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Ocean surface warming is commonly associated with a more stratified, less productive, and less oxygenated ocean. Such an assertion is mainly based on consistent projections of increased near‐surface stratification and shallower mixed layers under global warming scenarios. However, while the observed sea surface temperature (SST) is rising at midlatitudes, the concurrent ocean record shows that stratification is not unequivocally increasing nor is MLD shoaling. We find that while SST increases at three study areas at midlatitudes, stratification both increases and decreases, and MLD deepens with enhanced deepening of winter MLDs at rates over 10 m decade−1. These results rely on the estimation of several MLD and stratification indexes of different complexity on hydrographic profiles from long‐term hydrographic time‐series, ocean reanalysis, and Argo floats. Combining this information with estimated MLDs from buoyancy fluxes and the enhanced deepening/attenuation of the winter MLD trends due to changes in the Ekman pumping, MLD variability involves a subtle interplay between circulation and atmospheric forcing at midlatitudes. Besides, it is highlighted that the density difference between the surface and 200 m, the most widely used stratification index, should not be expected to reliably inform about changes in the vertical extent of mixing., Key Points Ocean observations show that surface warming is not definitively linked to a more stratified and less mixed oceanSubtle interplay between changes in circulation and atmospheric forcing contribute to find warmer and deeper MLDsHow our findings reconcile with the observation of a less productive and oxygenated ocean is discussed

From the last 1980s different initiatives have been carried out by the Spanish Institute of Oceanography (IEO) at its Oceanographic Centre of Santander to progressively increase its ocean observing system capabilities. From 1991, shelf and slope waters of the Southern Bay of Biscay are regularly sampled in a monthly hydrographic line north of Santander to a maximum depth of 1500m. In 2003, the Santander section was extended 90 miles offshore in the framework of a larger scale hydrographic and circulation monitoring program (VACLAN/COVACLAN projects, www.vaclan-ieo.es (REN 2003-08193-C03-01/MAR; CTM 2007-64600/MAR)). Measurements from the surface to the more than 5000 meters existing in the basin in deep hydrological standard sections are occupied twice a year including CTDO2, nutrients and currents from vessel-mounted ADCP and LADCP. In order to reinforce its contribution to the ocean surface monitoring, the IEO deployed in June 2007 an oceanic-meteorological buoy (AGL Buoy, 43°50.67' N; 003°46.20' W, and 40 km offshore, www.boya_agl.st.ieo.es/) in the southern Bay of Biscay. Previously, in August 1998 a satellite reception station was mounted at the IEO Santander Centre to receive SST images. Because of technical problems the station stop working in 2007, but a new reception station has been mounted in 2010, with FEDER funds and SST images of the surroundings areas are now available. Recently, a glider experiment has been performed in December 2010 to study the possibilities to repeat the Santander section using the new technology. All those observations are completed with the Santander tide gauge, operated by IEO since 1943. A complete view of oceanic processes is obtained through the combination of this set of monitoring programs, at the same time that the best way to optimize ocean observation is investigated.

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., Sí

From 1991, shelf and slope waters of the Southern Bay of Biscay are regularly sampled in a monthly hydrographical line north of Santander to a maximum depth of 1000m, as part of the IEO Radiales project. From 2003, a deep hydrological standard sections (part of the VACLAN project) is occupied twice a year extending the Santander section 90 miles offshore. Measurements include CTDO2, nutrients and currents from vessel-mounted ADCP and LADCP. The data base in the area hold by IEO is complemented with a current meters line moored in 2003 at 43deg48'N, 3deg 47'W sampling temperature, salinity and currents at the core of Central, Mediterranean and Labrador Sea water. On June 2007, an ocean-meteorological Buoy was moored at the Santander Section, 22 miles north of Santander at about 2850m depth, to complete the ocean information with the ocean-atmosphere interaction. The Santander section and AGL Buoy data highlight that heat and salt show a quite different behaviour in terms of their balance in the upper layers of the Bay of Biscay. At interannual and seasonal timescales temperature is mainly determined by the atmospheric forcing while salinity is more closely linked to advection processes. Finally, a severe storm developed in the Bay of Biscay on January 24, 2009 is described. The AGL buoy registered an individual maximum wave of 26.1m, the largest ever recorded by the Spanish buoy Networks.