Will deep water formation collapse in the North Western Mediterranean Sea by the end of the 21st century?
Iván M. Parras-Berrocal1, Rubén Vázquez1, William Cabos2, Dimitry V. Sein3, Oscar Álvarez1, Miguel Bruno1 and Alfredo Izquierdo1
1 Instituto Universitario de Investigación Marina (INMAR), University of Cádiz, Puerto Real, Cádiz, 11510, Spain.
2 Departmento de Física y Matemáticas, Universidad de Alcalá, Madrid, 28801, Spain.
3 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.
Corresponding author: Iván M. Parras-Berrocal (ivan.parras@uca.es)
Key Points:
Abstract
Deep water formation (DWF) in the North Western Mediterranean (NWMed) is a key feature of Mediterranean overturning circulation. Changes in DWF under global warming may have an impact on the regional and even on the global climate. Here we analyze the deep convection in the Gulf of Lions (GoL) in a changing climate using an atmosphere-ocean regional coupled model with a high horizontal resolution enough to represent DWF. We find that under the RCP8.5 scenario the NWMed DWF collapses by 2040-2050, leading to almost a 90% shoaling in the winter mixed layer depth by the end of the century. The collapse is mainly related to changes in sea water temperature and salinity of Modified Atlantic Water (MAW) and Levantine Intermediate Water (LIW) that strengthen the vertical stratification in the GoL. The stratification changes also alter the Mediterranean overturning circulation and the water, heat and salinity exchange with the Atlantic.
1 Introduction
The Mediterranean Sea is a semi-enclosed basin where evaporation exceeds precipitation and river run-off causing a deficit in the freshwater balance that is compensated by a net inflow of Atlantic Water (AW) through the Strait of Gibraltar (Bethoux and Gentili, 1999; Millot, 1999; Robinson et al., 2001; Sanchez-Gomez et al., 2011). AW flows through the western and into the eastern basin increasing its density and forming the Modified Atlantic Water (MAW). In winter, the oceanic conditions and the intense local air–sea interactions lead to open–sea intermediate and deep water convection of MAW in the Levantine basin producing the Levantine Intermediate Water (LIW) (Millot, 2014). Then, the LIW spreads westward over intermediate depths (150-600 m) flowing through the Sicily strait and into the Tyrrhenian Sea, reaching the northwestern Mediterranean approximately a decade after its formation (Millot, 2005). The LIW contributes to the Mediterranean outflow from Gibraltar to the Atlantic Ocean, forming the main thermohaline circulation cell of the Mediterranean (Lascaratos et al., 1993; Robinson et al., 2001; Vargas-Yáñez et al., 2012). Winter deep convection also takes place in the Northwestern Mediterranean Sea (NWMed; MEDOC-Group, 1970; Marshall and Schott, 1999; Durrieu de Madron et al., 2013), triggered by the actions of cold and dry regional winds of Mistral (northwesterly) and Tramontane (northerly) (Leaman and Schott, 1991; Somot et al., 2018). The succession of these winds episodes induces intense surface buoyancy loss associated to a rapid surface cooling and strong evaporation (Schott and Leaman, 1991; Seyfried et al., 2017). In the Gulf of Lions (GoL), the regional cyclonic circulation of MAW and of the underlying LIW (warmer and saltier) drives a doming of isopycnals that favors deep convection (Rhein, 1995; Houpert et al., 2016). The doming of isopycnals and the large surface buoyancy loss contribute to the deepening of the convection layer and to the formation of the Western Mediterranean Deep Water (WMDW). The WMDW formation is commonly described in three phases (Marshall and Schott, 1999; Waldman et al., 2017): the preconditioning phase, the intense mixing phase and the restratification-spreading phase.
Besides the main mechanisms commonly associated to deep water formation (DWF), Waldman et al. (2018) point that the intrinsic ocean variability, related to baroclinic instability of the cyclonic gyre, could determinate the occurrence or not of deep convection, especially in interannual time scale.
The NWMed DWF, which take place mainly in the GoL and in the Ligurian Sea (Margirier et al., 2020), may be affected by the warmer and dryer conditions expected at the end of the twenty-first century under IPCC scenarios (IPCC 2013; Darmakari et al., 2019; Soto-Navarro et al., 2020). In fact, the Mediterranean Sea is considered a “hot spot” climate change region (Giorgi, 2006), where the mean SST is projected to increases from 0.5ºC to 3.1ºC (e. g. Somot et al., 2006, 2008; Shaltout and Omstedt, 2014, Adloff et al., 2015, Darmakari et al., 2019; Parras-Berrocal et al., 2020, Soto-Navarro et al., 2020) by the end of the century. The warming may cause an increase in the stratification and thus in the reduction of deep convection (Somot et al., 2006), which is essential in sustaining the Mediterranean overturning circulation.
In the GoL, winter deep convection exhibits a strong interannual variability. The seasonal cycle of the mixed layer depth (MLD) shows very shallow values in summer due to surface warming; while in late winter the MLD presents a clear maximum, eventually deeper than 800-1000 m (D’Ortenzio et al., 2005; Somot et al., 2018).
The NWMed DWF has been largely studied (MEDOC-Group, 1970; Leaman and Schott, 1991; Durrieu de Madron et al., 2013; Houpert et al., 2016; Margirier et al., 2020) and many attempts to characterize DWF events in the NWMed using high temporal and spatial resolution numerical models have been made (Léger et al., 2016; Seyfried et al., 2017; Waldman et al., 2017; Somot et al., 2018). Most of these works have focused on the impact of atmospheric forcing and ocean preconditioning on the deep convection process. Margirier et al. (2020) found in glider observations that an increase in LIW temperature (0.3ºC) and salinity (0.08) limits the winter mixing, blocking the export of heat and salt to deeper layers. An analysis of the yearly maximum MLD in downscaled climate simulations in the framework of the Med-CORDEX project (Soto-Navarro et al., 2020) suggests a strong reduction in the intensity of DWF that is especially noticeable in the GoL, where the averaged maximum MLD of 2043 m will decrease in 1821 m under RCP8.5 by the end of the 21st century. Several studies have analyzed future climate change projections at regional scales (Somot et al., 2006; Shaltout and Omstedt, 2014; Adloff et al., 2015; Darmakari et al., 2019; Parras-Berrocal et al., 2020; Soto-Navarro et al., 2020) but only few of them investigate the DWF response to climate change, pointing to a strong reduction of deep convection (Somot et al., 2006; Adloff et al., 2015; Soto-Navarro et al., 2020). However, the causes of the reduction remain unexplored.
To tackle this issue, a climate change projection under the RCP8.5 scenario is dynamically downscaled with a regionally coupled model. The model appropriately simulates the present time and future climate in the NWMed region (Darmakari et al., 2019; Parras-Berrocal et al., 2020) and is part of the ensemble used in Soto-Navarro et al. (2020), showing the dramatic reduction of DWF intensity reported there. Basing on these premises, the objectives of this work are:
(i) to quantify the projected reduction of NWMed DWF.
(ii) to identify the mechanisms leading to that reduction.
(iii) to assess its impact on the change of the Mediterranean outflow properties into the Atlantic.
The regional coupled system and the experiments used in this work are described in section 2. Section 3 presents the results for the intensity of DWF, the contribution of atmospheric and hydrographic changes and the fluxes through the Strait of Gibraltar. Finally, section 4 contains the discussion and conclusions.
2 Description of Atmosphere Ocean Regional Coupled Model: ROM
We use the atmosphere-ocean regionally coupled model ROM (REMO-OASIS-MPIOM) developed by Sein et al. (2015). ROM comprises the REgional atmosphere MOdel (REMO; Jacob et al., 2001), the Max Planck Institute Ocean Model (MPIOM; Marsland et al., 2003; Jungclaus et al., 2013), the HAMburg Ocean Carbon Cycle (HAMOCC) model (Maier-Reimer et al., 2005), the Hydrological Discharge (HD) model (Hagemann and Gates, 1998, 2001), a soil model (Rechid and Jacob, 2006) and a dynamic/thermodynamic sea ice model (Hibler, 1979). The atmosphere and the ocean are coupled via OASIS3 (Valcke, 2013) coupler, while the other sub-models are treated as modules either of the atmosphere or the ocean. The model parameterizations and setup used here are described in Parras-Berrocal et al. (2020) who provide a detailed assessment of ROM-simulated present climate and future changes in the Mediterranean.
MPIOM is formulated on an orthogonal curvilinear Arakawa C-grid with a variable horizontal resolution of 7 km (south Alboran Sea) to 25 km (eastern Levantine Sea) in the Mediterranean, with 40 z-levels with increasing layer thickness with depth (Parras-Berrocal et al. 2020).
In this work, we use 1976-2005 as the historical reference period to define the DWF target area in the GoL (Figure 1), and we aim to offer an integrated vision of the impact of climate change on the NWMed DWF under the Representative Concentration Pathway 8.5 (RCP 8.5) for 2006-2099.