Ecological Consequences of Mesoscale Eddies and Planetary Waves
53
the subsequent spring bloom, because cells, though nutrient-replete, continue to be
intermittently mixed sufficiently deep as to become light-limited. Such a threshold does
not occur in lower latitudes where, in general, unusual seasonal deepening of the mixed
layer leads subsequently to enhanced strength of the seasonal bloom, by increasing the
nutrient supply. By this model, Follows and Dutkiewicz suggest mechanisms for the
control of the timing of the high latitude spring bloom and for coherent regional changes
in bloom strength observed in the subtropics.
Ecological Consequences of Mesoscale Eddies
and Planetary Waves
General maps of ocean circulation suggest that flow is linear and that free-drifting
particles, such as planktonic biota, will be transported parallel to the apparent axis. This
assumption is inherent in discussions of the biogeography of plankton and nekton, where
retention within oceanic gyres is generally assumed.
Satellite sensors now reveal that flow in the real ocean is far from linear and occurs
within a complex and constantly varying field of mesoscale eddies, both cyclonic and
anticyclonic, associated with a global field of planetary waves (reviewed by Chelton and
Schlax, 1996), previously observed only with difficulty because of their small signature at
the sea surface: a sea-level anomaly (SLA) of a only a few centimeters is associated with the
displacement of the thermocline of several tens of meters. Rossby waves are the dynamic
response at large scale to wind and buoyancy forcing on the eastern boundaries of ocean
basins; the velocity of their westward propagation is an inverse function of latitude.
Waves at the first baroclinic mode propagate slowly across the ocean basin, requiring
several months to cross the Pacific in low latitudes and several years at high latitudes.
Very close to the equator, most of the observed sea-level variability is associated with
eastward-propagating Kelvin waves, traveling along the equatorial wave guides. These
equatorially trapped waves may originate in wind events in the central part of ocean
basins or by the reflection of Rossby waves at the western boundary. The global field of
planetary waves may now, of course, be visualized in the SLA field obtained by TOPEXPOSEIDON and is the template for an important component in the sea-surface pattern
of chlorophyll. Physicists have commented on the unexpected nature of this relationship:
ecologists, if they had thought about the matter, would have predicted it. I shall have
frequent occasion to refer to it here.
It is now clear that flow in the ocean, as in the atmosphere, is everywhere dominated
by synoptic mesoscale eddies that extend deep into the water column, have lifetimes of
order 100 days and maximal rotational velocities of order 10 cm s
−1 . These are, it is trite
to repeat, the “weather systems” of the ocean and contain an order of magnitude more
energy than do the mean currents. Eddies are generated preferentially in regions of strong
horizontal gradients of properties, as, for instance, where coastal boundary currents turn
seaward. Flow around mesoscale eddies may be an order of magnitude faster than that
of the mean current.
Mesoscale eddies are, of course, both cyclonic and anticyclonic. In bowl-shaped,
cyclonic eddies, pressure is low below the sea surface so that the thermocline shallows
centrally, hence these are “cold-core” eddies. In anticyclonic features associated with
doming of the sea surface, pressure is high below the surface so that the thermocline
deepens centrally, resulting in a “warm-core” eddy. It is important to note the relative
scale of vertical motions likely to be observed centrally in an eddy in response to motion:
a few centimeters at the sea surface, a few meters at the thermocline, and a few tens
53
the subsequent spring bloom, because cells, though nutrient-replete, continue to be
intermittently mixed sufficiently deep as to become light-limited. Such a threshold does
not occur in lower latitudes where, in general, unusual seasonal deepening of the mixed
layer leads subsequently to enhanced strength of the seasonal bloom, by increasing the
nutrient supply. By this model, Follows and Dutkiewicz suggest mechanisms for the
control of the timing of the high latitude spring bloom and for coherent regional changes
in bloom strength observed in the subtropics.
Ecological Consequences of Mesoscale Eddies
and Planetary Waves
General maps of ocean circulation suggest that flow is linear and that free-drifting
particles, such as planktonic biota, will be transported parallel to the apparent axis. This
assumption is inherent in discussions of the biogeography of plankton and nekton, where
retention within oceanic gyres is generally assumed.
Satellite sensors now reveal that flow in the real ocean is far from linear and occurs
within a complex and constantly varying field of mesoscale eddies, both cyclonic and
anticyclonic, associated with a global field of planetary waves (reviewed by Chelton and
Schlax, 1996), previously observed only with difficulty because of their small signature at
the sea surface: a sea-level anomaly (SLA) of a only a few centimeters is associated with the
displacement of the thermocline of several tens of meters. Rossby waves are the dynamic
response at large scale to wind and buoyancy forcing on the eastern boundaries of ocean
basins; the velocity of their westward propagation is an inverse function of latitude.
Waves at the first baroclinic mode propagate slowly across the ocean basin, requiring
several months to cross the Pacific in low latitudes and several years at high latitudes.
Very close to the equator, most of the observed sea-level variability is associated with
eastward-propagating Kelvin waves, traveling along the equatorial wave guides. These
equatorially trapped waves may originate in wind events in the central part of ocean
basins or by the reflection of Rossby waves at the western boundary. The global field of
planetary waves may now, of course, be visualized in the SLA field obtained by TOPEXPOSEIDON and is the template for an important component in the sea-surface pattern
of chlorophyll. Physicists have commented on the unexpected nature of this relationship:
ecologists, if they had thought about the matter, would have predicted it. I shall have
frequent occasion to refer to it here.
It is now clear that flow in the ocean, as in the atmosphere, is everywhere dominated
by synoptic mesoscale eddies that extend deep into the water column, have lifetimes of
order 100 days and maximal rotational velocities of order 10 cm s
−1 . These are, it is trite
to repeat, the “weather systems” of the ocean and contain an order of magnitude more
energy than do the mean currents. Eddies are generated preferentially in regions of strong
horizontal gradients of properties, as, for instance, where coastal boundary currents turn
seaward. Flow around mesoscale eddies may be an order of magnitude faster than that
of the mean current.
Mesoscale eddies are, of course, both cyclonic and anticyclonic. In bowl-shaped,
cyclonic eddies, pressure is low below the sea surface so that the thermocline shallows
centrally, hence these are “cold-core” eddies. In anticyclonic features associated with
doming of the sea surface, pressure is high below the surface so that the thermocline
deepens centrally, resulting in a “warm-core” eddy. It is important to note the relative
scale of vertical motions likely to be observed centrally in an eddy in response to motion:
a few centimeters at the sea surface, a few meters at the thermocline, and a few tens