Felix Landerer (Jet Propulsion Laboratory, USA)

The Mediterranean sea level is influenced by ocean-atmosphere interactions that take place over the sea and the subtropical North Atlantic. While the seasonal cycle of sea level averaged over the Mediterranean Sea is mostly steric in nature, the non-seasonal (seasonal cycle subtracted) variability is barotropic (mass-related) and driven by wind forcing on the Atlantic side of the Strait of Gibraltar and over the strait itself. Large-scale atmospheric circulation patterns, determined by the phase of the North Atlantic Circulation (NAO), are characterized by winds that force water to flow in or out of the Mediterranean leading to month-to-month fluctuations of the basin-averaged sea level with an amplitude of up to 10 cm. The record largest fluctuations were observed by altimetry and GRACE satellites in January and December 2010 and in March 2013. We find that the non-seasonal variability of the Mediterranean sea level is significantly correlated (r ≈ −0.5) with the Atlantic Meridional Overturning Circulation (AMOC) measured by the RAPID array at 26.5°N. In particular, the aforementioned large sea level fluctuations correspond to ~50% reductions of the AMOC northward transport from its record average value of 16.9 Sv. We show that the observed relationship is due to the fact that both the Mediterranean sea level and the AMOC transport are to a great extent driven by the large-scale atmospheric circulation over the North Atlantic. An NAO-modulated strengthening/weakening of the subtropical gyre leads to an increase/decrease of the AMOC and to a decrease/increase of the Mediterranean sea level.