48
Martin V. ANGEL
the equator. As a result, in the Arabian Sea and to a
lesser extent in the Bay of Bengal, surface conditions
oscillate between being highly productive during the
south-west monsoon, when off-shore winds result in
coastal upwelling, and highly oligotrophic during the
north-east monsoon, when normal subtropical/tropical
conditions prevail. In the North-west Arabian Sea, this
creates a unique cycle in deep-sea conditions, in which
oxygen concentrations in subthermocline waters (down
to depths of 1000 m) oscillate between being almost
totally depleted towards the end of the south-west
monsoon, and ranging between 2 and 3 ml °
−1 during
the north-east monsoon.
The Arctic Ocean is the only truly polar ocean,
and is also a Mediterranean-type sea, being almost
entirely enclosed by land (see Chapter 8). Nearly
half of its total area of 14.06×10
6 km
2 consists of
broad areas of shallow continental-shelf seas which
are particularly extensive to the north of the Eurasian
continental landmass. Its greatest depth of 4400 m
occurs in the Fram Basin not far from the geographical
North Pole. It has a narrow and shallow connection
with the North Pacific via the Bering Strait, through
which there are only limited exchanges of water. In
contrast its connection with the North Atlantic is
broad and deep, allowing much freer exchanges of
water, which greatly influence the hydrography of
both oceans. The inflow of relatively warm Atlantic
Water (see above) pushes the southern boundary of the
winter pack-ice far to the north. This inflow is largely
balanced by the outflow of the East Greenland Current,
which carries cold water, pack-ice and icebergs far
to the south. Much of the Arctic Ocean remains
covered throughout the year with multi-year (up to
5 years old) pack-ice, which ranges in thickness
from 1.5 to 4 m. Recent evidence indicates that the
Arctic pack-ice is thinning at a rate which will
result in extensive areas becoming ice-free within
a few decades. Normally during summer the area
covered by pack-ice shrinks, but only by about 10%.
Voluminous outflows of fresh water from the great
Russian rivers create a stable haline stratification,
which keeps productivity relatively low throughout
much of the Arctic.
The Southern Ocean is very different. It is bounded
polewards by the continent of Antarctica, so that a
relatively small area lies to the south of the Antarctic
Circle (see Chapter 8). To the north it is open to
the other major oceans, and geographically cannot
be readily separated from them. Hydrographically,
its northern limit is the Antarctic Convergence (e.g.,
Foster, 1984). This is the major oceanic front at
which Antarctic Intermediate Water forms and sinks
as it spreads equatorwards, at its quasiequilibrium
depth of 1000 m below the subtropical water mass.
However, the exact position of the Convergence is not
geographically restrained, and varies both seasonally
and interannually (e.g., Peterson and Stramma, 1991).
The major feature of the circulation in the Southern
Ocean is the circumpolar current of the West Wind
Drift. This current developed about 35 million years
ago when circumpolar deep-water connections first
opened up as the Antarctic and Australasian continents
separated. The Antarctic Ice Cap developed only some
14 million years ago, but since then it has had
a major influence on the development of Southern
Ocean fauna, keeping the species sufficiently isolated
for a high degree of endemicity to have evolved
(Clarke, 1996). The extreme conditions have led
to the evolution of some special adaptations. For
example, some fishes have developed an antifreeze
in their blood and tissues, without which their blood
would turn to ice at temperatures below −1.5ºC.
The establishment of the circumpolar current appears
to have initiated a gradual cooling of the bottom
waters throughout the global ocean from about 10ºC
(during the Cretaceous) to the present 2º to 3ºC.
The flow of the circumpolar current is constrained
by the narrowness of the Drake Passage between the
tip of South America and the Antarctic Peninsula,
which has a profound influence on the general oceanic
circulation.
The extent of pack-ice in the Southern Ocean
fluctuates annually from about 20 million km
2 in the
austral winter to 5 million km
2 in summer (Gloersen
et al., 1992). Thus, in contrast to the Arctic, the areas
of multi-year pack-ice are limited, and mostly occur
in the Weddell Sea. It is in the Weddell Sea that
bottom-water formation is most active. Large tabular
icebergs are spawned from the broad ice-shelves of the
Ross and Weddell Seas, where the continental shelf is
unusually deep at 400–500 m, depressed isostatically by
the weight of the ice.
Surface waters of the Southern Ocean to the
south of the Convergence are always rich in plant
nutrients (Levitus et al., 1993). The current theory
to explain why the phytoplankton is never able to
exhaust the supplies of nitrates (as occurs in the North
Atlantic), is that there is insufficient iron available for
the phytoplankton to achieve maximum growth rates
Martin V. ANGEL
the equator. As a result, in the Arabian Sea and to a
lesser extent in the Bay of Bengal, surface conditions
oscillate between being highly productive during the
south-west monsoon, when off-shore winds result in
coastal upwelling, and highly oligotrophic during the
north-east monsoon, when normal subtropical/tropical
conditions prevail. In the North-west Arabian Sea, this
creates a unique cycle in deep-sea conditions, in which
oxygen concentrations in subthermocline waters (down
to depths of 1000 m) oscillate between being almost
totally depleted towards the end of the south-west
monsoon, and ranging between 2 and 3 ml °
−1 during
the north-east monsoon.
The Arctic Ocean is the only truly polar ocean,
and is also a Mediterranean-type sea, being almost
entirely enclosed by land (see Chapter 8). Nearly
half of its total area of 14.06×10
6 km
2 consists of
broad areas of shallow continental-shelf seas which
are particularly extensive to the north of the Eurasian
continental landmass. Its greatest depth of 4400 m
occurs in the Fram Basin not far from the geographical
North Pole. It has a narrow and shallow connection
with the North Pacific via the Bering Strait, through
which there are only limited exchanges of water. In
contrast its connection with the North Atlantic is
broad and deep, allowing much freer exchanges of
water, which greatly influence the hydrography of
both oceans. The inflow of relatively warm Atlantic
Water (see above) pushes the southern boundary of the
winter pack-ice far to the north. This inflow is largely
balanced by the outflow of the East Greenland Current,
which carries cold water, pack-ice and icebergs far
to the south. Much of the Arctic Ocean remains
covered throughout the year with multi-year (up to
5 years old) pack-ice, which ranges in thickness
from 1.5 to 4 m. Recent evidence indicates that the
Arctic pack-ice is thinning at a rate which will
result in extensive areas becoming ice-free within
a few decades. Normally during summer the area
covered by pack-ice shrinks, but only by about 10%.
Voluminous outflows of fresh water from the great
Russian rivers create a stable haline stratification,
which keeps productivity relatively low throughout
much of the Arctic.
The Southern Ocean is very different. It is bounded
polewards by the continent of Antarctica, so that a
relatively small area lies to the south of the Antarctic
Circle (see Chapter 8). To the north it is open to
the other major oceans, and geographically cannot
be readily separated from them. Hydrographically,
its northern limit is the Antarctic Convergence (e.g.,
Foster, 1984). This is the major oceanic front at
which Antarctic Intermediate Water forms and sinks
as it spreads equatorwards, at its quasiequilibrium
depth of 1000 m below the subtropical water mass.
However, the exact position of the Convergence is not
geographically restrained, and varies both seasonally
and interannually (e.g., Peterson and Stramma, 1991).
The major feature of the circulation in the Southern
Ocean is the circumpolar current of the West Wind
Drift. This current developed about 35 million years
ago when circumpolar deep-water connections first
opened up as the Antarctic and Australasian continents
separated. The Antarctic Ice Cap developed only some
14 million years ago, but since then it has had
a major influence on the development of Southern
Ocean fauna, keeping the species sufficiently isolated
for a high degree of endemicity to have evolved
(Clarke, 1996). The extreme conditions have led
to the evolution of some special adaptations. For
example, some fishes have developed an antifreeze
in their blood and tissues, without which their blood
would turn to ice at temperatures below −1.5ºC.
The establishment of the circumpolar current appears
to have initiated a gradual cooling of the bottom
waters throughout the global ocean from about 10ºC
(during the Cretaceous) to the present 2º to 3ºC.
The flow of the circumpolar current is constrained
by the narrowness of the Drake Passage between the
tip of South America and the Antarctic Peninsula,
which has a profound influence on the general oceanic
circulation.
The extent of pack-ice in the Southern Ocean
fluctuates annually from about 20 million km
2 in the
austral winter to 5 million km
2 in summer (Gloersen
et al., 1992). Thus, in contrast to the Arctic, the areas
of multi-year pack-ice are limited, and mostly occur
in the Weddell Sea. It is in the Weddell Sea that
bottom-water formation is most active. Large tabular
icebergs are spawned from the broad ice-shelves of the
Ross and Weddell Seas, where the continental shelf is
unusually deep at 400–500 m, depressed isostatically by
the weight of the ice.
Surface waters of the Southern Ocean to the
south of the Convergence are always rich in plant
nutrients (Levitus et al., 1993). The current theory
to explain why the phytoplankton is never able to
exhaust the supplies of nitrates (as occurs in the North
Atlantic), is that there is insufficient iron available for
the phytoplankton to achieve maximum growth rates