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.