Atlantic atmospheric rivers and the pulse of deep Mediterranean respiration

The Mediterranean is a saline sea because it exports moisture to the atmosphere1. Over the last million years, however, the Mediterranean has experienced abrupt declines in salinity that have collapsed the deep thermohaline circulation and caused bottom water anoxia each time the Earth has approached perihelion during the northern hemisphere summer2,3. For decades, this has been attributed to freshwater discharge from the Nile or increased precipitation in the Mediterranean basin2,3. However, the impact of past changes in the atmospheric moisture balance has been little explored. In this study, we show that these low salinity episodes were triggered by a reduced net export of moisture to the atmosphere due to enhanced mid-latitude Atlantic atmospheric water transport, affecting its salinity, deep ventilation and water exchange with the Atlantic. This conclusion is supported by the observation of low surface salinity events in the westernmost Mediterranean coinciding with events of heavy winter precipitation along the Atlantic Iberian Margin, which is the main gateway of moisture into the Mediterranean. These events were triggered by equatorward shifts of the main route of atmospheric rivers during persistent negative phases of the Arctic Oscillation at times of boreal summer insolation maxima.