36
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
water masses, (ii) with mesoscale eddies and filaments in the open ocean, but especially
associated with the edges of continental shelves, and (iii) in shallower seas, associated
with the effect of the semidiurnal tide.
Fronts and Frontal Systems
Oceanic Fronts and Eddy Streets
In the open oceans, we should distinguish between two kinds of frontal systems, although
the one is, in a sense, included in the other. These are (i) basin-scale zones of convergence
or divergence at the confluence between major current systems, and (ii) mesoscale filaments around, or shed from, mesoscale eddies that may occur as eddy streets lying across
almost entire ocean basins. Observations of enhanced levels of biomass are commonplace
in both categories, but it is at oceanic convergent fronts that the accumulation of biota
is greatest. These are the “siomes” of Japanese oceanographers and fishermen, where
turbulence at the confluence may disturb the wind-wave system and where the passive
aggregations of biota attracts an active concentration of pelagic fish, birds, and fisherfolk
(Sournia, 1994). At such fronts, accumulation of biota may depend on passive flotation/sinking rates, or on their individual swimming ability so as to avoid submergence
in the descending water mass (e.g., Owen, 1981).
The most important oceanic fronts for partitioning both biogeographic and ecological
processes are the Polar, Subtropical, and Equatorial frontal systems. Where the alignment
of the coasts permits it, examples of each occur in each hemisphere of each ocean,
appearing in basin-scale images of sea-surface elevation (Fig. 3.1) as linear fields of
300°
50°
30°
10°
C
C
R
R
16 January 2001
320°
340°
Fig. 3.1 Sea surface elevation ±30 cms of the North Atlantic Ocean, to demonstrate where mesoscale
eddy activity is most intense, and where the major frontal regions occur in this ocean: the data are from
TOPEX/POSEIDON radar altimeter data.
Source: University of Colorado.
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
water masses, (ii) with mesoscale eddies and filaments in the open ocean, but especially
associated with the edges of continental shelves, and (iii) in shallower seas, associated
with the effect of the semidiurnal tide.
Fronts and Frontal Systems
Oceanic Fronts and Eddy Streets
In the open oceans, we should distinguish between two kinds of frontal systems, although
the one is, in a sense, included in the other. These are (i) basin-scale zones of convergence
or divergence at the confluence between major current systems, and (ii) mesoscale filaments around, or shed from, mesoscale eddies that may occur as eddy streets lying across
almost entire ocean basins. Observations of enhanced levels of biomass are commonplace
in both categories, but it is at oceanic convergent fronts that the accumulation of biota
is greatest. These are the “siomes” of Japanese oceanographers and fishermen, where
turbulence at the confluence may disturb the wind-wave system and where the passive
aggregations of biota attracts an active concentration of pelagic fish, birds, and fisherfolk
(Sournia, 1994). At such fronts, accumulation of biota may depend on passive flotation/sinking rates, or on their individual swimming ability so as to avoid submergence
in the descending water mass (e.g., Owen, 1981).
The most important oceanic fronts for partitioning both biogeographic and ecological
processes are the Polar, Subtropical, and Equatorial frontal systems. Where the alignment
of the coasts permits it, examples of each occur in each hemisphere of each ocean,
appearing in basin-scale images of sea-surface elevation (Fig. 3.1) as linear fields of
300°
50°
30°
10°
C
C
R
R
16 January 2001
320°
340°
Fig. 3.1 Sea surface elevation ±30 cms of the North Atlantic Ocean, to demonstrate where mesoscale
eddy activity is most intense, and where the major frontal regions occur in this ocean: the data are from
TOPEX/POSEIDON radar altimeter data.
Source: University of Colorado.