3 Results
3.1 Intensity of deep water convection in the GoL
The model MLD is defined as the depth, where the density (ρ) has increased by 0.125 kg m-3 as compared to the value in the surface box (Wetzel et al., 2004). It is evident a strong reduction of yearly maximum MLD (MLDmax) over the GoL by mid-21st century under RCP8.5 emission scenario (Figure 2a). During 2006-2099, the model projects 15 events of deep convection, defined as a MLDmax deeper than 1000 m (Herrmann et al., 2010, Somot et al., 2018), with 6 episodes occurring in consecutive years between 2009 and 2014 (see supplementary material: Table S1). These events always correspond to winter months (February-March), when the intense mixing phase is activated.
Our results show a collapse of newly formed WMDW (starting from the 2040–2050 decade no convection is simulated), characterized by MLDmax shallower than 500 m (Figure 2a). This result holds independently of the criterion used for MLD definition. Further, we analyze the causes of this collapse.
3.2 Contribution of changes in the winter surface buoyancy loss
The role of the winter air-sea fluxes in the MLDmaxvariability is assessed with the help of the accumulated surface buoyancy loss (BL, eq. 2), defined as the time integral of the buoyancy flux (BF, eq. 1). In turn, the BF is calculated in terms of heat and freshwater fluxes (Marshall and Schott, 1999; Somot et al. 2018):
\(BF=g\bullet\left(\frac{{\alpha\bullet Q}_{\text{net}}}{\rho_{0}\bullet C_{p}}\ +\ \beta\bullet SSS\bullet\ FWF\right)\)(1)
\(BL\ (Y)=-\ \int_{T_{1}}^{T_{2}}\text{BF}\bullet dt\) (2)
where Qnet is the net surface heat flux (positive downward), FWF the net surface freshwater flux (positive downward), g is the gravitational acceleration (9.81 ms-2), α and β the thermal expansion and haline contraction coefficients (respectively calculated as a function of surface T and S), ρ0 the reference density of sea water 1025 kgm-3, Cp the specific heat capacity of sea water (equal to 4000 Jkg-1ºK-1) and SSS the sea surface salinity. Following Somot et al. (2018), the winter accumulated buoyancy loss (BL) (eq. 2) was computed for every year (Y) of the 2006-2099 from December of the previous year (T1) to March (T2) and averaged over the GoL.