38
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
(Wytki and Kilonsky, 1984). Between the 20
N ridge and the 10
N trough the NEC flows
westward. From the 10
N trough to the 5
N ridge the NECC flows eastward, while south
of this ridge the SEC flows westward. The near-equatorial trough represents a narrow
linear zone of cooler water, seen also clearly as a band of high chlorophyll concentration in
the relevant SeaWiFs image and of low temperatures in AVHRR images. Here, evidently,
we can use physical features unequivocally in our partition of the tropical ocean, in a
manner sensitive to the seasonal, zonal displacement of the equatorial current system,
because we can readily follow them in routine TOPEX-POSEIDON images.
The convergent Subtropical fronts are dominated by meandering flow along an eddy
street in which cyclonic and anticyclonic eddies alternate, once again readily observable
in the sea-level anomaly signature obtained by orbiting radar altimeters at around 40
latitude. These eddying flows lie along the poleward limbs of the subtropical gyres, so that
in the southern hemisphere the individual subtropical fronts of the Indian, Pacific, and
Atlantic Oceans form a quasicontinuous feature around the Southern Ocean. Although
it is, as we shall see, interrupted as it approaches the western coasts of South America
and Africa, passing equatorward, the Subtropical front or convergence is treated by most
authorities as a continuous feature. Indeed, it is so prominent in any global analysis of
sea surface features that I am constrained to consider it a province of the ecological
geographic system to be discussed in later chapters. As we shall see later, the geographies
of the Subtropical Convergence Fronts of the North Pacific and North Atlantic differ
strongly because of the strikingly different flow system in the subtropical gyres of the two
oceans. Recall that in the Atlantic most of the transport of the western boundary current
is lost to the Arctic Ocean, while in the Pacific essentially all flow passes directly eastward
across the ocean. The Azores Front, then, which is the Subtropical Convergence Front
of the North Atlantic, is a weak version of that of the North Pacific. There, it is often
referred to as the Transition Zone between the subtropical and subarctic gyres and is an
important feature in the biogeography of the North Pacific, selectively harboring some
species of, for instance, euphausiids.
Further poleward, the Subarctic and Polar Fronts must be crucial delimiters in any
ecological geography of the sea, but to map these we get less help from satellite imagery
because of cloudiness above 45–50
in winter, and because the radar altimeters of TOPEXPOSEIDON cannot deliver images from above latitude 60
. In the Southern Ocean, the
location of these fronts is a relatively simple annular arrangement (see Chapter 12),
but in the northern hemisphere there are significant differences between the Pacific and
Atlantic Oceans, once again because of the open passage between the Atlantic and the
Arctic Ocean.
The high-latitude fronts of the North Atlantic are both complex and confusingly
named. The north wall of the Gulf Stream in the northwest Atlantic forms a continuous
feature with a frontal system that lies SW-NE across the ocean toward the north of
Scotland. This is the Oceanic Polar Front of many authors but is the homologue of
the subpolar fronts of other oceans. Farther into the cold northern waters, the frontal
system that lies above the Iceland-Faeroes Ridge separates surface water originating in the
subtropical gyre from surface waters of the Barents Sea: it is this front that is most usually
termed the Polar Front in the North Atlantic. North of Iceland it is almost continuous
with another frontal system that can be traced continuously from the coast of Spitzbergen
to Greenland and around into the Labrador Sea. Truly polar water lies to the north of
this front, so I am proposing to treat this entire linear feature as the Polar Front of the
North Atlantic.
Finally, it will be well to note at this juncture that frontogenesis occurs at a variety
of scales in the ocean, in a manner that threatens to confuse us as we attempt to use
major frontal systems in biogeographical or ecological analysis. Consider the atmospheric
convergence that lies between the westerlies of the temperate zone and the easterly Trade
Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
(Wytki and Kilonsky, 1984). Between the 20
N ridge and the 10
N trough the NEC flows
westward. From the 10
N trough to the 5
N ridge the NECC flows eastward, while south
of this ridge the SEC flows westward. The near-equatorial trough represents a narrow
linear zone of cooler water, seen also clearly as a band of high chlorophyll concentration in
the relevant SeaWiFs image and of low temperatures in AVHRR images. Here, evidently,
we can use physical features unequivocally in our partition of the tropical ocean, in a
manner sensitive to the seasonal, zonal displacement of the equatorial current system,
because we can readily follow them in routine TOPEX-POSEIDON images.
The convergent Subtropical fronts are dominated by meandering flow along an eddy
street in which cyclonic and anticyclonic eddies alternate, once again readily observable
in the sea-level anomaly signature obtained by orbiting radar altimeters at around 40
latitude. These eddying flows lie along the poleward limbs of the subtropical gyres, so that
in the southern hemisphere the individual subtropical fronts of the Indian, Pacific, and
Atlantic Oceans form a quasicontinuous feature around the Southern Ocean. Although
it is, as we shall see, interrupted as it approaches the western coasts of South America
and Africa, passing equatorward, the Subtropical front or convergence is treated by most
authorities as a continuous feature. Indeed, it is so prominent in any global analysis of
sea surface features that I am constrained to consider it a province of the ecological
geographic system to be discussed in later chapters. As we shall see later, the geographies
of the Subtropical Convergence Fronts of the North Pacific and North Atlantic differ
strongly because of the strikingly different flow system in the subtropical gyres of the two
oceans. Recall that in the Atlantic most of the transport of the western boundary current
is lost to the Arctic Ocean, while in the Pacific essentially all flow passes directly eastward
across the ocean. The Azores Front, then, which is the Subtropical Convergence Front
of the North Atlantic, is a weak version of that of the North Pacific. There, it is often
referred to as the Transition Zone between the subtropical and subarctic gyres and is an
important feature in the biogeography of the North Pacific, selectively harboring some
species of, for instance, euphausiids.
Further poleward, the Subarctic and Polar Fronts must be crucial delimiters in any
ecological geography of the sea, but to map these we get less help from satellite imagery
because of cloudiness above 45–50
in winter, and because the radar altimeters of TOPEXPOSEIDON cannot deliver images from above latitude 60
. In the Southern Ocean, the
location of these fronts is a relatively simple annular arrangement (see Chapter 12),
but in the northern hemisphere there are significant differences between the Pacific and
Atlantic Oceans, once again because of the open passage between the Atlantic and the
Arctic Ocean.
The high-latitude fronts of the North Atlantic are both complex and confusingly
named. The north wall of the Gulf Stream in the northwest Atlantic forms a continuous
feature with a frontal system that lies SW-NE across the ocean toward the north of
Scotland. This is the Oceanic Polar Front of many authors but is the homologue of
the subpolar fronts of other oceans. Farther into the cold northern waters, the frontal
system that lies above the Iceland-Faeroes Ridge separates surface water originating in the
subtropical gyre from surface waters of the Barents Sea: it is this front that is most usually
termed the Polar Front in the North Atlantic. North of Iceland it is almost continuous
with another frontal system that can be traced continuously from the coast of Spitzbergen
to Greenland and around into the Labrador Sea. Truly polar water lies to the north of
this front, so I am proposing to treat this entire linear feature as the Polar Front of the
North Atlantic.
Finally, it will be well to note at this juncture that frontogenesis occurs at a variety
of scales in the ocean, in a manner that threatens to confuse us as we attempt to use
major frontal systems in biogeographical or ecological analysis. Consider the atmospheric
convergence that lies between the westerlies of the temperate zone and the easterly Trade