8
2 Frontal types
cells in the euphotic zone (Podestá 1990; Brandini et al. 2000). Numerical
simulations indicate that offshore flow in the bottom Ekman layer promotes
overturning over the continental shelf and detaches from the bottom at the
shelf break, where it mixes upward along sloping isopycnals, thus promoting
upwelling (Gawarkiewicz and Chapman 1992). High resolution hydrographic
observations across the Middle Atlantic Bight south of New England corroborate the model results (Barth et al. 1998; Houghton and Visbeck 1998).
More recently Matano and Palma (2008) proposed a mechanism by which as a
downwelling current flows along the continental slope in the direction of continental trapped waves (e.g. with the coast on the left (right) in the Southern
(Northern) Hemisphere), bottom friction and lateral diffusion spread the flow
onto the neighboring shelf, thus generating along-shelf pressure gradients and
a cross-shelf divergence that is compensated by shelf-break upwelling. Though
shelf break fronts are topographically trapped and generally reorganize in a
few days after being disrupted (Gawarkiewicz and Chapman 1992), there are
strong indications of frontal instability that by enhancing cross-shelf exchange
might further promote nutrient enrichment. Despite of their ubiquitous occurrence, the fertilization mechanisms of shelf-break fronts seem to be diverse
(Fig. 2.2b).
2.3 Upwelling Fronts
Wind driven currents that flow towards the Equator along the western coasts
of continents (e.g. Peru; California; Benguela; Canaries Currents) are driven
away from the coasts due to the Earth’s rotation, leading to coastal upwelling
of nutrient rich waters. Upwelling fronts frequently present strong seasonality
derived from the seasonal variability of prevailing upwelling-favorable winds.
For instance along the coast of California the onset of upwelling occurs during
the “spring transition” when southerly winds reverse to upwelling-favorable, and
last until late fall (Huyer 1983). The sloping isopycnals sustain an along-shore
baroclinic jet. As the upwelling fronts are located a few tens of km from shore,
the interaction between the frontal jet and coastal indentations promote frontal
instabilities and vertical motions. These eastern boundary upwelling ecosystems
are among the most productive regions in the oceans (Pauly and Christensen
2005). The upwelled waters move away from the coast by Ekman transport and
converge at certain distance offshore, so the upwelled water sinks. Upwelling
fronts form at this interface between shelf water and the cool, nutrient-rich water
brought to the surface during wind-driven coastal upwelling. This frontal region
is highly productive and planktonic organisms aggregate on the coastal side of
the front and large numbers of fish concentrate at that location (Mann and Lazier
2006) (Fig. 2.2c).
2 Frontal types
cells in the euphotic zone (Podestá 1990; Brandini et al. 2000). Numerical
simulations indicate that offshore flow in the bottom Ekman layer promotes
overturning over the continental shelf and detaches from the bottom at the
shelf break, where it mixes upward along sloping isopycnals, thus promoting
upwelling (Gawarkiewicz and Chapman 1992). High resolution hydrographic
observations across the Middle Atlantic Bight south of New England corroborate the model results (Barth et al. 1998; Houghton and Visbeck 1998).
More recently Matano and Palma (2008) proposed a mechanism by which as a
downwelling current flows along the continental slope in the direction of continental trapped waves (e.g. with the coast on the left (right) in the Southern
(Northern) Hemisphere), bottom friction and lateral diffusion spread the flow
onto the neighboring shelf, thus generating along-shelf pressure gradients and
a cross-shelf divergence that is compensated by shelf-break upwelling. Though
shelf break fronts are topographically trapped and generally reorganize in a
few days after being disrupted (Gawarkiewicz and Chapman 1992), there are
strong indications of frontal instability that by enhancing cross-shelf exchange
might further promote nutrient enrichment. Despite of their ubiquitous occurrence, the fertilization mechanisms of shelf-break fronts seem to be diverse
(Fig. 2.2b).
2.3 Upwelling Fronts
Wind driven currents that flow towards the Equator along the western coasts
of continents (e.g. Peru; California; Benguela; Canaries Currents) are driven
away from the coasts due to the Earth’s rotation, leading to coastal upwelling
of nutrient rich waters. Upwelling fronts frequently present strong seasonality
derived from the seasonal variability of prevailing upwelling-favorable winds.
For instance along the coast of California the onset of upwelling occurs during
the “spring transition” when southerly winds reverse to upwelling-favorable, and
last until late fall (Huyer 1983). The sloping isopycnals sustain an along-shore
baroclinic jet. As the upwelling fronts are located a few tens of km from shore,
the interaction between the frontal jet and coastal indentations promote frontal
instabilities and vertical motions. These eastern boundary upwelling ecosystems
are among the most productive regions in the oceans (Pauly and Christensen
2005). The upwelled waters move away from the coast by Ekman transport and
converge at certain distance offshore, so the upwelled water sinks. Upwelling
fronts form at this interface between shelf water and the cool, nutrient-rich water
brought to the surface during wind-driven coastal upwelling. This frontal region
is highly productive and planktonic organisms aggregate on the coastal side of
the front and large numbers of fish concentrate at that location (Mann and Lazier
2006) (Fig. 2.2c).