Understanding The Causes of Recent Warming of Mediterranean Waters. How Much Could be Attributed to Climate Change?

Abstract

During the past two decades Mediterranean waters have been warming at a quite high rate (De Madron et al., 2011) with the consequent scientific and social concern. This warming trend is observed in satellite data, in field sampling and in modeling simulations affecting surface and deep waters in the whole Mediterranean basin (Bethoux and Gentil, 1996; Nykjaer, 2009). The warming rate is, however, regionally different and also seems to change with time (Lelieveld et al., 2002) which has led to questioning what are the underlying causes of observed trends (Vargas-Yanez, 2008). Here we analyze available satellite information on sea surface temperature (SST) during the last 25 years with spectral techniques and find that more than half of the warming tendency during this period is due to a non-linear, wave-like tendency. Using a state of the art hydrodynamic model we perform a hindcast simulation and we obtain the SST evolution on the Mediterranean basin for the last 52 years. These SST results show clear sinusoidal tendency that follows the Atlantic Multidecadal Oscillation (AMO) during the simulation period. The acceleration of water warming during the 90’s seems, thus, to be caused by a superimposition of global warming with the positive phase of the AMO, while recent slow-down of the tendency is likely due to a shift in the AMO phase (Keenlyside et al., 2008). This change in AMO phase has been proposed to mask the effect of global warming in the forthcoming decades (Faurshou-Knudsen et al., 2011) and our results indicate that the same could be applicable to the Mediterranean Sea. Henceforth, natural multidecadal temperature oscillations should be taken into account to avoid an underestimation of anthropogenic-induced warming of the Mediterranean basin in the next future.JRC.H.1 - Water Resource