Corsica Channel Research Articles

A baseline for the monitoring of Mediterranean upper bathyal biogenic reefs within the marine strategy framework directive objectives

The Marine Strategy Framework Directive (MSFD, 2018) was adopted to achieve a Good Environmental Status (GES) in the EU's marine waters and to protect resources and ecosystem services, including the deep-sea waters and seafloor. The deep sea (below 200 m) is the largest biome on Earth, and its biodiversity plays a key role, despite being strongly threatened by several anthropogenic stressors, potentially affecting some ecosystem functions. Among the main deep-sea structuring species, the habitat-forming cold-water corals (CWCs) are known to form Vulnerable Marine Ecosystems (VMEs). In the Mediterranean Sea, in the last two decades, a significant effort has been devoted to unveiling the distribution, extension and ecological role of scleractinian CWCs (Madrepora oculata and Desmophyllum pertusum), but quantitative data on their conservation status are very limited. Because of their ecological importance and vulnerability, Italy has extended the implementation of the MSFD to the deep sea, carrying out specific monitoring programmes on these scleractinian CWCs. In this regard, this study established, for the first time, an extensive standardized baseline for national monitoring programmes focusing on two recently discovered upper bathyal areas dominated by scleractinian CWCs (Dohrn Canyon and Corsica Channel). The goal was to evaluate the ecological status of the bathyal biogenic reefs using traditional diversity and impact variables as well as the innovative photogrammetry tool. In both areas, the investigations have increased the knowledge about the extent of the habitat and new CWC assemblages in good health conditions were discovered. The pressure extent is limited with respect to the total extent of the CWC habitat, accounting for approximately 32% and 63% of the fishing-related items entangled on structuring species. The photogrammetric approach showed a deviation of about 33% in the measurement of the population size structures. Moreover, the use of a standardised operative protocol has been proposed to obtain reliable, coherent, and comparable data for future monitoring activities. This approach improved scientific knowledge for the two studied areas and it is pivotal in defining long-term monitoring activities useful to assess the effectiveness of specific protection measures, for these VMEs.

Facies created by the yellow coral Dendrophyllia cornigera (Lamarck, 1816): Origin, substrate preferences and habitat complexity

The yellow coral Dendrophyllia cornigera (Lamarck, 1816) is a NE Atlantic-Mediterranean scleractinian. It is considered a typical hard bottom species, generally reported on outcropping rocks from mesophotic to upper bathyal depths. Several evidences suggest that this species is able to tolerate a broad range of temperatures, which allows it to colonize numerous environments in a wide depth range.In the present study, we first provided a detailed ecological characterization of the D. cornigera dense aggregation thriving on the Mantice Shoal (NW Ligurian Sea, Mediterranean Sea). Information on substrate type and inclination, average extension and density, colonies size-class frequency distribution and associated fauna are reported. Then, we presented an extensive review of the available information on the ecology of this species, including 142 new ROV records from the Italian coast (40–1820 m). Results indicated that D. cornigera occurs on a wide range of substrates, including soft bottoms and hardgrounds (outcropping rocks, coralligenous rock and dead cold-water coral frameworks), with significant differences in colony density and size among different substrates.Dendrophyllia cornigera creates three main facies, each characterized by a specific combination of substrate, inclination, depth, and associated fauna. Scattered living colonies, as well as large thanatocoenoses, display a wide geographical and bathymetric distribution. Differently, the facies represented by dense meadows on horizontal soft-bottoms results rare, being reported only from the Mantice Shoal and the Amendolara Bank (Ionian Sea). The radiocarbon age of the thanatocoenoses varies between 400 (Corsica Channel) and 13000 (Vercelli Seamount) years before present.This study highlights the wide adaptability of D. cornigera in terms of environmental settings, changing the current view on the ecology of this species and providing essential insights for the implementation of international deep-sea habitat classification schemes and conservation measures.

Climate change signal in the ocean circulation of the Tyrrhenian Sea

Abstract. The Tyrrhenian Sea plays an important role in the winter deep water formation in the northwestern Mediterranean through the water that enters the Ligurian Sea via the Corsica Channel. Therefore, the study of the impact of the changes on the future climate on the Tyrrhenian circulation and its consequences represents an important issue. Furthermore, the seasonally dependent Tyrrhenian circulation, which is rich in dynamical mesoscale structures, is dominated by the interplay of local climate and the basin-wide Mediterranean circulation via the water transport across its major straits, and an adequate representation of its features represents an important modeling challenge. In this work we examine with a regionally coupled atmosphere–ocean model the changes in the Tyrrhenian circulation by the end of the 21st century under the RCP8.5 emission scenario, their driving mechanisms, and their possible impact on winter convection in the NW Mediterranean. Our model successfully reproduces the main features of the Mediterranean Sea and Tyrrhenian Basin present-day circulation. We find that toward the end of the century the winter cyclonic along-slope stream around the Tyrrhenian Basin becomes weaker. This weakening increases the wind work transferred to the mesoscale structures, which become more intense than at present in winter and summer. We also find that, in the future, the northward water transport across the Corsica Channel towards the Liguro-Provençal basin becomes smaller than today. Also, water that flows through this channel presents a stronger stratification because of a generalized warming with a freshening of upper and a saltening of intermediate waters. Both factors may contribute to the interruption of deep water formation in the Gulf of Lions by the end of the century.

Contrasted mixing efficiency in energetic versus quiescent regions: Insights from microstructure measurements in the Western Mediterranean Sea

Microstructure and CTD/LADCP measurements from the Western Mediterranean basin east of 5∘E revealed two types of dynamical regions (Ferron et al., 2017), contrasted in terms of current magnitude, vertical shear, stratification and turbulent kinetic energy dissipation rate: energetic regions (Corsica Channel, Egadi Valley and Sicily Channel) and quiescent regions (Ligurian Sea, around Sardinia, and Tyrrhenian Sea). On average, the current speed and the buoyancy frequency in the energetic regions were twice as large as in the quiescent regions, and the vertical shear was five times as large. Turbulence properties inferred from the microstructure measurements were also contrasted, dissipation rates in the energetic regions being two orders of magnitude larger than in the quiescent regions. The present study investigates the variability of the dissipation flux coefficient, a measure of the mixing efficiency, in a rich assortment of dynamical regimes. This dataset covers the full range of turbulence intensities observed in previous studies based on field measurements, direct numerical simulations, and laboratory experiments alike. The dependency of the dissipation flux coefficient as a function of turbulence intensity for the quiescent and energetic regions frames the previously observed lower and upper bounds, respectively. A contrasting behaviour was revealed between the two types of regions. In the quiescent regions, the dissipation flux coefficient linearly decreases on average by one order of magnitude with turbulence intensity increasing by four orders of magnitude. On the other hand, in the energetic regions the dissipation flux coefficient exhibits a nearly constant value over 4 decades of turbulence intensity, before decreasing for very strong turbulence intensities. In contrast with other studies, this dataset shows no relationship between the Richardson number and the dissipation flux coefficient. This may be due to inadequate vertical sampling resolution of the currents, or to the high diversity of sampled turbulent regimes, contrary to previous studies focused on a single type of dynamical region or framework (such as the thermocline or shear instabilities).

Aspects of the summer circulation in the eastern Ligurian Sea

Water exchanges between the Tyrrhenian Sea and the Liguro-Provençal basin occur through the Corsica Channel, a narrow passage between Corsica and the Elba Island, near the Italian coast. It has long been known that the Channel hosts a robust Tyrrhenian outflow in winter, which penetrates far into the Ligurian Sea, reaching the site where deep-water formation takes place. The summer circulation in the area, and, more generally, in the eastern part of the Ligurian Sea, is less well known. Several experimental and modelling works in the last decade have pointed out the presence of an anticyclone in the Channel area, sometimes referred to as Ligurian anticyclone, which could modulate the exchanges between the two basins in summer, and could strongly affect the dispersion properties in the south-eastern Ligurian Sea, with implications on the local, delicate marine environment. In this work, we focus on this structure, and on the role it plays in the local circulation, seeking information from long datasets of satellite observations (sea level, sea surface temperature and turbidity) that are now available. We find that the Ligurian anticyclone is a recurrent feature of the local circulation in summer. Its core is located between Cap Corse (the peninsula at the northern end of Corsica) and the Elba Island, but the anticyclone may extend till the Italian coast, and it may sometimes be more elongated in the latitudinal direction, and/or more displaced towards north. In some years, a companion anticyclonic vortex is found more to the north, in the north-eastern portion of the Ligurian Sea, but the robustness of this structure is unclear. Analysis of the ERA5 winds indicates that the Ligurian anticyclone is likely driven by an area of negative wind stress curl usually present near Cap Corse in summer, even though contributions from the surrounding circulation cannot be a priori excluded.

The “Corsica Channel Cold-Water Coral Province” (Mediterranean Sea)

Over 25 mounds have been identified in the Corsica Channel (Mediterranean Sea) through multibeam bathymetric mapping at depth of 400-430 m, with dimensions ranging from 70 to 330 m, achieving maximum heights of 25 m. Two such mounds were explored in detail by means of Remotely Operated Vehicle, revealing thick coral growth with a predominance of the branching scleractinian Madrepora oculata as main frame-builder, and subordinate Desmophyllum pertusum. The solitary species Desmophyllum dianthus and Javania cailleti add to the biodiversity here, which accounts for at least 50 macro- and megabenthic species. In consideration of the remarkable surface (ca. 3.4 km2) covered by living corals, their density and healthy appearance, and discontinuity with other major CWC occurrences in the Mediterranean Sea, we propose that this area represents a distinct CWC province in a sector already known for the presence of pre-modern CWC mounds. Noticeably, well-developed contourite drift systems occur in the Corsica Channel, lending support to their strict link with coral establishment at depth. The ecosystemic value of the new CWC province calls for proper conservation measures to ensure their present Good Environmental Status.

Short-term, linear, and non-linear local effects of the tides on the surface dynamics in a new, high-resolution model of the Mediterranean Sea circulation

The tides in the Mediterranean Sea are generally weaker than in other regions of the world ocean, but are locally intensified in passages with complex bathymetry, such as the Gibraltar Strait and the Sicily Channel. To date, a detailed understanding of their effects on the circulation, on the short time scales relevant to forecasting, is still missing, due to the lack of specific observations, and of basin-scale, numerical models explicitly accounting for the tidal forcing. The present investigation attempts to bridge this gap, using a newly developed forecasting model of the circulation of the Mediterranean Sea-Black Sea system that includes the effects of the main diurnal and semidiurnal astronomical tides. After validating the model barotropic tidal dynamics through comparison with the results of a tidal inversion software and historical data, a fully baroclinic run is analyzed, with focus on the Mediterranean Sea dynamics. The run covers the period 19 March–30 April 2018 and is initialized and forced with realistic fields produced by state-of-the-art operational models. After verifying that the modelled circulation and hydrology agree with available remote and in situ observations, attention is focused on short-time effects of the tidal forcing on the local circulation. Tides are found to significantly modulate transports not only across the Strait of Gibraltar and the Sicily Channel, as expected, but also across the Corsica Channel and the Otranto Strait. Tidal effects also modify relevant features of the circulation inside the basin, inducing local modulations of some of the main currents and exciting topographic waves that eventually get trapped over shallow bathymetric features, producing diurnal rotations of the flow patterns. Examples of the latter dynamics are found over the Adventure Bank and the Malta Plateau, in the Sicily Channel, and in the eastern portions of the Corsica Channel and the Otranto Strait. Furthermore, in several locations (Sicily Channel, Corsica Channel, Messina Strait, North Adriatic Sea), spectral analysis of the mean kinetic energy reveals the presence of spectral peaks corresponding to periods of about 8 and 6 h, which can only be interpreted as harmonics (overtides and compound tides) of the diurnal and semidiurnal tidal components, generated through non-linear interactions. This shows that, contrary to a widespread assumption, tidal effects in the Mediterranean cannot be linearly superimposed on the basin circulation.

On the dynamics in the southeastern Ligurian Sea in summer 2010

Drifters and a glider were operated in the southeastern Ligurian Sea to study the near-surface currents and water mass properties in summer 2010. Additional data were collected by a moored current meter in the Corsica Channel (CC). These in situ data were complemented by surface wind products, satellite images of ocean color and a Regional Ocean Modeling System (ROMS) numerical model that was implemented to simulate the local coastal dynamics. Southward currents were prevailing along the continental Italian coast, advecting filaments with a high optical signal coming from the Arno River. North of Elba Island, currents turned westward and northward in the vicinity of the CC. Further to the north they veered eastward, forming an anticyclonic circulation feature centered around Capraia Island. This general circulation picture was disrupted and reversed during events of sustained southerly winds occurring with a period of about a week. The near-surface currents in the CC and the anticyclonic circulation around Capraia Island showed the same weekly variations related to the local wind forcing. The ROMS model simulations agreed satisfactorily with the observations, in particular the strength of the Capraia anticyclonic circulation (quantified with the Capraia index) was confirmed to be strongly wind-dependent.

Zooplankton diel vertical migration in the Corsica Channel (north-western Mediterranean Sea) detected by a moored acoustic Doppler current profiler

Abstract. Diel vertical migration (DVM) is a survival strategy adopted by zooplankton that we investigated in the Corsica Channel using acoustic Doppler current profiler (ADCP) data from April 2014 to November 2016. The principal aim of the study is to characterize migration patterns and biomass temporal evolution of zooplankton along the water column. The ADCP measured vertical velocity and echo intensity in the water column range between about 70 and 390 m (the bottom depth is 443 m). During the investigated period, zooplanktonic biomass had a well-defined daily and seasonal cycle, with peaks occurring in late winter to spring (2015 and 2016) when the stratification of the water column is weaker. Zooplanktonic biomass temporal distribution in the whole water column is well correlated with biomass of primary producers, estimated with satellite data. Zooplanktonic blooming and non-blooming periods have been identified and studied separately. During the non-blooming period zooplanktonic biomass was most abundant in the upper and the deep layers, while during the blooming period the upper-layer maximum in zooplanktonic biomass disappeared and the deep layer with high zooplanktonic biomass became thicker. These two layers are likely to correspond to two different zooplanktonic communities. The evolution of zooplanktonic biomass is well correlated with chlorophyll, with phytoplankton biomass peaks preceding the upper-layer secondary production by a lag of about 3.5 weeks. Nocturnal DVM appears to be the main pattern during both periods, but reverse and twilight migration are also detected. Nocturnal DVM was more evident at mid-water than in the deep and the upper layers. DVM occurred with different intensities during blooming and non-blooming periods. One of the main outcomes is that the principal drivers for DVM are light intensity and stratification, but other factors, like the moon cycle and primary production, are also taken in consideration.

Current reversal and associated variability within the Corsica Channel: The 2004 case study

Flow reversals within the Corsica Channel, a strait East of Corsica, are investigated with a realistic, high-resolution (∼1.5km) numerical setup simulating the year 2004. The simulations compare well with available water mass transport estimates resulting in an annual mean of 0.49±0.49Sv. Similarly, the model agrees with hydrographic observations in the area, and current velocity measurements showing a flow in the Corsica Channel predominantly directed northward from the Tyrrhenian to the Ligurian Sea. On top of the well-documented Corsica Channel seasonal variability, a higher-frequency variability can be found throughout the year but more frequently during the summer season. This temporal variability, highest close to the western flank of the Channel, is of the order of a few days to a week and associated with reversals of the currents. We find that this variability is ascribed to periodic intrusions of the West Corsica Current on the Eastern side of the Island. Moreover, our findings suggest the importance, of a so-far neglected, across-channel variability of the meridional velocity throughout the entire 2004. This result potentially questions the single-mooring assumption that has always been at the center of the observational scheme. This assumption holds while looking at low-frequency/seasonal variability but fails when focusing on higher-frequency variability.

How important are diapycnal mixing and geothermal heating for the deep circulation of the Western Mediterranean?

AbstractThe dissipation rate of turbulent kinetic energy ε and the associated diapycnal turbulent mixing is inferred from a set of microstructure observations collected over several cruises from year 2012 to 2014. The geographical distribution of ε highlights several regions of enhanced levels of turbulence ranging from 10−9 to 10−6 W kg−1: the Sicily Channel, the Corsica Channel, and the Ligurian Sea. Elsewhere, ε was small, often below 10−10 W kg−1. Below 1300 m, geothermal heating provides three‐fold more buoyancy than small‐scale turbulence. Geothermal heating and turbulent diffusion provide enough buoyancy to balance 15% to 50% of a mean yearly deep water formation rate of 0.9 to 0.3 sverdrup (106 m3/s), respectively. The remaining part has to eventually overflow through the Strait of Gibraltar.

Analysis of surface circulation structures along a frequently repeated XBT transect crossing the Ligurian and Tyrrhenian Seas

A dataset of XBT (eXpendable BathyThermograph) temperature profiles collected by ships of opportunity along the Genova–Palermo route, since September 1999, is analyzed, together with altimetric observations and model results, with the purpose of identifying and characterizing robust circulation features along the track and investigating their variability. An anticyclone is found in the Ligurian Sea, just north of the Corsica Channel, not present in previous descriptions of the Mediterranean Sea circulation. It appears to be a recurrent feature, better defined and stronger in summer and in the beginning of autumn. In the northern part of the Tyrrhenian Sea, the well-known Bonifacio dipole shows a similar seasonality, in agreement with previous observations. However, the Bonifacio anticyclone also displays a strong interannual variability, not previously recorded, with significant variations in position and shape. In fact, the data suggest the existence of two distinct summer circulation regimes related to the position and shape of the Ligurian anticyclone. When the latter is wider, filling the entire region north of the Corsica Channel, the circulation in the northern Tyrrhenian Sea is isolated from that in the Ligurian Sea, in agreement with the common picture. However, the altimeter maps show that there are several cases in the last two decades in which the Ligurian anticyclone is small and displaced to the west, allowing an inflow through the Corsica Channel into the Tyrrhenian Sea. The two regimes appear to result from a delicate balance between the forcings acting in the two sub-basins and the topographic constraints.

Trials of an autonomous profiling buoy system

ABSTRACTIn June 2013 the Institute of Marine Sciences of the National Research Council of Italy (CNR-ISMAR) started the test phase of one of the few Mediterranean autonomous profiling systems, produced by NiGK Corporation, installed in a mooring configuration, transmitting daily hydrological vertical profiles in real time through satellite communication. The selected site is the Corsica Channel, a narrow passage between the Corsica and Capraia islands connecting the two main regions of the western Mediterranean: the Tyrrhenian and the Liguro-Provençal basins. During the test phase of this new instrument a continuous monitoring of the upper 181 m of the water column was performed, with more than 90% of successful satellite transmissions. The high-quality data received have been processed and analysed. The data collected confirmed our previous hydrological knowledge of the Corsica Channel, allowing us to observe in real time the variable and significant behaviour of the channel.

Mediterranean bottom-current deposits: an example from the Southwestern Adriatic Margin

Abstract The identification of bottom-current deposits is a key to understanding the long-term deep-sea circulation and its changes through geological times. The Southwestern Adriatic Margin (SAM) is a small Mediterranean sub-basin that represents a key site to study bottom-current deposits in a Mediterranean context and hence to improve our knowledge of changes in Mediterranean deep-water circulation during the recent geological past. The SAM is characterized by complex stratification and circulation related to an interaction between two south-flowing bottom water masses: the cold North Adriatic Dense Water (NAdDW), formed in the shallow northern Adriatic through cold wind forcing and winter heat loss, and the highly saline Levantine Intermediate Water (LIW), generated in the Eastern Mediterranean through intense evaporation and flowing along the slope in a depth range of 200–600 m. Chirp-sonar profiles, TOBI mosaics and sediment cores acquired along the SAM reveal distinctive sediment drift types (elongated, plastered and isolated drifts) and extensive fields of sediment waves. Non-depositional and erosional features related to bottom-current activity include moats between drifts and the steep slope, widespread upper-slope erosional areas and extensive furrowed areas, which are particularly developed where change in slope orientation blocks the current circulation. The distribution, morphology and size of bottom-current features along the SAM result from an interaction between current regime and slope morphology, characterized by structural highs perpendicular to the slope contour (e.g. Dauno Seamount), multiple slope incisions (e.g. Bari Canyon and slump scars) and extensive block-slide deposits. Morphobathymetric and seismic stratigraphic data on the SAM show that bottom-current deposits are best developed where the regional slope flattens seaward of a very steep, often erosional, upper slope. The roughness of the lower slope, in particular, seems to correlate with the complexity and decreasing size of the bottom-current deposits. Like other land-locked basins, the Adriatic underwent dramatic palaeogeographical and palaeoceanographic rearrangements during the Late Quaternary sea-level oscillations. Indeed, during the Last Glacial Maximum (LGM), most of the areas where NAdDW is formed today were subaerially exposed. Concurrently, during glacial times the LIW production was probably reduced compared with the present-day conditions. The SAM slope is a key site to study the impact of changing current regime on late Quaternary slope deposits. Other Mediterranean late Quaternary contourite deposits are either in water depths compatible with the LIW, particularly in the case of shallow sill basins (e.g. Sicily, Corsica Channel), or at the slope base reflecting the flow of Mediterranean deep waters. The SAM bottom-current deposits, instead, seems to record the changing interaction between two distinctive bottom-hugging currents along the same pathway.

Improved satellite altimetry in coastal systems: Case study of the Corsica Channel (Mediterranean Sea)

The fidelity of corrections and processing are critical for a realistic use of official altimetric products close to the coast. A new processing strategy, which starts from the TOPEX/Poseidon GDRs with the addition of improved corrective terms, is proposed and evaluated in the area of the Corsica Channel. Sea level anomalies agree with the coincident sea truth (bottom pressure and tide gauge) within 2–3 cm rms for seasonal and longer time scales. Analysis for almost ten years of coincident mooring and altimetric velocities shows that a substantial reduction of uncertainty to ∼4 cm s−1 may be possible after reasonable filtering of the noise introduced by more variable coastal sea surface states. The conclusion is that the altimetry success is still limited to seasonal time scales, and provided that the oceanographic signal ensures an adequate signature to be isolated from background noise.

Annual microplankton cycles in Villefranche Bay, Ligurian Sea, NW Mediterranean

Abundance and composition of microplankton were studied over a period of 2 years at two depths in Villefranche Bay (Ligurian Sea, NW Meditrranean Sea). Diatoms dominated the microplankton in late spring and autumn, whereas dinoflagellates composed the major part of the microplankton in summer. The silicoflagellate Dictyocha, fibula and the diatom Thalassionema frauenfeldii dominated in winter. Ciliates showed low variability throughout the year with the lowest abundance in February and an increase which coincided with the diatom maxima during autumn in both years. In 1998, the spring bloom (in May) was mainly composed of dinoflagellates near the surface and of diatoms in deeper layers. Subsurface diatom maxima were observed in August-September and November. In 1999, diatoms peaked in May both at the surface and at the depth of 50 m. They showed a strong maximum in October. Dinoflagellates and tintinnids showed maxima in early November. Comparisons with previous studies reveal that (i) changes in species composition have not been significant, (ii) the silicoflagellate's abundance is lower during the present study, (iii) the sequential spring bloom is composed of a pico-nanoplankton bloom in March and microphytoplankton in May, whereas in other western Mediterranean areas the spring microphytoplankton bloom is reported in February and March, (iv) high water transport through the Corsica channel coinciding with low or negative winter values of North Atlantic Oscillation (NAO) index are associated with the anomalous strong development of the spring diatom blooms in the Bay of Villefranche, whereas the usual trend is the lack of or weak development of the spring diatom bloom. This feature may determine the nature and the fate of primary production and the interannual variability in the relative importance of the microbial food web versus the microbial loop.

Etruscan archaeometallurgy record in sediments from the Northern Tyrrhenian Sea

The island of Elba has been known since antiquity as the major source of metals, especially iron, for the Etruscan civilisation. Unfortunately, the archaeometallurgic record of the island has been destroyed during our century so the present knowledge of this history is very poor and only historical sources constrain the timing. We present in this article indirect evidence of this activity buried in the sea deep that will shed new light on the archaeometallurgic history of the Western Mediterranean. Magnetic properties of marine sediments collected in the Corsica Channel imply enrichment in ferrimagnetic minerals and trace metals related to anthropic impact. Accelerator mass spectrometry 14C ages suggest that the contamination started about 4000 years ago; we attribute it to emissions of fly-ash from the Etruscan smelting technologies for metal production. These results show that archaeometallurgic activity started on Elba Island in the II Millennium BC, much earlier than previously assumed.

Sediment drifts of the Corsica Channel, northern Tyrrhenian Sea

Abstract A sediment drift complex, resulting from the activity of the northward flowing Levantine Intermediate Water, occurs in intermediate water depth (300-600 m) on the eastern flank of the Corsica Basin. Because of the effect of topo–graphic constriction, bottom currents are here accelerated, reaching velocities that are sufficient to erode, transport and redistribute fine-grained sediment. The sediment drift complex shows a variety of depositional and erosional features that appear very similar to oceanic examples. The development of such features appears to be mainly controlled by an interaction between the bottom current regime and slope topography. Seismic geometries and core data show that such features have grown since middle Pliocene time under a long-term stable bottom-current regime. Short-term variability of current effi–ciency, a concept including the combined effects of current speed, sediment availability and local topography, as a result of climate and sea-level changes, is recorded by the cyclical superposition of small-scale depositional units.

Modeling the Oceanic Circulation in the Area of the Strait of Sicily: The Remotely Forced Dynamics

In order to describe aspects of the baroclinic dynamics in the region of the Strait of Sicily a high-resolution multilayer numerical model has been implemented in a central Mediterranean region including the Tyrrhenian and the Ionian Seas. Three layers have been considered representing water of Atlantic origin (MAW), the Levantine Intermediate Water (LIW), and deep water of the Mediterranean. Quasi-stationary circulations representing the local manifestation of the large-scale Mediterranean conveyor belt are obtained [after an adjustment time of O(2 months)] by imposing steady fluxes along the remote open boundaries, in the absence of meteorological forcings. These circulations can be interpreted as possible dynamic scenarios of the seasonal variability in the Strait of Sicily. In the numerical simulations an inflow of MAW and an outflow of LIW through the Strait of Sardinia, an outflow of MAW and an inflow of LIW through the Ionian boundary, and an outflow of MAW through the Corsica channel are imposed, resulting in a vanishing total net transport in each layer. For realistic values of these transports the model captures the main features of the observed circulation, such as (i) the separation of the Algerian Current into two branches, one directed toward the Tyrrhenian Sea and the other entering the strait; (ii) a secondary bifurcation of MAW within the strait giving rise to a southward-moving current that follows the Tunisian continental slope and to a current that flows southeastward along the southern Sicilian coast and then northward along the southern Italian coasts (the so-called Atlantic‐Ionian Stream); (iii) a bifurcation of LIW at the strait level leading to a main current directed toward the Strait of Sardinia and to a weaker current that, after having crossed the strait, bends eastward and enters the Tyrrhenian Sea. Sensitivity experiments carried out by imposing different boundary fluxes have shed light on the functioning of the MAW and LIW bifurcations. First of all, for a given net transport of MAW and LIW through the strait (imposed indirectly by the boundary fluxes), the ratio Rmaw between the transport of MAW entering the Tyrrhenian Sea and that entering the strait is found to be virtually independent of the boundary-imposed Algerian Current transport. It is, on the contrary, determined by a local dynamic control, which selects the value Rmaw 0.43 for a net MAW/LIW strait transport of 61 Sv, in excellent agreement with observations. Second, for decreasing baroclinic transports the ratio Rmaw is found to decrease up to the limiting value Rmaw 0.2 (corresponding to the linear regime) for transports ,O(0.1 Sv). Finally, Rmaw is found to be very sensitive to the barotropic transport T through the strait, whereas the corresponding ratio for the LIW, Rliw, is virtually independent of T. For T 5 20.5 Sv, Rmaw 1.1 while for T 51 0.5 Sv, Rmaw decreases by one order of magnitude: Rmaw 0.1. In other words, a weakening of the LIW (or a strengthening of the MAW) net transport through the strait reduces the relative intensity of the Tyrrhenian branch of MAW, and vice versa. On the other hand, for values of T within the same range one always findsRliw 20.3. It thus appears that the local control exerted by the topography through the LIW potential vorticity budget forces the transport of the Tyrrhenian branch of LIW to be always ;1/3 of that directed toward the Strait of Sardinia.

Integrated use of altimeter and in situ data for understanding the water exchanges between the Tyrrhenian and Ligurian Seas

In this paper we examine the relationship between the seasonal and interannual variability observed in the water flow through the Corsica Channel and the sea level difference between the Tyrrhenian and Ligurian Seas. The steric contribution to the sea level difference, computed from historical hydrological data, is in good agreement with the stable presence of the seasonal signal in the water exchanges. We obtain the maximum steric difference in winter (∼16 cm) and the minimum in summer (∼2 cm). These values are consistent with the corresponding estimates of water volume transport (0.8 and <0.1 Sv in winter and summer, respectively). Also, TOPEX/Poseidon (T/P) satellite altimetry is shown to be capable of capturing the sea level difference anomaly between the two basins at seasonal and interannual scales. Accuracy of altimeter data in the study region has been checked using measurements from a bottom pressure recorder deployed in Capraia Island (rms difference is found to be ∼2 cm after application of a 30‐day half‐amplitude Gaussian filter). Because of the lack of an accurate geoid, the total water transport cannot be adequately monitored by satellite altimetry alone. However, the recovered signal can be directly related to the water transport anomaly through the channel. The resulting T/P signal can be considered as representing, to a great extent, the real steric variation induced by the net sea surface heat fluxes. The high correlation found between the water transport and sea level difference anomalies confirms the results obtained by the climatological hydrography and provides new evidence that interannual differences in the sea level and water transport can be related to the different heat flux balance in the Tyrrhenian and Ligurian Seas. Moreover, it demonstrates, for the first time in the Mediterranean, the potential of satellite altimetry for long‐term monitoring of the water exchanges between adjacent basins.