Dynamics and mixing of the Eastern Mediterranean outflow in the Tyrrhenian basin

Abstract

Hydrographic measurements in the Sicily Channel and in the southern Tyrrhenian Sea show that the water outflowing from the Eastern Mediterranean Basin enters the Tyrrhenian Sea directly, close to the Sicily coast. Two water types have been distinguished: the upper part constituted by the Levantine Intermediate Water (LIW), and the transitional Eastern Mediterranean Deep Water (EMDW) below. Current measurements taken about 50 km after the entrance into the Tyrrhenian evidence a well-developed mean flow from a depth of 500 m to the bottom. While the LIW flows at the known depth for this type of water (namely between 500 and 800 m), a considerable volume of transitional EMDW was seen to sink from a depth of about 300 m in the Sicily Channel to a maximum depth of 1850 m in the Tyrrhenian Sea, where it floats over the Tyrrhenian Deep Water (TDW). The sinking is a consequence of the higher density of the incoming water ( σ θ >29.10) with respect to that of the intermediate Tyrrhenian waters ( σ θ ∼29.00–29.05). The progressive sinking of the transitional EMDW along the principal route from the Sicily Channel to the Tyrrhenian Sea, can be observed in the evolution of the isotherms, isohalines and isopycnals. The isopycnals, in particular, clearly tend to follow the bottom slope. The mean hydrographic characteristics of this vein of Eastern Mediterranean water progressively change along the route, and two regions of strong mixing are identified. The first occurs immediately after the Sicily Channel, while the second is found following the entrance of the vein into the Tyrrhenian Sea. An estimation of the entrainment stress shows that it largely dominates over the acceleration term. When the bottom slope does not influence the water motion anymore, the mixing is entirely ascribable to molecular diffusion via finger instability. The transport of transitional EMDW through the Sicily Channel is estimated to be about 0.2–0.3 Sv, implying a salinity anomaly export of 0.05–0.075 psu per Sv to the deep layers of the Tyrrhenian Sea.