Large changes in the hydrographic structure of the Bay of Biscay after the extreme mixing of winter 2005

The extremely cold and dry winter of 2005 in southwestern Europe caused a profound transformation of the upper ocean hydrographic structure of the Bay of Biscay area, making it completely different from the previous decade. The strong local winter cooling resulted in the highest density flux estimated since the 1960s. The extreme buoyancy loss triggered the mixed layer to reach unprecedented depths affecting directly the level of local modal waters that are usually unconnected to air-sea interaction. The water column just below the climatological average mixed layer entered in a process of quick cooling that compensated in 2 years the 0.5°C gained in the period 1994–2004. Enhanced by a pronounced precipitation deficit the event caused concurrently a downward salt injection that made deeper levels of East North Atlantic Central Water begin a process of warming by isopycnal change, something never observed during the 1990s. As an overall result, the stratification of the upper permanent thermocline was dramatically reduced. The observed cold low stratification anomaly had a substantial spatial extent and remained for 2 years below the seasonal thermocline development, constituting a typical case of the reemergence mechanism, but was abruptly interrupted in the warmest winter on record of 2007. In addition to the hydrographic changes, the winter 2005 event had a notable effect on the marine ecosystem