Future sea level rise (SLR) constitutes a threat for the coastal environment and economies, which is liable to be further exacerbated by the superposition of waves, atmospheric surge, and tides. Climate scientists are therefore becoming more and more committed to improve projections of SLR as to both resolution and accuracy, and to reduce current uncertainties, at the same time increasing our understanding of such a challenging scientific problem and enabling more accurate risk assessments. The latter, in particular, suffer from the difficulty in accounting for the cascading effects from sea-level rise to actual coastal impacts, across a range of spatial and temporal scales that demand complex and differentiated approaches (Thiéblemont et al., 2019). Likewise, the lack of comprehensive projections of extreme sea levels (ESL) that include mean sea level (MSL), tides, waves, and storm surges is lamented by Vousdoukas et al. (2017), who present a first attempt to provide an impact-oriented, regional-scale evaluation of ESL for the European shoreline. Their effort, albeit still inadequate for the design of specific measures, can nevertheless help highlight vulnerabilities and emerging research issues. As a matter of fact, the authors denounce the limitations inherent to their approach, mainly arising from the inadequacy of the regional atmospheric projections available at the time, as well as those of the projections derived from coupled global OGCMs. In addition, they highlight that explicitly solving the nonlinear interactions between waves, storm surge, and tides should be a key element of any future effort to reliably model ESL, rather than continuing to resort to linear combinations of the ESL components resulting from independent simulations.
Besides being highly vulnerable to SLR due to the potentially disruptive impacts on its coastal economies (Jeftic et al., 1992), the Mediterranean basin is especially challenging from the scientific perspective, due to the inherent diversity of its geological history and the peculiar and complex features of its marine circulation and environment. In addition to the local dynamics and geological processes, interacting over a broad spectrum of scales, sea level in the basin is also constrained by the water mass exchange across the Strait of Gibraltar, which, in fact, regulates the hydraulic jump between the Mediterranean and the Atlantic Ocean and influences how sea level rise in the Atlantic is transmitted into the Mediterranean. In its turn, the connection with the Black Sea, through the Dardanelles, Sea of Marmara and the Bosphorus, couples the basin hydrology to the land-based hydrological cycle of a vast portion of continental Europe. Nevertheless, despite the recognized inability of coarse resolution GCMs to accurately represent the highly non-linear, small-scale processes in marginal seas such as the Mediterranean (Slangen et al., 2017a; Marcos and Tsimplis, 2008), the global sterodynamic (following the terminology proposed by Gregory et al., 2019) sea-level projections for the Atlantic area near Gibraltar, are still often used to estimate the basin's internal sea level (Thiéblemont et al., 2019).
At the global scale, the question is still open whether the strong trends observed in the last 25 years through satellite altimetry are part of longer-term tendencies, or reflect more recent changes in the atmosphere-ocean coupled system. Recent work by Dangendorf et al. (2019) provides evidence that the global trend accelerated at the end of the 1960s, and indicates that the resulting acceleration in the global sea level rise is linked to modifications of the southern hemispheric westerlies, leading to warming and circulation changes in the southern world ocean. Other factors have also been shown to play a key role, such as the change of terrestrial water storage, and the melting of ice sheets and glaciers, which has significantly increased in the last decades (see, e.g., Shugar et al., 2020, which focuses on the growth of glacial lakes, and King et al., 2020, where the mass loss from the Greenland Ice Sheet is analyzed). The work by Frederikse et al. (2020) provides a thorough review of the various contributions and their relative weight.
As to future scenarios, since 1995 the coordinated worldwide CMIP effort (Coupled Model Intercomparison Project) has provided constantly updated projections of future sea level rise using earth-system models, to which the contributions from continental glaciers and geologic processes are added offline. The Phase 5 models (CMIP5) used for the fifth Intergovernmental Panel on Climate Change (IPCC) assessment report (AR5, IPCC 2013) project a global sea level rise between 26 and 97 cm (overall likely range) at the end of this century, depending on the climate scenario, with pretty large inter-model spread. Such estimates were revised in the 2019 Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC), which assigns medium confidence to a likely range between 29 cm and 110 cm (IPCC, 2019). For the area of the North Atlantic in proximity of the Strait of Gibraltar, projections can range from about 40 cm to over 100 cm for the most pessimistic RCP8.5 scenario, with an ensemble mean of about 80 cm, and approximately from 30 cm to 80 cm for the intermediate RCP4.5 scenario, with an ensemble mean of 60 cm (Vousdoukas et al., 2017). On the other hand, Slangen et al., 2014 presented regional sea-level projections and their associated uncertainties up to the end of the 21st century, by combining model- and observation-based regional contributions of land ice, groundwater depletion and glacial isostatic adjustment, and CMIP5 projections of the changing ocean circulation, increased heat uptake and atmospheric pressure patterns, also accounting for gravitational effects due to mass redistribution, ending up with an estimate of about 70 cm for the Mediterranean Basin. However, while concluding that regional variations in sea-level can significantly differ from the global mean (up to 30% above and 50% below) and that the land ice contribution dominates the overall uncertainty, they also stressed the need for dedicated regional downscaling of each global climate scenario.
In general, the inherent uncertainty in SLR projections has induced researchers to adopt a variety of alternative estimates in their impact assessments. Thiéblemont et al. (2019) analyze the effects of a median estimate of about 80 cm and two different high-end scenarios, resulting from two extreme estimates for the sea-level equivalent of melting glaciers (i.e the the upper limit of the likely range and the “worst model” projections). Antonioli et al. (2017), while reviewing possible alternative ranges for the projected high-end SLR at 2100, use the 530–970 mm interval reported in the IPCC AR5, and Rahmstorf’s semi-empirical estimate of about 1.400 mm (Rahmstorf, 2007). Improved projections from the next generation of global models are expected by 2022, when the next IPCC assessment is to be delivered. However, as remarked above, the spatial resolution of current global models is not sufficient to provide realistic estimates of local sea level rise in areas, such as the Mediterranean, where crucial processes are far from being explicitly resolved, and hardly allow a reliable parameterization (Sannino et al., 2009).
Adloff et al. (2018) recently discussed the complexity inherent to projecting sea level in the Mediterranean, and analyzed the performances of four different regional hindcast simulations of the basin circulation (period 1980–2012), driven by realistic atmospheric forcing. The incorporation of improved sea level information at the Atlantic lateral boundary was found to significantly enhance the reliability of results, confirming that the correct representation of the interactions between the two basins is an important requirement for a successful numerical simulation. However, an adequate treatment of the complex, hydraulically driven dynamics across the Strait of Gibraltar was still missing, as well as a more refined treatment of the exchange between the Mediterranean Sea and the Black Sea through the Dardanelles Strait, at the model eastern boundary. In addition, model performances were mostly evaluated by computing basin averages, while altimeter data series clearly indicate that sea level rise has not been homogeneous in the basin over the last decades. By analyzing altimeter data over a 25 years period (1993–2017), Mohamed et al. (2019) showed that the observed increase in the average sea level anomaly (SLA) has been quite different in different sub-basins, ranging from a minimum of 1.95 mm/year, in the Ionian Sea, to a maximum of 3.73 mm/year, in the Aegean Sea. At a broader scale, the SLA positive trend appears to be significantly stronger in the Levantine basin than in the Western Mediterranean Sea, as well as characterized by distinctive features, with a fairly regular linear increase in the Western Mediterranean Sea, possibly attributable to the Atlantic contribution, and a more complex behavior in the Levantine basin. The latter, in particular, is liable to be influenced by the evolution of the Eastern Mediterranean Transient (EMT), the dramatic event occurred in the eastern Mediterranean at the beginning of the 1990s (Roether et al., 1996; Klein et al., 1999, Theocharis et al., 2002). The difficulty in separating climate-change-induced variations from the local circulation variability is therefore evident, as well as the role played by small-scale features, either attributable to internal variability or determined by atmospheric forcing, in generating long-lasting differences across the Mediterranean sub-basins, amplifying or mitigating the effects of global sea level rise.
Prompted by these considerations, we developed a regional ocean model for the long-term simulation of the Mediterranean Sea circulation (hereinafter MED16) which we used to obtain accurate projections of the Mediterranean sea level. The model represents the climate version of the high-resolution, tide-including ocean model described in Palma et al. (2020). The two models share the same computational domain, which includes the Black Sea, thus allowing to consistently compute water exchanges at the Dardanelles Strait and to avoid any ad hoc assumption at the Mediterranean eastern boundary. The climate version necessarily uses a coarser horizontal grid with respect to its operational counterpart, yet resolution is significantly increased in critical regions such as the Gibraltar, Dardanelles, and Bosphorus Straits. The explicit, high-resolution representation of inter-basin water exchanges at the boundaries constitutes a unique, distinguishing feature of the present implementation.
In the following, we analyze the hindcast of the Mediterranean circulation forced by the SMHI-RCA4 regional downscaling of the ECMWF ERA-Interim reanalysis data (Dee et al. 2011), and the historical and RCP8.5 scenario forced by the SMHI-RCA4 regional downscaling of the HadGEM2-ES global projection (Collins et al., 2011). The overall basin-scale model performance is assessed through comparison with observations and reanalysis data, with special focus on the model ability to reliably represent the local sea level height. In particular, comparison with data from tide gauges allows us to evaluate the model performance in coastal regions, where the impacts of sea level rise really affect human communities and economies, and need to be specifically assessed.
The paper is organized as follows. The main characteristics of the model and of its implementation are described in Sect. 2, whereas Sect. 3–5 are devoted to the detailed analysis of the simulations performed. In particular, Sect. 3 presents a validation of the hindcast and historical simulations in terms of transports at the main straits, hydrology, and circulation, obtained through comparison with observations and numerical reanalyses. The corresponding sea-level reconstructions are discussed in Sect. 4, and validated using satellite altimeter data and coastal tidal gauge observations. Section 5 then describes the results of the scenario simulation, highlighting future changes in the basin hydrology and circulation, and discussing the projected sea-level. Finally, conclusions are drawn in Sect. 6.