176
Chapter 9: The Atlantic Ocean
these places, it is observable as a cold signature in sea surface temperature images but,
interestingly, not always in the sea surface chlorophyll images.
Of more ecological interest, perhaps, than the general circulation of the semipermanent
gyral system is the more active series of coastal rim currents that proceed cyclonically
around the Mediterranean and are especially continuous around the western basin. This
flow is forced by topographic steering and perhaps by deflection of motion to the right,
or toward the coast, by the positive curl of the wind stress. Although this has long been
the accepted explanation for rim currents in both Mediterranean and Black Seas, the
actual forcing mechanism may be more complex, according to Saur et al. (1994).
Through the Straits of Gibraltar, Atlantic water enters the Mediterranean at the surface,
and this perpetual inflow attracted the attention of the ancient philosophers and was not
resolved until surprisingly late—toward the end of the 18th century: this was, with tidal
streams, the first problem in oceanography to attract scientific attention. Local fisherfolk,
of course, knew from the earliest times that a deep countercurrent was the answer—
although perhaps they did not know that there was a problem to be solved. The Atlantic
water passes eastward along the African coast as the density-driven, topographically locked
Algerian Current, which continues into the eastern basin through the Sicilian Channel
(Millot, 1992) as the Ionian-Atlantic stream. Instability in this coastal flow generates a
field of anticyclonic eddies and meanders, perhaps especially rich along the Algerian coast.
These are of order 50–100 km in diameter and drift eastward along the coast at a rate of
several kilometers per day. After a lifetime of several months they tend to enlarge and
depart progressively from the coast, often trailing a filament of upwelled water with them,
originating around their SW flanks. The Western Alboran Sea is occupied by a persistent
anticyclonic gyre forced by the orientation and topography of the Straits of Gibraltar. In
the eastern part of the Alboran Sea, circulation is more variable (Millot, 1987).
In the eastern Mediterranean basin, the already meandering Ionian-Atlantic stream at
the Sicilian Channel becomes a mid-Mediterranean jet that can be traced far into the
Levantine Basin passing between flanking cyclonic eddies (Cretan, Rhodian, and West
Cyprus) to the north and anticyclonic eddies to the south (Shikmona and Mersa Matruh).
The quasipermanent position of these eddies is determined by topography (POEM Group,
1992). A coastal anticyclonic loop current around the Ligurian Sea between Sicily and
Greece creates a series of mostly cyclonic eddies.
Although tidal streams in the Mediterranean are generally weak, the Venturi effect
over the shallow sill of the Straits of Messina produces tidal currents that are unusually
strong, and violent, local upwelling occurs. This is, of course, the whirlpool of Greek
antiquity lying between six-headed, dog-barking Scylla, and Charybdis, who swallowed
the sea and vomited it back again thrice daily.
Black Sea With its anoxic interior, the Black Sea is the most extreme case of a
meromictic basin in the present-day ocean (for modern reviews, see Murray, 1991, and
Özsoy and Ünlüata, 1998). Salinity nowhere exceeds 17‰, and excess precipitation
together with runoff from the rivers Danube, Dniester, and Don creates a surface lowsalinity layer overlying a halocline at about 100 m that is sufficiently strong to prevent
ventilation of the interior of the sea. Below the halocline, the basin lacks oxygen and
has high concentrations (increasing with depth) of dissolved H 2 S. Stratification includes
a summer thermocline at 10–30 m, shoaler than the main discontinuity that combines
nutricline, pycnocline, and oxycline.
The hydrology of the Black Sea depends strongly on its mass water budget because
of its near enclosure. Since precipitation and river discharge exceed evaporation, a fast,
permanent flow pours southward through the Bosphorus—making a ferry crossing at
Istanbul a memorable affair. Some Mediterranean water (representing <50% of the
outgoing flux) enters the Black Sea as a bottom flow along the narrow channel, though
Chapter 9: The Atlantic Ocean
these places, it is observable as a cold signature in sea surface temperature images but,
interestingly, not always in the sea surface chlorophyll images.
Of more ecological interest, perhaps, than the general circulation of the semipermanent
gyral system is the more active series of coastal rim currents that proceed cyclonically
around the Mediterranean and are especially continuous around the western basin. This
flow is forced by topographic steering and perhaps by deflection of motion to the right,
or toward the coast, by the positive curl of the wind stress. Although this has long been
the accepted explanation for rim currents in both Mediterranean and Black Seas, the
actual forcing mechanism may be more complex, according to Saur et al. (1994).
Through the Straits of Gibraltar, Atlantic water enters the Mediterranean at the surface,
and this perpetual inflow attracted the attention of the ancient philosophers and was not
resolved until surprisingly late—toward the end of the 18th century: this was, with tidal
streams, the first problem in oceanography to attract scientific attention. Local fisherfolk,
of course, knew from the earliest times that a deep countercurrent was the answer—
although perhaps they did not know that there was a problem to be solved. The Atlantic
water passes eastward along the African coast as the density-driven, topographically locked
Algerian Current, which continues into the eastern basin through the Sicilian Channel
(Millot, 1992) as the Ionian-Atlantic stream. Instability in this coastal flow generates a
field of anticyclonic eddies and meanders, perhaps especially rich along the Algerian coast.
These are of order 50–100 km in diameter and drift eastward along the coast at a rate of
several kilometers per day. After a lifetime of several months they tend to enlarge and
depart progressively from the coast, often trailing a filament of upwelled water with them,
originating around their SW flanks. The Western Alboran Sea is occupied by a persistent
anticyclonic gyre forced by the orientation and topography of the Straits of Gibraltar. In
the eastern part of the Alboran Sea, circulation is more variable (Millot, 1987).
In the eastern Mediterranean basin, the already meandering Ionian-Atlantic stream at
the Sicilian Channel becomes a mid-Mediterranean jet that can be traced far into the
Levantine Basin passing between flanking cyclonic eddies (Cretan, Rhodian, and West
Cyprus) to the north and anticyclonic eddies to the south (Shikmona and Mersa Matruh).
The quasipermanent position of these eddies is determined by topography (POEM Group,
1992). A coastal anticyclonic loop current around the Ligurian Sea between Sicily and
Greece creates a series of mostly cyclonic eddies.
Although tidal streams in the Mediterranean are generally weak, the Venturi effect
over the shallow sill of the Straits of Messina produces tidal currents that are unusually
strong, and violent, local upwelling occurs. This is, of course, the whirlpool of Greek
antiquity lying between six-headed, dog-barking Scylla, and Charybdis, who swallowed
the sea and vomited it back again thrice daily.
Black Sea With its anoxic interior, the Black Sea is the most extreme case of a
meromictic basin in the present-day ocean (for modern reviews, see Murray, 1991, and
Özsoy and Ünlüata, 1998). Salinity nowhere exceeds 17‰, and excess precipitation
together with runoff from the rivers Danube, Dniester, and Don creates a surface lowsalinity layer overlying a halocline at about 100 m that is sufficiently strong to prevent
ventilation of the interior of the sea. Below the halocline, the basin lacks oxygen and
has high concentrations (increasing with depth) of dissolved H 2 S. Stratification includes
a summer thermocline at 10–30 m, shoaler than the main discontinuity that combines
nutricline, pycnocline, and oxycline.
The hydrology of the Black Sea depends strongly on its mass water budget because
of its near enclosure. Since precipitation and river discharge exceed evaporation, a fast,
permanent flow pours southward through the Bosphorus—making a ferry crossing at
Istanbul a memorable affair. Some Mediterranean water (representing <50% of the
outgoing flux) enters the Black Sea as a bottom flow along the narrow channel, though