in February 1995 to 1.07 Sv in March 1996. This
increase of close to 0.1 Sv from 1995 to 1996 is
significant and could very well have implications
on the effects of the outflow in the water structure of the North Atlantic. Close monitoring of the
Gibraltar Exchange will certainly render valuable
information for climate studies of the Mediterranean Sea and the Global Circulation.
5.7.4 The effect of Mediterranean Water
outflow on the circulation of the North
Atlantic and the World Oceans
The Mediterranean Water exits the Strait of
Gibraltar as a single, dense plume that flows down
the northern continental slope of the Gulf of Cadiz
as a gravity-driven boundary current over complicated topography. As the outflow spreads northwestward along the southern Spanish and
Portuguese coasts, it slowly loses its high salinity
as it mixes with fresh North Atlantic Central
Water (Baringer and Price, 1997a; Bower et al.,
1997). By the time the flow reaches the vicinity of
Cape St Vincent it is neutrally buoyant (Ochoa and
Bray, 1991; Zenk and Armi, 1990). From this
point, part of the outflow is trapped along the continental slope flowing northward, while another
part flows westward into the ocean interior (Reid,
1994; Iorga and Lozier, 1999a). It is known that a
large portion of this westward branch is implicated in the formation of sub-mesoscale coherent
vortices, Meddies, that contain a core of warm
and salty Mediterranean Water (Käse and Zenk,
1996; Bower et al., 1997). Meddies are typically
20–100 km in diameter and 200–1000 m thick,
centred about 1000 m depth. Richardson et al.
(1989) estimated that 8–12 Meddies form each
year based on estimates of the number of coexisting Meddies and their average lifetime (2–3 years),
and that they may transport about 25% of the
salinity anomaly flux that comes through the Strait
of Gibraltar. Arhan et al. (1994) suggested that
Meddies may be responsible for more than 50% of
the zonal salinity flux at the level of the Mediterranean Water in the North Atlantic, based on
simultaneous observations of three Meddies along
a hydrographic section at 15°W, and previous
work showing that some Meddies drift westward at a speed five times the background flow.
Recently, Bower et al. (1997) have estimated a
Meddy formation rate of 15–20 Meddies per year
based on observations of RAFOS (reverse of
SOund Fixing And Ranging) floats seeded in the
Gibraltar outflow in the Gulf of Cadiz. This
Meddy production rate supports the idea that a
large portion of the observed zonal salinity anomaly flux, about 50%, is directly related to Meddies. With regard to the observational basis for the
pathways of the Mediterranean outflow, we do
not yet know how much of the outflow proceeds
north along the Iberian continental slope and how
much flows or circulates slowly westward from
the Strait. We cannot yet estimate with certainty
the outflow advected as Meddies versus how much
is advected in concentrated boundary currents, just
as we are still unclear on the mixing mechanisms
that erode the high-salinity core of the outflow
(Bryden and Webb, 1998). Quantifying and understanding these details are at the heart of being
able to answer with certainty the role played by
the Mediterranean outflow on the North Atlantic
circulation.
The outflow of the Mediterranean is only about
1 Sv. This is a relatively small transport compared
with those found in the North Atlantic, but its
salinity of 38.4‰ and temperature of 13°C are
extremely high compared with any other waters in
that depth range. The effect of these large contrasts in water mass properties is two-fold: first,
they help identify both a northward flow along the
eastern boundary to the Greenland–Scotland sill
and a westward flow across the Atlantic that turns
southward along the western boundary, reaching the Antarctic Circumpolar Current and the
Weddell Sea. Second, even at those distant places
where this water shallows or outcrops, such as
near Iceland and in the Weddell Sea, it retains
salinities high enough to form, when cooled
enough, the densest waters of the northern North
Atlantic and the Weddell Sea (Reid, 1994). Also,
the Mediterranean outflow is the source of the
mid-depth changes of heat and salt to the waters
entering the North Atlantic. As the Mediterranean
outflow pours down from the Strait of Gibraltar it
joins the subsurface waters flowing northward
along the eastern boundary. These northward
flows include waters from the South Atlantic, and
before they interact with the outflow they are relatively low in temperature and salinity. After the
encounter, these flows carry the outflow water
northward to about 40°N and then divide, a
portion continuing northward along the boundary
5.7 Mediterranean Water and Global Circulation
427
Candela
increase of close to 0.1 Sv from 1995 to 1996 is
significant and could very well have implications
on the effects of the outflow in the water structure of the North Atlantic. Close monitoring of the
Gibraltar Exchange will certainly render valuable
information for climate studies of the Mediterranean Sea and the Global Circulation.
5.7.4 The effect of Mediterranean Water
outflow on the circulation of the North
Atlantic and the World Oceans
The Mediterranean Water exits the Strait of
Gibraltar as a single, dense plume that flows down
the northern continental slope of the Gulf of Cadiz
as a gravity-driven boundary current over complicated topography. As the outflow spreads northwestward along the southern Spanish and
Portuguese coasts, it slowly loses its high salinity
as it mixes with fresh North Atlantic Central
Water (Baringer and Price, 1997a; Bower et al.,
1997). By the time the flow reaches the vicinity of
Cape St Vincent it is neutrally buoyant (Ochoa and
Bray, 1991; Zenk and Armi, 1990). From this
point, part of the outflow is trapped along the continental slope flowing northward, while another
part flows westward into the ocean interior (Reid,
1994; Iorga and Lozier, 1999a). It is known that a
large portion of this westward branch is implicated in the formation of sub-mesoscale coherent
vortices, Meddies, that contain a core of warm
and salty Mediterranean Water (Käse and Zenk,
1996; Bower et al., 1997). Meddies are typically
20–100 km in diameter and 200–1000 m thick,
centred about 1000 m depth. Richardson et al.
(1989) estimated that 8–12 Meddies form each
year based on estimates of the number of coexisting Meddies and their average lifetime (2–3 years),
and that they may transport about 25% of the
salinity anomaly flux that comes through the Strait
of Gibraltar. Arhan et al. (1994) suggested that
Meddies may be responsible for more than 50% of
the zonal salinity flux at the level of the Mediterranean Water in the North Atlantic, based on
simultaneous observations of three Meddies along
a hydrographic section at 15°W, and previous
work showing that some Meddies drift westward at a speed five times the background flow.
Recently, Bower et al. (1997) have estimated a
Meddy formation rate of 15–20 Meddies per year
based on observations of RAFOS (reverse of
SOund Fixing And Ranging) floats seeded in the
Gibraltar outflow in the Gulf of Cadiz. This
Meddy production rate supports the idea that a
large portion of the observed zonal salinity anomaly flux, about 50%, is directly related to Meddies. With regard to the observational basis for the
pathways of the Mediterranean outflow, we do
not yet know how much of the outflow proceeds
north along the Iberian continental slope and how
much flows or circulates slowly westward from
the Strait. We cannot yet estimate with certainty
the outflow advected as Meddies versus how much
is advected in concentrated boundary currents, just
as we are still unclear on the mixing mechanisms
that erode the high-salinity core of the outflow
(Bryden and Webb, 1998). Quantifying and understanding these details are at the heart of being
able to answer with certainty the role played by
the Mediterranean outflow on the North Atlantic
circulation.
The outflow of the Mediterranean is only about
1 Sv. This is a relatively small transport compared
with those found in the North Atlantic, but its
salinity of 38.4‰ and temperature of 13°C are
extremely high compared with any other waters in
that depth range. The effect of these large contrasts in water mass properties is two-fold: first,
they help identify both a northward flow along the
eastern boundary to the Greenland–Scotland sill
and a westward flow across the Atlantic that turns
southward along the western boundary, reaching the Antarctic Circumpolar Current and the
Weddell Sea. Second, even at those distant places
where this water shallows or outcrops, such as
near Iceland and in the Weddell Sea, it retains
salinities high enough to form, when cooled
enough, the densest waters of the northern North
Atlantic and the Weddell Sea (Reid, 1994). Also,
the Mediterranean outflow is the source of the
mid-depth changes of heat and salt to the waters
entering the North Atlantic. As the Mediterranean
outflow pours down from the Strait of Gibraltar it
joins the subsurface waters flowing northward
along the eastern boundary. These northward
flows include waters from the South Atlantic, and
before they interact with the outflow they are relatively low in temperature and salinity. After the
encounter, these flows carry the outflow water
northward to about 40°N and then divide, a
portion continuing northward along the boundary
5.7 Mediterranean Water and Global Circulation
427
Candela