The following notes are from RUSSO2009.
Being an epicontinental basin, the hydrology and dynamics of the NA are primarily influenced by meteorological
forcing, thermal variations and river runoff. Climatological studies (see CUSHMAN-ROISIN2001 and
citations therein) indicate that prominent weather situations in the NA include unperturbed weather or airflow
from the northwestern, northeastern and south-eastern quadrants (respectively Etesian, Bora and Sirocco winds;
these two last are the most frequent winds in the area and often trigger severe wind-storms). The NA receives
approximately 20 % of the total Mediterranean river runoff RUSSO1996, mainly from the Po River, average
flow rate approximately 1500 m
/y ARTEGIANI1981, RAICICH1994, with a pronounced seasonal variability, with peaks in spring and autumn (up to 8000 m
/s and minima in
summer (300-400 m/s ). To the south-east the Buna-Bojana river
shows an average discharge of 700 m/y. This leads to a net gain of fresh water.
In autumn, intense cooling and evaporation processes, usually associated with Bora wind events over the NA, create conditions for dense water formation during the winter VIBILIC2005.
Due to runoff and heating in the late spring and summer and to autumn-winter cooling, gradient currents are established within a cyclonic circulation system ZORE-ARMANDA1956, BULJAN1976, FRANCO1982, ORLIC1992, ARTEGIANI1997-I, ARTEGIANI1997-II, RUSSO1996, HOPKINS1999, POULAIN2001-CI consisting of an entering NW-ward current (the Eastern Adriatic Current, EAC), and an exiting SE-ward current (the Western Adriatic Current, WAC), that introduce warmer and saltier water into the sub-basin, while pushing fresher water towards the southern regions. The general circulation pattern in the NA is also largely affected by wind. Bora episodes can generate a transient double gyre circulation consisting of a cyclone north of Po delta and an anticyclone to the south, driving the upwind extension river plume filaments JEFFRIES2007; an anticyclonic circulation also develops along the southern Istrian coast POULAIN2001-NAS, POULAIN2001-CI, while Bora enforce flow in the WAC BOOK2007, URSELLA2006.
The NA Sea is one of the most biologically productive regions of the whole Mediterranean. The rate of oxygen
consumption due to biogeochemical processes is the largest of the entire Adriatic basin, with a maximum occurring
around the Po River delta area ARTEGIANI1997-II. This region can thus be regarded as a favourable
environment for the development of hypoxic conditions. The formation of a hypoxic bottom layer in wide areas
of the basin DEGOBBIS1993, DEGOBBIS2000 can cause major ecological problems such as the mass mortality
of marine animals, defaunation of benthic populations and a decline in fisheries production.
Hypoxia is usually defined as occurring in regions where dissolved oxygen concentrations are less
than 2 ml l
(equivalent to 2.8 mg l). This concentration is the lower tolerance limit for many
benthic species SIMUNOVIC1999, RABALAIS2000, WU2002.
The dynamics of the SAD is dominated by the presence of a quasi-permanent cyclonic gyre that in the winter season creates the conditions for the open-ocean convection and the production of dense and oxygenated waters. Studies show that two types of dense water formation processes occur during winter within the Adriatic Sea: the major portion of the Adriatic Deep Water (ADW) is formed through open ocean convection inside the Southern Adriatic Deep (SAD) within the cyclonic gyre, while the remaining dense water is formed on the continental shelf of the Northern and Middle Adriatic that moves southward and ultimately sinks to the bottom of the SAD OVCHINNIKOV1985,BIGNAMI1990-BARI, BIGNAMI1990-DWSAD, MRIZZOLI1991. The eastern margin is characterized by the influence of the incoming waters of Ionian origin which flow northward being restricted mainly to the continental slope. This area is interested by the Levantine Intermediate water (LIW) that occupies the layer between 150 and 600m.
The coastal zone of Albania and Montenegro in the eastern margin consists of a narrow shelf area North of the Strait of Otranto, with smooth bathymetry and with circulation features presumably determined by inflowing Ionian waters, by local winds, and by relatively large amounts of the Buna-Bojana river. The latter provide a strong contribution to the Adriatic freshwater budget, in a way that their influence in feeding the freshwater coastal zone is sometimes felt far downstream along the Croatian coast.
The current state of knowledge of oceanographic characteristics of the Albanian shelf is limited. Numerical simulations and satellite infrared images indicate that the circulation on the Albanian shelf responds strongly to the local wind forcing BERGAMASCO1996. More specifically, the northeasterly wind generates very intense coastal upwelling along the Albanian shoreline due to the sudden change of the coastline orientation in that area. Bora wind induces an undercurrent at intermediate depths near the Albanian shelf break, which is directed in the opposite direction of the Levantine Intermediate Water (LIW) inflow from the Ionian. Therefore, in addition to coastal upwelling, Bora in the Strait of Otranto weakens and occasionally blocks completely the LIW inflow.