The SRES scenarios
The socio-economic scenarios used in the present study are based on the Special Report on Emissions Scenarios (SRES). Among the three scenarios which have been performed, B1 is the most optimistic, A2 is the most pessimistic and A1B is intermediate in terms of gases emissions.
The A2 scenario has been developed in the frame of the Third Assessment Report (TAR) of the Intergovermental Panel on Climate Change (IPCC), and is one of the worst scenarios in terms of greenhouse gas emissions. It is based on the following asumptions: a continuously increasing population, a regionally oriented economic development and a world of self-reliant nations.
Future Mediterranean climate
Surface fluxes
As depicted in Table 2, E–P–R–B increases for all the projections for the period 2070–2099 compared with the historical period 1961–1990. Precipitation, runoff and Black Sea freshwater input tend to decrease whereas the evaporation increases, consistently with the studies by Mariotti et al. (2008), Sanchez-Gomez et al. (2009), Elguindi et al. (2011) and Dubois et al. (2012). To compensate the resulting enhanced water loss, the net water inflow at Gibraltar is intensified. These changes are strongly influenced by the choice of the scenario with the strongest effect under the A2 scenario, and the weakest effect under the moderate scenario B1, A1B being intermediate. As in Somot et al. (2006) and Dubois et al. (2012), we found a decrease of the surface net heat loss (Table 3) in all projections. Note that none of the scenarios shows a surface heat gain in 2070–2099 what means that the atmosphere still extracts heat from the Mediterranean Sea at the end of the twenty-first century. This change varies according to the scenario used, but also strongly depends on the chosen boundary conditions. The latest point will be investigated in Sect. 4.3.
We found a general decrease of the Mediterranean surface potential density (Fig. 11; Table 5) because the increase of the SSTs (Fig. 12) prevails on the increase of SSSs (Fig. 13). This is not true for B1-ARF where the surface density increases because the density gain from the saltening prevails on the density decrease from the warming. In the scenarios, the SSTs become warmer with a range between +1.7 and +3 °C (2070–2099 vs. 1961–1990, averaged over the Mediterranean) and this large spread reflects the sensitivity to the chosen scenario. The SST term has a low sensitivity to the boundary conditions due to the use of the SST relaxation term (Table 4; Fig. 12). Figures 14 and 15 represent the composites of the SST and SSS anomalies maximums and minimums: the largest or smaller anomaly out of the 6 scenario simulations is taken into account at each grid point. These figures show that the warming is not homogenous: the region of the Balearic Islands, the Northwest Ionian, the Aegean and Levantine Seas get warmer than the average (Fig. 14). Concerning the SSS, it generally increases with a range between +0.48 and +0.89 (Fig. 13) and the interannual standard deviation of the historical simulations is 0.07 in HIS and 0.08 in HIS-F. As for the SST, the SSS anomaly signal is not homogenous: the Aegean basin is getting saltier than the rest of the Mediterranean and both Balearic region and North Ionian display a weaker response, even a freshening in some simulations (Fig. 15). The choice of the scenario (experiments A2-ARF, B1-ARF and A1B-ARF) does not induce too strong variations in the SSS anomalies but the choice of the boundary conditions does (experiments A2, A2-F, A2-RF and A2-ARF), especially the choice of the Atlantic salinity (Fig. 13). The warming and saltening of the Aegean are mainly explained by the strong decrease of the net water input from the Black Sea, whereas the other strong patterns mainly refer to subsurface circulation changes (Sect. 4.2.3). In Figs. 14 and 15, it is worth noting that in most of the cases, the SST MAX map corresponds to the scenario A2-ARF, the SST MIN to B1-ARF, SSS MAX to A2-ARF and SSS MIN to A2.
Heat, salt and density contents
A warming and saltening are modelled in all the layers of the water column (Table 4), but this signal decreases through depth from an average of +0.7 and +2.6 °C at the surface to +0.4 and +1.0 °C in the 600 m-bottom layer. The penetration of the heat and salt anomalies from surface to depth varies according to the simulation, depending on the changes in convective areas, themselves influenced by the historical state of the vertical stratification and the associated MTHC. The changes of MTHC will be described in Sect. 4.2.4.
The density averaged over the whole water column increases in all the scenario simulations (Table 5). This also applies for the mean density of the layers 150–600 m and 600-bottom. However, the layer 0–150 m displays a density decrease in A2, A2-F and A2-RF: this signal is directly depending on the properties of Atlantic water entering in the Mediterranean which are lighter in these simulations. These density changes lead to changes in the vertical stratification of the basin in the future as displayed by the \(IS\) anomalies (Table 5; Fig. 16). Overall, the simulations with lighter Atlantic waters from GCM2 (A2, A2-F and A2-RF) represent a strong increase of the vertical stratification whereas A1B-ARF, B1-ARF and A2-ARF display a large decrease of \(IS\) in the eastern basin.
Surface circulation
Figure 17 displays anomalies of near-surface circulation at 34 m depth and shows a clear change in the Balearic region with the penetration of the Atlantic surface water toward the North, along the spanish coast, westward of the Balearic Islands. This feature is new if compared to HIS, or reinforced, when compared to HIS-F. However this change should be interpreted cautiously since the circulation of the historical simulation is not very realistic in this small region. In the Ionian basin, the “F” family simulations display a substantial modification of the trajectory of modified Atlantic waters into the eastern basin, which forms a “North Ionian jet”, similar to the surface circulation changes which occur during the EMT. These two large changes explain both surface warming and freshening (or weaker saltening) of the Balearic region and the North Ionian mentioned in Sect. 4.2.1.
THC characteristics and water masses dynamics
Figure 5 presents the picture of the MTHC in all the simulations. The MTHC future evolution in the scenarios is strongly related with the state of the vertical stratification in the historical simulations which is weaker in HIS than in HIS-F. The large differences between both historical states provide an interesting diversity for the analysis. We first analyse the results of the “F” simulations, whose atmospheric forcing comes from RCM4.
In the “F” family simulations, despite the general increase of the MTHC, there are strong changes in the areas where dense water is formed: the eastern basin becomes generally more productive and faces a similar situation as during the Eastern Mediterranean Transient. This EMT-like situation happens because the Aegean basin experiences a buoyancy loss in comparison with the historical period, this loss is related to both intensified winds and very low net water inflow from the Black Sea, which reduce the Aegean buoyancy. This situation is clearly depicted in Fig. 5 with the increase of the negative intermediate/deep cell between 27°E and 35°E, which represents the strong outflow of Aegean deep water into the Levantine basin through the straits of Kassos and Karpathos (not shown) as proposed by Beuvier et al. (2010). The intermediate/deep positive cell between 15°E and 27°E represents the outflow of Aegean deep water into the Ionian basin through the Strait of Antikythera (not shown).
To illustrate the EMT situation, we analyse the EMT-indexes (described in Sect. 3.6) averaged over the period 2070–2099 for each simulation (Fig. 18). The highest the index the more we simulate an EMT-like circulation. As expected, the historical simulations have an EMT-index set to 0 Sv, since both simulations depict the expected “standard” circulation, with no positive intermediate/deep vertical circulation cell in the Ionian basin. The “standard situation“ refers to a MTHC situation with a single source of deep water located in the Adriatic for the eastern Mediterranean. In the “F” family scenarios, the index increases with a range between 1.01 and 2.86 Sv. In the simulation A2, the index stays at 0 Sv with a vertical circulation which weakens but keeps the standard configuration. We aim to relate the EMT-index to the Aegean buoyancy. Figure 18 represents the relation between the EMT-index anomalies (scenario vs. historical period) and the Aegean buoyancy fluxes anomalies. From HIS to A2, the buoyancy fluxes of the Aegean basin increase, whereas they decrease from HIS-F to all the “F” scenario simulations. An Aegean full buoyancy decrease clearly pushes toward an EMT situation and is mainly expected in the future Mediterranean ocean climate (as seen in A2-F, A2-RF, A2-ARF, A1B-ARF and B1-ARF).
In Fig. 5, the negative very deep cell in the Ionian basin (between 15°E and 27°E) represents the deep circulation of the ADW which flows through Otranto, and cascades southwestward into the Ionian. The scenario simulations generally face a situation where the density difference between the ADW and the northwest Ionian water increases (in comparison with the historical period), therefore, the cascading depth of the ADW is increased from 1,265 m in HIS-F to a range between 1,861 and 2926 m in the “F” scenarios (Fig. 6). However, Fig. 6 also shows that the intensity of the circulation cell from the Adriatic into the northern Ionian weakens (but in B1-ARF) and tends to be restricted to the extreme North of the Ionian basin in A2-ARF and A1B-ARF.
From the “F” simulations analysis, we can state that we head towards an EMT-circulation in the future, but the ADW production is still active: the ADW is produced with a slightly lower intensity but the cascading goes deeper and is confined in the high northern Ionian due to the presence of enhanced Aegean outflow (negative cell in Fig. 6 for A2-ARF and A1B-ARF).
Concerning the changes occurring in the western basin, the dense water production in the Gulf of Lions decreases by the end of the century in all the scenario simulations “A2“ but remains similar to the historical state in A1B-ARF and increases in B1-ARF whose vertical stratification diminishes in this area (Figs. 16, 19). For the western Mediterranean, we can establish a direct link between the vertical stratification in the Gulf of Lions and the deep water production since the WMDW is formed locally.
If we focus on the differences between HIS and A2 (atmospheric forcing comes from RCM3), we find a decreasing MTHC in both eastern and western basins (Fig. 5), confirming results obtained by Somot et al. (2006) who used the same forcing but another ORCM and found a decrease in the surface density associated with a weakening of the thermohaline circulation, except for the Adriatic. This significative difference of future changes is related to the fact that the HIS simulation has a low vertical stratification and is very convective (especially in the eastern basin), due to very strong winter heat loss compared to HIS-F in the historical period. However, despite a decrease of the MTHC in A2 compared to its historical period, the high-convective initial state of HIS allows to bring the surface hydrographic anomalies to deeper layers from the beginning of the A2 simulation on. This explains the large anomalies found in the deepest layer despite moderate surface anomalies compared to the ”F“ family of scenario simulations.
Sea level changes
The model configuration does not consider changes in the Atlantic sea level, so from this ensemble we cannot estimate future changes of total sea level. However, the thermosteric component of sea level (the one related with expansion/contraction of the water column) can be estimated from the ensemble. In the former sections, we focused on changes in the formation of the water masses and their hydrographic properties in the future. These large changes have a strong impact on the mean thermosteric sea level (Fig. 20) which results from the heat content changes over the full water column. For the 2070–2099 period, a mean increase between 34 and 49 cm is simulated. This large spread is explained by differences in the convective areas and in the intensity of the convection, leading to differences in the transfer of the surface heat to the deepest part of the basin, thus impacting the basin heat content and the thermosteric sea level. It is important to highlight that this range of values (34–49 cm) only refers to the thermosteric component of sea level. To recover total sea level other components should be included (e.g. mass addition due to land ice melting, changes in the Atlantic circulation,...). From the recent 5th IPCC report (IPCC 2013), it seems that those components will be positive adding between 15 and 30 cm to the thermosteric component.
Sensitivity to boundary conditions
This section aims to identify and quantify the impact of the different boundary conditions on the obtained results for a given scenario. We examine the sensitivity of the future Mediterranean Sea to the prescribed hydrography of the Atlantic water, the river runoff and the surface fluxes. For that purpose, we only focus on A2 scenario simulations. In each of the analysed simulations, the combination of boundary forcing changes in such a way that we can isolate the sensitivity to each type of boundary forcing, by comparing simulations one to one. The uncertainty range explored in the present study can be small compared to the large spread displays among other regional and global models regarding the future changes in Atlantic hydrography, river runoffs and atmospheric fluxes. This point is discussed in Sect. 4.6.
Choice of atmospheric surface fluxes: A2 versus A2-F
The differences of atmospheric forcings between A2 and A2-F are substantial. For the 2070–2099 period compared to their respective historical period 1961–1990, the decrease of the net heat loss and increase in E–P water loss are larger in A2-F with +\(2.5\,\hbox {W}\,\hbox {m}^{-2}\) and +\(0.73\,\hbox {mm}\,\hbox {d}^{-1}\) compared to +\(2\,\hbox {W}\,\hbox {m}^{-2}\) and +\(0.40\,\hbox {mm}\,\hbox {d}^{-1}\) in A2. The heat and salt contents of the full water column do not increase similarly in A2 and A2-F: despite the larger E–P and heat fluxes anomalies in A2-F, the propagation of the salt anomalies in the deepest layer 600 m-bottom is larger in A2 (+0.31) than in A2-F (+0.26). As mentioned in Sect. 4.2.4, this happens because this simulation starts from a very convective initial state, and the surface anomalies strongly propagate already at the beginning of the A2 simulation, before the weakening of the MTHC.
Table 7 shows quite different state of the MLD compared to the historical period between A2 and A2-F. In A2, the maximum of the winter MLD, considered as a qualitative proxy for deep water convection, mainly decreases (except in the Aegean). In A2-F, the maximum of winter MLD increases in the eastern basin and decreases in the western basin. As stated in 4.2.3., the MTHC weakens basinwide in A2, whereas it increases in the eastern basin in A2-F. A2 does not face an EMT-like situation as A2-F but maintains a standard circulation with ADW as main source of eastern dense water, the ADW cell is weaker though (Figs. 5, 6). The warming signal of the deep layer 600-bottom is slightly larger in A2-F than if A2 despite a weaker transfer toward depth (Table 4), this is explained by a much larger surface signal in A2-F than in A2.
Table 7 Monthly maximum of the mixed layer depth (in m) and correponding month of occurence. Anomalies (2070–2099 vs. 1961–1990) are represented for the scenario simulations
Choice of rivers: A2-F versus A2-RF
The differences of river forcing between A2-F and A2-RF come from the use of different versions of the ARCM (RCM3 and RCM4) to obtain the runoff. The impact of the choice of river runoff on the oceanic state of the Mediterranean is negligible. For the 2070–2099 period, the total amount of runoff is decreased by 45 % in A2-F and by 42 % in A2-RF. This difference is mainly due to changes in the fresh water input from the Black Sea (treated as a runoff), which drops from \(8{,}709\,\hbox {m}^{3}\,\hbox {s}^{-1}\) for the 1961–1990 period to \(3{,}340\,\hbox {m}^{3}\,\hbox {s}^{-1}\) in A2-F and \(3{,}695\,\hbox {m}^{3}\,\hbox {s}^{-1}\) in A2-RF (2070–2099). This leads to mean SSS differences of 0.05 between A2-F and A2-RF. The 3D-circulation of the Mediterranean simulated for the future is similar between both simulations (Figs. 5, 6 and 7). Because A2-RF has larger runoff, the vertical stratification is greater than in A2-F, thus the surface heat anomalies do not penetrate as deep as in A2-F. This explains that the 3D averaged temperature anomaly in A2-RF (+0.93 °C) is lower than in A2-F (+1.04 °C) as seen in Table 4.
Choice of the Atlantic hydrography: A2-RF versus A2-ARF
The impact of the hydrographic properties of the Atlantic water on the oceanic state of the Mediterranean is assessed through the comparison of the simulations A2-ARF and A2-RF, whose Atlantic hydrographic conditions come from different GCM versions, GCM2 and GCM3 respectively. Figure 21 displays the anomalies of the Atlantic hydrographic properties in both simulations. Future Atlantic waters which enter in the Mediterranean basin are warmer and saltier than in the historical period. This signal is nevertheless larger in A2-ARF which displays a warming and saltening respectively 0.5 °C and 0.40 larger than in A2-RF for the 0–150 m layer of Atlantic waters for 2070–2099. This results in a smaller density decrease in A2-ARF (about \(-0.1\,\hbox {kg}\,\hbox {m}^{-3}\)) than in A2-RF (about \(-0.25\,\hbox {kg}\,\hbox {m}^{-3}\)), considering the same ocean layer (0–150 m) and the same time period (2070–2099 vs. 1961–1990). The hydrographic changes of the Atlantic buffer zone are advected through the Mediterranean, which displays mean SSS changes of +0.89 in A2-ARF and +0.64 in A2-RF for 2070–2099 (Table 4).
A2-RF and A2-ARF display quite different vertical circulation changes in the eastern Mediterranean. A2-ARF projects a more intense EMT situation for the future than A2-RF (Figs. 5, 18) because A2-RF has a larger surface density decrease from the incoming Atlantic water, thus a stronger vertical stratification than A2-ARF (Fig. 16). For the same reasons, the meridional overturning stream function of the Adriatic basin displays a deeper cascading of the ADW into the north Ionian in A2-ARF than in A2-RF (Fig. 6) and the reduction of dense water production in the Gulf of Lions is quite larger in A2-RF (Table 7; Fig. 19). This leads to large differences in the propagation of warming from the surface toward depth. For the same surface warming anomaly of 2.97 °C, A2-ARF has a more active transfer than A2-RF, as displayed by the heat content anomalies in the 600 m-bottom layer with the values of +1.14 °C for A2-ARF versus +0.68 °C in A2-RF (Table 4).
The hydrography of the Atlantic water is thus substantially impacting the new organization of water masses production. The changes are weaker in A2-RF whose lower surface density allows a stronger vertical stratification than in the simulation A2-ARF.
Sensitivity to the socio-economic scenario
The comparison between the simulations A2-ARF, A1B-ARF and B1-ARF provides some answers on the sensitivity to the choice of the socio-economic scenario. Table 4 shows that the surface salinity and temperature changes averaged over the Mediterranean basin are larger in A2-ARF than in A1B-ARF and B1-ARF, consistently with the greenhouse gases concentrations in each scenario. However, both heat and salt content changes over the full water column are found to be larger in A1B-ARF than in A2-ARF, reflecting a larger propagation of the surface heat anomaly to deeper layers in A1B-ARF. A1B-ARF is indeed having the strongest EMT-index among the whole family of scenario simulations (Fig. 18) but also keeps a deep cascading of ADW (Figs. 6, 18) and its maximum of MLD in the Gulf of Lion is not decreasing (unlike A2-ARF). All these features make that the simulation A1B reproduces the strongest vertical circulation at the end of the twenty-first century (Fig. 5), inducing thus the strongest advection of the surface changes toward deep layers. B1-ARF, as the moderate scenario, displays the weakest surface hydrographic changes, and the largest decrease of vertical stratification compared to its historical period. It shows a small increase of the dense water formation in the Gulf of Lions (Fig. 19), its EMT signal is the weakest among the 3 scenarios, and it maintains a strong ADW formation with an intense cell and a deep cascading into the Ionian basin (Figs. 5, 6).
Impact of the socio-economic scenario versus boundary conditions
With this large family of scenario simulations, we were able to identify to what extent the choice of the boundary conditions and socio-economic scenario impacts the oceanic changes in the future Mediterranean. The results are found to be especially sensitive to the choice of the Atlantic forcing and the socio-economic scenario. Depending on the prescribed Atlantic hydrography, the ocean model faces a vertical stratification which can modulate the response of the water masses dynamics with more or less dense water formation. The propagation of the heat from the surface through the depth is thus affected and a different response of the Mediterranean thermosteric sea level is expected (Fig. 20). From our qualitative analysis, we infer that the sensitivity to the choice of the Atlantic boundary conditions is at least of the same order as the sensitivity to the choice of the socio-economic scenario, if not larger. In Table 8, we quantify these differences by comparing the spread between the 3 simulations where only the socio-economic scenario differs (A2-ARF, A1B-ARF and B1-ARF), called \(\varDelta \hbox {Scen}\), with the spread between the 2 simulations with different Atlantic conditions only (A2-ARF and A2-RF), called \(\varDelta \hbox {Atl}\). For the heat and salt contents of the full water column averaged over the 2070–2099 period, \(\varDelta \hbox {Atl}\) is larger with a spread of +0.16 and +0.30 °C against +0.09 and +0.24 °C for \(\varDelta \hbox {Scen}\). The difference between \(\varDelta \hbox {Atl}\) and \(\varDelta \hbox {Scen}\) increases for the deepest layer with a spread of +0.19 and +0.46 °C for \(\varDelta \hbox {Atl}\) against +0.10 and +0.26 °C for \(\varDelta \hbox {Scen}\). The spreads for the two other boundary forcings, rivers (\(\varDelta \hbox {Riv}\)) and atmospheric fluxes (\(\varDelta \hbox {Atm}\)), are much smaller than \(\varDelta \hbox {Atl}\) and \(\varDelta \hbox {Scen}\). Table 8 also compares the different spreads of the EMT-index and confirms that the sensitivity to the Atlantic forcing is the largest.
Table 8 Sensitivity of different quantities to the following forcings: atmospheric fluxes (\(\varDelta \hbox {Atm}\)), river runoff (\(\varDelta \hbox {Riv}\)), Atlantic hydrography (\(\varDelta \hbox {Atl}\)) and socio-economic scenario (\(\varDelta \hbox {Scen}\)). The values are computed over the 2070–2099 period
Concerning the changes in thermosteric mean sea level for the 2070–2099 period (Fig. 20), \(\varDelta \hbox {Atl}\) displays a spread of 11 cm, with an increase between 34 and 45 cm. This large difference is driven by a weaker penetration of the heat anomaly in the A2-RF simulation, due to a weaker eastern convection compared to A2-ARF. \(\varDelta \hbox {Scen}\) has a spread of 9 cm with a range between 40 and 49 cm. \(\varDelta \hbox {Riv}\) and \(\varDelta \hbox {Atm}\) show respectively a spread of 4 and 3 cm.
This results highlight the relevance of the choice of boundary conditions for Mediterranean ocean projections. The choice of Atlantic conditions seems to be of highest importance and prevails on the choice of the socio-economic scenario.
Discussion on uncertainties among other models
In order to place our results in a wider context, we compare the uncertainty ranges explored in our study for each of the three boundary conditions with the large spread among global and/or regional models. From the study by Marcos and Tsimplis (2008), 10 GCMs used in CMIP3 show a large spread of changes in Atlantic hydrography computed for the buffer zone of NEMOMED8 model for the layer 0–150 m. The changes vary from −0.35 to +0.66 for salinity, and from −0.06 to +2.56 °C for temperature. This gives a range of 1.01 in salinity and 2.62 °C in temperature, which is larger than the uncertainty range used in our study (0.40 for salinity and 0.5 °C for temperature). Sanchez-Gomez et al. (2009) analysed Mediterranean runoff changes for the 2070–2099 period among various regional climate models and found a runoff decrease ranging from −5 to −43 %, and changes in Black Sea fresh water input from +25 to −102 %. In our study, the uncertainty ranges from −5 to −32 % for rivers and −25 to −61 % for the Black Sea input, thus is smaller than the ”possible“ range of uncertainty. For the total fresh water budget, Sanchez-Gomez et al. (2009) found changes from +20 to +60 % whereas our study investigates changes from +35 to +60 %. Finally, the study by Dubois et al. (2012) compiles model data from the CIRCE project with net heat budget changes ranging from +2 to +\(5.5\,\hbox {W}\,\hbox {m}^{-2}\). In our study we propose a smaller range of uncertainty which is between +1 to +\(3\,\hbox {W}\,\hbox {m}^{-2}\). The uncertainty range explored in our study is about 2–3 times smaller than the spread displayed among other regional and global models regarding the projected changes in Atlantic hydrography, river runoffs and atmospheric fluxes. Therefore, the spread of our ensemble must be considered as a lower bound for the actual range of uncertainties in Mediterranean Sea projections.