Towards very high resolution downscaling of climate scenarios for Mediterranean coastal areas

Simulating the response of local climate to the Earth global warming is a challenge. In coastal areas, it is mandatory to integrate the response of the sea to the progressive increase of the atmospheric temperature, the changes in the inland precipitation regimes and the surface wind field pattern. In this frame, the Mediterranean area is a benchmark, where atmosphere and sea interact with a high degree of coupling, besides to be a hot spot rising deep interest for its evolution.

Future climate scenarios for both atmosphere and sea over Mediterranean are already available form a rich set of regional climate simulation. Anyway, stakeholders require information on future climate having a spatial resolution higher than that characterizing the regional scale. Furthermore, many local process of interaction between atmosphere, hydrosphere and sea, especially along the coasts and in shallow waters, have a poor representation in the available model outputs. Downscaling is applied to regional climate and basin scale simulations with the aim to fill in the gap.

In this work, we present the application of a methodology to generate ensembles of high resolution simulations at the local scale, for the widely and commonly defined Global Warming Levels (GWLs: 1.5 °C, 2.0 °C, 3.0 °C, 4.0 °C). Using boundary conditions extracted from sets of EURO-CORDEX, MED-CORDEX and CMIP6 model outputs, ensembles of hydrological projections combined with hydrodynamic simulation of the Adriatic Sea shallow waters have been conducted to explore the air-rivers-lagoon-sea response to the GWLs across the XXI century. Hydrological simulations have been generated by means of the WRF-Hydro model, while the lagoon-sea ones through the SHYFEM numerical model.

From sensitive tests on the response of small spatial domains to the forcing and boundary conditions, we conclude that the effort to run very high resolution models covering continuously secular time range, is not an efficient approach. Instead, the quick response of the hydro-coastal system to the boundary and the forcing allows to generate a large set of local area sensitivity cases to the GWLs, with computational resources and times that are worth to be considered useful besides efficient.

This approach explores decadal time window ensembles of future scenarios, each belonging to the same GWL, with the spatial resolution requested by a wide spectrum of stakeholders on future river flows, temperature, salinity and level along the coasts, in the lagoons and in the open sea facing the coastal areas. In addition to the summary of the methodology, we present the results of its application to the northern Adriatic Sea, including the northeastern most lagoon.

This work has been conducted with the contribution from the EU co-financing in the frame of the Interreg Euro-MED Programme, thanks to the MedSeaRise Project, and the results achieved through the Interreg IT-HR AdriaClimPlus project.