Potential links between the North Atlantic Oscillation and decreasing precipitation and runoff on a Mediterranean area

In the Mediterranean region, the reduction in precipitation and warmer temperatures is generating a desertification process, with dramatic consequences for both agriculture and the sustainability of water resources. On the island of Sardinia (Italy), the decrease in runoff impacts the management of water resources, resulting in water supply restrictions even for domestic consumption. In the 10 Sardinian basins with a longer database (at least 40 complete years of data, including data from the past 10 years), runoff decreased drastically over the 1975–2010 period, with mean yearly runoff reduced by more than 40% compared to the previous 1922–1974 period. Trends in yearly runoff are negative, with Mann-Kendall τ values ranging from −0.39 to −0.2. Decreasing winter precipitation over the 1975–2010 period everywhere on Sardinia island has led to these decreases in runoff, as most yearly runoff in the Sardinian basins (70% on average) is produced by winter precipitation due to the seasonality typical of the Mediterranean climate regime. The trend in winter precipitation is not homogenous; the negative trend is higher (around −0.25) on the west Sardinian coast, becoming lower across the island toward the east coast (around −0.14). Winter precipitation is highly correlated with the North Atlantic Oscillation (NAO), a weather phenomenon in the North Atlantic Ocean that controls the direction and strength of westerly winds and storm tracks into Europe. High negative correlations (up to −0.45) between winter NAO index and winter precipitation are estimated along the west coast. Meanwhile, these correlations decrease east across the island toward the high mountain in the center of Sardinia, reaching the lowest values along the east coast (about −0.25). The generally decreasing correlation between winter NAO index and winter precipitation in the longitudinal direction (from the North Atlantic dipole to the east) here accelerates due to local-scale orographic effects that overlap the large-scale NAO impact on the winter precipitation regime, thus softening the precipitation reduction due to the NAO. Such local topographic effects that may attenuate large-scale climate change effects must be considered in water resource planning and management alongside such climate change effects related to large-scale circulations, such as NAO.