Chapter 9
The Atlantic Ocean
T
he symmetrical planetary forces that drive circulation and mixing can be modeled
with great precision for a landless world, but in reality they operate within irregular
basins shaped by the asymmetric drift of continental masses during geological
time: what we observe in the real ocean, therefore, departs very significantly from the
ideal state. So, the following chapters, each devoted to a single ocean basin, open with
a brief introduction to its basic geography because it is this that determines its water
circulation patterns and the distribution of turbulent energy.
It is remarkable how little emphasis is placed on the individual geography of each ocean
basin in most texts on oceanographic processes, whether these are physical, chemical, or
biological. Some knowledge of the geography of each ocean must be one of the most
important items in any oceanographer’s tool kit. With each ocean it is logical to include
discussion of its marginal seas so that, in the case of the well-endowed Atlantic, the
Arctic Ocean together with the Caribbean, Mediterranean, Black, and Baltic Seas will
be included. As Tomczak and Godfrey (1994) point out, this gives it by far the longest
latitudinal extent of any ocean basin—21,000 km from the Bering Straits, over the pole
and down to Antarctica.
The significant features of the geography of the Atlantic Ocean that influence its circulation are self-evident. South America extends much farther poleward into the Southern
Ocean than does Africa, and so creates a unique asymmetry in the surface temperature
fields of the South Atlantic that has no homologue in other oceans. Interruption of the
strong westerly flow of the Circumpolar Current induces a northward loop along the
continental edge east of Tierra del Fuego and of the Falklands plateau that continues
until it meets the southward flow of the warm western boundary current off Mar del
Plata. If the Falkland Islands arose from deep water, instead of from a shoal plateau,
the circulation pattern of the whole southern Atlantic would be different. Again, if Cape
Hatteras did not exist, the Gulf Stream would separate from the American continent at
another latitude and the ecology of the whole North Atlantic would be different from
what it is today. Perhaps most important, if the triangular protuberance of Brazil and
the Guianas lay farther north, the flow of the westward trade wind currents would be
more evenly divided between North and South Atlantic basins. Then, equatorial surface
water from both hemispheres would not, as it does now, flow almost entirely into the
Gulf Stream of the North Atlantic and, consequently, 10
C water would not penetrate to
60
N off Iceland.
In winter, between the Iceland low and the Azores high, wind stress (>250 ×
10
−2 dyn cm
−2 ) and heat flux (−40 to −60 W m
−2 y
−1 ) both take very high values
(Hellerman, 1967) and drive winter mixing to 750–900 m, especially at 50–60
N between
Ireland and Newfoundland. The spring bloom takes nitrate to very low levels in the
“nutrient hole” of the North Atlantic, which is a striking anomaly on the global nutrient
field. This deficit, which extends to 1000 m, is presumably maintained because export of
131
The Atlantic Ocean
T
he symmetrical planetary forces that drive circulation and mixing can be modeled
with great precision for a landless world, but in reality they operate within irregular
basins shaped by the asymmetric drift of continental masses during geological
time: what we observe in the real ocean, therefore, departs very significantly from the
ideal state. So, the following chapters, each devoted to a single ocean basin, open with
a brief introduction to its basic geography because it is this that determines its water
circulation patterns and the distribution of turbulent energy.
It is remarkable how little emphasis is placed on the individual geography of each ocean
basin in most texts on oceanographic processes, whether these are physical, chemical, or
biological. Some knowledge of the geography of each ocean must be one of the most
important items in any oceanographer’s tool kit. With each ocean it is logical to include
discussion of its marginal seas so that, in the case of the well-endowed Atlantic, the
Arctic Ocean together with the Caribbean, Mediterranean, Black, and Baltic Seas will
be included. As Tomczak and Godfrey (1994) point out, this gives it by far the longest
latitudinal extent of any ocean basin—21,000 km from the Bering Straits, over the pole
and down to Antarctica.
The significant features of the geography of the Atlantic Ocean that influence its circulation are self-evident. South America extends much farther poleward into the Southern
Ocean than does Africa, and so creates a unique asymmetry in the surface temperature
fields of the South Atlantic that has no homologue in other oceans. Interruption of the
strong westerly flow of the Circumpolar Current induces a northward loop along the
continental edge east of Tierra del Fuego and of the Falklands plateau that continues
until it meets the southward flow of the warm western boundary current off Mar del
Plata. If the Falkland Islands arose from deep water, instead of from a shoal plateau,
the circulation pattern of the whole southern Atlantic would be different. Again, if Cape
Hatteras did not exist, the Gulf Stream would separate from the American continent at
another latitude and the ecology of the whole North Atlantic would be different from
what it is today. Perhaps most important, if the triangular protuberance of Brazil and
the Guianas lay farther north, the flow of the westward trade wind currents would be
more evenly divided between North and South Atlantic basins. Then, equatorial surface
water from both hemispheres would not, as it does now, flow almost entirely into the
Gulf Stream of the North Atlantic and, consequently, 10
C water would not penetrate to
60
N off Iceland.
In winter, between the Iceland low and the Azores high, wind stress (>250 ×
10
−2 dyn cm
−2 ) and heat flux (−40 to −60 W m
−2 y
−1 ) both take very high values
(Hellerman, 1967) and drive winter mixing to 750–900 m, especially at 50–60
N between
Ireland and Newfoundland. The spring bloom takes nitrate to very low levels in the
“nutrient hole” of the North Atlantic, which is a striking anomaly on the global nutrient
field. This deficit, which extends to 1000 m, is presumably maintained because export of
131