towards Iceland and the rest turning with the
westward limbs of the large cyclonic and anticyclonic gyres. The northward part carries warm and
saline water that mixes with the Norwegian–
Greenland Sea Waters in the passages east and
west of Iceland. The westward part crosses the
Atlantic, and as it approaches the western boundary near 25–35°N it turns both northward, forming the deepest part of the Gulf Stream above
about 3000 m, and southward, joining the southward flow along the deep western boundary and
the eastward limb of the cyclonic gyre. As a result
of these gyral flows, the added heat and salt from
the Mediterranean outflow are spread both northward by the cyclonic gyre and southward along
the western boundary into the South Atlantic
Ocean, where its contribution makes the southward
flow warmer and more saline than the incoming
circumpolar waters to the east (Reid, 1994). However, recent analysis based on a new North
Atlantic climatology (Iorga and Lozier, 1999a,b)
does not support the presence of a westward current of Mediterranean Water that crosses the
North Atlantic. Instead, Iorga and Lozier’s inverse
model exhibits a westward current at ϳ35°N,
which derives its waters from the poleward, eastern boundary current off Africa rather than from
the Mediterranean outflow. This would imply that
the main mechanism for the westward propagation of the Mediterranean outflow is ‘diffusion’ by
meddies rather than a well-defined current. With
respect to the northward flow as an eastern
boundary current, their work does give evidence
for the presence of the northward branch of
Mediterranean Water penetrating past 55°N on its
way to the Nordic Seas. However, recent work by
McCartney and Mauritzen (2001) based on
regional water mass distributions and geostrophic
shear rejects Reid’s deep source hypothesis in
favour of a shallow source hypothesis. Rather than
a flow of deep Mediterranean Overflow Water
along the eastern boundary rising from depth to
feed the Nordic Seas, the inflow is supplied
directly by transformed North Atlantic Current
waters from the same depth range as the inflow.
This shallow source hypothesis is also supported
by the recent high-resolution ocean circulation
modelling work of New et al. (2000a), which proposes shallow sources composed of water masses
of western origin carried by branches of the North
Atlantic Current and the more saline Eastern
North Atlantic Water transported northwards
from the Bay of Biscay region in a ‘Shelf Edge Current’ around the continental margin. Clearly more
work is needed to identify if any of the proposed
sources of saline waters to the Nordic Seas is
exclusive of the others, or more likely the main
contributor during certain periods. In any event
what is well recognized is that the warm and saline
deep water flowing out of the North Atlantic
extends throughout the Antarctic, Indian and
Pacific Oceans, identified as a salinity maximum
that lies above the bottom over most of the southern hemisphere and projects back into the Atlantic
Ocean through the Drake Passage. The salinity
decreases along the path of flow but it remains
high enough where the saline layer shallows in the
Weddell Sea to contribute to the formation of the
densest abyssal water of the world’s open ocean
(Reid, 1994).
A possible line of research to investigate the
implication of the Mediterranean outflow on the
North Atlantic circulation is to use numerical simulations, though they must be validated through a
systematic comparison of their output with actual
observations. Recent modelling efforts have been
quite successful at reproducing the observed structure and have helped us to understand the dynamics governing the behaviour of the Mediterranean
salt tongue in the North Atlantic (Spall, 1999;
Stephens and Marshall, 1999). However, it seems
essential that the models should not only simulate
the mean observed features, but they should also
reproduce the observed transient behaviour of the
system. For climate change studies the models
should exhibit interannual and longer-term
variations of the same nature and magnitude as
are observed. Models useful for predicting
climate changes resulting from an increase in salinity of the Mediterranean outflow should capture
or parameterize accurately critical small-scale
processes, such as the pathways of the descending
outflow plume after it exits the Strait of Gibraltar
and its mixing. This poses a particular challenge
for numerical algorithms because the strong current is of such small scale and involves mixing
processes that occur on even smaller scales. Hence,
it is of outmost importance to establish some degree
of confidence in the models’ ability to represent
the real outflow characteristics by a thorough validation based on comparing the model simulations
with real data. Before model error estimates are
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
428
westward limbs of the large cyclonic and anticyclonic gyres. The northward part carries warm and
saline water that mixes with the Norwegian–
Greenland Sea Waters in the passages east and
west of Iceland. The westward part crosses the
Atlantic, and as it approaches the western boundary near 25–35°N it turns both northward, forming the deepest part of the Gulf Stream above
about 3000 m, and southward, joining the southward flow along the deep western boundary and
the eastward limb of the cyclonic gyre. As a result
of these gyral flows, the added heat and salt from
the Mediterranean outflow are spread both northward by the cyclonic gyre and southward along
the western boundary into the South Atlantic
Ocean, where its contribution makes the southward
flow warmer and more saline than the incoming
circumpolar waters to the east (Reid, 1994). However, recent analysis based on a new North
Atlantic climatology (Iorga and Lozier, 1999a,b)
does not support the presence of a westward current of Mediterranean Water that crosses the
North Atlantic. Instead, Iorga and Lozier’s inverse
model exhibits a westward current at ϳ35°N,
which derives its waters from the poleward, eastern boundary current off Africa rather than from
the Mediterranean outflow. This would imply that
the main mechanism for the westward propagation of the Mediterranean outflow is ‘diffusion’ by
meddies rather than a well-defined current. With
respect to the northward flow as an eastern
boundary current, their work does give evidence
for the presence of the northward branch of
Mediterranean Water penetrating past 55°N on its
way to the Nordic Seas. However, recent work by
McCartney and Mauritzen (2001) based on
regional water mass distributions and geostrophic
shear rejects Reid’s deep source hypothesis in
favour of a shallow source hypothesis. Rather than
a flow of deep Mediterranean Overflow Water
along the eastern boundary rising from depth to
feed the Nordic Seas, the inflow is supplied
directly by transformed North Atlantic Current
waters from the same depth range as the inflow.
This shallow source hypothesis is also supported
by the recent high-resolution ocean circulation
modelling work of New et al. (2000a), which proposes shallow sources composed of water masses
of western origin carried by branches of the North
Atlantic Current and the more saline Eastern
North Atlantic Water transported northwards
from the Bay of Biscay region in a ‘Shelf Edge Current’ around the continental margin. Clearly more
work is needed to identify if any of the proposed
sources of saline waters to the Nordic Seas is
exclusive of the others, or more likely the main
contributor during certain periods. In any event
what is well recognized is that the warm and saline
deep water flowing out of the North Atlantic
extends throughout the Antarctic, Indian and
Pacific Oceans, identified as a salinity maximum
that lies above the bottom over most of the southern hemisphere and projects back into the Atlantic
Ocean through the Drake Passage. The salinity
decreases along the path of flow but it remains
high enough where the saline layer shallows in the
Weddell Sea to contribute to the formation of the
densest abyssal water of the world’s open ocean
(Reid, 1994).
A possible line of research to investigate the
implication of the Mediterranean outflow on the
North Atlantic circulation is to use numerical simulations, though they must be validated through a
systematic comparison of their output with actual
observations. Recent modelling efforts have been
quite successful at reproducing the observed structure and have helped us to understand the dynamics governing the behaviour of the Mediterranean
salt tongue in the North Atlantic (Spall, 1999;
Stephens and Marshall, 1999). However, it seems
essential that the models should not only simulate
the mean observed features, but they should also
reproduce the observed transient behaviour of the
system. For climate change studies the models
should exhibit interannual and longer-term
variations of the same nature and magnitude as
are observed. Models useful for predicting
climate changes resulting from an increase in salinity of the Mediterranean outflow should capture
or parameterize accurately critical small-scale
processes, such as the pathways of the descending
outflow plume after it exits the Strait of Gibraltar
and its mixing. This poses a particular challenge
for numerical algorithms because the strong current is of such small scale and involves mixing
processes that occur on even smaller scales. Hence,
it is of outmost importance to establish some degree
of confidence in the models’ ability to represent
the real outflow characteristics by a thorough validation based on comparing the model simulations
with real data. Before model error estimates are
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
428