216
Chapter 9: The Atlantic Ocean
I discussed in Chapter 1, will give us difficulties in interpreting chlorophyll images, in
addition to the problem presented by CDOM. Where tidal flows are rapid, fines are
not deposited, and gravel and coarse sand dominate. Where wind stress at the surface
is strong, at least seasonally, fines are resuspended so that bottom deposits tend also
to be sandy. Marine geologists have long recognized a “mud line” at 70–80 m in this
province, below which one may expect to encounter bottom types dominated by fine,
muddy material. Thus, on the Aquitanian shelf, much of the solid material originating
in the effluents of the Loire and Gironde rivers is segregated in a muddy belt at midshelf
depths, the “Grande Vasière” of French benthic ecologists. Beyond 100 m in the northern
basin of the North Sea and in the Norwegian trough, mud also dominates. In shoaler
water, soft muddy sediments occur only in some restricted depressions off the Gironde,
in the central North Sea, and also in estuaries and some coastal embayments.
Defining Characteristics of Regional Oceanography
The hydrography of this province is dominated by tidal forces in the west, and by
freshwater buoyancy in the east; for this reason, I have chosen not to discuss tidal fronts
in a special section, as for some other provinces. As Simpson comments in his 1998
review of the Celtic Seas, the large tidal input there from the Atlantic represents about
12% of all tidal energy that is dissipated globally. The tidal wave, coming from the deep
ocean, interacts individually and differently with the topography of each of the many
basins comprising this province. Thus, tidal amplitude varies from almost zero at the
amphidromic points of the M 2 tide to >10 m in cone-shaped embayments such as the
Bristol Channel or the Gulf of St. Malo. Simpson also noted, “Particularly strong tidal
responses occur in the English Channel, the Irish Sea and the Bristol Channel with the
largest ranges in each case occurring on the right side of the basin when viewed from
the ocean.” In the Irish Sea, for example, 80% of the incident energy is dissipated, and
the frictional stress at the seabed may exceed 4 Pa and is largely in excess of 0.25 Pa,
which is equivalent to the surface stress of a wind of 13 m sec
−1 .
For the reasons briefly outlined in Chapter 3, those regions where tidal streams are
sufficiently strong that the mixing parameter log 10 h u
−3 remains in excess of a critical
value (around 2), summer stratification induced by surface heating does not occur. Mixed
and stratified regions are separated by convergent “tidal-mixing fronts” that are variable
in position during the lunar cycle (see Fig. 3.4). These fronts usually separate a cool
water mass from one with warm water above and cold below. Flow along such fronts is
associated with a two-cell transverse circulation, often marked by convergent slicks and
a linear cold temperature anomaly, and may break down into a series of mostly cyclonic
eddies. Convergence in tidal fronts has, as we have seen, important consequences for the
physical accumulation of biota.
Of course, during winter, when the open North Atlantic is mixed to a depth of several
hundred meters, the water column over this shelf is everywhere mixed to the bottom. It is
only after increasing irradiance in the spring has imposed stratification on the deeper parts
of the shelf that we may expect tidal fronts to occur. The progressive seasonal development
of stratification causes such fronts to move onshore in the spring and offshore in the
autumn, over a distance of >200 km in the western Channel (Pingree, 1975; Pingree and
Griffiths, 1978; Pingree et al., 1982). In summer, the principal tidally mixed regions are
the eastern Channel, the southern North Sea and the Irish Sea. Tidal fronts, therefore,
occur predictably in an arc across the southern North Sea from Yorkshire to the Friesian
coast, across the western entrance to the English Channel, and at northern and southern
mouths of the Irish Sea. The location of tidal fronts is sufficiently predictable that they
could be used for a regional partition of this province that would be sensitive to the
ecological consequences of stratification and mixing regimes respectively. In tidally mixed
Chapter 9: The Atlantic Ocean
I discussed in Chapter 1, will give us difficulties in interpreting chlorophyll images, in
addition to the problem presented by CDOM. Where tidal flows are rapid, fines are
not deposited, and gravel and coarse sand dominate. Where wind stress at the surface
is strong, at least seasonally, fines are resuspended so that bottom deposits tend also
to be sandy. Marine geologists have long recognized a “mud line” at 70–80 m in this
province, below which one may expect to encounter bottom types dominated by fine,
muddy material. Thus, on the Aquitanian shelf, much of the solid material originating
in the effluents of the Loire and Gironde rivers is segregated in a muddy belt at midshelf
depths, the “Grande Vasière” of French benthic ecologists. Beyond 100 m in the northern
basin of the North Sea and in the Norwegian trough, mud also dominates. In shoaler
water, soft muddy sediments occur only in some restricted depressions off the Gironde,
in the central North Sea, and also in estuaries and some coastal embayments.
Defining Characteristics of Regional Oceanography
The hydrography of this province is dominated by tidal forces in the west, and by
freshwater buoyancy in the east; for this reason, I have chosen not to discuss tidal fronts
in a special section, as for some other provinces. As Simpson comments in his 1998
review of the Celtic Seas, the large tidal input there from the Atlantic represents about
12% of all tidal energy that is dissipated globally. The tidal wave, coming from the deep
ocean, interacts individually and differently with the topography of each of the many
basins comprising this province. Thus, tidal amplitude varies from almost zero at the
amphidromic points of the M 2 tide to >10 m in cone-shaped embayments such as the
Bristol Channel or the Gulf of St. Malo. Simpson also noted, “Particularly strong tidal
responses occur in the English Channel, the Irish Sea and the Bristol Channel with the
largest ranges in each case occurring on the right side of the basin when viewed from
the ocean.” In the Irish Sea, for example, 80% of the incident energy is dissipated, and
the frictional stress at the seabed may exceed 4 Pa and is largely in excess of 0.25 Pa,
which is equivalent to the surface stress of a wind of 13 m sec
−1 .
For the reasons briefly outlined in Chapter 3, those regions where tidal streams are
sufficiently strong that the mixing parameter log 10 h u
−3 remains in excess of a critical
value (around 2), summer stratification induced by surface heating does not occur. Mixed
and stratified regions are separated by convergent “tidal-mixing fronts” that are variable
in position during the lunar cycle (see Fig. 3.4). These fronts usually separate a cool
water mass from one with warm water above and cold below. Flow along such fronts is
associated with a two-cell transverse circulation, often marked by convergent slicks and
a linear cold temperature anomaly, and may break down into a series of mostly cyclonic
eddies. Convergence in tidal fronts has, as we have seen, important consequences for the
physical accumulation of biota.
Of course, during winter, when the open North Atlantic is mixed to a depth of several
hundred meters, the water column over this shelf is everywhere mixed to the bottom. It is
only after increasing irradiance in the spring has imposed stratification on the deeper parts
of the shelf that we may expect tidal fronts to occur. The progressive seasonal development
of stratification causes such fronts to move onshore in the spring and offshore in the
autumn, over a distance of >200 km in the western Channel (Pingree, 1975; Pingree and
Griffiths, 1978; Pingree et al., 1982). In summer, the principal tidally mixed regions are
the eastern Channel, the southern North Sea and the Irish Sea. Tidal fronts, therefore,
occur predictably in an arc across the southern North Sea from Yorkshire to the Friesian
coast, across the western entrance to the English Channel, and at northern and southern
mouths of the Irish Sea. The location of tidal fronts is sufficiently predictable that they
could be used for a regional partition of this province that would be sensitive to the
ecological consequences of stratification and mixing regimes respectively. In tidally mixed