Hydrodynamic modelling of coastal seas: the role of tidal dynamics in the Messina Strait, Western Mediterranean Sea (original) (raw)
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Natural Hazards and Earth System Sciences Discussions, 2016
This work explored the importance of considering tidal dynamics when modeling the general circulation in the Messina Strait, a narrow passage connecting the Tyrrhenian and the Ionian Sea sub-basins in the Western Mediterranean Sea. The tides and the induced water circulation in this Strait are among the most intense oceanographic processes in the Mediterranean Sea. The quantification of these effects can be particularly relevant for operational oceanographic systems aimed to provide short term predictions of the main hydrodynamics in the Western Mediterranean sub-basins. A numerical approach based on the use of a high resolution hydrodynamic model was adopted to firstly reproduce both the tides propagation and the wind induced and thermohaline water circulation within the Strait and surrounding areas and secondly to quantify the role of the Strait dynamics on the larger-scale water circulation. The obtained results confirmed the importance of a correct representation of the hydrodyn...
On the relevance of tidal forcing in modelling the Mediterranean thermohaline circulation
Progress in Oceanography, 2015
The four dominant constituents of the semi-diurnal and diurnal tides have been implemented in a regional eddy-resolving Mediterranean version of the Massachusetts Institute of Technology general circulation model to assess the role played by tides on the simulated Mediterranean thermohaline circulation. To this aim we have compared two 10-year hindcast simulations differing only for the inclusion/omission of tidal forcing. Following the recent recommendations suggested by Sannino et al. (2014) both simulations use the same model having a substantial increment of the horizontal resolution in the region of the Strait of Gibraltar. The results suggest that application of explicit tidal forcing in a Mediterranean model has non negligible effects on the simulated circulation in addition to the expected intensification of local mixing processes. The western basin exhibits an immediate response to the different characteristics of the inflowing AW observable in the modified deep water convection processes in the Gulf of Lion. The inclusion of tidal forcing also induces changes in the intermediate circulation of the Tyrrhenian Sea bringing to a better representation of local structures and a reinforcement of the global thermohaline cell. LIW dispersal paths in the eastern basin are also affected by tides.
Journal of Physical Oceanography, 1999
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.
A two-dimensional tidal model for the Mediterranean Sea
Journal of Geophysical Research, 1995
The tidal propagation in the Mediterranean Sea is described through a highresolution, two-dimensional hydrodynamic model forced by the equilibrium tide and the incoming tide at the Strait of Gibraltar. The four most significant constituents, M2, S2, K1, and O1, are included in the model. Good agreement with a set of 63 coastal gauges is achieved. The significance of the equilibrium tide and the forcing at the open boundary is investigated. The incoming wave from the Strait of Gibraltar is important in tuning the tides in the whole of the Mediterranean. For the north Aegean Sea the solution without the forcing at Gibraltar results in a doubling of the amplitudes of the semidiurnal tides. western basin is connected to the Atlantic Ocean through the Strait of Gibraltar. Both straits are narrow enough to constrain atmospheric forced waves with periods up to a few days [Garrett and Majaess, 1984]. The Adriatic and Aegean Seas are connected to the eastern basin through the Straits of Otranto and Crete, respectively. The bathymetry of the Mediterranean Sea is quite complex with both the east and west basins being more than 3 km deep in places. The Adriatic Sea extends as a canal with NW-SE axis and can be divided into two basins with different bathymetric characteristics. The north basin slopes down regularly toward the southeast with depths, in general, less than 300 m. The southern basin is much deeper with maximum depths over 1200 m. The configuration of the Aegean Sea is very complex. Its communication with the eastern Mediterranean is through the straits east (Crete-Karpathos, Karpathos-Rhodes) and west (Kithira-Crete) of Crete. Defant [1961] divided the basin into two regions, one that starts from the eastern strait (Crete-Karpathos-Rhodes straits) and extends in a NW-SE axis up to the north coasts of Greece and another which starts at the western strait (Kithira-Crete strait) and is effectively isolated (as far as tidal regimes are concerned) from the northern Aegean by the Cyclades island complex. Apart from the strait of Gibraltar, two smaller openings of the Mediterranean to the Bosporous (NE Aegean) and the Suez (SE Mediterranean) channels are negligible for tidal propagation studies. Paper number 95JC01671. 0148-0227/95/95 J C-01671 $05.00 The tides of the Mediterranean are produced by the addition of the direct action of the equilibrium tide together with the part of the Atlantic tidal wave that can penetrate through the succession of straits [Pugh, 1987]. The interaction of the resulting wave with the bathymetry produces tidal components which exceed values of 10 cm only in certain areas, namely, the north Adriatic, the Gulf of Gabes, and the north Aegean. Enhanced tidal currents known since antiquity are produced in Messina Strait, between the Italian peninsula and Sicily, and in Euripus Strait, between Evoia Island and mainland Greece. It is also worth noting that the second barotropic natural mode of oscillation of the Mediterranean basin is close to the frequency of the semidiurnal tides, thus complicating the propagation of these tidal waves [Schwab and Rao, 1983]. The propagation of the tide through the Strait of Gibraltar has long been a point of disagreement. Several authors [Harris, 1897; Stemeck, 1916; Villain, 1949; Lafuente et al., 1990] state that the incoming Atlantic tide sets up some sort of standing wave in the western basin. On the other hand, Maloney and Bums [1958, p. 2] consider the tide in the Mediterranean as a direct response to forcing by the tide-generating potential and state that "... the erection of a barrier across the Strait of Gibraltar would have little or no effect on the overall tide of the Mediterranean Sea." Defant [1961], in his extensive review of the subject, stated that a great amount of tidal energy penetrates through the Strait of Gibraltar. The employment of simple calculations [Candela et al., 1990] revealed that about 94% of the incident Atlantic wave is reflected at the entrance of the Strait of Gibraltar and that the incoming energy could account for a uniform amplification of 1.4 cm of the equilibrium signal (about 10%). Candela et al. [1990], from analysis of tidal currents in the Strait of Gibraltar, found a net input of energy for the M 2 component of 8 x 108 W but, in view of the associated standard deviation of 10 9 W, concluded that this was consistent with zero net flux. Existing hydrodynamic numerical models do not succeed in describing the tidal propagation in all areas, due to lack of tide gauge data, coarse spatial resolution, unreliable bathymetry, or a combination of these factors. Indeed, only four numerical models covering the whole of the Mediterranean are known to the authors. The first [Dressier, 1980] (hereinafter referred to as DR80), with a resolution of 1/3 ø, involves only the M 2 component. The second model [Vincent and Canceill, 1993; 16,223 16,224 TSIMPLIS ET AL.: TIDAL MODEL FOR THE MEDITERRANEAN SEA 45 40 35 30 I I I lOOO t' 3000 I lO 20 Figure la. Bathymetric chart of the Mediterranean. P. Canceill et al., Barotropic tides in the Mediterranean Sea from a finite element numerical model, submitted to Journal of Geophysical Research, 1993] (hereinafter these are referred to as CA93), with a spatial resolution of 10 to 20 km, could not reproduce the characteristics of the tide by direct forcing alone and therefore had to constrain its solutions at several points to succeed in describing the major characteristics of the tides. Although this may be a useful assimilation technique, especially for use in altimetric studies, it limits any conclusions on the physics of propagation of the tides in the Mediterranean and depends heavily on the reliability of the harmonic parameters at the forcing points. Another model using an assimilation technique based on tide gauge and altimetric data, with a resolution of 1/2 ø, excludes the Aegean Sea [Sanchez et al., 1992] (hereinafter referred to as SA92). Recently, another model based on tide gauge and altimetric data assimilation techniques has also become available through Internet (L. H. Kantha, personal communication, 1994), and a model by C. J. Lozano and J. Candela (manuscript in preparation, 1995) for the M 2 tide only, but which deals with the tidal energetics as well as the tidal propagation, will be available shortly (D. E. Cartwright, personal communication, 1995). The models present conflicting conclusions regarding the role of the Strait of Gibraltar and tidal propagation. DR80 found small differences in the east basin when the Gibraltar Strait was closed, thus concluding that the tides there are due 45 40 35 30 71 Lions 10 12 21 29 42 50 5 "• 11 13 26 ß 36 • ,58 9 37 •
Tidal models of the Mediterranean sea
1994
In order to evaluate the vertical displacement of the Sea Surface Topography due to the ocean tides, we have made the modeling of the four main constituents M2, 52, 01 and Kl for the whole Mediterranean Sea and the local model N2 corresponding to the western area. This paper concludes with a brief discussion of some results of the computed surface tide.
Assessment of recent tidal models in the Mediterranean Sea
Ocean tides especially in closed sea areas can deviate considerably from the theoretical values due to unequal water depths and to the fact that the continents impede the movement of water. Satellite altimetry enabled the development of improved tidal models event in closed sea areas, by assimilating altimeter data into hydrodynamic models. The Mediterranean Sea due to its morphology is an ideal test field for the assessment of tidal models, based on this technique. An attempt to assess the recent tidal models TPXO.6, GOT00.2 and NAO.99b was based on (a) an inter-comparison of tidal heights computed from the three models at different time moments on the same 15 t × 15 t grid covering the Mediterranean and (b) on the comparison of the statistics of a crossover analysis of nearly 2.3 years of JASON-1 altimeter data (Cycle 1-86), before and after the tidal correction, using the three tidal models. The inter-comparison in terms of mean value and standard deviation of the differences between the tidal heights on the 15 ~ × 15 ~ grid resulted in mean values up to 3 mm and standard deviations ranging from 16 to 26 mm. However, maximum values of differences exceed several din. On the other hand, the statistics of the crossover analysis showed a 17% decrease of the standard deviation of the JASON-1 crossover differences after the tidal correction. These results show a good agreement between the three tidal models.
Numerical simulation of the interannual variability of the Mediterranean Sea upper ocean circulation
Geophysical Research Letters, 1997
Numerical simulations reveal that variations in wind stress and heat fluxes can induce significant interannual fluctuations in the circulation of the upper layers of the Mediterranean. From January 1980 to November 1988, the atmosphere shows changes in the structure and magnitude of the surface winds and in the air temperatures which induce modifications in the upper ocean structure and currents. The model prediction of the interannual fluctuations of the Sicily Strait baroclinic westward volume transport is in agreement with observations and the variability is explained as a function of the wind curl forcing in the region. The current anomalies persist for many months after a Winter atmospheric anomalous disturbance has occurred over the basin. The Eastern Mediterranean basin is the area where the interannual ocean response is most pronounced.
Tidal and subtidal currents in the Strait of Sicily
Journal of Geophysical Research, 2004
Current measurements, from shipboard and moored instruments, from the Strait of Sicily are examined to evaluate the importance of tidal and subtidal currents during summer 2000. The results show that the principal tidal components (M2, K1, S2, and O1) are particularly important in the western shelf and sill region but are significantly less important over the eastern shelf and almost negligible in the central part of the strait. There is a significant internal tide in the narrowest passage through the western sill. The residual (nontidal) currents describe the main pathways and local structures of Atlantic Water as it crosses the strait. The residual currents are compared with the geostrophic velocity calculated from simultaneous hydrographic measurements.