Hydrographical variability on the French continental shelf in the Bay of Biscay, during the 1990s

Abstract

A synthesis of 9 years of hydrographic measurements, acquired during approximately 25 cruises (1992–2000) on the French continental shelf of the Bay of Biscay, is presented. The main focus is upon salinity distribution, as it is little known in this area. The whole of the data set covers 11 months of the year, with some sampling fields repeated several times a year, for multiple years. This time-series allows temperature and salinity distributions to be studied together with their seasonal, inter-annual and mesoscale variability. The seasonal trend in temperature can be described in three stages, which are in agreement with past studies. Thermal stratification occurs between May and mid-September in a layer ∼50 m in thickness, whereas the water column remains homogenous from January to the beginning of April. The mid-September–December period corresponds to the progressive destruction of the thermocline. In contrast, the salinity distribution displays two main seasonal patterns. From March to June, the haline stratification is strong; this is in response to high river runoff and relatively low vertical mixing. In autumn, stratification decreases because of low river discharge and wind-induced vertical mixing. Surface salinity increases by up to 1 psu inshore of the 100-m isobath, from spring to end of summer. The salinity gradient runs cross-shore in spring and along-shore in summer: this is due to the mean wind direction during the previous 6 months (SW from September to March, NW from March to September). Nevertheless, this seasonal signal could be biased by the high inter-annual variability. Indeed, the monthly extent of low surface salinity ( S<35) varied between years, this is driven by river runoff over the previous 3–6 months and short-term wind events (∼10 days), particularly when they are upwelling-favourable. Forcing at these time-scales are responsible also for mesoscale structures, such as lower salinity lenses and wind-induced coastal upwelling off Southern Brittany. In the deeper layers the inter-annual variability of a denser water structure named (the “Cold Pool”) is also investigated. Consequently, the inter-annual, seasonal and mesoscale variability in salinity patterns are caused by (i) river runoff from the Loire and Gironde, that reduces surface salinity locally; and (ii) the wind that influences the location of low salinity water masses. Mesoscale structures and the observed links between inter-annual and seasonal variability, raise problems in relation to the interpretation of in situ data, making it impossible to establish the actual mean distribution. There is a pressing need within the Bay of Biscay, for sampling at higher spatial and temporal resolution.