The monitoring program Radiales (https://www.seriestemporales-ieo.net/) by
the Spanish Institute of Oceanography, has been providing hydrographical and biogeochemical series in marine waters around Spain on a monthly basis from early 90's. The
proximity of the shelf-break in front of the city of Santander (SE Bay of Biscay) allowed
tracking intermediate and deep waters along the standard section perpendicular to this
city for three decades (sampling was limited to 1000 meter until late 2007, then extended
to 1500 m, and full-depth 2400 m since 2014). From the start of the sampling in nearly 90`s,
subsurface waters showed unabated warming and salt-increase. Warming was linked to
isopycnal sinking (heave) during the 90`s and early 00`s until the occurrence of very
strong winter mixing in 2005 that shifted quickly the salinity down to lower East North
Atlantic Central Waters (ENACW) levels (ca. 400 m). Overall, warming and salt-increase
at the core of ENACW added up to 0.3ºC and 0.08 in salinity within only two and a bit
decades. In 2014, the upper central waters showed freshening and cooling for the first
time in the series, a process that enhanced in the following years especially in salinity that
currently (2021) presents the lowest value of the overall timeseries. This shift in regional
hydrography follows the large salinity drop observed in the subpolar gyre around 2012
and its subsequent expansion downstream into the subtropical gyre and subarctic seas.
This regime shift implies that subsurface environmental conditions in the region have returned back to 90`s state, contrasting to the uppermost waters which continue to show
large positive anomalies. The effects of this cold and freshwater inflow in the regional
circulation of southern Biscay are discussed
