The temperature evolution over the past 40 years in the Bay of Biscay (North-East Atlantic) is investigated from an in situ data analysis, completed with a satellite SST (Sea Surface Temperature) analysis over the last 20 years. The in situ dataset is an interannual version of the
BoByClim climatology, covering the Bay of Biscay area with a 10-km horizontal resolution and a 5-m vertical step. The satellite dataset is the European COastral Sea Operational Observing and Forcast System Program (ECOOP) daily analysis, covering the IBIROOS (Iberian-Biscay-Irish Sea) area with a 4-km resolution. The study area (43°N-50°N/12°W-1°W) extends over the intersection of the domains covered by these two analyses.
In the 0–200 m layer of this domain, a heat budget has been computed from an
ocean circulation model over the period 1965-2004. Heat is essentially
imported from the west by the North Atlantic Drift, then exported southward and northward, or transferred downward into deeper layers. The annual average of air-sea flux is weak, with a zero isoline crossing the domain from the northwest to the southeast. Ocean and atmosphere, forming a coupled system, have comparable contributions and may both result in a heat sink or
source. At the interannual scale, variability is mainly controlled by the
air-sea flux, with a secondary contribution from oceanic transport. A strong
warming of $\sim $0.2 °C/decade for the period 1965-2004 is apparent from
the surface down to the 200-m depth. The decadal evolution is consistent
with several analyses over the North Atlantic. Meanwhile, warming is
approximately twice faster than in the whole basin. This trend is not
steady, as a cooling period occurred until the early 1970's, and the
temperature increase has been significantly stronger during the last 20
years ($\sim $0.3 °C/decade between 0 and 100 m, from 1986 to 2005). The
trend and the interannual variability are maximal over the northern part of
the shelf. The typical duration of interannual anomalies is two years and
their penetration depth is ~100 m, although they occasionally exceed
200 m. The interseasonal anomalies are also stronger within the mixed layer
(root-mean-square, RMS = 0.6 °C at the surface) than below (RMS = 0.3 °C at 100 m). Their geographic distribution is radically different between
the surface and the deep levels. A seasonal dependence of the warming rate
is evident in the upper layer, as the temperature trend is stronger in summer than in winter (by a factor of 2 at the surface). Some of the potential implications for the ecosystem are discussed, particularly in terms of species distribution and dynamics of species exploited by fisheries.