical Control of Ecological Processes
of meters at the main pycnocline. Thus, the very small SLAs observed by TOPEXPOSEIDON indicate much larger vertical displacement, deeper in the water column. It
is a trite experiment to match images of sea-surface chlorophyll with sea-level anomaly
maps from TOPEX-POSEIDON and to observe the match between chlorophyll patches
and the centers of cyclonic eddies; indeed, a University of Colorado site now matches
the two data sets for you, within rectangles and for dates of your specification.
Motion of an eddy depends on where it lies in relation to the axis of major current.
Those intimately associated with the flow of the Gulf Stream near its origin, for example,
travel eastward while those lying farther from the axis travel west and north as a result
of their interaction with the general field of potential vorticity (Flierl and McGillicuddy,
2002). In regions especially active in eddy formation, such as along the Gulf Stream and
Kuroshio fronts, meanders of the main jet may become pinched off, to enclose a parcel
of the adjacent water mass. These are the cold-core and warm-core “rings,” having a
larger dimension than simple mesoscale eddies, of order 200–300 km. Cold-core rings,
enclosing slope water surrounded by a jet of warm Gulf Stream water, form southeast
of the jet current five to eight times a year and may persist for up to 2 years; warmcore rings are more rapidly wrecked against the shelf edge and have shorter life spans
(Wiebe et al., 1976). The subtropical front of the North Atlantic (see Chapter 3) is
probably typical of midocean frontal systems (Pingree, 1997; Pingree and Sinha, 2000);
here, westward motion of Rossby waves can be observed in TOPEX-POSEIDON imagery
at about 32–34
N, carrying eddies having SLA signatures above background. The larger
features may be followed westward, crossing the mid-Atlantic Ridge, over periods of
several months.
Because eddies create vertical motion, a different response of phytoplankton to irradiance and nutrients is to be anticipated in these features and in the surrounding ocean.
Early studies of SLA and temperature in the Sargasso Sea showed a correspondence
between cold (warm) temperature anomalies and higher (lower) pigment anomalies.
Phytoplankton in the oligotrophic ocean that are not growing at their maximum specific growth rate may be stimulated by such “eddy-pumping” (Falkowski et al., 1991).
Especially in low latitudes, the lifting of nutrients from below into the photic zone in
cold-core eddies stimulates an instantaneous response of the phytoplankton that is at first
linear; later, the accumulating nutrients decrease rapidly as the phytoplankton enters—at
least theoretically—a phase of exponential growth. McGillicuddy and Robinson (1997)
suggested that in the subtropical gyre of the North Atlantic the contribution of the fertilizing effects of mesoscale eddies is significantly larger than the combined entrainment
of nutrients into the mixed layer during winter convection, and by thermocline mixing
and wind-driven transport. Eddy pumping may therefore be the dominant nutrient flux
in such regions.
In anticyclonic eddies the process is more complex and marginal, and enhanced
chlorophyll concentrations occur preferentially in the high-velocity region encircling the
eddy, where elevation of the pycnocline (and hence of the nutricline in most situations)
is associated with a strongly baroclinic structure (Yentsch and Phinney, 1985; Lohrenz
et al., 1993). Vortex contraction may strengthen the upwelling of nutrients, especially on
the anticyclonic side of the transient jets (Woods, 1988). This effect is usually limited to
patches on the 10-km scale, which serves to explain the beaded string of high chlorophyll
often observed around anticyclonic eddies in chlorophyll images. The same effect is
observed on the filaments shed by mesoscale eddies, especially on their curved rather
than their straight sections (Tranter et al., 1983). Production within warm-core rings in
the Agulhas eddy field may be limited by convective instability, but around the edge of
such rings stability and enhanced productivity is conferred by the warm water of the ring
overlying the cooler water in which it is embedded (Dower and Lucas, 1993). The same,
of course, may occur elsewhere.
Chapter 4: Physical Control of Ecological Processes
of meters at the main pycnocline. Thus, the very small SLAs observed by TOPEXPOSEIDON indicate much larger vertical displacement, deeper in the water column. It
is a trite experiment to match images of sea-surface chlorophyll with sea-level anomaly
maps from TOPEX-POSEIDON and to observe the match between chlorophyll patches
and the centers of cyclonic eddies; indeed, a University of Colorado site now matches
the two data sets for you, within rectangles and for dates of your specification.
Motion of an eddy depends on where it lies in relation to the axis of major current.
Those intimately associated with the flow of the Gulf Stream near its origin, for example,
travel eastward while those lying farther from the axis travel west and north as a result
of their interaction with the general field of potential vorticity (Flierl and McGillicuddy,
2002). In regions especially active in eddy formation, such as along the Gulf Stream and
Kuroshio fronts, meanders of the main jet may become pinched off, to enclose a parcel
of the adjacent water mass. These are the cold-core and warm-core “rings,” having a
larger dimension than simple mesoscale eddies, of order 200–300 km. Cold-core rings,
enclosing slope water surrounded by a jet of warm Gulf Stream water, form southeast
of the jet current five to eight times a year and may persist for up to 2 years; warmcore rings are more rapidly wrecked against the shelf edge and have shorter life spans
(Wiebe et al., 1976). The subtropical front of the North Atlantic (see Chapter 3) is
probably typical of midocean frontal systems (Pingree, 1997; Pingree and Sinha, 2000);
here, westward motion of Rossby waves can be observed in TOPEX-POSEIDON imagery
at about 32–34
N, carrying eddies having SLA signatures above background. The larger
features may be followed westward, crossing the mid-Atlantic Ridge, over periods of
several months.
Because eddies create vertical motion, a different response of phytoplankton to irradiance and nutrients is to be anticipated in these features and in the surrounding ocean.
Early studies of SLA and temperature in the Sargasso Sea showed a correspondence
between cold (warm) temperature anomalies and higher (lower) pigment anomalies.
Phytoplankton in the oligotrophic ocean that are not growing at their maximum specific growth rate may be stimulated by such “eddy-pumping” (Falkowski et al., 1991).
Especially in low latitudes, the lifting of nutrients from below into the photic zone in
cold-core eddies stimulates an instantaneous response of the phytoplankton that is at first
linear; later, the accumulating nutrients decrease rapidly as the phytoplankton enters—at
least theoretically—a phase of exponential growth. McGillicuddy and Robinson (1997)
suggested that in the subtropical gyre of the North Atlantic the contribution of the fertilizing effects of mesoscale eddies is significantly larger than the combined entrainment
of nutrients into the mixed layer during winter convection, and by thermocline mixing
and wind-driven transport. Eddy pumping may therefore be the dominant nutrient flux
in such regions.
In anticyclonic eddies the process is more complex and marginal, and enhanced
chlorophyll concentrations occur preferentially in the high-velocity region encircling the
eddy, where elevation of the pycnocline (and hence of the nutricline in most situations)
is associated with a strongly baroclinic structure (Yentsch and Phinney, 1985; Lohrenz
et al., 1993). Vortex contraction may strengthen the upwelling of nutrients, especially on
the anticyclonic side of the transient jets (Woods, 1988). This effect is usually limited to
patches on the 10-km scale, which serves to explain the beaded string of high chlorophyll
often observed around anticyclonic eddies in chlorophyll images. The same effect is
observed on the filaments shed by mesoscale eddies, especially on their curved rather
than their straight sections (Tranter et al., 1983). Production within warm-core rings in
the Agulhas eddy field may be limited by convective instability, but around the edge of
such rings stability and enhanced productivity is conferred by the warm water of the ring
overlying the cooler water in which it is embedded (Dower and Lucas, 1993). The same,
of course, may occur elsewhere.