Dense low‐salinity outflow from the Adriatic Sea under mild (2001) and strong (1999) winter conditions

Abstract

Quasi‐synoptic temperature, salinity, and oxygen measurements in the Adriatic and northern Ionian Seas (Mediterranean Sea) during two different winter conditions are presented. In addition to the confirmation of previous knowledge about deepwater formation in the eastern Mediterranean, measurements give more insight into processes in the Adriatic Sea in winter and, especially, into the fate of the dense low‐salinity water masses transported by the West Adriatic Current (WAC) and its spreading into the Ionian Sea. A mild winter season without dense water production in the shallow areas of the Adriatic Sea was encountered during a cruise in February 2001. Dense water outflow, restricted to the deeper parts of the Strait of Otranto, was nevertheless present during that mild winter and also in previous records of a cruise in autumn 1999. Thus, even in times of no deepwater production in the southern Adriatic Sea, observations reveal a continuous flow of Adriatic Deep Water (ADW) through the Strait of Otranto and downslope in the northern Ionian Sea. Strong winter conditions are represented by a cruise carried out in February 1999. At that time, cold and fresh water with density higher than that of south ADW was observed on the Italian shelf. The dense coastal current is continued around Cape Santa Maria di Leuca at the heel of the Italian boot. In the Gulf of Taranto, where the width of the shelf rapidly decreases, dense coastal water is released to depth and transformed by intrusion and mixing with ambient water. Plumes and patches with horizontal extensions smaller than distances of a station grid are resolved by towed measurements in the 200 m upper layer with a multisensor chain. Products of coastal water transformation may be found at any depth on the western slopes of the Ionian Sea. The new observations of dense water carried by the WAC and plunging down to the level of neutral buoyancy in the Gulf of Taranto seem to confirm the hypothesis that the WAC could be the origin of cold lenses found below the thermocline in the central Ionian Sea [Sellschopp and Onken, 2000].

Water mass characteristics in the deep layers of the western Ionian Basin observed during May 2003

“…In accordance with previous observations [ Sellschopp and Álvarez , 2003] our investigation identifies intense mixing occurring in the region of sharp topographic variations within the Gulf of Taranto as responsible for the strong dilution of the ADW observed in the western part of the Ionian basin. It also identifies a second path for the southern spreading of ADW, located further east, where smaller mixing is observed.…”

“…Obviously, using such a numerical model, we did not intend to address the whole complexity of the near‐bottom dynamics in that region: we just used the model to corroborate our conjecture based on previous work and on our observations. As the bottom waters present in the region are characterized by density fields confined in a rather narrow range, different scenarios are not excluded: It is possible indeed that, during “very cold” years (during which, e.g., denser water is produced in the Adriatic basin than the one we observed), both paths may contribute to form the deep waters of the abyssal plain of the central Ionian basin (see also the discussion by Sellschopp and Álvarez [2003]). In this case, however, water entrainment (to which determination the density contrast between bottom current and overlying ambient water fundamentally contributes) seems still to constitute the major control of the pathway of dense water descending on the slope.…”

Temperature and productivity influences on U37K′ and their possible relation to solar forcing of the Mediterranean winter

“…This forcing signal may be imported by the dense but low‐salinity West Adriatic Current into the Gulf of Taranto. The current influences productivity and mixing on the Adriatic side of the Gulf of Taranto and its influence on the Gulf significantly differs between mild and severe winters [ Sellschopp and Álvarez , 2003]. The forcing may influence the properties of the current itself and/or modulate the winter mixing and productivity in the Gulf.…”

Core-top calibration of δ18O and δ13C of G. ruber (white) and U. mediterranea along the southern Adriatic coast of Italy

“…In the Gulf of Taranto we find complex water mass distributions (Sellschopp and Álvarez, 2003) because of the influence from the less saline water masses coming from the Adriatic Sea and flowing in a small band along the coast of the Gulf of Taranto, and the water masses coming from the Ionian Sea which have a higher temperature and salinity.…”