Hydrological cycle of the Mediterranean-Black Sea system

Abstract

The Mediterranean-Black Sea system consists of two semi-enclosed basins connected by the Turkish Straits. In turn, the Mediterranean Sea is connected to the Atlantic Ocean through the narrow Strait of Gibraltar. The hydrological cycle of the system is driven by fresh water exchanges between the atmosphere, continents and oceans, and by salty water mass exchange among the ocean basins. Monitoring such water fluxes, especially its time evolution, is essential to understand the water cycle in the region, which is very sensitive to global climate changes and influences the variability of the Atlantic Meridional Overturning Circulation (AMOC), which in turn affects the global climate. In this study, we have estimated the hydrological cycle of the Mediterranean-Black Sea system from the time-variable gravity observations performed by the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellites, and precipitation and evaporation from ERA5 atmospheric reanalysis data for the period 2002–2020. In the Black Sea, rivers introduce an average water volume of 391 ± 12 km3/year, one third of which escape through the atmosphere and two thirds go to the Mediterranean Sea. In the latter, 1787 ± 23 km3/year are lost via net evaporation. The rivers runoff (502 ± 27 km3/year), and the inflow of Atlantic waters (1020 ± 56 km3/year; 0.0323 ± 0.0018 Sv), finally restore the Mediterranean water budget. The balance is not reached instantaneously, and this delay introduces a seasonal variability in all the fluxes. In particular, the net water flux from the Atlantic Ocean increases up to 2660 ± 111 km3/year in August/September, and reverses to − 407 ± 140 km3/year in April/May. On top of the climatology, the mean annual Atlantic water flux varies significantly between 706 and 1262 km3/year.