Dr Elham Baktash

Dr Jérôme Capitolis

Dr Lionel Tinat

Dr Clément Larquet

Tsou Hsi

Camille Chan Chang

Jean-Jacques Gallet

Dr Fabrice Bournel

Clément Sanchez

Sophie Carenco

David Portehault
email: david.portehault@sorbonne-unversite.fr

Prof T H C Chan C Chang

Dr S Sanchez

Dr D Carenco

Portehault

Dr J.-J Gallet

Dr J.-J Gallet

Differential reactivity of rutile and anatase TiO2 nanoparticles: synthesis and surface states of nanoparticles of mixed valence Magnéli oxides

Keywords: titanium oxides, Magneli, nanoparticles, surface states, Near-Ambient-Pressure XPS

Magnéli phases TinO2n-1 (3 < n ≤ 10) are mixed Ti 4+ /Ti 3+ oxides with high electrical conductivity. When used for water remediation or electrochemical energy storage and conversion, they are nanostructured and exposed to various environments. Therefore, understanding their surface reactivity is of prime importance. Such studies have been hindered by carbon contamination from syntheses. Herein we address this synthetic and characterization challenge through a new approach towards 50 nm carbon-free Ti4O7 and Ti6O11 nanoparticles. To do so, we take advantage of the different reactivities of rutile and anatase TiO2 nanoparticles versus H2, to use the former as precursors of TinO2n-1 and the latter as diluting agents. This approach is combined to silica templating in order to restrain particles growth. The surface reactivity of the Magnéli nanoparticles under different atmospheres was then evaluated quantitatively with synchrotron radiation-based X-ray photoelectron spectroscopy, highlighting oxidized surfaces with lower conductivity than the core. This finding sheds a new light on the charge transfer occuring in these materials.

Introduction

Titanium oxides TinO2n-1 (4 ≤ n ≤ 10), so-called titanium Magnéli phases, were discovered in the 1950's [1] and experience since the 2010's a renewal of interest owing to their large conductivity and good stability, related to the mixed Ti 4+ /Ti 3+ valences and to their oxide nature, respectively. These properties have prompted the study of Magnéli phases in the late years as conductive scaffolds to be used in harsh conditions as cathodes for aprotic lithium-air [2- 4] and lithium-sulfur batteries, [5][6][7][8] anodes for alcohols oxidation [9,10] and microbial fuel cells [11] as well as water remediation membranes. [12,13] Superconductivity has also been recently evidenced in an epitaxially grown Magnéli phase. [14] Most of these studies not only take benefit from the conductivity of Magnéli phases, especially metallic Ti4O7, they also rely on their surface properties. For instance, many authors attribute the enhanced cyclability of Ti4O7-sulfur electrodes of Li-S batteries to the chemical bonding between sulfur atoms and Ti atoms at the surface of the Ti4O7 electrodes. [5][6][7] Likewise, Ti4O7 is considered as electrocatalyst [2] or electrocatalyst support [3,4] for air electrodes of Li-air batteries owing to their surface stability. For these reasons, increased surface-to-volume ratio is sought through nanostructuration, which emphases the importance of deep scrutinization of the composition and reactivity of Magnéli oxide surfaces. Although X-ray Photoelectron Spectroscopy (XPS) has shown that Ti 4+ and Ti 3+ coexist at the surface of Magnéli materials, [4,6,9,11,15] only two reports suggest surface oxidation of Ti4O7 materials, [2,8] but precise evaluation of surface states is lacking, so that the conduction and sorption properties of few nm-thick surface layers remain mostly a black box. Nanostructured Magnéli phases can be produced by several approaches, including reduction of TiO2 by Zr oxygen sponges [16] or plasmas. [17] However, most of the materials reported above are produced by two main separate ways. The first pathway relies on carbothermal reduction of TiO2 at ~800-1100 °C. It unavoidably produces carbon residues, accounting for 2 to ~20 wt. % of the samples. [5,7,8,[18][19][20][21] These residues confine the oxide during the transformation of TiO2 into Magnéli phases and enable recovering nanostructured materials. However, carbonaceous moieties are present at the surface of the Magnéli material. They can interfere in the oxide-electrolyte or oxidesulfur/polysulfide interactions, thus making it difficult to draw conclusions about the respective role of the surfaces of the oxide and of the carbonaceous components. The second approach uses reduction of stoichiometric TiO2 by dihydrogen at elevated temperature ~800-1100 °C. [3,4,6,11,13] In this route, crystal growth is difficult to restrain and leads to materials with submicronic length scale. [6,11,13] Attempts to limit grain growth by silica templating [3,4,15] encountered only limited success, leading mostly to 100-300 nm spheres [3,4] after dissolution of the silica coating.

Herein we propose an alternative method to produce carbon-free Magnéli nanoparticles (Figure 1) by reduction of a low cost commercial TiO2 product, so-called P25, a mixture of nanoparticles of anatase and rutile TiO2 polymorphs. In order to avoid extensive crystal growth during the crystallization of Magnéli phases, the approach takes benefit from silica templating and from the mechanism of formation of Magnéli phases, which occurs only in the rutile component through topotactic transformation. Therefore, anatase particles act only as diluting agents to further limit particle sintering. On the opposite, anatase particles can be selectively dissolved during a post treatment with hydrofluoric acid, in order to isolate the Magnéli nanoparticles. We then use these carbon-free nanoparticles as models to assess surface reactivity. Surface states are quantitatively probed by synchrotron radiation-based XPS for the pristine nanoparticles and after exposure to different atmospheres.

Results and Discussion

Synthesis and characterization of carbon-free Magnéli nanoparticles

Commercially available P25 titania was used as titanium precursor. This powder contains a mixture of anatase and rutile TiO2 (80 wt. % anatase and 20 wt. % rutile) with ~35 nm particle size as shown by transmission electron microscopy (TEM) (Figure 2a). Silica-coated TiO2 nanoparticles were prepared via the Stöber method [22] from tetraethyl orthosilicate as silica precursor added to an alcoholic suspension of P25 in basic conditions. TEM (Figure 2b) confirms that TiO2-SiO2 core-shell nanoparticles are obtained, with an 8 nm-thick silica shell showing lower contrast than the crystalline TiO2 core. A similar SiO2 shell is observed when grown on pure rutile particles (Figure S1). These particles were dried and the powders were heated at 1050 °C under reducing atmosphere (H2/Ar, 5/95 vol. %) for 3, 6, 9 and 12 h. TEM (Figure 2c-e) shows that nanoparticles are obtained at 6 h and 12 h, still with surrounding silica, also observed from rutile nanoparticles (Figure S1). HRTEM (Figure 2d, f) indicates that the particles are made of Ti6O11 and Ti4O7 at 6 and 12 h, respectively. This is confirmed by powder X-Ray Diffraction (XRD, Figure 3) that also indicates minor amounts of crystalline anatase TiO2 with Ti4O7. As expected, increasing the reduction time yields further reduction, from the semiconductor Ti6O11 Magnéli phase to metallic Ti4O7 (Figures 2 and3). The stability of anatase, not the most stable polymorph in the bulk phase, suggests that some anatase nanoparticles are sufficiently stable to stand harsh thermal conditions. Indeed, anatase becomes the most stable polymorph of TiO2 for nanoparticles smaller than ~10 nm. [23] Then, in the size range of P25 particles, the relative stabilities of anatase and rutile are close and yield coexistence of both polymorphs. As anatase particles growth is restricted by the silica coating, the smallest TiO2 particles are stable in the anatase polymorph and do not convert into TiO2 rutile. Rutile is a crucial intermediate for the formation of Magnéli phases through topotactic transformation by formation of shear planes (Figure 1), [18,19,24] so that hindering its formation also hinders the formation of Magnéli phases. Both silica and anatase dissolve in an hydrofluoric acid solution, while rutile TiO2 phase does not react. [25] Because of the structural similarity between rutile and Magnéli phases, one can expect similar inertness versus hydrofluoric acid. Therefore, the samples were treated with an aqueous HF solution of diluted hydrofluorhydric acid (5 % vol.) to remove anatase and silica by-products. After washing, anatase remained only as minor impurity, while the Magnéli crystal structures were preserved (Figure 3). No particle smaller than 10 nm could be distinguished by TEM (Figures 4 and5) and silicon was not anymore detected by XPS. HRTEM images and Selected Area Electron Diffraction (SAED) patterns are fully indexed along the Ti6O11 (Figure 4) and Ti4O7 (Figure 5) structures, respectively. The size distributions (Figures 4 and5) for both Ti6O11 and Ti4O7 samples indicate that the nanoparticle size increased after treatment at high temperature, with distribution maxima for diameters of ca. ~50 nm. The occurrence of minor amounts of particles larger than 80 nm suggests that at 1050 °C, the silica coating experiences local rearrangements and does not act as a perfect barrier against particle growth. However, severe particle growth of the titanium oxide particles is clearly limited thanks to the combined action of silica and anatase particles against sintering. Indeed, a reference sample produced by heat treatment for 12 h of pure rutile nanoparticles embedded in silica yield particles larger than 100 nm in average (Figure S1). Hence, non-reactive anatase nanoparticles are crucial for hindering Magnéli particles sintering through a dilution effect. Overall, the Magnéli oxides Ti6O11 and Ti4O7 can be obtained as crystalline nanoparticles of 50 nm, respectively.

In order to assess the surface reactivity of the silica-free Magnéli nanoparticles versus different atmospheres, we have probed the average oxidation state of titanium near the surface by X-ray Photoelectron Spectroscopy (XPS) induced by synchrotron radiation. Adjusting the incident energy enabled tuning the probed depth and then retrieving depth profiles of Ti oxidation state. [26]

Surface state of as-obtained Magnéli nanoparticles

The Ti6O11 and Ti4O7 nanoparticles were analysed after synthesis and purification with hydrofluoric acid in their initial state (Figures 4 and5). The survey XPS spectra are provided as Figure S2. The photon energies were fixed at 560, 735 and 950 eV, corresponding for Ti2p to electron kinetic energies of 100, 390 and 490 eV, then to depths of analysis (Inelastic Mean Free Path IMFP) of 0.6, 0.9 and 1.3 nm, hence 2, 3 and 4 TiO6 octahedra, respectively. [27] Spectra of the Ti 2p region of the fresh nanoparticles (Figures 6a andS3 for IMFP 0.6 nm, Figures S4 and S5 for all IMFPs) show two unambiguous contributions: one for Ti(IV) species (green) and one for Ti(III) species (brown), each of them a doublet with a splitting of about 5.7 eV. The two contributions are separated by 1.6 eV, in agreement with previous reports. [28,29] The fitting procedure (details in the experimental section and SI) allows calculating the average Ti oxidation state through each probed depth (Figure 6b). Due to the fairly low amount of Ti(III) near the surface, the precision on the average oxidation state is ± 0.1. The expected oxidation states of Ti4O7 and Ti6O11 are respectively 3.50 and 3.67. We observe higher oxidation state than expected: 3.8 for Ti4O7 and a gradient from 3.9 to 3.7 for Ti6O11, from the very surface to a depth of ~1.3 nm for Ti6O11. These results are confirmed by the O 1s region of the XPS spectra (Figure S6). This behaviour is in agreement with two previous works qualitatively showing surface oxidation of Magnéli materials produced by carbothermal reduction, hence containing carbon residues. [2,8] Here we not only show that surface oxidation occurs even with carbon-free Magnéli phases, but we also quantify the average Ti oxidation state. Although the XPS measurements are performed on a powder deposited on a substrate, [26] one can propose a simple model of the depth profile of the Ti oxidation state, based a simple 3-layers model from the three depths probed by XPS (Figure 7). [26] Note that the Ti oxidation state most probably evolves through a gradient from the surface to core, so that the layered model shown here is a simplification. Ti 3+ is present at the surface of Ti4O7 and Ti6O11, ensuring charge transfer during interfacial events occurring in thermoelectric energy conversion, [18] electrochemical water remediation [12,13] or charge storage. [2][3][4][5][6][7][8] However, the low Ti 3+ amount in the first three octahedra layers corresponds to a composition closer to Ti10O19 than Ti4O7 or Ti6O11, hence a semiconducting layer instead of a metallic phase as expected from Ti4O7. This result suggests that accurate evaluation of sorption properties must take into account such surface oxidation. [6] Given XPS measurement uncertainties (Figure 7), the buried layer corresponding to the fourth TiO6 octahedra layer reaches compositions close to the nominal compounds Ti4O7 and Ti6O11, respectively. A gradual decrease of the average oxidation state deeper into the particle core is likely to occur and yield the Ti4O7 and Ti6O11 detected by XRD.

Surface reactivity of Magnéli nanoparticles

In order to assess the surface reactivity of the Magnéli phases, we exposed the nanoparticles to oxidizing conditions. O2 (0.2 mbar) was introduced in the chamber and the samples were heated up to 390 °C. Spectra were collected after cooling the samples and pumping the gas atmosphere (Figure 6a for IMFP 0.6 nm and Figures S7 and S8 for all IMFPs). The average oxidation state has been evaluated from the fit of these curves (dotted lines on Figure 6b). For the first two Ti atom layers (IMFP = 0.6 nm), complete conversion of Ti(III) to Ti(IV) was observed for both Ti4O7 and Ti6O11. Deeper below the surface (Figures 6b and7), more reduced states were observed. Therefore, close to the core, some Ti(III) species survived the oxidizing treatment and the surface layer of TiO2 acted as a passivation layer. Note that at similar temperatures but higher pressure of oxygen (1 bar), thermogravimetric analysis shows that Ti4O7 experiences complete oxidation into TiO2. [18] We attempted to reduce back the surface Ti(IV) species into Ti(III) by exposing the nanoparticles to 1 bar of H2 under heating up to 350 °C for 10 minutes. The samples were again analyzed after cooling and outgasing (Figure 6a for IMFP 0.6 nm, other IMFPs not shown). No significant change in the oxidation states was observed, indicating that in such conditions, surface oxidation is not reversible and reduction would need higher temperatures.

Conduction properties

Because XPS depends strongly on the ability of the material to extract the charges produced by incident X-ray radiation, the technique is suitable to sort qualitatively different materials in terms of electric conductivity and electronic structure. Each Ti 2p spectrum was calibrated over Au 4f spectrum, collected within minutes. Within the spectral resolution of the beamline (<0.1 eV), charging effects were observed in the Ti 2p region. At each step of the thermal treatment, the Ti 2p3/2 binding energy (B.E.) of the Ti(IV) species was used as an indicator of this charging effect (Figure 8).

For each incident energy, B.E. for Ti4O7 shows a negligible variation, between 0 and 0.2±0.1 eV (Figure 8a). For Ti6O11 (Figure 8b) these variations are between 0.1 and 0.4 eV, the main effect being observed from the initial state to the oxidized state, where the variation is between 0.3 and 0.4 eV, which represents significant surface charging. Charging effects observed here are thus compatible with the higher conductivity reported for Ti4O7 than for Ti6O11, in the bulk state. [30] More important, this study also shows that even if Ti4O7 is oxidized at the surface under oxygen at 390 °C, it is still more conductive (higher B.E.) than oxidized Ti6O11, and a fortiori TiO2. Thus, if surface oxidation impacts the first three TiO6 octahedra layers at the surface of the particles, Magnéli phases and especially Ti4O7 maintain high conductivity. Note also that after treatment under H2, the conductivity is increased (Figure 8), although no significant change in the surface oxidation state of Ti is observed. This evolution could arise from the formation of small amounts of Ti 3+ within the uncertainty range of the XPS measurements. The conductivity enhancement could also be related to the creation of a disordered surface layer, well documented for black anatase produced by reacting H2 with stoichiometric TiO2 in the same temperature range. [31,32]

Conclusions

In this work we have reported two advances. We have first described a simple approach to produce carbon-free nanoparticles of Magnéli phases with diameter approx. 50 nm and controlled stoichiometry. This achievement was made possible by combining silica templating and the large difference in reactivity of two different TiO2 polymorphs: while rutile TiO2 nanoparticles transform into Magnéli phases upon reduction, anatase TiO2 nanoparticles do not yield Magnéli phases and can act as diluting agents to avoid severe sintering of the Magnéli nanoparticles. On the opposite, anatase is more reactive towards hydrofluoric acid than Magnéli phases, so that it can be selectively dissolved together with the silica template and produce freestanding Magnéli nanoparticles. Such nanoparticles without surface carbonaceous moieties appear as excellent models to assess quantitatively the surface reactivity of Magnéli nanostructures. Sensitivity to oxidation under ambient conditions has been evidenced for the first surface layers of TiO6 octahedra. Note that this surface oxidation could also be partly related to short exposure to aqueous diluted hydrofluoric acid in order to dissolve silica and anatase components. This question should be addressed in the future. These results should enlighten the properties of interfaces between Magnéli materials and various electrolytes or electrode materials, such as sulphur or lithium oxides, [2,5,6] in order to design advanced energy storage and remediation devices.

Experimental Section

Materials. P25 titania, tetraethyl orthosilicate (TEOS) (99.999%), ammonia aqueous solution (28-30%) and ethanol (99.5%) were purchased from Sigma-Aldrich. Pure rutile nanoparticles were purchased from SSNano. All chemicals were reagent grade and used without further purification.

Synthesis. 1.0 g of P25 and 1.34 mL of ammonia solution were added into 10 mL ethanol. The mixture was dispersed using ultrasonication for about 30 min. 1.9 mL of TEOS was added dropwise to the dispersion during 45 min. The concentration of TEOS after addition into the dispersion was 0.85 M. The reagents molar ratio correspond to Si/Ti = 0.68. After stirring overnight, the reaction mixture was centrifuged at 15000 rpm for 10 min. The supernatant was removed, the resulting core-shell TiO2-SiO2 nanoparticles were washed with ethanol by three cycles of centrifugationredispersion and then dried overnight at room temperature under vacuum. In the next step, TiO2-SiO2 nanoparticles were heated to 1050 °C at 10 °C•min -1 under reducing atmosphere (H2/Ar, 5:95 vol.) for different periods to yield silica-coated Magnéli particles. Finally some selected samples were treated with an HF solution 5 vol. % for 15 min. They were then washed three times with MilliQ water and dried under vacuum at room temperature overnight.

Powder X-Ray Diffraction (XRD) was performed on a D8 Bruker diffractometer operating at Cu-Kα radiation in the Bragg-Brentano configuration. The XRD patterns were indexed along the ICSD cards 9852, 9161, 35121 and 16297 for anatase TiO2, rutile TiO2, Ti6O11, Ti4O7.

Transmission Electron Microscopy (TEM) was performed on a Tecnai

Spirit microscope operating at 120 kV. Samples were prepared by dropcasting of a suspension of nanoparticles on a carbon-coated copper grid. SAED patterns were calibrated according to the pattern of a sample made of gold nanoparticles. The patterns were indexed along the crystallographic references listed above in the XRD section.

X-ray photoelectron spectroscopy (XPS)

. XPS experiments were performed on the NAP-XPS end-station of Sorbonne Université set on the TEMPO beamline at SOLEIL synchrotron (Saint-Aubin, France). The hemispheric electron analyzer is a Phoibos 150 NAP apparatus (SPECS). The samples were prepared by dip-coating. Nanoparticles suspended in ethanol were deposited on a silicon wafer coated with a 50 nm thick gold layer. The XPS spectra were calibrated according to the Au 4f7/2 signal at 84.0 eV. Using CasaXPS software package, the Ti 2p regions were fitted with two doublets Ti 2p3/2/Ti 2p½ corresponding to Ti(III) and Ti(IV) (see supporting information). For both contributions the background was fitted by a Shirley background and the components by a Gaussian/Lorentzian product GL(30) line shape. Details of the fitting constraints are given in supporting information. The average oxidation state for Ti in the probed layer was then evaluated according to the following formula:

In this formula, ∆ox mean is the average oxidation state for Ti in the probed layer. ATi(III) and ATi(IV) are the area of the 2p 3/2 peak of titanium at oxidation state III and IV, respectively.

A new route to nanoparticles of TinO2n-1 mixed valence oxides with controlled size distribution is described, by combining a templating strategy with the different reactivities of two TiO2 polymorphs. Synchrotron radiation-based photoelectron spectroscopy reveals for the first time the surface reactivity of these carbon-free nanoparticles.

Elham Baktash, Jérôme Capitolis, Lionel Tinat, Clément Larquet, Tsou Hsi Camille Chan Chang, Jean-Jacques Gallet, Fabrice Bournel, Clément Sanchez, Sophie Carenco, David Portehault* Differential reactivity of rutile and anatase TiO2 nanoparticles: synthesis and surface states of nanoparticles of mixed valence Magnéli oxides ((Insert TOC Graphic here; max. width: 11.5 cm; max. height: 2.5 cm))

Figure 1 .

 1 Figure 1. Scheme of the carbon-free process developed herein for the synthesis of freestanding nanoparticles of Magnéli phases. The approach combines silica templating, selective transformation of rutile TiO2 into Magnéli phases, inability of the anatase polymorph to transform into Magnéli phases and selective solubilisation of silica and anatase into an aqueous solution of hydrofluoric acid.

Figure 2 .

 2 Figure 2. TEM images of (a) TiO2 P25 nanoparticles, (b) TiO2-SiO2 coreshell particles. TEM (c) and HRTEM (d) of Ti6O11 particles embedded into silica after thermal treatment (6 h). TEM (e) and HRTEM (f) of Ti4O7 particles embedded into silica after thermal treatment (12 h). Black arrows highlight silica, also shown in zooms-in Figure S1.

Figure 3 .

 3 Figure 3. Powder XRD patterns of P25 titanium oxide particles in their pristine state and embedded into silica after heat treatment under H2 (5 vol. % in argon) during 3 h, 6 h, 9 h and 12 h. The samples obtained at 6 h and 12 h have been treated in hydrofluoric acid 5 vol. % in order to recover Ti6O11 and Ti4O7 nanoparticles, respectively. The peaks are indexed according to anatase TiO2 (), rutile TiO2 ( ▌), Ti6O11 (), Ti4O7 ().

Figure 4 .

 4 Figure 4. (a) TEM image, (b) SAED pattern, (c) HRTEM image and (d) size distribution of Ti6O11 nanoparticles obtained after hydrofluoric acid treatment. The SAED pattern and HRTEM image are indexed along the Ti6O11 structure.

Figure 5 .

 5 Figure 5. (a) TEM image, (b) SAED pattern, (c) HRTEM image and (d) size distribution of Ti4O7 nanoparticles obtained after hydrofluoric acid treatment. The SAED pattern and HRTEM image are indexed along the Ti4O7 structure.

Figure 6 .

 6 Figure 6. (a) Ti 2p X-ray photoelectron spectra of Ti4O7 and Ti6O11 nanoparticles collected with a photon energy of 560 eV (IMFP 0.6 nm corresponding to the first top TiO6 octahedra layer). Ti(IV) species are in green and Ti(III) species in brown. Spectra were recorded at the initial state, after exposure to O2 (0.2 mbar) at 390 °C and then after exposure to H2 (1.0 bar at 350 °C). Average oxidation states (b) of both samples are plotted for the initial state (plain lines) and the oxidized state (dotted lines) as a function of the IMFP (depth analysis).

Figure 7 .

 7 Figure 7. Depth profiles of the average oxidation state of titanium for Ti4O7 and Ti6O11 nanoparticles in their initial state (a) and after oxidation under 0.2 mbar of O2 at 390 °C (b). The oxidation states are evaluated from the Ti 2p X-ray photoelectron spectra at different incident energies, eg. different IMFP values, hence different probed thicknesses. The three layers from the surface to the core correspond to the first and second (0.6 nm), third (0. 3 nm) and fourth (0.4 nm) TiO6 octahedra layers, respectively.

Figure 8 .

 8 Figure 8. Binding energies (B.E.) of Ti(IV) 2p3/2 species for (a) Ti4O7 and (b) Ti6O11. The darker, intermediate and lighter lines stand for the data collected at 950, 735 and 560 eV, respectively. The corresponding IMFPs are given. The measurement error of B.E. is 0.1 eV.

Acknowledgements

The authors acknowledge funding from Collège de France and CNRS. This work was also funded by the Domaine d'Intérêt Majeur of Région Ile de France on functional oxides (DIM Oxymore. SOLEIL synchrotron is acknowledged for beamtime and financial support under the proposal 20160135.

A typical result of the fitting procedure is detailed below, on the example of Ti4O7 in the pristine state: The top graph shows the residues for the three spectra. From bottom to top: 560, 735 and 950 eV, corresponding to 0.6, 0.9 and 1.3 nm probed depths (IMFPs), respectively. Experimental data are dots while components, backgrounds and envelopes are lines. The top graph shows the residues for the three spectra.


Siham Douadi

Olivier Sandre

Valérie Cabuil

Bassam Hamdoun

Maud Save

Bernadette Charleux

SYNTHESIS OF HIGH MOLAR MASSES POLY(n-BUTYL METHACRYLATE)-b-POLYSTYRENE DIBLOCK COPOLYMERS BY ATRP. FORMATION OF LAMELLAR PHASES IN THIN FILMS

Introduction

Diblock copolymers are known to self-organize in ordered domains. The morphology of this microphase separation depends essentially on the relative amount of each immiscible block in the copolymer (e.g. spherical, cylindrical, gyroidal and lamellar phases). 1 The lamellar nanoscopic structures can serve as templates for the selective incorporation of inorganic nanoparticles 2, , 3 4 for instance to get nanopatterned surfaces or for new applications such as wave guides. 5,6 In most of the studies concerning diblock copolymer self-assembly in lamellar periodic structures, diblock copolymers were synthesized by anionic polymerization which is a suitable technique to prepare well-defined high molar mass diblock copolymers. 7,8 In the present study, our aim is the synthesis of high molar mass poly(n-butyl methacrylate)-b-polystyrene diblock copolymers by Atom Transfer Radical Polymerization (ATRP). Compared to anionic polymerization, controlled radical polymerization (CRP) is a more versatile technique which requires less drastic experimental conditions. Our results highlight the self-assembly of the symmetric poly(nbutyl methacrylate)-b-poly(styrene) diblock copolymer synthesized by ATRP into a lamellar ordered structure after an annealing treatment.

Experimental

Materials. Styrene (S) and n-butyl methacrylate (nBMA) (Aldrich products) were distilled prior to polymerization to remove any trace of inhibitor. N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA), CuBr, CuBr 2 , CuCl, CuCl 2 and ethyl-2-bromoisobutyrate (EBrIB) (Aldrich products) were used as received.

Diblock copolymer synthesis by ATRP. The typical polymerization recipes were as follows (see Table 1). For the synthesis of the P(nBMA) first block, the copper powder was first introduced under nitrogen into the polymerization flask. A solution containing n-butyl methacrylate, PMDETA and EBrIB was separately degassed for 30 minutes by nitrogen bubbling and transferred into the polymerization flask under a nitrogen flow via a doubletipped stainless steel needle. The reactor was immediately immersed in an oil bath thermostated at 100°C. The P(nBMA) was precipitated in methanol, dried and used as a macroinitiator for the diblock copolymer synthesis via the following typical procedure. P(nBMA) homopolymer and PMDETA were mixed with styrene and the viscous solution was degassed by nitrogen bubbling. The polymerization of styrene started when the copper-catalyst was added under a nitrogen flow into the polymerization flask thermostated at 100°C. The diblock copolymer was precipitated in methanol. Any residual P(nBMA) first block was removed by extraction with hexane.

Analytical Techniques. 1 H NMR (250 MHz) analysis of the crude polymers were performed in CDCl 3 in 5 mm tubes at room temperature using an AC250 Brüker spectrometer. The average molar mass and molar mass distribution of the copolymers were obtained by size exclusion chromatography (SEC) in THF with a 1 mLmn -1 flow rate using a triple detection system (Viscotek, TriSEC model 302). The Viscotek apparatus was composed of two mixed bead columns (PLgel, mixed C, 5µm) and one low mass column (PLgel, 5 µm, 100 Å) thermostated at 40°C. Three online detectors were used, a refractometer, a viscometer (Wheaston bridge used to measure the intrinsic viscosity online) and a light scattering detector working at 90° angle. The value of dn/dc used for the P(nBMA) was 0.077.

Sample preparation. Thin P(nBMA)-b-PS films were spin-coated onto freshly cleaved mica sheets from a toluene solution. The copolymer films were slowly dried two days at 25°C and subsequently annealed for 68h at 165 °C. The film thickness was approximately 1 µm as estimated from the optical fringes near the film border.

Atomic Force Microscopy AFM images were taken in dry tapping mode with a Nanoscope III (DI).

Results and Discussion

Very few articles describe the synthesis of poly(n-butyl methacrylate)-bpolystyrene diblock copolymers by Controlled Radical Polymerization (CRP). 9,10 The only example available in the literature concerns the synthesis of P(iBMA)-b-P(S) diblock copolymer by ATRP in miniemulsion with an overall molar mass below 60 000 g/mol. Besides, these diblock copolymers were polluted by residual macroinitiator. Here, we report the synthesis of well-defined P(nBMA)-b-PS diblock copolymers synthesized by ATRP in bulk. The experimental conditions were optimized to reach the highest macroinitiator efficiency (100%, Table 3) with an overall molar mass of the diblock copolymer up to 112 000 g/mol and a polydispersity index of 1.5. a The polymerizations were all carried out in bulk except the ones marked by a star * which were conducted in 25 v/v benzonitrile solution..

The CRP mechanism is based on a activation-desactivation equilibrium between dormant and active species. In the case of ATRP, this equilibrium is established through redox reactions using a copper catalyst. For the synthesis of the diblock copolymers, the choice of the first P(nBMA) block was governed by the necessity to get the highest ratio between the initiation rate and the propagation rate and hence, to limit any residual macroinitiator inside the diblock copolymer. It is known that the k p K eq ratio for the methacrylic monomers is higher than the one for styrene polymerization (k p K eq = 1.1 × 10 -3 M -1 s -1 for MMA and 3.6 × 10 -5 M -1 s -1 for styrene polymerization at 90°C using EBrIB as an initiator 11 ) and consequently the most favored experimental conditions are fulfilled when starting by the polymerization of nBMA. From the results gathered in Table 2 concerning the polymerization of nBMA by ATRP, it is obvious from the polydispersity indexes that the polymerization was better controlled in bulk than in 25% v/v benzonitrile solution (Table 2). In similar experimental conditions, Madruga et al. 12 reported different results as an increase of the initiation efficiency was observed when the nBMA polymerization by ATRP was performed in benzonitrile solution. Concerning the P(nBMA)-b-PS diblock copolymer synthesis (see Table 3), the macroinitiator exhibited a lower efficiency when terminated by a bromine rather than by a chlorine atom, with CuCl as a catalyst for the styrene polymerization. This could be explained by a higher amount of PnBMA dead chains formed in the first step in the presence of CuBr as a catalyst rather than CuCl, which is in accordance with a higher activation rate with CuBr catalyst. 13 The final P(nBMA)-b-PS diblock copolymer displayed a narrow molar mass distribution and a symmetrical structure (P(nBMA) 425 -b-PS 490 ) when using PnBMA-Cl as macroinitiator (see Figure 1). On the contrary, for the diblock copolymer prepared from the brominated-P(nBMA) macroinitiator, a shoulder at the low molar masses was observed on the SEC chromatogram, which is characteristic of residual first block (Figure 2). The macroinitiator could be removed by cleaning the copolymer using n-hexane as a selective solvent for P(nBMA) and the final composition is P(nBMA) 405 -b-PS 460 . The well-defined nearly symmetric P(nBMA) 425 -b-PS 490 diblock copolymer (expt 6) with a styrene fraction in the copolymer of 0.53 was chosen to prepare thin films onto mica surface and study the ability of this copolymer to form multilayered structure. After annealing, a thin film of the P(nBMA) 425 -b-PS 490 diblock copolymer spread on mica and observed by AFM (Figure 3) exhibited: i) nearly flatness over most of the surface area, with a RMS roughness of 1.6 nm; ii) round features of diameters ranging from 50 to 200 nm, which were either higher or lower than the average surface level. These structures called respectively "islands" and "holes" are typical of a multilayered order with lamellae parallel to the substrate. 14,15 The histogram of the heights measured on many islands (51) on large field AFM views leads to 4 groups: 4.5-7.5 nm (34), 9.5-13.5 nm (8), 15-17 nm (8) and 21 nm (1), giving an estimate of the lamellar period about 6 nm, probably underestimated due to the softness of the polymer film.

Conclusions

Well-defined nearly symmetric P(nBMA)-b-PS diblock copolymers with molar masses up to 110 000 g/mol and polydispersity indexes below 1.5 were successfully synthesized by ATRP. An increase of the macroinitiator efficiency was observed when using CuCl catalyst for the first P(nBMA) block synthesis instead of CuBr. The first evidence of a lamellar ordered structure in the thin film formed by a diblock copolymer synthesized by controlled radical polymerization was given by the presence of islands and holes in AFM images.

Figure 1 .

 1 Figure 1. SEC of the P(nBMA) 425 macroinitiator (thin line, expt 3) and the P(nBMA) 425 -b-PS 490 diblock copolymer (bold line, expt 6).

Figure 2 .

 2 Figure 2. SEC of the P(nBMA) 405 -b-PS 460 (left, expt 5) and the P(nBMA) 425b-PS 490 (right, expt 6) diblock copolymers before (thin line) and after (bold line) hexane extraction.

Figure 3 .

 3 Figure 3. Tapping AFM images and cross-section analysis of round defects in the lamellar structure of a P(nBMA) 425 -b-PS 490 diblock copolymer film. The hole depth and the island height are respectively 6.1 and 6.8 nm (±1.6 nm).

Table 1 . Experimental conditions for the synthesis of P(nBMA) macroinitiator and P(nBMA)-b-PS diblock copolymers by ATRP.

 1

	Expt Monomer	Initiator	Catalyst	[M]:[Initiator]:	Time
		a			[Catalyst]:[PMDETA]	(h)
	1	nBMA	EBrIB	CuBr/CuBr 2	400:1:1:1	1h30
				0.8/0.2
	2	nBMA *	EBrIB	CuBr/CuBr 2	330:1:0.9:0.9	7h
				0.8/0.2
	3	nBMA	EBrIB	CuCl/CuCl 2	400:1:1:1	2h
				0.8/0.2
	4	nBMA *	EBrIB	CuCl/CuCl 2	390:1:1:1	7h
				0.8/0.2
	5	S	P(nBMA)-Br	CuCl	665:1:1:1	15h
			(from expt 1)
	6	S	P(nBMA)-Cl	CuCl	690:1:1:1	16h
			(from expt 3)

Table 2 . Results of the P(nBMA) homopolymer synthesis.

 2

	Expt from Table 1	Conv. a (%)	Theoretical b g/mol n M	SEC (RI) n M	n w M M	(triple) c SEC n M	f d
				g/mol		g/mol
	1	87	49680	44290	1.19	57520	0.86
	2	66	31165	43240	1.53	56200	0.55
	3	84	47975	46110	1.18	60860	0.79
	4	98	54540	54950	1.47	71435	0.76

a Conversions were determined by

1

H NMR analysis. b M n (theoretical) = M EBrIB + [nBMA] 0 /[EBrIB] 0 × conversion × M nBMA c

M n (triple) corresponds to the molar masses obtained from the triple detection system (see the experimental part). d initiator efficiency f = M n (theoretical)/ M n (SEC, triple).

Table 3 . Results of the P(nBMA)-b-PS diblock copolymer synthesis.

 3

	Expt from Table 1	Conv. (%)	n theoretical M g/mol	n SEC M (RI) a	n SEC M (RI) b
				g/mol	g/mol

Acknowledgement. CNRS, University Paris 6 and Islamic University of Liban are acknowledged for their financial support and J.J. Rousseau for scientific discussions.


Milena Menna
email: mmenna@inogs.it

Pierre-Marie Poulain

Daniele Ciani
email: daniele.ciani@cnr.it

Andrea Doglioli
email: andrea.doglioli@univ-amu.fr

Giulio Notarstefano
email: gnotarstefano@inogs.it

Riccardo Gerin
email: rgerin@inogs.it

Marie-Helene Rio

Rosalia Santoleri
email: rosalia.santoleri@cnr.it

Adam Gauci
email: adam.gauci@um.edu.mt

Aldo Drago
email: aldo.drago@um.edu.mt

New insights of the Sicily Channel and southern Tyrrhenian Sea variability

Keywords: Sicily Channel, spatial and temporal variability, mesoscale eddies

The dynamics of the Sicily Channel and southern Tyrrhenian Sea is highly influenced by the seasonal variability of the Mediterranean basin-wide circulation, by the interannual variability of the numerous mesoscale structures present in the Channel, and by the decadal variability of the adjacent Ionian Sea. In the present study, all these aspects are investigated using in-situ (Lagrangian drifter trajectories and Argo float profiles) and satellite data (Absolute Dynamic Topography, Sea Level Anomaly, Sea Surface Temperature, wind products) over the period 1993-2018. The availability of long time series of data and high resolution multi-sensor surface currents, allow us to add new details on the circulation features and on their driving mechanisms and to detect new permanent eddies not yet described in literature. The structures prevailing in winter are mainly driven by wind, whereas those prevailing in summer are regulated by topographical forcing on surface currents. The strength of the surface structures located at the western entrance of the Ionian Sea and of the mesoscale activity along the northern Sicily coast is modulated by the large-scale internal variability. The vertical hydrological characteristics of these mesoscale eddies are delineated using the Argo float profiles inside these structures.

Introduction

Thanks to its location in the centre of the Mediterranean Basin, the Sicily Channel (SC) plays a crucial role in connecting the western and eastern Mediterranean basins and modulating their exchange of the surface and intermediate waters [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF][START_REF] Schroeder | Rapid response to climate change in marginal sea[END_REF]. The SC is characterised by a complex bottom topography, with submarine ridge and shallow banks, and it is delimited to the north by the Tyrrhenian Sea and the Sicily coast, to the south by the Libyan coast, to the west by the Tunisia coast and to the east by the Ionian Sea (Figure 1a). Its circulation can be schematized on the vertical as a two-layer exchange with an eastward flow of Atlantic Water (AW) superposed to a westward flow of intermediate water, dominated by the Levantine Intermediate Water (LIW) [START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF]. Microstructure measurements taken in the SC show that it is a hotspot for turbulent mixing [START_REF] Ferron | How important are diapycnal mixing and geothermal heating for the deep circulation of the Western Mediterranean?[END_REF][START_REF] Vladoui | Turbulence in the Sicily Channel from microstructure measurements[END_REF]. Consequently, the SC is a key area for the Water 2019, 11, x FOR PEER REVIEW 2 of 24 regulation of salt exchanges between Eastern and Western basins, with an impact on the deep-water formation processes [START_REF] Millot | Circulation in the Mediterranean Sea[END_REF].

Circulation in the upper layer of the SC and in the southern Tyrrhenian Sea is mainly dictated by the large-scale Mediterranean thermohaline circulation, the wind-driven currents along the shelf, the upwelling events off Sicily, the sub-basin scale and mesoscale permanent and quasi-permanent structures [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF][START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF][START_REF] Lermusiaux | Features of dominant mesoscale variability, circulation patterns and dynamics in the Strait of Sicily[END_REF][START_REF] Béranger | The dynamics of the Sicily Strait: A comprehensive study from observations and models[END_REF][START_REF] Poulain | Surface circulation in the central Mediterranean Sea as deduced from Lagrangian drifters in the 1990s[END_REF]. The recent results of AW circulation schemes, derived by numerical model simulation [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF][START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF], are summarized in Figure 1b (both permanent and seasonal circulation structures are depicted with the same colour in Figure 1b; more details on the time scales and variability of these structures are available in Section 3; acronyms are defined in Table1). The sub-basin scale structures are characterised by a prominent seasonal variability [START_REF] Poulain | Surface circulation in the central Mediterranean Sea as deduced from Lagrangian drifters in the 1990s[END_REF][START_REF] Astraldi | The role of straits and channels in understanding the characteristics of Mediterranean circulation[END_REF][START_REF] Poulain | Surface geostrophic circulation of the Mediterranean Sea derived from drifter and satellite altimeter data[END_REF][START_REF] Pinardi | Mediterranean Sea large-scale low-frequency ocean variability and water mass formation rates from 1987 to 2007: a retrospective analysis[END_REF] associated with large wind stress fluctuations [START_REF] Molcard | Numerical investigation of the Sicily Channel dynamics: density currents and water mass advection[END_REF]. The numerous mesoscale structures located in the SC are mainly driven by the instability of the large scale circulation, by the the interactions between currents and bathymetry and by the direct wind forcing [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF]. superimposed with the schematic surface circulation adapted from [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF] and [START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF]; acronyms are listed in Table 1.

Nowadays, the availability of long time series of in-situ and satellite data and of sophisticated statistical techniques, allow us to add new details on the mesoscale features and on their driving mechanisms. In this study, Lagrangian drifter trajectories, Argo float profiles and satellite data (Absolute Dynamic Topography, Sea Level Anomaly, Sea Surface Temperature, wind products) are used to describe the surface circulation of the SC and the southern Tyrrhenian Sea over the period 1993-2018. The simultaneous use of all these datasets leads to overcome the intrinsic limitations of each of them, e.g. the accurate but discontinuous spatial and temporal drifter sampling and the low accuracy of altimetry gridded data in the identification of the mesoscale field [START_REF] Amores | Up to what extent can we characterize ocean eddies using present day gridded altimetric products?[END_REF]. Moreover, the knowledge of the mesoscale field is ameliorated using the multi-sensor currents (defined hereafter as 'optimal currents') derived from the merging of the satellite altimetry data and the Sea Surface Temperature (SST) fields [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF]. This product enables to improve the description of eddy dynamics and non geostrophic dynamical features [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF]. The vertical structures and the hydrological characteristics of the mesoscale eddies are delineated using the Argo float profiles inside these structures. All these data and products enhance the knowledge on the temporal variability of the mesoscale structures, with detection of new features not yet described in the literature, and the addition of new insights on the formation mechanisms of these structures.

Materials and Methods

The datasets used for this study are:

 The OGS Mediterranean drifter dataset in the SC and Southern Tyrrhenian Sea, composed of 377 drifter tracks collected between 1993 and 2018 (Figure 1b). Drifter data were retrieved from the OGS own projects, but also from databases collected by other research institutions and by international data centres (Global Drifter Program, SOCIB, CORIOLIS, MIO, etc.). These data were cleaned from potential outliers and elaborated with standard procedures (editing, manual editing and interpolation; [START_REF] Menna | The OGS Mediterranean drifter database: 1986-2016[END_REF][START_REF] Menna | Detecting the drogue presence of SVP drifters from wind slippage in the Mediterranean Sea[END_REF]). In particular, we use the low-pass filtered and interpolated (6 hours) drifter tracks which represent the near-surface currents between 0 m and 15 m depth.

 The daily (1/8º Mercator projection grid) Absolute Dynamic Topography (ADT) and correspondent Absolute Geostrophic Velocities (AGV) derived from altimeter and distributed by CMEMS in the period 1993-2018 (product user manual CMEMS-SL-QUID_008-032-051). The ADT was obtained by the sum of the sea level anomaly and a 20-years synthetic mean estimated by [START_REF] Rio | Computation of a new mean dynamic topography for the Mediterranean Sea from model outputs, altimeter measurements and oceanographic in situ data[END_REF] over the 1993-2012 period.

 The Argo float vertical profiles of temperature and salinity from the upper 2000 m of the water column and the horizontal currents displacements at the parking depth. In the Mediterranean Sea, the Argo floats are generally programmed to execute 5-day cycles with a drifting depth of 350 m (parking depth). Additionally, they alternate the profiling depth between 700 m and 2000 m (see the MedArgo program in [START_REF] Poulain | MedArgo: a drifting profiler program in the Mediterranean Sea[END_REF]). When a float drifts in a shallow area and touches the ground, it can increase its buoyancy to get away from bottom, or can stay there until it is time to ascent (depending on how it is programmed).

Information about grounding events is contained in the Argo float trajectory file. Among all the data available in the Mediterranean Sea, we selected of the part of the Argo floats trajectories which correspond to a float entrapped in the mesoscale structures of the SC and southern Tyrrhenian Sea. These data were used to define the vertical hydrographic peculiarities of the mesoscale features. Details about the missions of the seven floats selected for this work are listed in Table 2.

 The optimal currents, estimated by [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF] and presently available in the period 2012-2016. This product was used to confirm the occurrence of the mesoscale structure derived from altimetry and to estimate their interannual variability. Indeed, the optimal currents are based on the synergy of the daily 1/8° Copernicus CMEMS altimeter-derived geostrophic velocities (data ID: SEALEVEL_MED_PHY_L4_REP_OBSERVATIONS_008_051) and the daily 1/24° CMEMS sea-surface temperatures for the Mediterranean Sea (data ID: SST_MED_SST_L4_REP_OBSERVATIONS_010_021). The optimal reconstruction method is based on the inversion of the ocean heat conservation equation in the mixed layer [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF]. The principles of the optimal currents are thoroughly described in [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF][START_REF] Rio | Improved global surface currents from the merging of altimetry and Sea Surface Temperature data[END_REF][START_REF] Piterbarg | A simple method for computing velocities from tracer observations and a model output[END_REF].

Such method takes advantage of the high-resolution spatial temporal gradients of the satellite-derived SST to improve the temporal (1 day) and spatial (1/24°) resolution of the altimeter derived geostrophic currents at the basin scale. The reconstruction method of the optimal currents yielded positive improvements for both the components of the motion in the SC [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF].

 The Cross-Calibrated, Multi-Platform (CCMP) V2.0 ocean surface wind velocity which were downloaded from the NASA Physical Oceanography DAAC for the period July 1993 -May 2016 [START_REF] Atlas | A crosscalibrated, multiplatform ocean surface wind velocity product for meteorological and oceanographic applications[END_REF]. These products were created using a variational analysis method to combine wind measurements derived from several satellite scatterometers and microwave radiometers. The temporal resolution of the CCMP product is six hours, the spatial resolution is 25 km (level 3.0, first-look version 1.1).

Drifter velocities were divided in bins of 0.25°x0.25° and pseudo-Eulerian statistics were computed over the period 1993-2018 and qualitatively compared with the ADT derived from altimetry. The mean current field was also estimated in the period 2012-2016 using the optimal currents. The seasonal variability of the drifter, altimetry and optimal currents fields was estimated by dividing the dataset in two extended seasons: the extended summer corresponding to May-October, and the extended winter to November-April, as suggested by [START_REF] Poulain | Surface circulation in the central Mediterranean Sea as deduced from Lagrangian drifters in the 1990s[END_REF].

The CCMP six-hourly gridded analyses were used to quantify the wind stress and the vertical component of the wind stress curl [curl τ]z over the study area:

      10
; , ,

y x y x D w w z curl C u v U x y              (1)
where (τx,τy) are the wind stress components, ρ (1.22 Kg/m 3 ) is the density of air, (uw,vw) and U10 are the components and the magnitude of the wind speed at 10 meters, respectively, and CD is the drag coefficient already used in the Mediterranean Sea by [START_REF] Shabrang | Long-term variability of the southern Adriatic circulation in relation to North Atlantic Oscillation[END_REF][START_REF] Menna | Decadal variations of circulation in the Central Mediterranean and its interactions with the mesoscale gyres[END_REF]: Wind stress and wind stress vorticity fields were used to speculate on the link between the wind variations and the seasonal and/or interannual variability of mesoscale structures.

Monthly means of the AGV and optimal currents fields were used to estimate the relative vorticity (𝜻), defined as the vertical component of the velocity field curl:

𝜻 = 𝝏𝑽 𝝏𝒙 - 𝝏𝑼 𝝏𝒚 ; (3)
where U and V are the velocity components. The resulting current vorticity fields were spatially averaged in the regions of the main mesoscale structures listed in Table 1 and filtered (13-month moving average) in order to remove the seasonal and intra-annual variations.

Results

Mean currents and wind fields

The qualitative comparison between the ADT and drifter data shows that the two datasets fit rather well (Figure 2a), and allow us to update the pseudo-Eulerian current maps described in [START_REF] Poulain | Surface circulation in the central Mediterranean Sea as deduced from Lagrangian drifters in the 1990s[END_REF] and enhancing the schematic circulation maps published by [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF] and [START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF] the drifter and altimetry data (Figure 2a), 2012-2016 for the optimal currents (Figure 2b) and 1993-2016 for the wind (Figures 2c,2d). The time periods are rather long and the statistics rather robust to consider the average in Figure 2a comparable with those in Figures 2c and2d and representative of the mean conditions in the SC. The optimal currents are available over a reduced period, and are therefore not directly compared with the other datasets. They are rather used to bring out some aspects of the current field that are not obvious using only drifters and altimetry data. The structures emphasized in white in Figures 2a and2b, are here described for the first time or with different shapes and positions with respect to those schematized in Figure 1b. The Atlantic Tunisian Current (ATC) originates from the branch of the Algerian Current (AC) that enters in the SC and flows southward between Pantelleria Island and the Tunisian coast [START_REF] Poulain | Surface circulation in the central Mediterranean Sea as deduced from Lagrangian drifters in the 1990s[END_REF]. It shows a complex pattern only partially described by the previous model studies. Indeed, the ATC splits in two branches at about 36.5°N (Figure 2a): one branch continues to move southward along the Tunisian coast (shown in white color), whereas another branch moves eastward south of Pantelleria

Island. At about 35.5°N and 13°E the ATC splits another time: a part of the current forms the Bifurcation Atlantic Tunisian Current (BATC), the other part turns southward towards the Libyan coast at ~ 13°E. The latter branch describes the Atlantic Libyan Current (ALC; [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF][START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF]), that move westward towards the Gulf of Gabes and eastward along the Libyan coast (Figures 1b,2a).

Drifter and altimetry data confirm the well known meandering pattern of the Atlantic Ionian Stream (AIS) and outline the edge of the Maltese Channel Crest (MCC) located on the Maltese Plateau. The Medina Gyre (MG) is located on the west and/or southwest side of the Malta Island in agreement with the scheme of [START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF]. The region where the ATC splits and forms the BATC is characterized by a sudden reduction of depth due to the shelf extension (see the location of MG in Figure 1a) that probably facilitates the cyclonic rotation of the surface currents around 35°-36°N and 13°-14.5°E. It is interesting to note that [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF] located the MG in a different position southeast of Malta (see Figure 2 of [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF]). South of the MG, altimetry and drifter data detect another permanent cyclonic mesoscale structure that has never been described before in the literature; hereafter we will refer to this structure as the Southern Medina Gyre (SMG) (emphasized in white Figure 2a). The Libyan Shelf Break Vortex (LSBV) is well described only by altimetry data (Figure 2a) because of a scarce quantity of drifter tracks along the Libyan coast; this structure appears meridionally elongated and squeezed along the Libyan coast, showing a different shape and location with respect to [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF].

The higher spatial resolution of the mean circulation derived by optimal currents (2012-2016; Figure 2b) permits a more detailed description of the mesoscale structure of the SC and the southern Tyrrhenian Sea. The anticyclonic structure, clearly visible along the northern coast of Sicily and located between the Aeolian Islands and Cape Gallo (38.1-38.5°N; 13.5-15°E), was observed by [START_REF] Aulicino | Results of the first wave glider experiment in the southern Tyrrhenian Sea[END_REF] in September 2012 but it was not described by these authors; hereafter it will be defined as the Northern Sicily Anticyclone (NSA). Another mesoscale anticyclone is located at the entrance of the SC (37.5-38°N; 11.5-12°E) and represents a kind of watershed between the waters entering the Tyrrhenian Sea (Bifurcation Tyrrhenian Current -BTC) and those entering the SC; hereafter we will define it as the Sicily Channel Anticyclone (SCA). A third mesoscale anticyclone is squeezed between the MG and Malta Island (35.5°-36° N; 14°-14.5° E) and we will define it hereafter as the Southern Maltese Anticyclone (SMA). Similarly, southeast of the Ionian Shelf break Vortex (ISV), there is another steady cyclonic structure hereafter defined as the Southern Ionian Shelf break Vortex (SISV). In addition, the optimal currents reveal new information on the shape of the Sidra Gyre (SG), which appears as a large anticyclone that involves two smaller anticyclonic structures (Figure 2b), and confirms the existence of the SMG (Figure 2b). The cyclonic circulation in the region of the Adventure Bank Vortex (ABV; 37-37.5°N; 12-13°E) is detected by the three datasets (drifter, altimetry and optimal currents) but the vortex is not well resolved by any of them; it appears more like a cyclonic meander rather than a vortex (Figure 2, upper panels).

In the study area the mean wind stress is oriented to the east in the southern Tyrrhenian Sea and to the southeast in the SC (Figure 2c) with amplitude (range of values between 0 and 0.04 Nm -2 ) and directions in agreement with the results of [START_REF] Pinardi | Mediterranean Sea large-scale low-frequency ocean variability and water mass formation rates from 1987 to 2007: a retrospective analysis[END_REF]. The regions mostly impacted by the wind stress are located in the band north of 35°N and south of the southern Sicily coast (Figure 2c). The rotating motion induced by the wind (wind stress curl Figure 2d) is cyclonic (positive) along the southern and eastern Sicily coasts and on the Malta plateau, whereas it is anticyclonic (negative) along the Tunisia coast and in the southern Tyrrhenian Sea (along the northern Sicily coast). We can speculate that the wind field plays an important role in shaping the sub-basin circulation (e.g. the branch of the ATC that moves southward along the Tunisia coast) and in defining the sense of rotation of the mesoscale structures located around the Sicily coast (e.g. the ABV, the Messina Rise Vortex -MRV, the ISV and the NSA).

Seasonal variability of currents and wind fields

The seasonal variability of the drifter-derived and optimal current fields is shown in Figure 3 together with the altimetry data. Results substantially confirm the paths described in [START_REF] Poulain | Surface circulation in the central Mediterranean Sea as deduced from Lagrangian drifters in the 1990s[END_REF], but add new insights.

The BTC is a permanent feature as shown by [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF], and it is stronger in winter (Figures 3b and3d;

winter speeds larger than 20 cm/s; mean summer speeds of ~ 10 cm/s), whereas the NSA is much more intense in summer (Figures 3a and3c; mean winter speeds of ~ 5 cm/s; mean summer speeds of ~ 10 cm/s). Along the northern coast of Sicily the drifter data describe a westward current during summer, with the consequent inflow of surface water in the SC (Figure 3a), whereas the coastal currents moves eastward during winter (Figure 3b). This summer westward current (mean speeds of ~ 10 cm/s) was already described in [START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF] and defined as the Tyrrhenian Sicilian Current (TSC). The TSC is not identified by the optimal currents (Figures 3c and3d).

The ATC is part of the permanent pattern of the SC, in agreement with [START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF] and it is more intense and more meandering in nature during the extended winter (Figures 3b,3d; maxima winter speeds of ~ 30 cm/s; maxiama summer speeds of ~ 20 cm/s); it is interesting to note that [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF] described it as a winter structure. The cyclonic Pantelleria Vortex (PV) is observed only during the extended winter (Figures 3b,3d; speeds of 10-15 cm/s), in agreement with the optimal currents and with [START_REF] Jouini | The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation[END_REF] and in disagreement with [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF], that describe this as part of the permanent pattern. The MCC is stronger during the extended summer (speeds larger than 15 cm/s), as well as the AIS (Figures 3a,3c; mean winter speed of ~ 5 cm/s; mean summer speeds of ~ 15 cm/s). The SG is stronger and larger in size during summer (Figures 3a and3c; mean winter speeds of ~ 7 cm/s; mean summer speeds of ~ 12 cm/s; winter longitudinal extension of ~ 100 km; summer longitudinal extendion larger than 250 km), in agreement with the results of [START_REF] Ciappa | Surface circulation patterns in the Sicily Channel and Ionian Sea as revealed by MODIS chlorophyll images from 2003 to 2007[END_REF] that report the SG seasonal expansion in summer and contraction in winter. The ISV and the MRV are permanent structures, in agreement with [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF], and they are stronger in winter (Figures 3b,3d; mean winer speeds of ~ 9 cm/s; mean summer speeds of ~ 6 cm/s); also the SISV is a permanent structure more intense in winter, according to the optimal currents (Figures 3d; mean winer speeds of ~ 10 cm/s; mean summer speeds of ~ 5 cm/s). The BATC is predominant during winter south of the MG (Figures 3b and3d; maxima winer speeds of ~ 20 cm/s; maxima summer speeds of ~ 10 cm/s), in agreement with [START_REF] Sorgente | Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion[END_REF]. The ALC is stronger in winter (Figures 3b and3d; speeds larger than 15 cm/s), whereas the LSBV is stronger in summer (Figures 3a,3c;

speeds larger than 25 cm/s).

The wind stress and the wind stress curl are more intense in winter (Figure 4) influencing the seasonal variability of some sub-basin currents such as the BTC, the ALC, the BATC; the winter intensification of the BATC and its interaction with the topography lead to an intensification of the MG. The wind stress does not impact the circulation structures which are more intense in summer (MCC, AIS, NSA, SG), when the wind stress and the wind stress curl are weakened. The seasonal variability of these structures is presumably related to other forcings such as the instability of the surface currents and the interaction with the topography. The behaviour of the AIS and of the MCC confirms this insight:

the AIS is stronger in summer (Figure 3a), when the wind stress is weaker (Figure 4a) and its meander on the Malta Plateau (the MCC) is anticyclonic, although the vorticity induced by the wind in this region is substantially cyclonic (Figure 4c). Along the northern coast of the Sicily, the amplitude of the wind stress and the anticyclonic vorticity induced by the wind are stronger in winter (Figure 4), whereas the strengthening of the NSA is observed in summer (Figures 3a and3c). This result suggests that the wind influences the sense of rotation of the surface circulation in the NSA, but other forcings modulate the strength of the seasonal and interannual variability of this circulation structure. The summer intensification of the LSBV appears to be instead related to the intensification of the cyclonic wind stress curl along the western Libyan coast (Figure 4c). The SG shows a pronounced longitudinal extension in summer (Figure 3a), when the wind stress is weaker but the vorticity induced by the wind is essentially anticyclonic in the southeastern region of the SC (Figure 4a); its southern margin is oriented parallel to the Libyan coast following the 400 m isobath. Ionian (Figure 5). In the region located between Pantelleria and Malta islands (35.5°N-37°N, 11°E-15°E), the surface currents are samller than the mean field during the anticyclonic mode (Figure 5a; current anomalies are oriented in an opposite direction with respect to the mean field) and larger during the cyclonic mode (Figure 5b). Along the northern coast of the Sicily, the NSA is reduced in intensities with respect to the mean field during the anticyclonic mode (Figure 5a; the current anomalies are oriented ciclonically) and increased during the cyclonic mode.

The largest variations are observed east of 15°E, in agreement with the results of [START_REF] Menna | Decadal variations of circulation in the Central Mediterranean and its interactions with the mesoscale gyres[END_REF]. The MRV and the SISV are less intense than the mean currents during the anticyclonic mode (Figure 5a) and slightly more intense during the cyclonic mode (Figure 5b). The AIS tends to be deflected towards the northern Ionian during the anticyclonic circulation mode (northeastward currents along the Sicily eastern coast) and the MIJ is reduced in intensity with respect to the mean currents (Figure 5a).

During the cyclonic circulation mode the AIS feeds the MIJ, that shows larger intensities with respect to the mean, and the currents are mainly directed southwestward along the western coasts of the Ionian Sea (Figure 5b).

Interannual variability and vertical structure of the quasi-permanent mesoscale eddies in the Sicily Channel and southern Tyrrhenian Sea

The main quasi-permanent mesoscale eddies of the SC and southern Tyrrhenian are analysed in term of their interannual variability, using the time series of spatially averaged vorticity fields derived both from altimetry and optimal currents data. In the period in which the optimal currents are available the accuracy of the vorticity derived by the AGV is generally improved, showing larger complexity in the temporal variability of the signal (see Figures 6,[START_REF] Béranger | The dynamics of the Sicily Strait: A comprehensive study from observations and models[END_REF][START_REF] Poulain | Surface geostrophic circulation of the Mediterranean Sea derived from drifter and satellite altimeter data[END_REF]. The thermohaline properties and the vertical extension in the water column of these mesoscale eddies is studied using the Argo float profiles.

Southern Tyrrhenian Sea and Sicily Channel entrance

The analysis of the vorticity field in the areas of the NSA (38.1-38.5°N; 13.5-15°E) and SCA (37.5-38°N;

11.5-12°E) confirms that the anticyclonic nature of these regions persists with time (Figure 6). A more accurate analysis of the temporal evolution of the vorticity in the NSA shows quasi-decadal variations of the intensity of the vorticity field that coincide with the inversions of the surface circulation in the northern Ionian. The black dashed lines in Figure 6 give an indication of the mean vorticity values during each anticyclonic/cyclonic circulation mode. The anticyclonic vorticity of the NSA is reduced during the anticyclonic circulation modes of the northern Ionian (1993-1996, 2006-2010, 2017-2018), whereas it is enhanced during the cyclonic circulation modes (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2011)(2012)(2013)(2014)(2015)(2016). This result supports the relationship suggested by [START_REF] Rubino | Tidal record as liquid climate archives for largescale interior Mediterranean variability[END_REF]

Malta plateau

The surface circulation in the region of the Malta Plateau is strongly influenced by the flow of the AIS that forms a large anticyclonic meander defined as MCC. The permanent anticyclonic vorticity in this region (36.1-36.8°N; 14-15°E) is confirmed by the time series of vorticity field (Figure 8). At local scale, this meander can sporadically create an anticyclonic gyre on the Malta plateau [START_REF] Drago | CALYPSO -An operational network of HF radars for the Malta-Sicily Channel[END_REF]. In the framework of the Italia-Malta Calypso Project [START_REF] Drago | CALYPSO -An operational network of HF radars for the Malta-Sicily Channel[END_REF], about 38 drifters were deployed on the Malta plateau between December 2012 and September 2016. These drifters were captured by the anticyclonic gyre on the plateau in two specific deployments: December 2012 and March 2014 (Figure 9). In December 2012-January 2013, drifters are captured by the anticyclonic gyre for about four weeks (approximately between 14 December 2012 and 10 January 2013) before being transported out of the plateau (Figure 9a). This gyre is also confirmed by the trajectory of the float WMO 6901044 (Figure 10a) and by HF radar measurements [START_REF] Capodici | Validation of HF radar sea surface currents in the Malta-Sicily Channel[END_REF]. On 22 March 2014, six drifters were deployed in the area and were trapped in the gyre for about 2 weeks (Figure 9b). Drifters allow to estimate the radius and the rotation period of the structure considering the centroids computed from all the closed loops of the drifter trajectories. The radius spanned between 11 and 27 km and the period increased from 4.1 to 8.4 days, in coincidence with an increasing the distance from the centre. 12.5°-14°E). This behaviour is probably related to the wind-stress, which is more intense in the Malta region than in the southern SC (Figure 2c). The vorticity of SMA increases with time over the considered period. The float WMO 6903242, became entrapped in the anticyclonic SMA in mid-September 2018, therefore describes five loops around the eddy core before being captured by the eastward BATC (Figure 12a). It had a short cycling period (see Table 2) and its trajectory indicates the mean near surface displacements (0-180 m). The subsurface density distribution clearly shows the net differences between the water masses located east (potential density smaller than 24.8 kg/m 3 ) and south (potential density larger than 26 kg/m 3 ) of the of SMA (Figure 12b). The SMA extends down to a depth of about 40 m (Figure 12c). At the end of October 2018 the float left the cyclonic structures and moves eastward encountering surface waters of eastern origin, denser than 1026 kg/m 3 . The float WMO 1900629, coming from the Libyan coast, was entrapped in the cyclonic SMG at the end of 2007 (Figure 13a). This structure is denser than the surrounding waters (Figure 13b). Since in the region of the SMG the maximum depth of the sea bottom is of about 400 m (Figure 1a), and since the cyclonic trajectory of the float WMO 1900629 (Figure 13a) represents the displacements of the currents at the parking depth of 350 m (see Table 2), we can conclude that the cyclonic structure affects the entire water column in this area. 2), its displacements represent the current at 350 m depth due to the bathymetry of the LSBV region (see Figures 1a and14b). We can conclude that the cyclonic structure affects the entire water column in this area (Figure 14b).

Ionian cyclones

Figure 15a shows the time series of the vorticity field along the eastern coast of Sicily (MRV and ISV area -36.5°-38°N; 15°-16°E) and in the region of the SISV. Figure 15a show an inconsistency between the vorticity derived from AGV and those derived from the optimal currents in the region of the SISV.

From the optimal currents validation carried out by [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF], this structure lies in an area where the method degrades the quality of the surface currents (in particular of the meridional component) when compared with the AGV(see Figure 10 of [START_REF] Ciani | A synergetic approach for the space-based sea surface currents retrieval in the Mediterranean Sea[END_REF]). Therefore the optimal currents could not be consistent in the description of SISV. The MRV and ISV are wind-driven structure [START_REF] Lermusiaux | Features of dominant mesoscale variability, circulation patterns and dynamics in the Strait of Sicily[END_REF][START_REF] Menna | Decadal variations of circulation in the Central Mediterranean and its interactions with the mesoscale gyres[END_REF] and their interannual variability is related to the wind-stress along the eastern coast of Sicily (Figures 15b and15c): e.g. lower values of current vorticity in the MRV-ISV regions are related to lower wind-stress and lower wind stress curl. The SISV (35.5°-36.6°N; 15°-16°) is not influenced by the wind-stress vorticity, whereas it appears to be influenced by the quasi-decadal reversal of the northern Ionian (see black dashed lines in Figure 15a that give an indication of the mean vorticity value during each anticyclonic/cyclonic circulation mode) in the period 1993-2010. After 2010, the vorticity of the currents is no longer consistent with the decadal variability and seems to be rather linked to some other phenomena, that currently are not detectable from our datasets.

The float WMO 1900954, coming from the eastern Ionian, was entrapped in the SISV in December 2017 (Figure 16a) and shown an increase of density with respect to the water located north of this mesoscale structure (Figure 16b). Even if the float WMO 1900954 has a parking depth of 1000 m (see Table 2), its displacements represent the current at about 500 m depth due to the bathymetry of the SISV region (see Figures 1a and16b). The vertical extension of the SISV is about 100 m (Figure 16b).

Discussion and Conclusions

The main findings of this work are summarized in Figure 17, where a schematic diagram of the surface circulation, based on the mean circulation map depicted in Figure 2b, is presented. In Figure 17a the main circulation structures are classified according to their seasonal variability, whereas in Figure 17b they are identified based on the main forcing factor that determines them.

The basin scale circulation is essentially oriented according to the wind stress direction (northwest -southeast; Figures 2a,2c,17). Most of the main sub-basin scale current systems and mesoscale structures are permanent but affected by a strong seasonal variability (Figure 17a); only the TSC and the PV show a seasonal incidence and occur exclusively in summer and winter, respectively. The BTC, ATC, BATC and ALC show a winter intensification (Figure 17a) concurrent with the intensification of the wind stress (Figures 3 and4); for this reason they are classified as wind-driven features in Figure 17b. The relationship between the sub-basin scale structures and the wind stress variability was already suggested by [START_REF] Molcard | Numerical investigation of the Sicily Channel dynamics: density currents and water mass advection[END_REF][START_REF] Pinardi | Baroclinic wind adjustment processes in the Mediterranean Sea[END_REF]. Other sub-basin scale and mesoscale structures -e.g. the meandering AIS, the SG, and the MCC -are stronger in summer, when the wind stress is weaker. Hence, we exclude the direct impact of the wind on the variability of these structures. Rather, we suggest that they are influenced by other forcings such as the instability of the surface currents and/or their interaction with the complex and relative shallow bottom topography (Figure 17b), characterised by continental shelves and channels.

The MG and SMA are located in a highly dynamic region (high level of Eddy Kinetic Energy, see Figure 6 of [START_REF] Menna | Decadal variations of circulation in the Central Mediterranean and its interactions with the mesoscale gyres[END_REF] and Figure 9a of [START_REF] Poulain | Surface geostrophic circulation of the Mediterranean Sea derived from drifter and satellite altimeter data[END_REF]) characterized by the split of the ATC, whose eastward branch forms the BATC, and by a sudden reduction of depth (Figure 1a). These factors probably interact in facilitating the eddies formation and retention. From these considerations, the MG and SMA are influenced by the surface current dynamics and/or topography (Figure 17b). They are stronger in winter because the sub-basin currents, involved in their formation (ATC and BATC), are most intense and energetic in this season (Figure 17a).

The ABV, the MRV and the ISV are located along the southern and the eastern coast of Sicily where the wind stress curl is steadily cyclonic (Figures 2d,4c,4d). Their strength and their interannual variability are influenced by the temporal evolution of the wind stress amplitude and vorticity (Figures 15,17b).

The SMG is located on the margins of the African shelf break (Figures 1a, 2a, 2b) and its vertical extension affects the entire water column (maximum depth of 400 m) as documented by the float WMO 1900629 (Figure 13); the wind stress is weak and dimly cyclonic in this region (Figure 2 lower panels). Therefore, we define the SMG in Figure 17b as strongly influenced by the interaction between the surface currents and the topography.

The mesoscale structures located in regions where the influence of the wind stress is lower can be forced by the large-scale internal variability of the ocean. An example of this interaction is the SISV, whose interannual variability is connected to the decadal variability of the surface circulation in the northern Ionian (Figure 15a). However, the most representative example of the influence of internal forcing on the variability of a mesoscale structure is the NSA. This anticyclone, although located in the southern Tyrrhenian Sea, is affected by the decadal variability induced by the adjacent Ionian Sea (Figure 6). This result, as suggested by [START_REF] Rubino | Tidal record as liquid climate archives for largescale interior Mediterranean variability[END_REF], opens to a new interpretation of the link between different Mediterranean sub-basins and underlines the importance of internal processes on the variability of the mesoscale structures.

In summary, the surface circulation in the SC and southern Tyrrhenian Sea is characterized by multi-scale spatial and temporal variability. The main spatial scales involved are the basin, subbasin and mesoscale. The main temporal scales involved are the seasonal, interannual and

Figure 1 .

 1 Figure 1. (a) Bathymetry of the SC (100 m, 200 m, 400 m, 600 m, 1000 m, 2000 m isobaths) and geographical references. (b) Low-pass filtered drifter trajectories in the SC between 1993 and 2018

Figure 2 .

 2 Figure 2. Mean drifter currents (a) in spatial bins of 0.25°x0.25° (blue vectors) superimposed on mean maps of absolute dynamic topography (colours) during the period 1993-2018; mean optimal currents (b) in spatial bins of 1/12° (vectors; one vector every two grid point is plotted) superimposed on mean maps of absolute dynamic topography (colours) during the period 2012-2016. The structures emphasized with white arrows in panels (a) and (b) are new or with different shapes and positions with respect to those described in Figure 1b; the structures already known are highlighted with pink arrows. (c) Mean map of the wind stress amplitude (colours) and direction (vectors) and (d) wind stress curl over the period 1993-2016.

Figure 3 .

 3 Figure 3. Pseudo-Eulerian drifter statistics (blue vectors) superimposed on mean maps of the absolute dynamic topography (colours) for the (a) extended summer and (b) extended winter. The schematic circulation structures superimposed on the currents field are coloured with red arrows (a) when they are most intense in summer and with light blue arrows (b) when they are most intense in winter. Mean optimal currents in spatial bins of 1/12° (blue vectors: one vector every two grid point is plotted) superimposed on mean maps of absolute dynamic topography (colours) during the period 2012-2016 for the (c) extended summer and (d) extended winter.

Figure 4 .

 4 Figure 4. Mean map of the wind stress (upper panels) amplitude (colours) and direction (vectors) and (d) wind stress curl (lower panels) over the extended summer (a, c) and extended winter (b, d).

3. 3

 3 Decadal variations Decadal variations are emphasized by removing the mean ADT and AGV (1993-2018) fields from the interannual composite mean over the time periods characterised by the anticyclonic (1993-1996, 2006 -2010 and 2016-2017) and cyclonic (1997-2005 and 2011 -2016) circulation modes in the northern

Figure 5 .

 5 Figure 5. Mean maps of absolute geostrophic velocity anomalies (vectors) superimposed to the absolute dynamic topography anomalies (colours) during a) anticyclonic (1993-1996, 2006-2010, 2017-2018) and b) cyclonic (1997-2005 and 2011-2016) circulation modes in the northern Ionian.

  between the large-scale interior ocean variability in the central Mediterranean Sea and the local dynamics. More specifically, these authors suggest a link between the inversions of the surface circulation in the northern Ionian and the local tidal observations in the area of the Strait of Messina. The present work shows that not only the Strait of Messina, but all the coastal area adjacent to the northern Sicily coast, can be influenced by the variability attributed to the large scale dynamics of the central Mediterranean.

Figure 6 .

 6 Figure 6. Time series of the spatially averaged, low pass filtered (13-months) vorticity field over the regions of the NSA and SCA. Dashed-dotted lines are referred to the vorticity field derived from the AGV; Continuous lines are related to the vorticity field derived from optimal currents in 2012-2016. Dashed black lines show the average values of the vorticity over each anticyclonic/cyclonic period of the Northern Ionian Gyre.

  ). It is interesting to note that the intermediate currents in the region of the NSA, during the period covered by the float WMO 6900981, flow in an opposite direction (cyclonic displacements; Figure7a) with respect to the mean surface currents (Figure2b). The diagram in Figure7bshows a gap of the float profiles between 1 September 2012 and 20 October 2012. Despite this gap, the float remains confined to the eastern part of the NSA, from which it moves away only in December 2012. Unfortunately, we have no floats entrapped in the SCA.

Figure 7 .

 7 Figure 7. Map of the trajectory (black line) and profile positions (first profile: magenta dot; other profiles: black dots) of the float WMO 6900981 superimposed on the mean map of the ADT (between 18-Apr-2012 and 3-Jan-2013) (a), and contour diagram of the potential density versus depth and time (b).

Figure 8 .

 8 Figure 8. Time series of the spatially averaged, low pass filtered (13-months) vorticity field over the regions of the Malta plateau. Dashed-dotted lines are referred to the vorticity field derived from the AGV; continuous lines are related to the vorticity field derived from optimal currents in 2012-2016.

Figure 9 .

 9 Figure 9. Trajectories and deployment positions (black dots) of the drifters deployed on the Malta Plateau in December 2012 (a) and March 2014 (b).

Figure 10 .

 10 Figure 10. Map of the trajectory (black line) and profile positions (first profile: magenta dot; other profiles: black dots) of the float WMO 6901044 superimposed on the mean map of the ADT (between 16-Dec-2012 and 30-April-2013) (a), and contour diagram of the potential density versus depth and time (b).

3. 4 . 3

 43 South of MaltaThe time series of the vorticity fields obtained at the locations of the mesoscale eddies south and south-west of Malta are shown in Figure11. The MG (35°-35.5°N; 13°-14°E) and the SMA (35.5°-36°N; 14°-14.5°E), two adjacent structures located south-west of the Maltese Islands, show larger variabilities of the vorticity field with respect to the SMG (34°-35°N; 13°-14°E) and LSBV (33°-33.5°N;

Figure 11 .

 11 Figure 11. Time series of the spatially averaged, low pass filtered (13-months) vorticity field over the regions of MG, SMA, SMG and LSBV. Dashed-dotted lines are referred to the vorticity field derived from the AGV; continuous lines are related to the vorticity field derived from optimal currents in 2012-2016.

Figure 12 .

 12 Figure 12. Map of the trajectory (black line) and profile positions (first profile: magenta dot; other profiles: black dots) of the float WMO 6903242 superimposed on the mean map of the ADT (between 12-Sep-2018 and 11-Nov-2018) (a), map of the potential density measured at 20 m depth (b), and contour diagram of the potential density versus depth and time (c).

Figure 13 .

 13 Figure 13. Map of the trajectory (black line) and profile positions (first profile: magenta dot; other profiles: black dots) of the float WMO 1900629 superimposed on the mean map of the ADT (between 27-Aug-2007 and 23-Feb-2008) (a), and contour diagram of the potential density versus depth and time (b).

Figure 14 .

 14 Figure 14. Map of the trajectory (black line) and profile positions (first profile: magenta dot; other profiles: black dots)of the float WMO 1900948 superimposed on the mean map of the ADT (between 19-Jul-2015 and 28-Feb-2016) (a), and contour diagram of the potential density versus depth and time (b).

Figure 15 .

 15 Figure 15. a) Time series of the spatially averaged, low pass filtered (13-months) vorticity field over the regions of the MRV-ISV and SISV. Dashed-dotted lines are referred to the vorticity field derived from the AGV; continuous lines are related to the vorticity field derived from optimal currents in 2012-2016; b) time series of the monthly, spatially averaged low-pass filtered (13-months) wind-stress and c) wind stress vorticity in the MRV region.

Figure 16 .

 16 Figure 16. Map of the trajectory (black line) and profile positions (first profile: magenta dot; other profiles: black dots) of the float WMO 1900954 superimposed on the mean map of the ADT (between 20-Oct-2016 and 7-Mar-2017) (a), and contour diagram of the potential density versus depth and time (b).

  and summarized in Figure 1b. The averages were made over different periods in accordance with the availability of data: 1993-2018 for

	Water 2019, 11, x FOR PEER REVIEW	5 of 24

Table 1 .

 1 List of acronyms used in this paper

	Geographical names
	SC	Sicily Channel
	Water masses
	AW	Atlantic Water
	LIW	Levantine Intermediate Water
	Currents
	AC	Algerian Current
	AIS	Atlantic Ionian Stream
	ALC	Atlantic Libyan Current
	ATC	Atlantic Tunisian Current
	MIJ	Mid-Ionian Jet
	BTC	Bifurcation Tyrrhenian Current
	BATC	Bifurcation Atlantic Tunisian Current
	TSC	Tyrrhenian Sicilian Current
	Gyres and eddies
	ABV	Adventure Bank Vortex
	ISV	Ionian Shelf break Vortex
	LSBV	Libyan Shelf Break Vortex
	MG	Medina Gyre
	MCC	Maltese Channel Crest
	MRV	Messina Rice Vortex
	NSA	Northern Sicily Anticyclone
	PV	Pantelleria Vortex
	SCA	Sicily Channel Anticyclone
	SG	Sidra Gyre
	SISV	Southern Ionian Shelf break Vortex
	SMG	Southern Medina Gyre
	SMA	Southern Maltese Anticyclone
	Physical properties
	ADT	Absolute Dynamic Topography
	AGV	Absolute Geostrophic Velocities

Table 2 .

 2 List of selected Argo float with dates and positions of the first and the last profile considered in this work, parking and profiling depths and the cycle period of each instrument.

	Float WMO	First profile	Last profile	Parking depth (m)	Profile depth (m)	Cycle Period (days)

Acknowledgments: The authors would like to thanks all the people who have deployed drifters and made their data available in the Mediterranean Sea in the period 1993-2018. We acknowledge Antonio Bussani for his

Funding: This research was mainly funded by the Italian Ministry of Education, University and Research as part of the Argo-Italy program, and partly funded by the Italia-Malta Programme -Cohesion Policy 2007-2013, European Union Regional Development Funds (ERDF) through the CALYPSO and CALYPSO FO projects,. The program MISTRALS of CNRS funded the drifters deployed during the PEACETIME cruise (Guieu C., Desboeufs K., 2017, RV Pourquoi pas ?, https://doi.org/10.17600/17000300).

decadal scales. In this work, the complexity of the SC current system was investigated by combining different in-situ data and satellite products. Results provide an updated picture of the surface circulation, detecting new mesoscale features and describing their temporal variability and strength in relation to the main external and/or internal forcings. The winter strengthening of the wind stress directly influence most of the structure stronger in this season.

The structures stronger in summer and/or located in high dynamical regions are mainly driven by the instability of the surface current and/or by their interaction with the bottom topography.

In the regions where the influence of the external forcings is weaken, the large-scale internal variability of the adjacent Mediterranean basins can influence the local dynamics. technical support and his work in the production of the Mediterranean drifter dataset, and Elena Mauri for her constructive comments. We thank the two anonymous reviewers for their constructive comments on the manuscript.

Conflicts of Interest:

The authors declare no conflict of interest.