No abstract

On 24 and 25 October 1995, high-resolution oceanographic measurements were carried out in the Strait of Messina by using a towed conductivity-temperature-depth chain and a vessel-mounted acoustic Doppler current profiler. During the period of investigation the surface water of the Tyrrhenian Sea north 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 surface water of the Ionian Sea spread as a surface jet into the Tyrrhenian Sea, whereas during southward tidal flow heavier surface water of the Tyrrhenian Sea spread, after having sunk to a depth of about 100 m, as a subsurface jet into the Ionian Sea. Both jets had the form of an internal bore, which finally developed into trains of internal solitary waves whose amplitudes were larger north than south of the strait sill. These measurements represent a detailed picture of the tidally induced internal dynamics in the Strait of Messina during the period of investigation, which contributes to elucidate several aspects of the general internal dynamics in the area: 1) Associated with the tidal flow are intense water jets whose equilibrium depth strongly depends on the horizontal density distribution along the Strait of Messina; 2) although climatological data show that a large horizontal density gradient in the near-surface layer along the Strait of Messina exists, its reversal can occur; 3) fluctuations in the larger-scale circulation patterns that determine the inflow of the modified Atlantic water into the Eastern Mediterranean Sea can be responsible for this reversal. As the tidally induced internal waves reflect the variability in the horizontal density distribution along the Strait of Messina, it is suggested that from the analysis of synthetic aperture radar imagery showing sea surface manifestations of internal waves in this area fluctuations of larger-scale circulation patterns in the Mediterranean Sea can be inferred.

The investigation is based on data collected between the eastern Algerian Basin and the Strait of Sicily and in the southern Tyrrhenian Sea. The major pathways of water masses are identified by the core method and geostrophic currents are derived from the objectively analysed density field. Between the Sardinia Channel and the Strait of Sicily, the large-scale circulation of Modified Atlantic Water and Winter Intermediate Water is found to be cyclonic. Inflow into the gyre occurs via the Sardinia Channel by means of a boundary current attached to the Algerian coast, and from the northern Tyrrhenian. The outflow is accomplished via the Strait of Sicily and to the Tyrrhenian. The Levantine intermediate water flow resembles that of Modified Atlantic Water/Winter Intermediate Water in the southern Tyrrhenian, but it is opposed in the Strait of Sicily and off Tunisia. Outflow to the Algerian Basin occurs south of Sardinia. In the eastern Algerian Basin the flow direction of all water masses is eastward close to the Algerian shelf. Farther offshore, Modified Atlantic Water flows mainly southwest whereas the Levantine Intermediate Water is opposed to that supporting northward transport along the Sardinian shelf. The large-scale flow of all water masses is perturbed by mesoscale eddies. The impact of topographic obstacles is investigated. © 2001 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS Résumé − Masses d'eau et circulation entre le bassin algérien et le détroit de Sicile en octobre 1996. Cette étude est basée sur l'utilisation de données océanographiques collectées entre le bassin algérien et le détroit de Sicile et dans la partie sud de la mer Tyrrhénienne. Entre le canal de Sardaigne et le détroit de Sicile, la circulation à grande échelle des eaux modifiées de l'Atlantique et des eaux intermédiaires d'hiver est identifiée comme cyclonique. L'apport en flux dans la rotation se fait principalement par le canal de Sardaigne, grâce à un courant intense le long de la côte algérienne et le long de la mer Tyrrhénienne septentrionale. L'écoulement s'effectue par le détroit de Sicile et vers la mer Tyrrhénienne. Le flux des eaux intermédiaires du Levant ressemble à celui de l'ensemble eau modifiée de l'Atlantique/eau intermédiaire d'hiver dans la partie sud de la mer Tyrrhénienne. Dans le détroit de Sicile et près des côtes de Tunisie, le flux de l'eau intermédiaire du Levant est opposé ; l'écoulement vers le bassin algérien s'effectue au sud de la Sardaigne. Dans la partie est du bassin algérien, toutes les masses d'eau sont dirigées vers l'est, près du plateau algérien. Au large et à l'ouest de la Sardaigne, l'eau modifiée de l'Atlantique s'écoule principalement vers le sud ouest alors que l'eau intermédiaire du Levant s'écoule, à l'opposé, vers le nord le long du plateau de la Sardaigne. Le flux à grande échelle de toutes les masses d'eau est perturbé par les tourbillons. L'impact des obstacles topographiques est étudié.

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].

Acoustic data measured in the ocean fluctuate due to the complex time-varying properties of the channel. When measured data are used for model-based, geo-acoustic inversion, how do acoustic fluctuations impact estimates for the seabed properties? In May 1999 SACLANT Undersea Research Center and TNO-Physics and Electronics Laboratory (FEL), conducted a shallow-water experiment on the Adventure Bank off the southwest coast of Sicily, Italy to assess the effects of a time-varying ocean on acoustic propagation and geo-acoustic inversion. A favorable area for acoustic propagation was identified which had slight internal wave activity and a weakly range-dependent bathymetry with sand-like bottom properties. Oceanographic and acoustic measurements were performed continuously over a 3-day period. Broadband (0.2–3.8 kHz) acoustic signals from a bottom-moored source were transmitted over fixed paths and received on a moored vertical hydrophone array. During the transmissions extensive environmental measurements (e.g., sound speed, current, sea-surface waveheight, etc.) were made to correlate the time-varying environmental and acoustic data. Modeled acoustic data show time variability which agrees with the measurements. Results illustrate severe problems when modeling shallow-water acoustic propagation at ranges beyond a few kilometers in the frequency band considered. Further, the acoustic fluctuations in time caused erroneous time variability in inverted seabed properties.