48
B. Manca et aI.
the Adriatic Sea. This paper refers mainly to the
overall seasonal variability of the dynamics of
the Adriatic Sea, focussing on the influence of the
fresh water input and dense water formation on
the general circulation patterns. The coastal currents and interior jets connecting the sub-basin
gyres, and the water exchange regime between
the northern, middle and southern basins are
shown, and compared using direct current measurements.
Measurements
Four multidisciplinary surveys were made every
three months (May, August, November 1995, and
February 1996) at stations located in four areas
(Fig. 1), each of them traversed by three parallel
transects: in the northern shallow zone (the
Rimini transects), in the middle Adriatic (the
Giulianova transects), over the steep continental
slope of the southern basin (the Gargano transects), and at the southern opening to the Ionian
Sea (the Otranto transects). Temperature and
salinity profiles were collected with a Seabird
SBE-911 plus CTD system coupled with a 24-botde General Oceanics rosette sampler. Water samples were analysed and SIS digital reversing thermometer readings were taken to provide data for
the calibration of salinity and temperature; samples were also collected for biogeochemical
determinations. Continuous Eulerian current
measurements were conducted by means of current meter moorings deployed along the same
sections.
Results and Discussion
Seasonal Thermal, Salinity and Density Cycles
The seasonal thermal cycle in the northern and
middle Adriatic Sea is shown by the space-averaged vertical profiles of temperature computed
separately for the Rimini and the Giulianova
transects (Fig. 2), In winter, the most distinguishing feature is the noticeable decrease of
temperature at the surface due to the maximum
heat loss of the seasonally cold waters. The cooling effect involves the entire water column in
the northern shelf area and reaches a depth of
80-100 m in the central basin. A vertically
homogeneous cold and dense water mass,
namely the NADW, is formed in the northern
shelf area (° 0 > 29.40 kg·m- 3 ) by turbulent convective movements that reduce the vertical density gradients. A further, progressive decrease of
temperature in the bottom layer of the central
basin occurs in spring. This is due to the advection of the cold NADW which, formed during
the previous winter, reaches the Giulianova sections later on.
In spring and summer, the upper thermocline becomes more pronounced. reaching a
maximum depth of 50 m. The NADW is topped
by the warm surface layer and is entrained
southwards by the western coastal current. The
NADW has been identified by Manca and
Giorgetti (1998) as a sub-surface cold core water
« 13 0c) flowing close to the western shelf in the
southern basin throughout the year.
In the southern Adriatic Sea, the layer below
the seasonal thermocline is subjected to a thermal variability (not shown) mosdy related to the
lateral advection of the NADW and to the inflow
of the Mediterranean water. However, the relatively fresh water from the northern basin flows
along the Italian coast leaving the open-sea
waters exposed to buoyancy losses during winter. An overturning of the water column occurred
during winter 1996 down to 600 m. This horizon
was marked by the isopycnal 29.18 kg·m- 3
(Manca and Bregant 1998).
The seasonal salinity and density distributions along the Rimini transect are shown in
Figs. 3 and 4, respectively. The salinity field consistently shows two-layer haline stratification
with important variations typified by the freshwater river discharges occurring throughout the
year. The major Po river discharge with repetitive impulses of about 4000 m 3 ·s· 1 occurs from
15 April to 15 July 1995. affecting the corresponding spring and summer surveys. The
salinity front separating the coastal water from
the open-sea water extends from the sea surface
to the western shelf and further, in spring (Fig.
3a). The fresh water flows southwards in a narrow coastal boundary region of about 30-40 km
up to the southernmost section at the Otranto
Strait. The return flow is constituted by the
saline core water at the interior of the basin that
presents a vertical homogeneity right down to
the bottom. In the northern basin, preliminary
evidence seems to indicate that the presence of
less saline water further to the east may very
likely be associated with meandering current
B. Manca et aI.
the Adriatic Sea. This paper refers mainly to the
overall seasonal variability of the dynamics of
the Adriatic Sea, focussing on the influence of the
fresh water input and dense water formation on
the general circulation patterns. The coastal currents and interior jets connecting the sub-basin
gyres, and the water exchange regime between
the northern, middle and southern basins are
shown, and compared using direct current measurements.
Measurements
Four multidisciplinary surveys were made every
three months (May, August, November 1995, and
February 1996) at stations located in four areas
(Fig. 1), each of them traversed by three parallel
transects: in the northern shallow zone (the
Rimini transects), in the middle Adriatic (the
Giulianova transects), over the steep continental
slope of the southern basin (the Gargano transects), and at the southern opening to the Ionian
Sea (the Otranto transects). Temperature and
salinity profiles were collected with a Seabird
SBE-911 plus CTD system coupled with a 24-botde General Oceanics rosette sampler. Water samples were analysed and SIS digital reversing thermometer readings were taken to provide data for
the calibration of salinity and temperature; samples were also collected for biogeochemical
determinations. Continuous Eulerian current
measurements were conducted by means of current meter moorings deployed along the same
sections.
Results and Discussion
Seasonal Thermal, Salinity and Density Cycles
The seasonal thermal cycle in the northern and
middle Adriatic Sea is shown by the space-averaged vertical profiles of temperature computed
separately for the Rimini and the Giulianova
transects (Fig. 2), In winter, the most distinguishing feature is the noticeable decrease of
temperature at the surface due to the maximum
heat loss of the seasonally cold waters. The cooling effect involves the entire water column in
the northern shelf area and reaches a depth of
80-100 m in the central basin. A vertically
homogeneous cold and dense water mass,
namely the NADW, is formed in the northern
shelf area (° 0 > 29.40 kg·m- 3 ) by turbulent convective movements that reduce the vertical density gradients. A further, progressive decrease of
temperature in the bottom layer of the central
basin occurs in spring. This is due to the advection of the cold NADW which, formed during
the previous winter, reaches the Giulianova sections later on.
In spring and summer, the upper thermocline becomes more pronounced. reaching a
maximum depth of 50 m. The NADW is topped
by the warm surface layer and is entrained
southwards by the western coastal current. The
NADW has been identified by Manca and
Giorgetti (1998) as a sub-surface cold core water
« 13 0c) flowing close to the western shelf in the
southern basin throughout the year.
In the southern Adriatic Sea, the layer below
the seasonal thermocline is subjected to a thermal variability (not shown) mosdy related to the
lateral advection of the NADW and to the inflow
of the Mediterranean water. However, the relatively fresh water from the northern basin flows
along the Italian coast leaving the open-sea
waters exposed to buoyancy losses during winter. An overturning of the water column occurred
during winter 1996 down to 600 m. This horizon
was marked by the isopycnal 29.18 kg·m- 3
(Manca and Bregant 1998).
The seasonal salinity and density distributions along the Rimini transect are shown in
Figs. 3 and 4, respectively. The salinity field consistently shows two-layer haline stratification
with important variations typified by the freshwater river discharges occurring throughout the
year. The major Po river discharge with repetitive impulses of about 4000 m 3 ·s· 1 occurs from
15 April to 15 July 1995. affecting the corresponding spring and summer surveys. The
salinity front separating the coastal water from
the open-sea water extends from the sea surface
to the western shelf and further, in spring (Fig.
3a). The fresh water flows southwards in a narrow coastal boundary region of about 30-40 km
up to the southernmost section at the Otranto
Strait. The return flow is constituted by the
saline core water at the interior of the basin that
presents a vertical homogeneity right down to
the bottom. In the northern basin, preliminary
evidence seems to indicate that the presence of
less saline water further to the east may very
likely be associated with meandering current