5.7.1 Marginal seas
Marginal seas bordering the world’s oceans make
important contributions to the global thermohaline
ocean circulation through their exchange of water
with the major oceanic basins. Their principal
effect is related to processes of water mass formation that occur within these bordering seas where
heat and/or net freshwater losses are intensified
due to their confined nature. It is well known that
some of the intermediate and all of the deep water
masses of the world’s oceans are produced as the
result of intense air–sea exchange in marginal seas
(Warren, 1981a). The water masses formed in the
marginal sea typically enter the open ocean as a
dense outflow through a restricted channel or
strait. One of the best known outflows is that from
the Mediterranean Sea, which enters the North
Atlantic through the Strait of Gibraltar as the lower
layer of a two-way exchange flow (Bryden et al.,
1994; Bower et al., 1997). After rounding Cape
St Vincent, at the southwestern corner of the Iberian
Peninsula, the outflow water begins to spread into
the eastern North Atlantic to form a warm and
salty tongue that extends westward from Portugal
across the eastern Atlantic (Wüst and Defant,
1936; Worthington, 1976; Lozier et al., 1995).
This tongue is one of the most prominent features
of the North Atlantic hydrography at intermediate
depths, and its high-salinity water has been implicated in the preconditioning of the North Atlantic
Deep Water formation (Reid, 1978).
This chapter deals with the Mediterranean Sea
and its influence on the world’s ocean thermohaline circulation and therefore on climate. It
describes water formation processes within the
Mediterranean Sea, outlining the characteristics of
well-known water formation sites and the evidence
of recent shifts in water formation locations within
the sea. Then it focuses on the characteristics of
the exchange flows between the Mediterranean
and the North Atlantic Ocean through the Strait
of Gibraltar based on historical as well as recent
(1994–96) flow measurements that reveal significant seasonal and interannual variability of the
exchange flows, and in particular of the Mediterranean outflow into the North Atlantic. This outflow is then the subject of a detailed description
since it determines the way the Mediterranean
Water mixes with the surrounding waters in the
North Atlantic, creating a warm and saline tongue
of water that can be identified throughout the
whole North Atlantic at a depth of about 1100 m
(Fig. 5.7.1). It is fundamental for numerical
models investigating the influence of the Mediterranean Water on the thermohaline circulation of
the World Ocean to reproduce or parameterize the
mixing processes in the outflow accurately, since
they determine the properties and the depth of
penetration of the Mediterranean Water in the
North Atlantic. The chapter ends with a discussion
of observational hydrographic evidence, mainly
put forth in Reid’s (1994) study, which supports
the hypothesis that the Mediterranean contribution to the Nordic and Labrador Seas allows the
formation of deep waters in the North Atlantic, or
at least that without this contribution the waters
formed would not be dense enough to penetrate to
the depths they presently do.
5.7
Mediterranean Water and Global Circulation
Julio Candela
419
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
Marginal seas bordering the world’s oceans make
important contributions to the global thermohaline
ocean circulation through their exchange of water
with the major oceanic basins. Their principal
effect is related to processes of water mass formation that occur within these bordering seas where
heat and/or net freshwater losses are intensified
due to their confined nature. It is well known that
some of the intermediate and all of the deep water
masses of the world’s oceans are produced as the
result of intense air–sea exchange in marginal seas
(Warren, 1981a). The water masses formed in the
marginal sea typically enter the open ocean as a
dense outflow through a restricted channel or
strait. One of the best known outflows is that from
the Mediterranean Sea, which enters the North
Atlantic through the Strait of Gibraltar as the lower
layer of a two-way exchange flow (Bryden et al.,
1994; Bower et al., 1997). After rounding Cape
St Vincent, at the southwestern corner of the Iberian
Peninsula, the outflow water begins to spread into
the eastern North Atlantic to form a warm and
salty tongue that extends westward from Portugal
across the eastern Atlantic (Wüst and Defant,
1936; Worthington, 1976; Lozier et al., 1995).
This tongue is one of the most prominent features
of the North Atlantic hydrography at intermediate
depths, and its high-salinity water has been implicated in the preconditioning of the North Atlantic
Deep Water formation (Reid, 1978).
This chapter deals with the Mediterranean Sea
and its influence on the world’s ocean thermohaline circulation and therefore on climate. It
describes water formation processes within the
Mediterranean Sea, outlining the characteristics of
well-known water formation sites and the evidence
of recent shifts in water formation locations within
the sea. Then it focuses on the characteristics of
the exchange flows between the Mediterranean
and the North Atlantic Ocean through the Strait
of Gibraltar based on historical as well as recent
(1994–96) flow measurements that reveal significant seasonal and interannual variability of the
exchange flows, and in particular of the Mediterranean outflow into the North Atlantic. This outflow is then the subject of a detailed description
since it determines the way the Mediterranean
Water mixes with the surrounding waters in the
North Atlantic, creating a warm and saline tongue
of water that can be identified throughout the
whole North Atlantic at a depth of about 1100 m
(Fig. 5.7.1). It is fundamental for numerical
models investigating the influence of the Mediterranean Water on the thermohaline circulation of
the World Ocean to reproduce or parameterize the
mixing processes in the outflow accurately, since
they determine the properties and the depth of
penetration of the Mediterranean Water in the
North Atlantic. The chapter ends with a discussion
of observational hydrographic evidence, mainly
put forth in Reid’s (1994) study, which supports
the hypothesis that the Mediterranean contribution to the Nordic and Labrador Seas allows the
formation of deep waters in the North Atlantic, or
at least that without this contribution the waters
formed would not be dense enough to penetrate to
the depths they presently do.
5.7
Mediterranean Water and Global Circulation
Julio Candela
419
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER