The Hydrological Cycle in the Mediterranean Region and Implications for the Water Budget of the Mediterranean Sea

Abstract

The hydrological cycle in the Mediterranean region is analyzed focusing on climatology and interannual to interdecadal variability, in particular long-term changes related to the well-established North Atlantic Oscillation (NAO) teleconnection. Recent atmospheric reanalyses and observational datasets are used: precipitation, evaporation, and moisture flux from 50 yr of NCEP's and 15 yr of ECMWF's reanalyses; precipitation from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) and the East Anglia University Climate Research Unit (CRU) datasets; and evaporation from the University of Wisconsin?Milwaukee (UWM) Comprehensive Ocean?Atmosphere Data Set (COADS). A budget analysis is performed to study contributions to the freshwater flux into the Mediterranean Sea, including atmospheric as well as river discharge inputs. The total river discharge is derived using historical time series from Mediterranean Hydrological Cycle Observing System (MED-HYCOS) and Global Runoff Data Center (GRDC) archives. Mediterranean-averaged precipitation during the period 1979?93 has an annual mean ranging among datasets from 331 to 477 mm yr?1, with a seasonal cycle amplitude of ?700 mm yr?1. Evaporation is estimated in the range of 934?1176 mm yr?1 with a seasonal cycle amplitude of ?1000 mm yr?1. The excess of evaporation over precipitation gives an annual mean Mediterranean Sea water loss ranging among datasets approximately from 500 to 700 mm yr?1. The annual mean river discharge is 100 mm yr?1, somewhat smaller than previous estimates using similar approaches. Water loss to the atmosphere and riverine inputs combined lead to an estimated Mediterranean freshwater deficit of about 500 mm yr?1, consistent with most oceanographically based estimates of the water flux from the Atlantic Ocean at the Gibraltar Strait. On interannual to interdecadal timescales, during the period 1948?98, the Mediterranean atmospheric winter water deficit is positively correlated with the NAO and has been increasing due to the long-term positive anomalies of the NAO since the early 1970s. Precipitation, which is also significantly correlated with the NAO, appears to be mostly responsible for this since no significant correlation is found for evaporation. Over the 50-yr period the Mediterranean atmospheric water deficit increased by about 24% in the winter season, and by 9% annually. Given the pattern of the NAO-related precipitation anomalies, this change is likely to have occurred primarily north of 35°N. The results presented here suggest that in response to the changes in the freshwater flux significant variations in the characteristics of Mediterranean waters and the Gibraltar flux may also have occurred during this period, mostly driven by the influence of the NAO.