174
Chapter 9: The Atlantic Ocean
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The Mediterranean Sea comprises two deep basins, connected with the Atlantic Ocean
across a sill depth of only 290 m at the Straits of Gibraltar. The Black Sea is landlocked
except for its connection with the Mediterranean through the Bosphorus; this connection
is slender, having a shore-to-shore width of only 725 m at the choke point, and a
midchannel sill depth of only 40 m.
The Mediterranean is characterized by rather narrow continental terraces that accommodate the deltas of a few large rivers: Nile, Po, Rhone, and Ebro; extensive shelf areas
occur only in the northern half of the Adriatic, and on the eastern coast of Tunisia where
the shelf widens to about 300 km in the Gulf of Gabes. Most of the shelf is rocky or
carries quartz sand deposits with a biogenic calcareous component, and mud is restricted
to coastal embayments. Some mud occurs, of course, on either side of the Nile delta, and
here the edge of the shelf conforms to the line of the out-built delta.
The coasts of the Black Sea are steep-to only in the southern part of the basin. To the
north and west, there are extensive shelf areas that are, in general, much muddier than
those of the Mediterranean. The peninsula of the Crimea stands on a wide shelf that
reaches 200 km wide off Odessa; the edge of this shelf runs east-west, so almost directly
across the northern part of the Black Sea. The effects of the great deltaic region, and
of the effluents, of the Danube and Dniester on the western coast must be integrated
into an understanding of this sea. The Sea of Azov is a very shallow continental shelf
embayment, almost completely isolated from the main basin of the Black Sea, to the east
of the Crimean peninsula.
In both Mediterranean and Black Seas the principal currents are marginal, adjacent to
the coastline or shelf break, so that coastal and shelf-break fronts are a common feature
that isolate this flow from the slower gyral and subgyral circulations. Tidal effects, mostly
semidiurnal, are small in both the Mediterranean and Black Seas, because of the relatively
small size of each basin.
Defining Characteristics of Regional Oceanography
It will be convenient to discuss the Mediterranean and Black Seas separately because
of their very different characteristics. The Mediterranean is an evaporative basin constrained by a shallow sill at the Straits of Gibraltar, whereas in the Black Sea the salt
balance is approximately in equilibrium despite its shallow sill. The fast and continuous
surface flow from the Black Sea into the Mediterranean through the narrow passages of
the Bosphorus and Dardanelles is a unique feature in the circulation of the ocean. The
Danube contributes two-thirds of the 304 km
3 of freshwater that enters the Black Sea
annually. One of the most useful and comprehensive accounts of Black Sea ecology is
still that of Caspers, to be found in Hedgepeth (1957); this is particularly useful now as
a reference for the near-pristine state of a now highly modified ecosystem.
Mediterranean Sea It is in winter, when the zonal band of the westerlies lies farthest
equatorward, that the planetary wind field is most effective in the Mediterranean; wind
stress at the surface is strongest and most uniformly distributed from December to March.
For most of the year, although the wind-stress climatology is dominated by northwesterly
winds, the dominant effects are local and orographic. In winter there are local irruptions
of especially strong northerly winds, cold and dry, that in the western Mediterranean are
the adiabatic “mistral” winds channeled down the Rhone valley, while the similar “bora”
is not appreciated by the residents of Venice and Trieste.
In the absence of large-scale geostrophic circulation, the physical oceanography of the
Mediterranean is strongly influenced by the local wind fields (Theocaris et al., 1998). The
circulation of surface water is characterized especially in the eastern basin by a number
Chapter 9: The Atlantic Ocean
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The Mediterranean Sea comprises two deep basins, connected with the Atlantic Ocean
across a sill depth of only 290 m at the Straits of Gibraltar. The Black Sea is landlocked
except for its connection with the Mediterranean through the Bosphorus; this connection
is slender, having a shore-to-shore width of only 725 m at the choke point, and a
midchannel sill depth of only 40 m.
The Mediterranean is characterized by rather narrow continental terraces that accommodate the deltas of a few large rivers: Nile, Po, Rhone, and Ebro; extensive shelf areas
occur only in the northern half of the Adriatic, and on the eastern coast of Tunisia where
the shelf widens to about 300 km in the Gulf of Gabes. Most of the shelf is rocky or
carries quartz sand deposits with a biogenic calcareous component, and mud is restricted
to coastal embayments. Some mud occurs, of course, on either side of the Nile delta, and
here the edge of the shelf conforms to the line of the out-built delta.
The coasts of the Black Sea are steep-to only in the southern part of the basin. To the
north and west, there are extensive shelf areas that are, in general, much muddier than
those of the Mediterranean. The peninsula of the Crimea stands on a wide shelf that
reaches 200 km wide off Odessa; the edge of this shelf runs east-west, so almost directly
across the northern part of the Black Sea. The effects of the great deltaic region, and
of the effluents, of the Danube and Dniester on the western coast must be integrated
into an understanding of this sea. The Sea of Azov is a very shallow continental shelf
embayment, almost completely isolated from the main basin of the Black Sea, to the east
of the Crimean peninsula.
In both Mediterranean and Black Seas the principal currents are marginal, adjacent to
the coastline or shelf break, so that coastal and shelf-break fronts are a common feature
that isolate this flow from the slower gyral and subgyral circulations. Tidal effects, mostly
semidiurnal, are small in both the Mediterranean and Black Seas, because of the relatively
small size of each basin.
Defining Characteristics of Regional Oceanography
It will be convenient to discuss the Mediterranean and Black Seas separately because
of their very different characteristics. The Mediterranean is an evaporative basin constrained by a shallow sill at the Straits of Gibraltar, whereas in the Black Sea the salt
balance is approximately in equilibrium despite its shallow sill. The fast and continuous
surface flow from the Black Sea into the Mediterranean through the narrow passages of
the Bosphorus and Dardanelles is a unique feature in the circulation of the ocean. The
Danube contributes two-thirds of the 304 km
3 of freshwater that enters the Black Sea
annually. One of the most useful and comprehensive accounts of Black Sea ecology is
still that of Caspers, to be found in Hedgepeth (1957); this is particularly useful now as
a reference for the near-pristine state of a now highly modified ecosystem.
Mediterranean Sea It is in winter, when the zonal band of the westerlies lies farthest
equatorward, that the planetary wind field is most effective in the Mediterranean; wind
stress at the surface is strongest and most uniformly distributed from December to March.
For most of the year, although the wind-stress climatology is dominated by northwesterly
winds, the dominant effects are local and orographic. In winter there are local irruptions
of especially strong northerly winds, cold and dry, that in the western Mediterranean are
the adiabatic “mistral” winds channeled down the Rhone valley, while the similar “bora”
is not appreciated by the residents of Venice and Trieste.
In the absence of large-scale geostrophic circulation, the physical oceanography of the
Mediterranean is strongly influenced by the local wind fields (Theocaris et al., 1998). The
circulation of surface water is characterized especially in the eastern basin by a number