The well known changes in the deep thermohaline circulation of the eastern Mediterranean Sea, the so‐called Eastern Mediterranean Transient (EMT), which modified the outflow characteristics through ...the Sicily Strait, led to significant changes in the western Mediterranean Sea since the early 90's. In spring 2005 an oceanographic survey, carried out in the central part of the western basin, showed the presence of a recently formed layer of western Mediterranean deep water, spreading at the bottom of the whole Algero‐Provençal Basin. It was characterized by unusual θ‐S shapes, as its temperature, salinity and density were higher with respect both to the resident deep waters and to the climatological values. The possible influence of the EMT on the deep water formation processes occurred in the Gulf of Lions in the previous winter is here evidenced, even taking into account other data sets previously collected in the western Mediterranean.
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.
The dynamics of the Sicily Strait are investigated in a comprehensive study using recent in situ observations, Ocean General Circulation Model (OGCM) simulations and process model results. In ...particular, results of a very high-resolution Mediterranean model (MED16) developed in the context of the Mercator project are described.
The circulation in the Sicily Strait can be described as a two-layer exchange of Atlantic Water (AW) and Eastern Mediterranean Outflow Water (EOW).
The Algerian Current brings AW (the upper layer) eastwards and splits into two branches at the entrance to the Sicily Strait, one flowing to the Tyrrhenian Sea, the other into the Sicily Strait. The later is composed of two streams, referred to as the Atlantic Ionian Stream (AIS) and the Atlantic Tunisian Current (ATC). In winter, the ATC signature is more pronounced. In summer, the AIS is associated with a number of well-known semi-permanent features including the intermittent northward extension of the AIS (called NAIS) at the Ionian shelf break, which seems to be driven by the surface density contrast between waters of the Sicilian and the Ionian basins. New intermittent patterns are shown by the MED16 simulations, for example an anticyclonic eddy (called MCA) south of the Malta Channel and an anticyclonic gyre associated to the NAIS (called NAISA). Upwelling is well developed in August, with temperature lower than 24°C along the western and southern coasts of Sicily. The seasonal transport of the AW through the strait has a maximum value
(1.4
Sv)
in winter, and a minimum value at the end of summer
(0.8
Sv)
.
In the lower layer, at the exit of the Sicily Strait, the EOW, mainly composed of Levantine Intermediate Water (LIW), outflows into the Tyrrhenian Sea and veers to the right. Then, as well known, the LIW flows along the northern coast of Sicily before reaching the Sardinia Channel. A new LIW path from the Sicily Strait towards the Sardinia Channel is shown by the MED16 model, and is due to intermediate eddy shedding at the exit of the Sicily Strait.
The topography of the sill in the Sicily Strait plays a major role in the circulation. A combination of barotropic/baroclinic double Kelvin waves on both side of the sill provides a mechanism, for splitting the Algerian Current.
Recent studies have definitively shown the high sensitivity of the Mediterranean Sea to the effects produced by the large-scale atmospheric systems. The rapid response of this basin, when compared ...with the ocean time scales, makes the interannual variability of the circulation an important signal that, in some regions, may prevail over the annual cycle. We have also to consider that the distinct sub-basins, often subjected to different atmospheric and dynamic regimes, produce specific water masses that can reach all the Mediterranean regions as far as the adjacent Atlantic Ocean by a connecting network provided by the internal straits and channel. In this scenario an important role is played by the Strait of Sicily dividing the two principal sub-basins, the eastern and the western Mediterranean. A long-term monitoring of the hydrographic properties of water masses across this strait, initiated in the second half of the 1980s, permitted the identification of the principal interannual variations in the water exchange between the Ionian Sea and the Tyrrhenian Sea. It was possible to follow the evolution of the water mass characteristics under the influence of the climatic transient, which modified the production of both the intermediate and the deep water in the eastern Mediterranean. A consequence was an increase of density in the Sicily westward outflow, which produced a cascade in the deep Tyrrhenian with a remarkable deep injection of heat and salt. Furthermore, moving from the impact on the Tyrrhenian Sea, the possible effects on the western Mediterranean basin are discussed.
Hydrographic observations repeated from 1993 to 1999 in the central Mediterranean Sea, between the Sicily Strait and the Sardinia Channel, allowed us to define the water masses exchanged between the ...Eastern and the Western Mediterranean and the long-term variability of their properties. Besides the well known Modified Atlantic Water (MAW) and the Levantine Intermediate Water (LIW), other water masses are involved in this exchange: the waters of the upper deep layer in the Ionian Sea (transitional Eastern Mediterranean Deep Water (tEMDW)), which form a density current flowing at the bottom, and, from the Western Mediterranean, the Western Mediterranean Deep Water and a stream of LIW that has re-circulated in that basin (old LIW). The sub-surface water masses flow into the Tyrrhenian Sea, an intermediate basin, where they are subject to intense mixing, which modifies them or even makes them disappear. Accordingly, the outflow is formed by LIW and by the modified Tyrrhenian Deep Water (mTDW), both of them contributing to the exchange with a stream directed to the western Mediterranean. An interesting aspect indicated by the hydrographic time series is that, although the temperature and salinity of LIW in the Sicily Strait showed a prevailing trend towards lower values consistent with the changes produced by the recent climatic transient in the Eastern Mediterranean, the temperature and salinity of mTDW increased progressively throughout the whole period. This apparent anomaly was related to the behaviour of tEMDW in the Tyrrhenian Sea. While sinking at the Tyrrhenian entry, tEMDW represents a source of heat and salt for the colder and less saline resident waters at depth, thus progressively raising their temperature and salt content. The small, long-term tendency of this was intensified by the arrival of the new waters produced by the climatic transient. In our opinion, the effects of this process may add a component to the well known positive trend affecting the temperature and salinity of deep waters of the western Mediterranean.
Straits in the Mediterranean Sea form an important network from which one can determine the characteristics of the water exchange between all the constituent sub-basins. This includes the definition ...of water masses and water transport and their time variability. From 1994, all the major straits in the Mediterranean Sea (Gibraltar, Sicily, Otranto, Balearic Sea Straits, Cretan Arc Straits and Corsica) were subject to long term observations as part of various research projects. Besides adding new elements to the knowledge of internal strait conditions, the data sets collected allow us to propose a fairly consistent representation of the Mediterranean circulation and budgets in key points within the basin. The amplitude of the annual water transport measured at these straits was about 1 Sv and it appears to be modulated by a significant low-frequency and seasonal variability. For the first time, a seasonal component was identified at Gibraltar, thus raising new questions on the actual state of the Mediterranean. Also, the very likely existence of a significant interannual component was documented. In the Corsica Channel, this component was found to be related to the interannual variability of the North Atlantic Oscillation. The observations in the Cretan Arc Straits have provided a more comprehensive representation of the recent changes in the Eastern Mediterranean thermohaline cell. It is noteworthy that the effects of these changes have been observed both in the Otranto and Sicily Straits, and are now affecting the adjacent sea regions. The presence of a stream of Modified Atlantic Water in the Balearic Sea Channels indicates that part of the Atlantic inflow may be diverted directly into the northern region of the Western Mediterranean. Finally, data gathered in the Sardinia Channel indicate that the Central Mediterranean region plays a critical role in controlling exchanges between the Eastern and the Western Mediterranean, while it is emphasized that the Tyrrhenian Sea area plays a role in strongly modifying some of the water masses that contribute to the large scale basin circulation. Their mixing creates new water types which modify the currently known pattern and composition of the Mediterranean circulation.
Hydrographic and current meter data, gathered in different periods in the Strait of Sicily and in the southern Tyrrhenian Sea, allow the outflow characteristics from the eastern toward the Western ...Mediterranean basin to be analyzed. The evolution through the strait of a deep vein of dense water coming from the Eastern Mediterranean is described, together with the dynamic interaction with overlying layers.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
The principal hydrographic structures and dynamic conditions of the northern Tyrrhenian Sea are studied through the comparison of in situ field data and a modeling simulation. The field observations ...are long‐term Eulerian and Lagrangian current measurements associated with two hydrographic campaigns conducted in late summer and in winter. The data indicate that a well‐marked cyclonic gyre affecting the water column from the surface to the bottom is present in the northern Tyrrhenian Sea in both periods. The gyre undergoes significant seasonal changes in both shape and size. Following the indications of past studies, a numerical quasi‐geostrophic model driven by the wind jet blowing from the Strait of Bonifacio was able to reproduce the principal gyre characteristics. However, the wind jet alone does not explain all the gyre features, especially its seasonal variation. A study of the model results and field data indicates that these variations are linked to the seasonal variability of the northward surface current flowing along the eastern boundary of the Tyrrhenian Sea.
A current time series spanning from 1985 to 1996 in the Corsica Channel shows that seasonal and interannual variabilities are stable components of the circulation of the northern part of the Western ...Mediterranean Sea. For the first time these characteristics are related to the state of the North Atlantic Oscillation during winter. We have found that this large‐scale control is mainly maintained through the air‐sea interaction processes which take place in the Northern Basin. These results provide new elements for a consistent explanation of the major features of the basin circulation, such as the seasonal increase of the boundary current and Deep Water Formation.
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