CHAPTER 4
Hydrological Characteristics and Dynamics of the Northern Adriatic
during Late Summer and Autumn 1997
V. Cardin and L. Ursella
ABSTRACT
Horizontal and vertical distributions of the hydrological data monitored during the autumn
campaign of the PRISMA 2 project in the Northern Adriatic in 1997 show the presence of various mesoscale cyclonic and anticyclonic structures, as well as a strong coastal front separating
water of fluvial origin from that of the open sea. The surface layer, in the southern part of the
studied area, presents higher density and salinity than in the northern part due to the cyclonic
nature of the basin-scale Adriatic general circulation. On the contrary, the presence of a low
density core in the bottom layer probably indicates an anticyclonic circulation. The application
of a statistical method, cluster analysis, to temperature and salinity data have permitted the
individuation of three groups with distinct characteristics, corresponding to three different
water masses. Their spatial extension is in concordance with the oceanographic conditions
already observed in the Northern Adriatic. The dynamic characteristics were obtained from
ship-mounted ADCP current measurements to which the "detiding" method of Candela has
been applied. The steady part, in agreement with the general circulation of the Adriatic Sea,
indicates currents flowing southwards along the Italian coast; additionally there are indications
of a sub-basin cyclonic circulation to the north of the Po river mouth. On the contrary, the low
frequency residual current field shows a variable spatial structure, with currents alternating
northward and southward, that broadly agrees with the horizontal and vertical distribution of
the thermohaline parameters. Finally, it is observed that where water of fluvial origin is present!
the residual current field indicates southward flow. The M2 tidal ellipses, as expected, are
polarised in the centre of the basin along the NW-SE direction, while along the boundaries, they
follow the coastline orientation.
Introduction
The Adriatic Sea is an elongated basin located
between the Italian peninsula and the Balkans.
Its northern end is very shallow and gently sloping, and can be divided into a shallower region
where our study was carried out, and a deeper
region, separated by the 40-m isobath. This part
of the Adriatic, as also the rest of the basin, is
subject to strong meteorological forcing producing a clear seasonal variability in circulation
(Arregiani et al. 1997a), The river runoff is particularly strong in the northern area, affecting
the circulation through buoyancy input. Due to
this particular characteristic, the entire Adriatic
Sea is considered as a dilution basin with an
average annual fresh water gain of about 1metre,
since precipitation and evaporation almost compensate each other (Raicich 1996). In the baroclink circulation of the Adriatic Sea currents and
gyres can be identified, whose strength and
occurrence vary seasonally mainly in the northern and middle basins. During autumn, the western and eastern coastal areas are characterised
by an intensification of the alongshore currents
and generally the circulation is cyclonic presenting three main cyclonic gyres in the northern,
middle and southern basins (Arregiani et al.
1997b). During spring and summer, the water
column is strongly stratified due to a vertical
temperature gradient, and fresh water inputs
mainly affect the surface layer. In autumn and
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/(., 34010 Sgoruro (TS), Italy
F.M. Faranda, L. Guglielmo, G. Spezie (eds)
Mediterranean Ecosystems: Structures and Pro.:esses
@ Springer-Verlag Italia 2001
Hydrological Characteristics and Dynamics of the Northern Adriatic
during Late Summer and Autumn 1997
V. Cardin and L. Ursella
ABSTRACT
Horizontal and vertical distributions of the hydrological data monitored during the autumn
campaign of the PRISMA 2 project in the Northern Adriatic in 1997 show the presence of various mesoscale cyclonic and anticyclonic structures, as well as a strong coastal front separating
water of fluvial origin from that of the open sea. The surface layer, in the southern part of the
studied area, presents higher density and salinity than in the northern part due to the cyclonic
nature of the basin-scale Adriatic general circulation. On the contrary, the presence of a low
density core in the bottom layer probably indicates an anticyclonic circulation. The application
of a statistical method, cluster analysis, to temperature and salinity data have permitted the
individuation of three groups with distinct characteristics, corresponding to three different
water masses. Their spatial extension is in concordance with the oceanographic conditions
already observed in the Northern Adriatic. The dynamic characteristics were obtained from
ship-mounted ADCP current measurements to which the "detiding" method of Candela has
been applied. The steady part, in agreement with the general circulation of the Adriatic Sea,
indicates currents flowing southwards along the Italian coast; additionally there are indications
of a sub-basin cyclonic circulation to the north of the Po river mouth. On the contrary, the low
frequency residual current field shows a variable spatial structure, with currents alternating
northward and southward, that broadly agrees with the horizontal and vertical distribution of
the thermohaline parameters. Finally, it is observed that where water of fluvial origin is present!
the residual current field indicates southward flow. The M2 tidal ellipses, as expected, are
polarised in the centre of the basin along the NW-SE direction, while along the boundaries, they
follow the coastline orientation.
Introduction
The Adriatic Sea is an elongated basin located
between the Italian peninsula and the Balkans.
Its northern end is very shallow and gently sloping, and can be divided into a shallower region
where our study was carried out, and a deeper
region, separated by the 40-m isobath. This part
of the Adriatic, as also the rest of the basin, is
subject to strong meteorological forcing producing a clear seasonal variability in circulation
(Arregiani et al. 1997a), The river runoff is particularly strong in the northern area, affecting
the circulation through buoyancy input. Due to
this particular characteristic, the entire Adriatic
Sea is considered as a dilution basin with an
average annual fresh water gain of about 1metre,
since precipitation and evaporation almost compensate each other (Raicich 1996). In the baroclink circulation of the Adriatic Sea currents and
gyres can be identified, whose strength and
occurrence vary seasonally mainly in the northern and middle basins. During autumn, the western and eastern coastal areas are characterised
by an intensification of the alongshore currents
and generally the circulation is cyclonic presenting three main cyclonic gyres in the northern,
middle and southern basins (Arregiani et al.
1997b). During spring and summer, the water
column is strongly stratified due to a vertical
temperature gradient, and fresh water inputs
mainly affect the surface layer. In autumn and
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/(., 34010 Sgoruro (TS), Italy
F.M. Faranda, L. Guglielmo, G. Spezie (eds)
Mediterranean Ecosystems: Structures and Pro.:esses
@ Springer-Verlag Italia 2001