is where convective processes reaching depths of
more than 2000 m have been extensively documented since the Mediterranean Ocean Convective
(MEDOC) experiment (MEDOC Group, 1970).
The water mass distribution in the Western
Mediterranean comprises three layers. In the upper
layer, and originating mostly from the inflow
through the Strait of Gibraltar, is the Modified
Atlantic Water (MAW). Between 150 to 500 m
depth, a warm and salty layer is found, referred to
as Levantine Intermediate Water (LIW), which is
formed by shallow convection in the Eastern
Mediterranean Basin and then slowly spreads into
the Western Mediterranean through the Strait of
Sicily. Beneath the LIW layer, the basin is filled
with near-homogeneous Western Mediterranean
Deep Water (WMDW). It is this WMDW that is
formed by deep convection processes in the Gulf
of Lions and which contributes importantly to
the characteristic of the Mediterranean outflow
(Stommel et al., 1973; Kinder and Parrilla, 1987).
Based on a newly composed hydrographic
climatology, Krahmann (1997) estimates a
WMDW production rate in the northwestern
Mediterranean of 1.8<0.610
13 m
3 yr
91
, corresponding to 0.6<0.2 Sv. The average yearly
WMDW production is made up of 1.310
13 m
3 of
LIW and 0.510
13 m
3 of MAW. Thus, the formation rate of LIW and slight variations of its characteristics have implications on the WMDW
formation rate and its variability. Actually, it is
becoming clear from recent observations that deep
water formation is not a process that recurs every
year with certainty and regularity. The intensity of
convection shows great variability from one year to
the next and from one decade to another. For
example, in the Gulf of Lions, 1969, the year of the
first MEDOC experiment (MEDOC Group, 1970),
was a year of strong convection, but convection in
1971 was not as strong (Gascard, 1978). Vigorous
deep convection to 2200 m returned in 1987 causing
a very homogeneous water body of potential temperature 12.79°C and salinity 38.45‰ (Leaman
and Schott, 1991). Convection reached only to
1700 m in 1991 and did not mix the water column
as thoroughly (Schott et al., 1996). Fluctuations in
the composition and possibly also the volume of
the Mediterranean outflow are the result of the
variability in WMDW formation, coupled to the
LIW formation variability (Nittis and Lascaratos,
1998). Recent findings that in the last decade an
influx of Aegean Sea water has replaced 20% of
the deep waters of the Eastern Mediterranean,
which were previously only formed in the Adriatic
Sea (Roether et al., 1996), also affect such fluctuations.
Apart from these observed interannual variations in deep and intermediate water formation,
there is a well-documented increase in the salinity
of the deep waters of the Western Mediterranean
over the past 40 years (Lacombe et al., 1985;
Leaman and Schott, 1991). Observations also suggest that in the past 10 years there has been a
jump in the salinity of the newly formed deep
waters in the Eastern basin (Roether et al., 1996).
It has been argued that this salinity increase has
resulted from the diversion of the Nile and Russian rivers for irrigation so that the effective net
evaporation over the Mediterranean basin has
increased. Application of hydraulic control models
then project that the overall Mediterranean salinity will increase by about 0.13‰ over the next 100
years or so (Rohling and Bryden, 1992). However,
it remains unclear what the implications of such
an increase in salinity would be on the overall circulation of the Mediterranean and North Atlantic.
5.7.3 Outflow of Mediterranean Water at
the Strait of Gibraltar
The Strait of Gibraltar is the Mediterranean’s only
communication with the World Ocean; it is about
60 km long, 15 km wide at its narrowest section
(Tarifa narrows) and only 280 m deep at its main
sill (Fig. 5.7.3). There are three main components
to the flow (Lacombe and Richez, 1982; Candela,
1991): a tidal, mainly barotropic flow, with magnitudes of up to 2.5 m s
91 (Candela et al., 1990); a
barotropic subinertial component driven by atmospheric pressure fluctuations within the Mediterranean and with magnitudes close to 0.4 m s
91
(Candela et al., 1989); and a baroclinic subinertial
component driven by the internal pressure gradient due to the density difference between the
Mediterranean and the Atlantic Waters, with magnitudes of about 0.5 m s
91
, and likely to be
hydraulically controlled (Armi and Farmer, 1988).
Therefore, the Strait is dynamically very energetic
with tidal, subinertial and long-term currents
all being of significant amplitude. This situation
makes studying the exchange flows particularly
difficult, requiring long and careful measurements
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
422
more than 2000 m have been extensively documented since the Mediterranean Ocean Convective
(MEDOC) experiment (MEDOC Group, 1970).
The water mass distribution in the Western
Mediterranean comprises three layers. In the upper
layer, and originating mostly from the inflow
through the Strait of Gibraltar, is the Modified
Atlantic Water (MAW). Between 150 to 500 m
depth, a warm and salty layer is found, referred to
as Levantine Intermediate Water (LIW), which is
formed by shallow convection in the Eastern
Mediterranean Basin and then slowly spreads into
the Western Mediterranean through the Strait of
Sicily. Beneath the LIW layer, the basin is filled
with near-homogeneous Western Mediterranean
Deep Water (WMDW). It is this WMDW that is
formed by deep convection processes in the Gulf
of Lions and which contributes importantly to
the characteristic of the Mediterranean outflow
(Stommel et al., 1973; Kinder and Parrilla, 1987).
Based on a newly composed hydrographic
climatology, Krahmann (1997) estimates a
WMDW production rate in the northwestern
Mediterranean of 1.8<0.610
13 m
3 yr
91
, corresponding to 0.6<0.2 Sv. The average yearly
WMDW production is made up of 1.310
13 m
3 of
LIW and 0.510
13 m
3 of MAW. Thus, the formation rate of LIW and slight variations of its characteristics have implications on the WMDW
formation rate and its variability. Actually, it is
becoming clear from recent observations that deep
water formation is not a process that recurs every
year with certainty and regularity. The intensity of
convection shows great variability from one year to
the next and from one decade to another. For
example, in the Gulf of Lions, 1969, the year of the
first MEDOC experiment (MEDOC Group, 1970),
was a year of strong convection, but convection in
1971 was not as strong (Gascard, 1978). Vigorous
deep convection to 2200 m returned in 1987 causing
a very homogeneous water body of potential temperature 12.79°C and salinity 38.45‰ (Leaman
and Schott, 1991). Convection reached only to
1700 m in 1991 and did not mix the water column
as thoroughly (Schott et al., 1996). Fluctuations in
the composition and possibly also the volume of
the Mediterranean outflow are the result of the
variability in WMDW formation, coupled to the
LIW formation variability (Nittis and Lascaratos,
1998). Recent findings that in the last decade an
influx of Aegean Sea water has replaced 20% of
the deep waters of the Eastern Mediterranean,
which were previously only formed in the Adriatic
Sea (Roether et al., 1996), also affect such fluctuations.
Apart from these observed interannual variations in deep and intermediate water formation,
there is a well-documented increase in the salinity
of the deep waters of the Western Mediterranean
over the past 40 years (Lacombe et al., 1985;
Leaman and Schott, 1991). Observations also suggest that in the past 10 years there has been a
jump in the salinity of the newly formed deep
waters in the Eastern basin (Roether et al., 1996).
It has been argued that this salinity increase has
resulted from the diversion of the Nile and Russian rivers for irrigation so that the effective net
evaporation over the Mediterranean basin has
increased. Application of hydraulic control models
then project that the overall Mediterranean salinity will increase by about 0.13‰ over the next 100
years or so (Rohling and Bryden, 1992). However,
it remains unclear what the implications of such
an increase in salinity would be on the overall circulation of the Mediterranean and North Atlantic.
5.7.3 Outflow of Mediterranean Water at
the Strait of Gibraltar
The Strait of Gibraltar is the Mediterranean’s only
communication with the World Ocean; it is about
60 km long, 15 km wide at its narrowest section
(Tarifa narrows) and only 280 m deep at its main
sill (Fig. 5.7.3). There are three main components
to the flow (Lacombe and Richez, 1982; Candela,
1991): a tidal, mainly barotropic flow, with magnitudes of up to 2.5 m s
91 (Candela et al., 1990); a
barotropic subinertial component driven by atmospheric pressure fluctuations within the Mediterranean and with magnitudes close to 0.4 m s
91
(Candela et al., 1989); and a baroclinic subinertial
component driven by the internal pressure gradient due to the density difference between the
Mediterranean and the Atlantic Waters, with magnitudes of about 0.5 m s
91
, and likely to be
hydraulically controlled (Armi and Farmer, 1988).
Therefore, the Strait is dynamically very energetic
with tidal, subinertial and long-term currents
all being of significant amplitude. This situation
makes studying the exchange flows particularly
difficult, requiring long and careful measurements
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
422