The salinity of the inflow in Gibraltar decreases (-9.0·10-3 psuyr-1) due to the freshening of the inflowing Atlantic waters. This fresher Atlantic water is the main contributor to the surface freshening in the GoL. Otherwise, the outflow waters become saltier (+2.0 ·10-3psuyr-1) because of the salinization of LIW and deeper waters as shown in Figure 3, thus the salt transport through the Strait to Atlantic Ocean increases (Table 1).
4 Discussion and Conclusions
The response of the NWMed deep water formation to climate change has been explored in Soto-Navarro et al. (2020). The authors found in an ensemble of six Regional Climate Models a robust dramatic decrease of DWF by the end of the 21st century. However, the mechanisms involved were not revealed. As the time evolution of the DWF is strongly model dependent, single model runs can be used to identify in a physically consistent way the causes of this collapse. Therefore, we study these mechanisms in one of the simulations used in Soto-Navarro et al. (2020). Specifically, we analyze the simulation with the regionally coupled system ROM under the RCP8.5 scenario.
Our simulation is able to reproduce the several DWF episodes during the present period: deep convection occurs every year from 2009 to 2014 (Figure 2a and Table S1), what is statistically in good agreement with observations (Houpert et al., 2016; Somot et al., 2018; Margirier et al., 2020).
Starting from the early 2040s, we find a dramatic reduction of MLDmax in the GoL (Figure 2a). The maximum MLD for the present climate (1976-2005) is 2385 ± 630 m while for 2070-2099 under the RCP8.5 scenario is 297± 47 m. That is, the MLDmaxsimulated by ROM experiences a reduction of almost 90% in the GoL. These values are in close agreement with values reported in Soto-Navarro et al. (2020).
Exploring the possible mechanisms (t. e. air-sea fluxes and ocean preconditioning) that lead to this dramatic MLDmaxdecrease we find that the air-sea fluxes do not seem to be the factor responsible for the DWF collapse, as the buoyancy loss does not change significantly (Figure 2b). Changes in the buoyancy loss also cannot explain the changes in DWF strength, as strong or weak DWF events can happen with similar BL values. In agreement with Somot et al. (2018), our results show that in any case no deep water is formed (MLDmax < 1000 m) when BL < 0.6 m2s-2 during the 2006-2013 period.
Our results indicate that the changes in properties of the upper and intermediate water masses affecting the ocean preconditioning are key in the DWF collapse. In our simulation, the temperatures of MAW and LIW are projected to increase 2.6ºC and 2.3ºC, respectively (Figure 3a and 3b). In turn, the salinity decreases slightly (-0.01 psu) on the surface, while deeper waters become saltier (Figure 3c and 3d). As shown previously in Parras-Berrocal et al. (2020), the warming and freshening signal of MAW comes to the GoL from the Eastern Atlantic, while the increase of LIW temperature and salinity is propagated from its formation area in the Eastern Mediterranean. These changes increase the vertical density gradient between the MAW and LIW, strongly reducing the vertical mixing between these water masses. This is reflected in the stratification index for the 0-1000 m water column (Figure 3e), which from 2040s onward experiences a positive trend (0.02 m2s-2y-1). Our results do not show a significant change in atmospheric fluxes, changes in MAW and LIW characteristics play the main role in the DWF collapse in the NWMed. The recent study of Margirier et al. (2020) lends some support to our hypothesis: they found in glider and other platforms profiles collected over 2007-2017 that an abrupt jump of LIW temperature and salinity provoked a strong reduction of vertical mixing in the NWMed in 2014. The authors concluded that under those conditions, stronger atmospheric forcing is needed to trigger deep convection. Amitai et al. (2021) have also recently demonstrated that LIW characteristics play a key role in enabling or disabling the deep convection in the GoL. In agreement with Margirier et al. (2020) and Amitai et al. (2021), our results indicate that the projected deep water formation collapse in the 21st century is controlled by the change in LIW characteristics, but we found also that changes in MAW properties play a role. In order to assess the relative contribution of LIW and MAW to the deep water formation collapse we compare the SI calculated from spatially and temporally averaged vertical profiles in the GoL in four cases (Figure S2): (i) using the values corresponding to the pre-collapse period (2006-2041); (ii) using the values corresponding to the post-collapse period (2070-2099); and creating additionally two synthetic profiles, (iii) one containing the pre-collapse characteristics of MAW (0-200 m depth) with the post-collapse properties of deeper layers (200-1000 m depth; Figure S2g), and (iv) a second one with the post-collapse MAW and pre-collapse deeper layers. As seen in Figure S2, there is a clear increase in the post-collapse SI (see also Fig. 3e), but interestingly the SI for the (iv) situation (1.20 m2s-2, Figure S2h) is greater than for the (iii) (0.97 m2s-2, Figure S2g). This results suggest that in the future the change in MAW properties will play a role at least as relevant as LIW in the deep water formation collapse. This collapse of NWMed deep water formation seems to have an impact on the ventilation and on the thermohaline circulation of the Mediterranean Sea.
Both inflow and outflow water transport at the Strait of Gibraltar show a decreasing trend which is larger in the outflow, increasing the net flow (Table 1). Moreover, the incoming surface Atlantic jet will transport more heat and less salt into the Mediterranean basin, causing the hydrographic changes of surface waters (MAW) in the GoL represented in Figure 3. At the same time, the salinization of intermediate (LIW) in the Mediterranean Sea contributes to the increase of MO salinity. As a result, the MO will be warmer and saltier by the end of the 21st century. During the period with DWF episodes (2006-2041) the warming of MO is gradual and does not present noticeable changes, however after the collapse of DWF it accelerates reaching an increase rate of 0.034ºCyr-1 (Figure S1). Then, the collapse of DWF could be one of the driving factors of the changes in characteristics of fluxes at Gibraltar Strait which reflects changes in the Mediterranean overturning circulation at large scale.
Acknowledgments and Data
I. M. Parras-Berrocal, O. Álvarez and M. Bruno were supported by the Spanish National Research Plan through project TRUCO (RTI2018-100865-BC22). R. Vázquez was supported through a doctoral grant at the University Ferrara and University of Cádiz. W. Cabos have been funded by the Spanish Ministry of Science, Innovation and Universities, the Spanish State Research Agency and the European Regional Development Fund, through grant CGL2017-89583-R. D. Sein was supported in the framework of the state assignment of the Ministry of Science and Higher Education of Russia (0128-2021-0014). The model data are available online (at https://doi.org/10.5281/zenodo.5151396).
The authors declare that they have no conflict of interest.
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