Evidence for the influence of Atlantic-Ionian stream fluctuations on the tidally induced internal dynamics in the Strait of Messina

On 24 and 25 October 1995, high-resolution oceanographic measurements were carried out in the Strait of Messina by using a towed conductivity-temperat ure-depth chain and a vessel-mounted acoustic Doppler current profiler. Dur ing the period of investigation the surface water of the Tyrrhenian Sea nor th of the strait sill was heavier than the surface water of the Ionian Sea south of the strait sill. As a consequence, during northward tidal flow sur face water of the Ionian Sea spread as a surface jet into the Tyrrhenian Se a, whereas during southward tidal flow heavier surface water of the Tyrrhen ian Sea spread, after having sunk to a depth of about 100 m, as a subsurfac e jet into the Ionian Sea. Both jets had the form of an internal bore, whic h finally developed into trains of internal solitary waves whose amplitudes were larger north than south of the strait sill. These measurements repres ent a detailed picture of the tidally induced internal dynamics in the Stra it of Messina during the period of investigation, which contributes to eluc idate several aspects of the general internal dynamics in the area: 1) Asso ciated with the tidal flow are intense water jets whose equilibrium depth s trongly depends on the horizontal density distribution along the Strait of Messina; 2) although climatological data show that a large horizontal densi ty gradient in the near-surface layer along the Strait of Messina exists, i ts reversal can occur; 3) fluctuations in the larger-scale circulation patt erns that determine the inflow of the modified Atlantic water into the East ern Mediterranean Sea can be responsible for this reversal. As the tidally induced internal waves reflect the variability in the horizontal density di stribution along the Strait of Messina, it is suggested that from the analy sis of synthetic aperture radar imagery showing sea surface manifestations of internal waves in this area fluctuations of larger-scale circulation pat terns in the Mediterranean Sea can be inferred.