Climatic Impacts of a Warmer Mediterranean Sea Simulated by the Fully Coupled Community Earth System Model, Version 2 (CESM2)

This study presents a set of process-oriented sensitivity experiments using the state-of-the-art fully coupled Community Earth System Model, version 2 (CESM2), for the first time to isolate the role of Mediterranean sea surface temperature (MedSST) anomalies from regional to global scales. Ten-member ensembles of historical simulations (Control) and two Mediterranean Pacemaker experiments [Mediterranean Pacemaker (MedP) and MedP 138C (MedP3)] are performed for the 1980–2014 period. MedP aims to achieve an accurate representation of MedSST evolution by applying bias corrections to the model-simulated fields, while MedP3 introduces a 138C perturbation to MedP as an idealized experiment to explore the sensitivity of the climate to substantially warmer MedSSTs. The impacts of the bias correction are evaluated by comparing MedP and Control with observational-based data, while the climatic impacts of a warmer Mediterranean are assessed by comparing MedP3 with MedP. MedP improves the representation of surface temperatures over large parts of the Euro-Mediterranean region. Relatedly, there are not only improvements in precipitation patterns over Anatolia, the Balkans, and Gibraltar but also degradations in central Europe and the southern Mediterranean basin. In MedP3, warmer MedSSTs enhance upward motion and induce anomalous cyclonic surface winds, increasing atmospheric instability and intensifying precipitation. This precipitation enhancement is largely confined to the Mediterranean Sea and adjacent northern land areas, particularly the Anatolian, Italian, and Balkan peninsulas. At the hemispheric scale, MedP3 produces a Northern Hemisphere warming response. The associated interhemispheric thermal contrast shifts the intertrop-ical convergence zone slightly northward, resulting in increased precipitation in the northern tropics. Overall, our study shows that changes in MedSSTs could have climatic impacts on both regional and global scales.