Warming trend in the western Mediterranean deep water

Abstract

THE western Mediterranean Sea comprises three water masses: a surface layer (from 0 to ∼150 m depth), an intermediate layer (∼150–400 m) issuing from the eastern basin, and a deep water mass at depths below 400 m. The deep water is homogeneous and has maintained a more or less constant temperature and salinity from the start of the century until recently¹. Here we report measurements from the Medatlante cruises of December 1988 and August 1989, which show the deep layer to be 0.12 °C warmer and ∼0.03 p.s.u. more saline than in 1959. Taking these data together with those from earlier cruises, we find a trend of continuously increasing temperatures over the past three decades. These deep-water records reflect the averaged evolution of climate conditions at the surface during the winter, when the deep water is formed. Consideration of the heat budget and water flux in the Mediterranean^2,3 leads to the possibility that the deep-water temperature trend may be the result of greenhouse-gas-induced local warming.