Effects of the Western Mediterranean Transition on the resident water masses: Pure warming, pure freshening and pure heaving

Winter 2004-05 marks the beginning of the Western Mediterranean Transition, involving a change in the physical properties of the water masses in the Western Mediterranean. Temperature and salinity decreased in the intermediate water and increased suddenly in the bottom layer. All these changes are related to strong deep water convection events in winters 2004-05 and 2005-06 in the Gulf of Lions and the Ligurian subbasin respectively. A CTD time series collected at the DYFAMED station (Ligurian subbasin) has been analysed to study the effect of the ventilation of the water column due to the large volume of new deep water formed in both winters. Also the impact of the new saltier and warmer deep water mass formed in both winters on the resident deep and intermediate waters has been evaluated. Temperature and salinity changes have been decomposed into changes along isopycnals and vertical displacements of isopycnals (pure warming, pure freshening and pure heaving according to the nomenclature in Bindoff and McDougall, 1994). The results of this analysis show that the formation of a large volume of new deep water caused the upward displacement of the resident deep water. Therefore, the decrease of temperature and salinity in the intermediate water was not only due to actual water mass changes, but also to the uplifting of Western Mediterranean Deep Water. Isopycnals situated at the bottom of the water column before the Western Mediterranean Transition became cooler and fresher after the event. A combination of pure warming and salting is the most plausible mechanism explaining this temperature and salinity decrease observed on isopycnals. (C) 2012 Elsevier B.V. All rights reserved