Climate change signal in the ocean circulation of the Tyrrhenian Sea

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


Introduction
The Mediterranean Sea is a semi-enclosed basin with an only connection to the open ocean via the narrow and shallow Strait of Gibraltar ( 14 km width, 300 m depth). It is connected to the Black Sea through the Marmara-Bosphorus system and is divided into the Eastern and Western Mediterranean by the Strait of Sicily. Despite its small size, the Mediterranean Sea presents a thermohaline circulation which is driven by negative surface heat and freshwater budgets i.e., it experiences a net heat and water loss to the atmosphere (e.g., Jordà et al., 2017), with deep and intermediate-depth open convection at specific sites in response to winter cooling (Bergamasco and Malanotte-Rizzoli, 2010). The surface component of the thermohaline circulation, the Atlantic Waters (AWs) flow into the Mediterranean Sea through the Strait of Gibraltar and then reach the Eastern Mediterranean via the Sicily Strait. On its way to the east, the AWs experience a surface buoyancy loss due to evaporation. In the Levantine basin, water and heat loss due to air-sea fluxes drives intermediate-depth convection (down to 150-350 m) forming the so-called Levantine Intermediate Water, LIW (Wüst, 1961;Lascaratos et al., 1993;LIWEX group, 2003). This salty water mass, the LIW, flows to the west and then to the Atlantic Ocean, along with other Mediterranean intermediate and deep waters (Millot 2009(Millot , 2019 through the deepest portion of the Strait of Gibraltar. In turn, the increased salt content of the LIW preconditions surface waters for deep water formation in winter in locations such as the Gulf of Lions, the Adriatic or the Aegean (MEDOC Group, 1970).
In the Western Mediterranean, the Tyrrhenian Sea is well known for its complex configuration and bathymetry.
Furthermore, its basin-scale circulation presents a strong seasonal variability and a very active, also seasonal-dependent, mesoscale activity which are set by the interaction of the water exchange through the Strait of Sicily, the Corsica Channel and the Strait of Sardinia (see their location in Fig. 1), as well as by local atmospheric fluxes (e.g., Krivosheya and Ovchinnikov, 1973;Korres et al., 2000). The Tyrrhenian water column is composed by Modified Atlantic Water (MAW) at the surface, LIW at intermediate depths and Tyrrhenian Deep Water (TDW) at deeper levels (Astraldi et al., 2002;Vetrano et al., 2004). Observations and modeling works indicate a cyclonic winter circulation occupying the whole basin starting from a stream of MAW that enters the Tyrrhenian through the Sardinia Strait (Krivosheya and Ovchinnikov, 1973;Artale et al., 1994;Zavatarelli and Mellor, 1995), which is interrupted in summer (Artale et al., 1994;Gasparini et al., 2008). Below MAW, the LIW crosses the Sicily Strait and gets into the Tyrrhenian Sea, where flows affected by the cyclonic circulation.
At the Corsica channel, part of LIW deflects southward along the eastern coast of Corsica and Sardinia, while the rest continues flowing northward to the North Western Mediterranean forming the so-called Ligurian current (Sciascia et al., 2019). Thus, the outflow of LIW through the Corsica Channel to the Liguro-Provençal region is important as it may directly impact the so-called preconditioning phase (MEDOC Group, 1970) of deep water formation in the Gulf of Lions . Regarding the mesoscale circulation, permanent and recurrent structures have been spotted over the last years thanks to the use of satellite images and high-resolution ocean circulation models (Béranger et al., 2004;Vetrano et al., 2010;Iacono et al., 2013;Napolitano et al., 2016;de la Vara et al., 2019). In particular, mesoscale circulation has been found to be especially active in summer.
In the current context of climate change, the Mediterranean Sea has been identified as one of the regions in which the warming signal is amplified. Thus, this sea is widely considered as a climate change "hot-spot" for the scientific community (Giorgi, 2006). This projected warming may have an impact on the air-sea fluxes, deep water convection (Parras-Berrocal et al., 2020) and in the overall Mediterranean Sea circulation (Soto-Navarro et al., 2020). As stressed above, the Tyrrhenian Sea shows seasonal variability and features recurrent structures that are heavily dependent on the large-scale Mediterranean circulation and the local air-sea fluxes. Therefore, the impact that the changes induced by the future warming in the ocean and the atmosphere may have on the circulation in that region is of particular interest. Also, waters that exit the Tyrrhenian in the Gulf of Lions (Astraldi et al., 1999). As climate change simulations with state-of-the-art global models do not have the necessary horizontal resolution, the best tool available for the study of changes in the regional ocean circulation and the corresponding driving mechanisms are the Atmosphere-Ocean Regionally Coupled Models (AORCMs; e.g., Rummukainen 2016). Only the oceanic and atmospheric components of regional models can reach the necessary horizontal resolution to simultaneously capture basin-scale and mesoscale processes in this region (e.g., Somot et al., 2006). The main aim of this work is to study the impact of climate change on the variability of the surface circulation of the Tyrrhenian Sea by the end of the century, as well as to identify the implied driving mechanisms and to examine the possible impact of these changes on winter convection in the NW Mediterranean Sea. To that end we use the regionally-coupled climate model ROM (Sein et al. 2015), which has been extensively adopted to study aspects related to the climate of the Mediterranean Sea (e.g., Darmaraki et al., 2019;Parras-Berrocal et al., 2020). This article is structured as follows. In Section 2, the model setup is described. In Section 3, the present-day and future geostrophic circulation of the Tyrrhenian basin and changes in the transport across the Sardinia Strait and the Corsica Channel are studied. In Section 4, the causes and the possible consequences of the reported changes are analyzed. The Conclusions are offered in Section 5. Figure 1: Bathymetry of the Mediterranean Sea adopted in ROM, in m, together with the oceanic grid (only 1 out of 4 lines are drawn). "WMed" and "EMed" refer to the Western and Eastern Mediterranean Sea, respectively. The arrows next to "Sar" and "Cor" indicate the location of the transects in which the transport across the Sardinia Strait and the Corsica Channel are measured (their coordinates are indicated in Figs. 4 and 5 of the manuscript). "Tyrr" stands for Tyrrhenian, "GoL" for Gulf of Lions and "LPB" to Liguro-Provençal basin.
MPIOM is a well-established global ocean model that has not only been extensively used for global climate studies as part of the different versions of MPI-ESM (Max-Planck-Institut für Meteorologie Earth System Model; Giorgetta et al., 2013), but also for regional studies (e.g. Mathis et al., 2018;Izquierdo and Mikolajewicz 2019;Liu et al., in press). MPIOM has an orthogonal curvilinear grid that, in this configuration, achieves a maximum horizontal resolution of about 7 km (mesoscale eddy-rich) near the Gibraltar Strait, decreasing down to 25 km at the eastern coast of the Mediterranean. On the vertical, MPIOM counts with 40 unevenly spaced z-levels with a variable thickness that ranges from 16 m at the surface to 650 m near the seafloor. Unlike most of the regional models devoted to the study of the Mediterranean Sea, the water exchange at Gibraltar and the Dardanelles in ROM is not parameterized and Atlantic water properties are not relaxed towards climatological values in the areas adjacent to the straits.
In REMO a rotated grid with the center of the model domain located around the equator is used. It has a constant horizontal resolution of 0.25º and a total of 27 hybrid levels. The atmospheric domain includes both the North Atlantic and the Mediterranean Sea (see Fig. 1S). The global Hydrological Discharge model (HD, Hagemann and Dümenil Gates, 2001) computes river runoff at 0.5º resolution and is coupled to both the ocean and the atmosphere.
OASIS (Valcke et al., 2003) is used to couple REMO and MPIOM and atmospheric and oceanic fields are exchanged with a 3-hour coupling time step (see Sein et al., 2015). For more details on the ROM setup we refer the reader to Cabos et al. (2020). ROM has been shown to be able to provide a regional signal that is significantly different from the one provided by the driving model (Limareva et al., 2017;Cabos et al., 2018;Sein et al., 2020).
To study the impact of climate change on the mesoscale circulation of the Tyrrhenian, we use a simulation in which ROM is forced by the low-resolution version of MPI-ESM under the high emission RCP8.5 CMIP5 scenario (Taylor et al., 2012), which is expected to have the strongest impact on the Tyrrhenian circulation. ROM and MPI-ESM share the same oceanic component, but the MPIOM setup used in MPI-ESM differs from the setup used in ROM. In particular, MPI-ESM has the higher horizontal resolution in the North Atlantic deep water formation areas, with a minimum grid spacing of about 15 km around Greenland (Jungclaus et al., 2006). In ROM, the higher resolution areas are located near the North America

Results
In this section we focus on the geostrophic circulation in order to use available gridded products derived from satellite altimetry for validation. First we show the present-day  Mediterranean-scale geostrophic circulation simulated by ROM. Then, we concentrate on the validation of the present-day geostrophic circulation of the Tyrrhenian Sea and study how its basin-scale and mesoscale-size circulation changes by the end of the century (2070-2099). Additionally, we also examine present-day and future vertical sections of velocity, salinity and temperature at the Sardinia Strait and the Corsica Channel.

Present-day Mediterranean geostrophic circulation
Here we study the ability of ROM to represent the seasonal, present-day geostrophic circulation of the Mediterranean Sea in comparison to high-resolution altimeter data from AVISO (www.marine.copernicus.eu). AVISO provides multimission altimeter gridded sea-surface heights and derived variables over the Mediterranean Sea with a horizontal resolution of 1/8° × 1/8°. The reason why we focus on the surface geostrophic circulation is twofold. On the one hand, a complete validation of the ability of ROM to reproduce the main features of the Mediterranean Sea characteristics has already been done in Parras-Berrocal et al. (2020). On the other hand, an adequate representation of the main features of the present-day circulation is required in order to study the future circulation and determine the causal mechanisms responsible for the changes in the geostrophic circulation by the end of the 21 st century.
In winter ROM captures the main features of the Mediterranean geostrophic circulation inferred from AVISO (Figs. 2A and 2C). This is, AWs entering the Mediterranean Sea at the surface and the development of the so-called Alboran anticyclonic gyres. AWs keep on flowing along the African coast on their way to the east forming the Algerian current, characterized by a large mesoscale activity (Millot, 1985). Part of this AWs, modified in the Algerian basin, flow north to the Ligurian and Provençal areas, enter the Tyrrhenian Sea and the remaining waters go through the Sicily Strait to the Eastern Mediterranean (Ovchinnikov, 1966). Then, MAW continues to flow along-slope to complete a cyclonic gyre around the Mediterranean Sea. Also there are areas are characterized by large negative values of SSH associated to the sinking of surface waters due to convection.
In summer, ROM also reproduces well the main characteristics of the Mediterranean circulation from AVISO

Present-day and future geostrophic circulation in the Tyrrhenian Sea
In winter, in agreement with AVISO and previous modeling and observational works, the large-scale circulation pattern that arises from ROM points to a basin-wide, cyclonic stream around the Tyrrhenian Sea east of this narrow strait separating Corsica and Sardinia and is related to the wind funneling through the Strait of Bonifacio (Artale et al., 1994;Vigo et al., 2019). The second one is a recirculation structure off Sardinia at the southern boundary of the Tyrrhenian Sea. These structures have been identified as permanent or quasi-permanent features of the Tyrrhenian (Artale et al., 1994;Rio et al., 2007;Rinaldi et al., 2010;Vetrano et al., 2010;Iacono et al., 2013;Napolitano et al., 2016).
In summer, the basin-scale circulation presents important differences with respect to the winter situation and ROM is able to capture the main features of the typical summer pattern (Figs. 3D and 3E). For instance, the winter basin-wide cyclonic circulation around the Tyrrhenian does not arise. Whilst, as in winter, MAW enters the basin through the southern part of the Sardinia Strait, the development of several dynamical structures to the east of the basin reverses the circulation along the southern and eastern coasts of the Tyrrhenian, hampering the direct connection between the southern and northern openings. In general, more dynamical structures develop than in winter. The Bonifacio Gyre stretches zonally and the water transport across the Corsica Channel becomes smaller (Astraldi and Gasparini 1992). The recirculation structure near Sardinia becomes more defined. The anticylonic circulation becomes predominant to the southeast of the Tyrrhenian, over the Sicilian coast (Iacono et al., 2013;Napolitano et al., 2016;de la Vara et al., 2019).
In the projected scenario, the winter circulation patterns are qualitatively similar to those found in the present-day simulation (Figs. 3A-3C). However, the Tyrrhenian-basin cyclonic circulation becomes weaker and the previously two  Fig. 3, the surface circulation pattern is substantially different from that in winter, with a weaker and shallower MAW stream entering the Tyrrhenian through the center of the strait and outflow currents elsewhere in the studied depth range (Fig. 4C). Also, the water column in the first 200 meters is more stratified and lighter than in winter due to summer insolation. In future climate, current velocities experience a generalized decrease, but there is a notable intensification of velocities in the depth range and location of the present LIW, with a maximum from about 150 to 400 m depth by the Sardinia shelf slope (Fig. 4D). It should be noted that, regardless of the season, in future climate water density decreases throughout the first 400 meters (Figs. 3A-3D). This is related to the generalized temperature increase in the Mediterranean and the freshening of the MAW (Parras-Berrocal et al., 2020).
Present-time velocities in the Corsica Channel are positive (outflowing) all over the water column and feature a maximum over 0.06 m/s at the surface in winter (Fig. 5A). This implies that, as seen in Figs. 3A-3C, part of the MAWs that flow around the Tyrrhenian do not complete the cyclonic gyre around the basin and exit via the Corsica Channel. The core of this outflow is located at the west of the channel within the upper 30 m and velocities decrease gradually with depth. In the future, the same pattern remains but velocities become weaker and the maximum, which is close to 0.04 m/s (Fig. 5B), is not located at the surface but further down close to a depth of 200 m. This is consistent with the results found in the Sardinia Strait i.e., the MAW stream that enters the Tyrrhenian weakens in the future. This is also in line with the weakening of the cyclonic stream around the Tyrrhenian basin and the enhancement of dynamic structures with a shift towards the summer geostrophic circulation pattern. In summer, at present, as can be seen in but negative values arise at shallow levels, with values of a magnitude greater than 0.02 m/s (Fig. 5D). Again, density experiences a future decrease all over the water column.
From these results we can conclude that, in future climate, in winter, MAW enters the Tyrrhenian at a slower pace than at present. This is consistent with the slowdown of the along-slope, cyclonic stream that borders the Tyrrhenian in this season, as seen in Figs. . This is also the case for the MAW entering Liguro-Provençal basin via the Corsica Channel, as illustrated in Figs. 5A and 5B. This northward branch of the stream that flows close to the Corsica coast weakens significantly in the future.
We now examine the changes in the water properties i.e., temperature and salinity across the Sardinia Strait and Corsica Channel (Figs. 6-9). Presently, winter temperatures in the upper 200 m of the Sardinia Strait vary between 15.5ºC to the south to 14ºC to the north (Fig. 6A). This meridional temperature gradient relates to the fact that MAW enters the Tyrrhenian to the south and exits the basin to the north after completing a cyclonic gyre. Down from that d epth, temperature is quite homogeneous and exhibits values close to about 14ºC. In summer, the water column is, as expected, stratified due to the development of the seasonal thermocline, but the meridional temperature gradient is less marked due to the decrease in the MAW inflow into the Tyrrhenian. Values are close to 20ºC at the surface and about 14ºC at a depth close to 100 m (Fig.   6C). From that depth downwards, temperature is close to 14ºC or less. In the future, the winter and summer vertical structure of the water column is qualitatively similar to that at present, but with warmer temperatures (Figs. 6B and 6D). In winter, surface temperature ranges between 17.5 and 17ºC at the surface and from 150 to 200 m depth water temperature is close to 16ºC. In summer, surface temperature is even warmer than 22ºC. From that depth downwards, temperature is about 16ºC.
At present, in the Corsica Channel, in winter, temperature is quite homogeneous and takes values close to 14ºC with values slightly higher at the surface (Fig. 7A). In the future, winter temperature is homogeneous as well, but shows a nearly 2ºC increase all over the water column (Fig. 7B). In summer, the water column is thermally stratified with values close to 20ºC at the surface and close to 14ºC below the thermocline down to the seafloor (Fig. 7C). In the future summer, the water column remains stratified but warmer, with values close to 24ºC at the surface and values close to 16ºC from about 100 m depth (Fig. 7D).
Focusing now on salinity we observe that, at present, in the Sardinia Strait, salinity is lower at the surface, with a marked north-south gradient and is more homogeneous from about 250 m (Fig. 8A). Specifically, in winter, surface salinity is close to 37.5 psu to the south and closer to 38 psu to the north of the strait. These results are in line with those from Beranger et al. (2004) and de la Vara et al. (2019). This salinity gradient reflects a side-by-side channel flow which is due to the entrance of fresher MAW from the Algerian Current to the south of the Tyrrhenian which completes a cyclonic gyre around the basin and exits the Tyrrhenian to the north of the strait with a higher salinity. Further deep, from about 250 m, salinity is close to 38.4 psu, which is typical from LIW (e.g., Astraldi et al., 2002). In the future, the winter salinity pattern is qualitatively similar to that at present, but values are different (Fig. 8B). On the one hand, MAW that enters and exits the Tyrrhenian, respectively, are slightly fresher than today, which may be related to the North Atlantic influence as previously has reported in Parras-Berrocal et al. (2020) and Soto-Navaro et al. (2020). On the other hand, LIW is saltier than today, with a LIW core with a salinity close to 38.8 psu.
In summer, at present, the entrance of MAW to the Tyrrhenian via the Sardinia Strait is more limited and the cyclonic stream around the basin is limited to the southern Tyrrhenian Sea, so that the north-south salinity gradient across the Sardinia Strait is more modest than in winter and thus isohalines are less tilted i.e., more horizontal (Fig. 8C). From about 150 m depth, salinity varies less and takes values close to 38.4 psu. In the future, MAW also becomes fresher than at present. LIW becomes, as in winter, saltier than today, with values close to 38.8 psu for the LIW core (Fig. 8D).
In the Corsica Channel, at present, in the winter situation, salinity is lowest at the surface and increases with depth ( Fig. 9A). Its value is close to 38 psu at the surface (see e.g., Béranger et al., 2005;de la Vara et al., 2019) and close to 38.4 psu at the bottom of the channel, which are values close to those from Béranger et al. (2005). In the future, salinity stays similar or lower at the surface, but waters become saltier from about 100 m to the seafloor, reaching up to 38.8 psu (Fig. 9B).
In summer, the situation is qualitatively like that reported for the winter season (Figs. 9C and 9D). These changes in salinity make a strong contribution to the reinforcement of the stratification of the waters that enter the Liguro-Provençal basin, as will be tackled in the Discussion.

Discussion
At this point it is interesting to understand the causes and consequences of the projected changes in the Tyrrhenian Sea geostrophic circulation and water transport across its main straits.

Drivers for the future changes in the geostrophic circulation of the Tyrrhenian
Here we investigate the role of winds in the changes of the geostrophic circulation projected for the end of the century, as seen in the previous section. The surface wind pattern over the Tyrrhenian Sea presents a clear seasonality. We observe that in present time winter the there is a predominance of westerly winds (Fig. 10A). The strongest wind speeds occur to the east side of Sardinia and Bonifacio straits, due to wind funneling (see e.g., Gérigny et al., 2015). These areas of values between 4 and 5 m/s (Fig. 10B). The wind slackening found near the entrance of the Sardinia Strait may be an important trigger for the slowdown of the cyclonic stream around the basin which is fed by MAW.
In summer, winds are far weaker than in winter, with an overall decrease of the magnitude of the westerlies (Fig.   10C). Once more, the greatest wind speeds arise to the east of the Bonifacio Strait, with values close to 2 m/s. In this season, high wind speeds also occur to the southeast of the Tyrrhenian, where two important dynamical structures are found (see Figs. 3D and 3E). As found for the winter season, the future pattern of surface wind is qualitatively similar to the present one, but with remarkably lower wind speeds (Fig. 10D).
We now study the relationship between the wind representation and the ocean circulation. To that end, we compute the mean wind work in the Tyrrhenian e.g., Renault et al., 2016). This is calculated using the following expression: where ρo is the mean value of the sea water density assumed 1020 kg/m Gyre is found. Negative values are also present in the recirculation area near the Sardinia Strait. In the future, qualitatively, the same basin-scale pattern remains, but positive and negative values of the wind work become remarkably higher in the Bonifacio Gyre with up to 8 cm 3 /s 3 in that region (Fig. 11B). The future enhancement of the wind work, especially in the areas where the dynamical structures appear is thus not caused by wind speed intensification. The enhancement of the wind work may be related to the weakening of the cyclonic stream around the Tyrrhenian, which makes stronger the transference of kinetic energy from the wind to the Bonifacio Gyre and could be responsible for its intensification and increased extension. The strength of the Bonifacio Gyre is determined by the energy input by the wind stress and its interaction with the cyclonic jet, which employs part of the wind energy for the shedding of smaller-scale eddies, constraining the size of the gyre. In the future, the weakening of the stream makes it more stable, reducing the shedding, which is reflected by the increase in size of the Bonifacio Gyre and its strengthening.
As shown in Fig. 11C, at present, the wind work in summer is mainly positive except for the southeast of the Tyrrhenian, where positive and negative side-by-side values appear due to the development of a characteristic summer dynamical structure and in the Bonifacio Gyre (Figs. 3D-3F; e.g., de la Vara et al., 2019;Napolitano et al., 2016). It takes smaller values than in winter due to the wind slackening. In the future, the wind work does not present important changes, but increases especially in the Bonifacio Gyre area (Fig. 11D). The reasoning behind this is that, again, a decrease in the water transport of MAW across the Sardinia Strait increases the effectiveness of the transference of kinetic energy from the wind to these dynamical structures.

Implications for the water exchange with other basins
We now study how the changes in the water properties across the Tyrrhenian straits in the future can have a possible impact on adjacent areas. In particular, we focus on the Liguro-Provençal basin, the circulation and hydrology of which is strongly affected by waters that exit the Tyrrhenian Sea via the Corsica Channel and this can have a concomitant effect on the deep convection over the Gulf of Lions (Astraldi et al., 1999).
From the above analysis we can thus conclude that in both the Sardinia Strait and the Corsica Channel the water column presents, qualitatively, the same structure as today but waters experience a generalized warming which is in line with Parras et al. (in preparation). Also in agreement with that work, surface waters tend to be fresher than today, whilst intermediate-depth waters will be saltier, due a freshening of the Atlantic surface waters that enter the Mediterranean, as well as by an enhanced evaporation that causes intermediate waters to be saltier than today. The prevailing effect of the warning leads a generalized density decrease all over the sections studied (Figs. 4 and 5). This reduction is more pronounced at the surface due to the simultaneous warming and freshening, and at intermediate-depths, the density reduction is more modest because both temperature and salinity show an increase. A clear implication of this is that the water column becomes more stratified in the future. The waters flowing from the Tyrrhenian towards the Liguro-Provençal basin play a first-order 19 405 410 415 importance role in shaping the stratification of the area and thus in the preconditioning for deep water convection in the Gulf of Lions (e.g., Schroeder et al., 2010). Our results suggest that the combination of the decrease of the volume and the strengthening of the stratification of the waters that flow through the Corsica Channel towards the Liguro-Provençal basin could be one of the contributing factors to the projected interruption of the deep water formation in this area in the future (Margirier et al., 2020;Parras-Berrocal et al. (in preparation)).

Conclusions
In this work we examine the possible changes in the Tyrrhenian future surface circulation under the RCP8.5 emission scenario with the atmosphere-ocean regionally coupled model ROM. Special attention is given to the mechanisms that drive the changes by the end of the 21 st century and the possible implications in the Liguro-Provençal basin. The Tyrrhenian Sea is a challenging region to study the impact of global warming on the oceanic circulation: it presents complex seasonal-dependent circulation patterns and rich mesoscale activity which is controlled by the interaction of local climate and the large-scale Mediterranean circulation through the water transport at its main straits. We show that ROM is capable of successfully reproducing the main features of this circulation. The main conclusions of this study regarding circulation changes in future can be summarized as follows:  In winter, the cyclonic stream around the Tyrrhenian becomes weaker compared to that at present due to a reduced water transport across the Sardinia Strait, while the Bonifacio Gyre and the recirculation area near the Sardinia Strait intensify. This means that the winter pattern becomes less developed than at present, which is consistent with a warming climate.
 In summer, the dynamical structures become more intense than at present and, as today, the cyclonic stream around the Tyrrhenian basin breaks down. Therefore, the summer pattern becomes more intensified than at present.  The intensification of the dynamical structures in winter is related to the weakening of the cyclonic stream around the Tyrrhenian Sea which is, in turn, caused by a reduction in the inflow of MAW into the basin. This weakening is associated with the increase of the magnitude of the wind work over the main dynamical structures, which are intensified and enlarged.
 In the future, the northward water transport across the Corsica Channel towards the Liguro-Provençal basin becomes smaller than today. Also, water that flows through this channel presents a stronger stratification than at present due to a generalized warming with a saltening of intermediate waters. These changes potentially contribute to the interruption of deep water formation in the Gulf of Lions by the end of the century.