This work explores the importance of considering tidal dynamics when modelling the general circulation in the Messina Strait, a narrow passage connecting the Tyrrhenian and the Ionian subbasins 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 subbasins. A numerical approach based on the use of a high-resolution hydrodynamic model was followed to reproduce the tides propagation and the wind-induced and thermohaline water circulation within the Strait and in surrounding areas. A set of numerical simulations was carried out 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 hydrodynamics in the Messina Strait even when focusing on predicting the water circulation in the external sea traits. In fact, model results show that tidal dynamics deeply impact the reproduction of the instantaneous and residual circulation pattern, waters thermohaline properties and transport dynamics both inside the Messina Strait and in the surrounding coastal and open waters.
In the XII chapter of the Odyssey, before the landing to Trinacria Island, Ulysses and his crew leaving the Circe refuge, experienced the wrath of Scylla and Charybdis, with great loss of men and ships (Homer, VI B.C.). Homer's poem, describing the intense vortices and heavy currents generated by the tides (Scylla and Charybdis) in the Messina Strait (Western Mediterranean Sea, hereafter MS), can be considered as one of the first examples of grey literature in physical oceanography.
The tides and the induced water circulation in this Strait (Fig. 1a and b) are among the most interesting oceanographic processes in the Mediterranean Sea, and not only because of Homer's epic.
The intense current speeds and the high variability of tidal phases and frequencies lead us to consider this area as one of the most energetic in terms of momentum and impulse all over the basin (Hopkins et al., 1984). This is why, in recent years, several research activities were carried out to investigate how tidal dynamics in this area can be exploited to produce renewable energy (Coiro et al., 2013).
Although notorious, the dynamic of the Strait is not fully addressed in scientific literature, with only few and old studies describing the water circulation in both theoretical and experimental terms (Hopkins et al., 1984; Cescon et al., 1997) and very few recent investigations using numerical modelling techniques (Androsov et al., 2002a, b). In particular, while tidal dynamic inside the Strait has been studied and described by many authors starting from the early XX century (Vercelli, 1925, 1926; Defant, 1940, 1961; Bossolasco and Dagnino, 1957; Massi and Sallusti, 1979; Mosetti, 1988), the effects of the Messina tidal in- and outflow on the outer open ocean thermohaline water circulation are still unaddressed in scientific literature. In particular, both recent and old studies focused mainly on describing the behaviour of Tyrrhenian and Ionian waters flowing through the Strait (Bossolasco and Dagnino, 1959; Androsov et al., 2002b) and on the generation of internal waves (Brandt et al., 1997, 1999) without quantifying the role of MS tidal dynamics in modifying the outer circulation pattern.
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 subbasins. Most of these ocean prediction systems (Tonani et al., 2008, 2015; Oddo et al., 2009; Pinardi et al., 2010; Sorgente et al., 2011) are not suitable to accurately reproduce the Strait dynamics mainly due to numerical grid limitation, such as orthogonality and spatial resolution generally not appropriate to describe the fine scale coastal features. As a consequence, their operational outputs, including water temperature, salinity and currents fields, are produced ignoring the contribution of the tidal exchanges within the Strait which can be capable of modifying the aforementioned fields as well as the water mass budgets between the subbasins.
Consequently, the question is how big the effect of accurately capturing the tidal dynamics and the finer scale processes is on the model reproduction of the general circulation in this area. This issue is particularly relevant in the case of MS, which is characterized by intense tidal dynamics (quite a unique case in the Mediterranean Sea), where tides are generally weak and have a low influence on the circulation (Sannino et al., 2015).
In this work, a numerical approach using a high-resolution hydrodynamic model based on finite elements method was proposed to reproduce both the tidal propagation and the wind-induced and thermohaline circulation in the Strait and surrounding areas and to quantify the role of the Strait dynamics on the outer water circulation. Three different scenarios, characterized by different model forcings, were investigated in order to identify the weight of each single contribution (tides, thermohaline and wind) to the main hydrodynamics in the area of interest.
The paper is organised as follows: a brief description of the MS study area, including the morphological and oceanographic features, is reported in Sect. 2. An overview of the applied methods including the description of the adopted numerical model and of the three simulated scenarios is reported in Sect. 3. In Sect. 4, the differences between the three scenarios results are analysed highlighting the importance of reproducing the tidal dynamics in the MS. Finally in Sect. 5, the concluding remarks are presented.
The Messina Strait (Fig. 1a, b) is a narrow and deep channel
connecting two Mediterranean subbasins: the Tyrrhenian and the Ionian Sea.
The Strait is comprised approximately between 37.9–38.3
The astronomic tides represent the main forcing driving the water circulation
inside the MS, which occurs mainly along the major axis. The water vertical
displacement varies between 0.2 and 0.3 m, which are the typical values of
tidal amplitudes in the Western Mediterranean Sea. Despite the very low
amplitudes, the water flow inside the Strait is very intense reaching up to
2.5 m s
The flow inside the Strait is directed northward during the flood phase and southward during the ebb phase. The interaction between the intense currents and the channel topography and bathymetry gives rise to the formation of inertial eddies and strong horizontal current shears generally located at the lee sides of both Sicily and Calabria main capes.
The Strait connects two subbasins with different oceanographic features. The northern part of the Strait opens on the Southern Tyrrhenian Sea which is characterized by the presence of a west to east flow located between 50 and 200 m, carrying the surface Atlantic water toward the northern part of the subbasin moving geostrophically along the Italian coast (Krivosheya, 1983; Astraldi and Gasparini, 1994; Millot, 1999; Vetrano et al., 2004). The Southern part of the Strait opens on the Western Ionian Sea characterized by a surface flow carrying the Ionian waters from the southern Calabria coastlines along the eastern side of Sicily coastlines southward to the Sicily channel.
The tidal dynamics and the water exchange in the Messina Strait and their influence on the general water circulation in the Southern Tyrrhenian and Ionian Sea are investigated following a numerical approach.
A high-resolution hydrodynamic model (SHYFEM, hereafter; Umgiesser et al., 2004), based on the finite element method applied to Messina Strait, Southern Tyrrhenian Sea and part of the Ionian Sea was adopted to reproduce the main hydrodynamic inside the Strait and in the surrounding open sea areas. This model is part of the operational system MeSOS (Messina Strait Operational System), developed under the framework of TESSA (Technologies for the Situational Sea Awareness) project funded by the Italian Ministry for Environment, aiming to develop an innovative operational platform for the sea awareness in the Mediterranean Sea.
SHYFEM was nested into a lower-resolution open ocean model, the Tyrrhenian Sicily Channel sub-Regional Model (TSCRM, hereafter) applied to the whole Tyrrhenian Sea and Sicily Channel in order to properly account for the lateral open boundary conditions.
The TSCRM sub-regional ocean model covers the area from 8.98 to
16.5
The nesting between SHYFEM and TSCRM was carried out following the procedure in Cucco et al. (2012a, b) which allows to reproduce in high details both the outer general circulation and the coastal hydrodynamics (Melaku et al., 2015; Marras et al., 2015).
The following section provides a description of SHYFEM, the numerical core of the system, its application to the MS, the adopted set-up and the three numerical experiments performed to achieve the proposed objectives.
SHYFEM is a 3-D hydrodynamic model based on the finite element method that resolves the shallow water equations integrated over each layer in their formulations with water levels and transports. The model has been applied with success in several applications and case studies in the Mediterranean Sea basin aimed to investigate and predict both open ocean and coastal hydrodynamics and to evaluate their mutual interactions (Bellafiore et al., 2008, 2011; Cucco et al., 2009; Melaku et al., 2012; Ferrarin et al., 2013a, 2014; Simeone et al., 2014; Umgiesser et al., 2014; Cucco and Umgiesser, 2015; Quattrocchi et al., 2016; Cucco et al., 2016).
It uses finite elements for horizontal spatial discretizations,
SHYFEM forcing data, including open boundary conditions (OBC) and surface boundary conditions (SBC) adopted in the three numerical experiments TDO, THO and TTC.
The GOTM (General Ocean Turbulence Model), a turbulence closure model described
in Burchard and Petersen (1999), was used for the computation of the vertical
viscosity
The hydrodynamic model is coupled with an advection and diffusion numerical module that takes into account the salt and thermal balance equations to simulate the transport of both passive tracers and the salinity and temperature in the domain. Details of numerical treatment are reported in Umgiesser et al. (2004).
SHYFEM uses finite elements unstructured mesh for representing the model domain. A grid composed of about 23 000 nodes and 45 000 triangular elements was implemented. The mesh reproduces the MS, the south-eastern Tyrrhenian Sea and part of the western Ionian Sea. In Fig. 1a, the extent of the model domain is depicted by the area with the bathymetric details. The elements size and shape distribution is modulated accounting for both the distance from the MS and the basin morphological features. In particular, the spatial resolution varies between 50 m inside the MS and 2 km in the outer areas, corresponding to the mesh size of the TSCRM open ocean model (Fig. 1c).
Two open boundaries were defined at the northern and southern mesh borders located in the inner Tyrrhenian Sea and in the Ionian Sea respectively (Fig. 1a). The model domain was vertically discretized with 30 zeta levels ranging between 5 and 800 m. At the closed boundaries, only the normal velocity is set to 0, whereas the tangential velocity is a free parameter corresponding to a full slip conditions. The same model parameter set-up defined in Ferrarin et al. (2013b) was adopted.
Three different simulations, with three different set-ups defining three different scenarios, were carried out. In the first scenario (Tide Only, TDO, hereafter), SHYFEM was used to reproduce the tidal propagation in the MS and surrounding areas.
In the second scenario (atmospheric and thermohaline forcing only; THO, hereafter), the model was used to reproduce the wind-induced and thermohaline circulation in the area by downscaling the sub-regional model (TSCRM) solutions to the high-resolution model (SHYFEM) domain.
In the third scenario (tide, thermohaline and atmospheric contributions; TTC, hereafter), SHYFEM was used to reproduce the general circulation in the area induced by both thermohaline and wind forcing and by astronomic tides. For all scenarios, simulations referred to the period between 1 January 2014 and 31 December 2015.
In Table 1, the summary of the forcing data adopted for the three scenarios
is provided. Open boundary conditions for TDO scenario consisted in a set of
2-year time series of hourly water levels data extracted from a regional
tidal model of the Mediterranean and Black Sea
(
For THO scenario, the open boundary conditions consisted in a set of hourly
time series of water levels, temperature (
Similarly, for the TTC scenario, the same ocean and meteorological data were used as open boundary conditions with the exception of the water levels which, in this case, were derived for each point of the mesh by the sum of the tidal elevations (adopted in TDO scenario) and the sea surface elevations computed by TSCRM (adopted in THO scenario).
The model was applied to simulate the 3-D water circulation and the
In the following, in the first part, the tidal dynamic inside the MS, as reproduced by the TDO scenario, was described in terms of water currents, fluxes and residual circulation. Subsequently, the role of the tides in modulating the wind-induced and thermohaline circulation was investigated both inside and outside the MS by comparing the THO and TTC scenarios results. Residual circulation, water fluxes, temperature and salinity distribution inside and outside the Strait as well as the transport properties were computed to quantify the influence of the tidal forcings.
Two different sections inside the MS, A–B and C–D, in Fig. 1c were used to compute the fluxes through the Strait and to investigate the vertical variability of the main hydrodynamics.
Amplitude and phase of main harmonics observed (
In the TDO scenario, SHYFEM was applied to reproduce the propagation of tides inside the MS and surrounding areas. The water levels computed at the tidal stations both outside (stations 1, 2 and 3 in Fig. 1a) and inside the Strait (stations form 4 to 10 in Fig. 1b) were compared with observations to estimate the model accuracy in predicting the tidal waves propagation in the domain.
For each station, harmonic analysis was applied to extract the amplitudes and phases of each main tidal constituent. The main harmonics observed amplitudes and phases were estimated by Vercelli (1925) during the homonym oceanographic cruise in 1922 and more recently reported by Brandolini et al. (1980) and Androsov et al. (2002a). In this work we refer to the values from Brandolini et al. (1980) for stations from 1 to 6 and from Androsov et al. (2002a) for stations from 7 to 10. We considered the tidal signals as composed by four semidiurnal waves (M2, S2, N2 and K2), by three diurnal waves (K1, O1 and P1) and by the M4 as compound tide, which are the major components of the tides in the Mediterranean Sea and in the MS (Vercelli, 1925; Sannino et al., 2014).
In Table 2, the computed and observed amplitudes and phases of the main components are reported. For stations inside the MS most of tidal energy, around 40 % of the total amplitude, is contained in the M2-wave, around 19 % in the S2-wave and around the 14 % of the total energy in the K1-wave. The obtained results are in line with the observed energy balance, in which M2-wave accounted for 38 %, S2-wave for 16 % and K1 for 13 % of the total water level.
Considering the amplitude of the M2-wave, the root mean square error (RMSE) between model results and observations computed for the whole set
of stations was on average about 1.7 cm with discrepancies ranging between
0.1 and 3.3 cm. As for the amplitudes, also the computed phases mainly
agree with observations, with average discrepancies between model results and
observed values estimated to be around 40
Considering the main diurnal K1, the RMSE between the observed and computed
amplitudes is about 0.3 cm with values varying between 0.1 and 0.8 cm. The
phases also agree with observations with average discrepancies between
observed and computed values around 40
For the main compound tide, M4-wave, the experimental dataset was incomplete, with observations available only for station 5 (Punta Pezzo) on the eastern side of the MS. For this station, the computed amplitude is in line with observations with an estimated discrepancy less than 1 cm. Even if not quantifiable by direct comparison with experimental data, the computed M4 amplitudes vary inside the MS following the same features described by Androsov et al. (2002a), with higher values in proximity of the sill on the eastern side of the MS (stations 9 and 5) and in proximity of Capo Peloro (station 4) on the western side of the MS. On average, considering the whole set of stations and tidal components, the model results reproduced the observed amplitudes with a RMSE of 0.78 cm, which is fair and acceptable for this type of analysis.
Vertically averaged water currents fields computed at the maximum
flow during ebb phase
The water circulation induced by the tides in the Strait develops mainly
along the main axis of the channel, with a northward flow during the flood
and southward flow during the ebb phase. Figure 2 shows the vertically
averaged water current fields computed at the maximum flow during both flood
(Fig. 2a) and ebb phases (Fig. 2b) of a spring tidal cycle. Numerical results
evidenced that, for the sill area, the ebb flow (southward) is generally more
intense than the flood flow (northward). Along the C–D sections the maximum
current speed computed during the spring tidal cycle was about
2.4 m s
Water fluxes computed through the sections C–D from the TDO scenario predicted maximum flux values of about 0.4 Sv in spring tides during ebb flows. On average, the ebb (southward) and the flood fluxes (northward) were about 0.1 Sv with differences between the two tidal phases of about 0.01 Sv and a dominance of southward flows.
During maximum inflow and outflow, advective eddies are generated at the lee side of the main capes. In particular, during the ebb phase (Fig. 2b) an inertial cyclonic feature is generated south of Punta Pezzo (station 5), whereas anticyclonic eddies are produced at the southern side of Capo Peloro in proximity of Ganzirri and Faro (stations 4 and 7) and south of Messina harbour (station 10). Contrarily, during the maximum flood flow (Fig. 2a) a small cyclonic eddy is generated north of Punta Pezzo on the Calabria side, whereas an intense anticyclonic eddy is formed outside of the strait by the intense outflow north of Capo Peloro (station 4). Similar inertial features were described by Cescon et al. (1997), which measured a strong inertial anticlockwise eddies south of Punta Pezzo, at station 5, migrating from the coast to the inner Strait during the ebb flow (see Fig. 2 in Cescon et al., 1997).
These features were not described by previous modelling works as in Androsov et al. (2002a), whose results highlighted only the flow vorticity generated by the bending of the Strait (see Fig. 12 in Androsov et al., 2002a). In that case, in fact, the low model spatial resolution was not adequate to solve the quasi-inertial features generated by the flow–morphology interactions.
During the slack phases at maximum and minimum tides the circulation pattern is less homogeneous and complicated flow geometries arise from the interaction between the out-flowing and the in-flowing water masses.
The aforementioned advective features are the main factors influencing the residual tidal flow inside the Strait. In Fig. 3a the Eulerian residual current field computed for the whole set of simulated synodic months and vertically averaged is reported for the MS area.
Vertically averaged residual circulation induced by tides in MS
The intensities of the residual speeds are not homogeneous inside the MS,
with values ranging between a few cm s
Computed winter residual current field from the THO scenarios
results (
Computed summer residual current field from the THO scenarios
results (
Among the presence of the several cyclonic and anticyclonic structures there is also evidence of a fixed circulation pattern which transfers water masses from the Tyrrhenian Sea along the Calabrian coast to the Sicilian coast in proximity of the sill and then southward to the Western Ionian Sea. This was also found in Androsov et al. (2002a) and validated through empirical observations by Mosetti (1988).
Figure 3b shows the vertical distribution of the residual circulation along
the sections A–B as computed for each model layer. The residual flow is not
vertically homogeneous with differences in both direction and intensity. Two
main cells, converging in proximity of the sill with an upward flow on both
Tyrrhenian and Ionian side, dominate the velocity components of the residual
currents along the section. In particular, while the northern cell is
continuous, connecting the outer deep layers to the inner surface layers, the
southern cell is incomplete and characterized by the presence of convergence
zone in the surface layers close to the sill. The component of the residual
flow orthogonal to the section is characterized by the presence of an
inversion of the current direction between the surface layers and the bottom
layers on both side of the sill. The highest residual speeds are found within
the top 50 m depth. The intertidal variability of the residual circulation is
generally low with an average standard deviation of the residual current
intensity of about 0.05 m s
The dominant role of tidal forcing in both ruling the exchanges between the two subbasins and modulating the hydrodynamics inside the MS was evidenced by the TDO scenario results.
The quantification of the tidal contribution to the general circulation both
inside and outside the Strait was investigated by comparing the THO (without
tidal forcing) and the TTC (with tidal forcing) scenario results. In both
scenarios the seasonal variations of temperature (
The comparison was carried out considering the water circulation and the
fluxes through the Strait, the
The comparison between the instantaneous current fields obtained from the THO
and TTC scenarios is trivial. In fact, the differences between the speeds
intensities in the two cases were higher than a magnitude order. Speed values of some m s
In Figs. 4 and 5 the residual currents computed for winter and summer seasons as the algebraic averages of the three-dimensional hourly current speeds obtained for winter (January, February and March) and summer months (July, August and September) during the 2 years of simulation are reported for both scenarios. Upper panels (a and b in Figs. 4 and 5) refer to the residual circulation obtained as the vertical averages of the residual velocities computed between the surface and the 50 m depth layer. Bottom panels (c and d in Figs. 4 and 5) refer to the residual field computed at the 100 m depth layer, which include the deepest part of the MS sill and where Tyrrhenian and Ionian deeper waters signals can be detected. Left (b and d in Figs. 4 and 5) and right (a and c in Figs. 4 and 5) panels depict the results obtained without and with the tidal contribution respectively.
Inside the MS, during the winter period, the wind and thermohaline forcings
only (THO scenario) generate a residual circulation mainly homogeneous in
space and directed southward with average speeds around 0.1 m s
Water fluxes through sections C–D computed for 45 days during
winter
Considering the TTC scenario results, the surface residual circulation
obtained for winter months (Fig. 4b) is strongly modulated by the residual
tidal current field (Fig. 3a) as evidenced by the presence of anticyclonic
and cyclonic gyres within the Strait. Similar values, around
0.6 m s
During the summer period, the wind-induced and thermohaline residual
circulation (THO scenario) at the surface (Fig. 5a) is less intense, with
peak speeds around 0.3 m s
As for the winter period, when tidal forcing is included (TTC scenario), the surface residual current pattern inside the MS (Fig. 5b) differs mainly for the presence of anticyclonic and cyclonic structures generated by the residual tidal flow. A general reduction of the residual current speeds is recorded at both surface and deep layers and both inside and outside the MS. In this case, the tidal action, while preserving the general pattern, tends to modify the two main residual flows, the northward and southward one, both inside and outside the MS. This is particularly evident at the 100 m depth layer outside the MS in the Tyrrhenian Sea (Fig. 5d), where the coastal current along the Calabria side is stronger and sharper than the one obtained from THO scenario results (Fig. 5c). In contrast, in the Ionian waters, the tidal forcing tends to disrupt the southward coastal flow by reducing its maximum speed and by smoothing the horizontal velocity gradients. Finally, the tidal action reduces the extension of the divergence areas inside the MS at both surface and deeper depths.
Water fluxes through sections C–D were computed during both winter and summer months from the THO and TTC scenarios results. The winter period included January, February and March, whereas the summer period included the months of July, August and September of both simulated years. As an example, in Fig. 6 the time series of the computed fluxes are reported, for reason of clarity, only for 45 days during both summer and winter 2015 and for the THO and TTC scenarios.
The magnitude of the computed fluxes differs between the two scenarios, with peak values higher than 0.4 Sv obtained when including the tides (blue lines in Fig. 6a and b) and values always less than 0.2 Sv when not (red lines in Fig. 6a and b). As expected, the differences between the TTC and the THO net fluxes are always different from 0. This is evident when considering the 24 h mobile averages computed from the TTC results (black line in Fig. 6a and b) where values, even when generally following the trend, are always differing from the computed THO fluxes. TTC and THO scenarios differ mainly for the net flux budget between the inflows and the outflows.
In Table 3 the average positive, negative and net fluxes computed during the winter and summer periods are reported for both scenarios. Also in this case, the differences between the THO and TTC results are found for both the northward and the southward fluxes, with TTC average values always greater than THO one, with values ranging between 0.084 and 0.04 Sv. This is particularly clear for the winter period when the less intense density gradients, leading to a lower contribution to the general circulation, tend to increase the differences between the two scenarios.
From both scenarios, the computed net flux is negative in winter (indicating a prevailing southward average flow) and positive in summer (indicating a prevailing northward average flow). The tidal forcing tends to smooth these discrepancies (TTC-THO in Table 3) with a reduction of 0.042 Sv to the net southward flux in winter and a reduction of 0.039 Sv to the net northward flux in summer.
Water, salt and heat fluxes through sections C–D computed for winter
and summer periods from THO and TTC scenario results and differences
(TTC–THO). Water and salt fluxes are expressed in Sv, heat fluxes in
10
Vertical distribution of the average water salinity expressed in
PSU computed along the sections A–B from THO (
Surface water temperature computed during the summer period: average
values from the TTC simulation results
The temporal and spatial variability of the water salinity and temperature was computed for both the TTC and THO scenarios and for the whole considered period. Summer and winter thermohaline properties of the water masses were also analysed and compared to each other.
From the TTC scenario results, the vertically averaged salinity in the MS area varied between 37.7 PSU, for the Tyrrhenian waters in the northern part, and 38.6 PSU, for the Ionian waters in the southern side, with average differences between winter and summer periods of about 0.5 PSU. The variation of the salinity fields due to the tidal contribution in the MS is, on average, less than 0.2 PSU with a general increment of salty water during summer and a decrease during the winter periods.
Figure 7 shows the winter and summer average salinity distributions computed from the THO and the TTC scenario results along sections A–B. In the winter period (Fig. 7a and b), due to the negative net flux budget across the Strait which indicates a prevailing southward flow, the saltier Ionian deep waters are mainly confined to the southern side of the sill. The action of tides, favouring the exchanges at the interface, tends to increase the outflow of saltier Ionian deep waters on the northern side of the Strait (Fig. 7b). This is confirmed by the computation of the average net salt fluxes through sections C–D (Table 3) that indicates a general decrease of the southward flux component, estimated around 1.6 Sv, when considering the tidal contribution. The differences in the vertical structures between the two scenarios are mainly due to the tidal residual current field, which is characterized by a convergence zone in proximity of the sill (Fig. 3b) that tends to foster the rising of the deeper and saltier waters.
Summer average water temperature along sections A–B computed from THO
and TTC scenarios (
Monthly average distribution of tracers release during winter (upper panels) and summer (lower panels) from THO (left panels) and TTC scenario results (right panels). Coloured areas refer to values with 10 % the initial tracer concentration. Red and blue lines refer to Ionian tracked waters at bottom and surface layers. Green and yellow lines refer to Tyrrhenian tracked bottom and surface layers water.
In summer period, the average northward net flux (Table 3) causes the partial
overflow of the Ionian lower surface waters across the sill into the northern
side of the MS. While tidal exchanges generally tend to counteract this
process, as confirmed by the negative differences of
During winter, the water
During summer, due to the strong heat fluxes and the intense large-scale
thermohaline circulation, the
In Fig. 8b the differences between the average surface
Inside the MS, the tides tend to reduce the surface
Off the northern mouth, in the Tyrrhenian Sea, the tides induce an average
increase of
The vertical distributions of the average summer
The heat fluxes through sections A–B was computed for both summer and winter
periods (see Table 3), revealing in the Tyrrhenian basin an average net heat
loss of about 2.7
The action of tides tends to reduce the discrepancies between northward and
southward flows with a similar contribution found in the analysis of the
water fluxes. In fact, the ratios between the TTC and THO net values are
about 0.35 for the winter period (see Table 3;
The transport of water masses through the MS is a fundamental process to be investigated in order to understand the importance of this narrow passage on the hydrodynamics of the surrounding open and coastal seas areas.
The effects of tidal action on the exchanges through the MS were evaluated considering its role in changing the dominance of Tyrrhenian and Ionian waters in the Ionian and Tyrrhenian subbasins respectively. Specifically, passive tracers were monthly released with unitary concentration in the southern part of the MS, south of station 10, for tracking Ionian waters, and in the northern part of the basin, north of stations 4 and 8, for tracking Tyrrhenian waters. The trajectories of both tracers were simulated for the whole considered period in both TTC and THO scenarios. The winter and the summer monthly average distribution of the tracer [C] were then analysed.
In Fig. 10, the 0.1 [C] contour lines for the surface (5 m depth) and bottom layers are reported for both the Tyrrhenian (blue and red lines in Fig. 10) and Ionian tracers (green and yellow lines in Fig. 10) and for winter (Fig. 10a and b) and summer (Fig. 10c and d) as obtained from the THO and TTC scenarios (Fig. 10a and c).
The 10 % contour lines for both surface and bottom layers indicate that
during winter (Fig. 10a and b), the tidal action favours the outflow
of Ionian waters into Tyrrhenian Sea and tends to reduce the presence of
Tyrrhenian waters into the Ionian Sea. For both tracers, the highest
variations between the two scenarios are found in the surface waters. In
particular, for the Ionian tracer, the tides increase the surface with an average
[C] of about 14 % for surface layer and about the 3 % for bottom layer.
Similarly, the reduction of the area with Tyrrhenian tracer [C] greater than
10 % is higher for surface waters, with about the
In contrast, during summer (Fig. 10c and d), the contour lines indicate that tidal forcing generates the outflow of the Tyrrhenian waters into the Ionian Sea and tends to foster the propagation of Ionian waters northward. In particular, for Ionian waters, the reduction of the surface with tracer [C] greater than 10 % was higher in deeper layers, up to 40 % at the 200 m depths, and lower for surface layers, about 19 % at 5 m depth. This is in line with the results obtained by the analysis of the residual circulation (Fig. 5b); in fact, the tidal action, while reducing the residual current fields in the area, tends to maintain and intensify the strong coastal current on the eastern side of the basin. This is evidenced by the fact that in the case of tide, even if the 10 % contour lines is less extended, the average tracer [C] in the surface layers of the Calabria coastal areas are always higher than those found in the THO scenario. In contrast, for the Tyrrhenian tracer, the increment of the area with [C] higher than 10 % induced by the tidal action is higher for surface layers, around 35 % at 5 m, and lower for deeper layers, with 5 % at 100 m.
The focus of this work was the investigation of the effects of the reproduction of the tidal dynamics on modelling the general circulation in the Messina Strait, a narrow passage connecting the Tyrrhenian and the Ionian Sea subbasins in the Western Mediterranean Sea.
The adopted approach highlighted the role of the tides in modulating the local circulation patterns and allowed to quantify the effects of the Strait tidal hydrodynamics on the circulation of the surrounding coastal waters. These aspects are particularly important when dealing with operational oceanography systems designed to provide short-term predictions that, in turn, can be used by other end-user applications such as decision support systems for maritime safety (Mannarini et al., 2016a, b) or simply to provide basic fields for marine forecasting applications.
These operational oceanographic systems are generally characterized by low spatial resolution, on the order of some kilometres, which is too coarse to reproduce the tidal dynamics and the general circulation inside this narrow passage. As a consequence, these systems are also not suitable to make predictions over a large extent of the surrounding open sea areas where the influence of the Strait dynamic was found to be important. For this area, in fact, the correct reproduction of the MS tidal dynamics deeply impacts on the prediction of the instantaneous and residual circulation pattern, the thermohaline properties and transport dynamics.
Considering the exchanges between the two subbasins through the MS, the correct reproduction of the tidal propagation in the Strait can generate changes in water fluxes ranging between 800 %, when considering the instantaneous values, and 60 % when estimated from the residual flows.
Although these values slightly influence the overall flux budgets of the two
subbasins, the increase in current speeds in the Strait due to tides, as well
as the intensification of the interactions between water masses with
different thermohaline properties, lead to intense modifications of the
current and thermohaline fields in Tyrrhenian, Sicily channel and southern
Ionian coastal waters. In fact, considering the Tyrrhenian and Sicily Channel
water mass budgets, while the net fluxes through the MS are up to 2 orders of
magnitude lower than the monthly average fluxes through the Sardinian and
Sicily Straits, with 10
This is particularly evident from the water temperature analysis in the outer
coastal areas, where a correct reproduction of the MS dynamics led to changes
in surface
The obtained results confirmed the importance of reproducing correctly the MS dynamics even when focusing on predicting the hydrodynamics of the external sea areas. This is not only the case of Messina Strait and Tyrrhenian and Ionian Sea and Sicily Channel subbasins; in fact, most of the open ocean operational systems applied to areas with complicated geometry, including narrow passages and straits, are not suitable to correctly reproduce the hydrodynamics of coastal waters and consequently their role in influencing the open ocean current pattern. In our study case, the water circulation inside the Strait was particularly important in shaping the outer seas current patterns and thermohaline features. Therefore, the use of numerical procedures, including both open ocean and high-resolution coastal hydrodynamic models, is highly suggested even for operational oceanographic systems focused on predicting the open ocean hydrodynamic fields.
This research was funded by the project TESSA (Technologies for the Situational Sea Awareness) project funded by the Italian Ministry for Environment. Edited by: I. Federico Reviewed by: two anonymous referees