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.