Fronts and Frontal Systems
41
side. So, not only is physical accumulation of plankton and nekton to be anticipated at
the front, but the relatively stable, shallow pycnocline region just seaward of the front
generally supports the most intense production of phytoplankton.
Tidal Fronts and River Plumes of the Shelf Seas
Stratification of coastal seas is forced not only by solar heating at the surface, but also
by coastal freshwater run-off or by an excess of precipitation over evaporation. Several
processes contribute to the development of fronts between stratified and nonstratified
regions of the shelf: most important, perhaps, is vertical shear within the tidal streams,
together with the effect of baroclinic eddies of semidiurnal frequency.
Where a river enters the sea, a plume of relatively fresh, often silt-laden water flows
from the estuary. If the coast is insufficiently low-lying to permit the development of a
delta that will disperse the effluent, the effluent will pass seaward as a buoyant plume above
the coastal water mass. In the absence of entrainment at the interface, the buoyant plume
would spread indefinitely, but this does not occur, of course, and it takes dimensions
appropriate to the seasonal pattern of discharge. As the horizontal interface between the
river water above and shelf water below progressively shoals and breaks the surface, a
rather distinct frontal boundary forms around the edges of the plume. Although rivers
entering the sea may be so small as to be trivial, or so large as to dominate the regional
hydrography, all will be found to have plumes that follow these principles. Off very small
rivers, a plume may form only at certain states of the tide.
The injection of land-based nutrients by river plumes into coastal seas may lead
to biological enhancement there, although this is not always the case and biological
enrichment may have more to do with divergence in the zone of interaction between the
two water types than with riverborne nutrients. At the boundary between regions where
tidal stress fully mixes the water column and where the river plume enforces stability,
estuarine fronts may be formed between stratified and mixed water. These, in general,
lie parallel to the axis of the estuary. But the most important effect of tidal streams is to
break down the inherent stratification of shelf seas, temporally or permanently, and over
relatively large areas.
When the semidiurnal tide encounters shoaling water, as over the continental slope,
the amplitude of the tidal wave and its horizontal velocity progressively increase. At
some depth, usually shoaler than the continental edge, vertical turbulence produced by
friction between the tidal stream and the seabed is sufficiently enhanced (when added to
turbulence produced by wind stress at the sea surface) as to overturn seasonal thermal
stratification of the water column. As an aside, John Simpson likened the frictional effect
of tidal streams to “hurricane force winds blowing regularly twice each day”: no wonder,
then, that stratification is so regularly broken down on continental shelves. Where this
occurs, the tidally mixed and stratified regions of the shelf are separated by a frontal
region that migrates semidiurnally and also seasonally because the area of vertically mixed
water over the shelf is usually larger during seasons of higher-than-average wind stress.
Although there exist more complex formulations, it is convenient to use maps of the
parameter log 10 h/u
3 (where h is water depth and u
3 is the tidal current at the surface)
to locate the transition between mixing and stratification. For most shelf areas, assuming
that the surface heating rate is spatially uniform, the critical value of this parameter is
between 1.8 and 2.0. More simply, a tidal stream of 1 m sec
−1 in 100 m of water falls
within this range and therefore mixes (Fig. 3.4).
Phytoplankton observed at shelf sea fronts may exceptionally result from accumulation,
but growth in situ is the more normal case: Franks and Chen (1996) show that, at the
tidal front which encircles Georges Bank, off New England, the distribution of nutrients
mirrors that of productivity. They show that the strong tidal mixing over the central bank
41
side. So, not only is physical accumulation of plankton and nekton to be anticipated at
the front, but the relatively stable, shallow pycnocline region just seaward of the front
generally supports the most intense production of phytoplankton.
Tidal Fronts and River Plumes of the Shelf Seas
Stratification of coastal seas is forced not only by solar heating at the surface, but also
by coastal freshwater run-off or by an excess of precipitation over evaporation. Several
processes contribute to the development of fronts between stratified and nonstratified
regions of the shelf: most important, perhaps, is vertical shear within the tidal streams,
together with the effect of baroclinic eddies of semidiurnal frequency.
Where a river enters the sea, a plume of relatively fresh, often silt-laden water flows
from the estuary. If the coast is insufficiently low-lying to permit the development of a
delta that will disperse the effluent, the effluent will pass seaward as a buoyant plume above
the coastal water mass. In the absence of entrainment at the interface, the buoyant plume
would spread indefinitely, but this does not occur, of course, and it takes dimensions
appropriate to the seasonal pattern of discharge. As the horizontal interface between the
river water above and shelf water below progressively shoals and breaks the surface, a
rather distinct frontal boundary forms around the edges of the plume. Although rivers
entering the sea may be so small as to be trivial, or so large as to dominate the regional
hydrography, all will be found to have plumes that follow these principles. Off very small
rivers, a plume may form only at certain states of the tide.
The injection of land-based nutrients by river plumes into coastal seas may lead
to biological enhancement there, although this is not always the case and biological
enrichment may have more to do with divergence in the zone of interaction between the
two water types than with riverborne nutrients. At the boundary between regions where
tidal stress fully mixes the water column and where the river plume enforces stability,
estuarine fronts may be formed between stratified and mixed water. These, in general,
lie parallel to the axis of the estuary. But the most important effect of tidal streams is to
break down the inherent stratification of shelf seas, temporally or permanently, and over
relatively large areas.
When the semidiurnal tide encounters shoaling water, as over the continental slope,
the amplitude of the tidal wave and its horizontal velocity progressively increase. At
some depth, usually shoaler than the continental edge, vertical turbulence produced by
friction between the tidal stream and the seabed is sufficiently enhanced (when added to
turbulence produced by wind stress at the sea surface) as to overturn seasonal thermal
stratification of the water column. As an aside, John Simpson likened the frictional effect
of tidal streams to “hurricane force winds blowing regularly twice each day”: no wonder,
then, that stratification is so regularly broken down on continental shelves. Where this
occurs, the tidally mixed and stratified regions of the shelf are separated by a frontal
region that migrates semidiurnally and also seasonally because the area of vertically mixed
water over the shelf is usually larger during seasons of higher-than-average wind stress.
Although there exist more complex formulations, it is convenient to use maps of the
parameter log 10 h/u
3 (where h is water depth and u
3 is the tidal current at the surface)
to locate the transition between mixing and stratification. For most shelf areas, assuming
that the surface heating rate is spatially uniform, the critical value of this parameter is
between 1.8 and 2.0. More simply, a tidal stream of 1 m sec
−1 in 100 m of water falls
within this range and therefore mixes (Fig. 3.4).
Phytoplankton observed at shelf sea fronts may exceptionally result from accumulation,
but growth in situ is the more normal case: Franks and Chen (1996) show that, at the
tidal front which encircles Georges Bank, off New England, the distribution of nutrients
mirrors that of productivity. They show that the strong tidal mixing over the central bank