Figure 2. Interannual time series (2006-2099) of RCP8.5 projections of (a) yearly maximum MLD (in m) and (b) integrated buoyancy loss (BL, in m2s-2) averaged over the GoL. The black line corresponds to the total buoyancy loss, the red dashed line is the heat-related term, and the blue line is the freshwater-related term.
The BL over the GoL along the 2006-2099 period does not show a significant trend and has a mean value of 0.75 ± 0.12 m2s-2. The BL is widely dominated by the heat-related term (mean value of 0.66 ± 0.12 m2s-2). The time series of yearly BL and the heat contribution are well correlated (r=0.99) and of the same order of magnitude, while the freshwater term is one order of magnitude smaller and its Pearson correlation coefficient with the BL is 0.61. Our RCP8.5 simulation shows episodes of MLDmax deeper than 1000 m only when the BL is above 0.63 m2s-2. Although the simulated stronger convective episodes (2014, 2031 and 2033) show a BL ranging from 0.84-0.90 m2s-2, there are years with MLDmax < 200 m such as 2066, 2067 and 2087, with values of BL > 0.90 m2s-2. This indicates that the BL is not the only factor determining the intensity of the deep water convection and that other factors such as ocean preconditioning could also contribute to the DWF (Margirier et al., 2020).
3.3 Contribution of hydrographic changes in the water column
To analyze the stratification of the water column we have calculated the stratification index (SI); lower values of SI correspond to a less stratified water column. The SI has been previously employed in multiple Mediterranean studies (L’Hévéder et al. 2013, Somot et al. 2018, Margirier et al. 2020) and it is defined as follows (Turner, 1973):
SI=\(\int_{0}^{h}N^{2}\text{zdz}\) (3)
where N is the Brunt-Väisälä frequency (\(N^{2}=\frac{g}{\rho_{0}}\frac{\partial_{\rho}}{\partial_{z}})\), z is the depth, ρ the potential density and h the maximum depth of integration which we have chosen to be 1000 m depth.
Under the RCP8.5 scenario, temperature increases through the whole water column (Figure 3a). The detected warming that originally takes place at the surface is transferred progressively to deeper layers (Parras-Berrocal et al., 2020). In the upper layer (0-200 m) the temperature is expected to experience a warming of 2.6ºC, while in the intermediate (200-600 m) and 600-1000 m layers will warm by 2.3ºC and 0.8ºC (Figure 3b), respectively. By the end of 21stcentury, the MAW flowing in the upper layer of the GoL is projected to slightly freshen (-0.01 psu) while the intermediate (0.4 psu) and 600-1000 m layers (0.2 psu) tend to get saltier (Figure 3c and 3d). The expected increase in temperature and salinity accelerates from 2040 at 200-600 m (Figure 3b and 3d). The 200-600 m depth range corresponds to the equilibrium depth of LIW (Menna and Poulain, 2010) in the western Mediterranean.