Thermohaline variability in th Bay of Biscaycauses and physical implications

Zusammenfassung

There is a wide consensus among climatologist and oceanographers that the climate system has undergone rapid changes during the last decades of the 20th century and the beginning of the 21st. The most remarkable variation is the increase in temperature both in oceans and atmosphere. Ocean plays a key role to mitigate global warming since it absorbs the vast majority of the heat gained by the Earth. So, small changes in ocean`s properties can lead to unforeseeable consequences. Numerous studies showed that these variations are highly dependent on temporal and spatial scales, in such a way that changes are more marked at regional scale. The Bay of Biscay is a semi-enclosed sea located in the North Atlantic Ocean, off the north coast of Spain and the west coast of France. Its bathymetry is characterized by a wide continental shelf along the French coast, especially in its northern part (more than 100 km), and a narrower one along the Spanish coast. The Biscay Abyssal plain, with depths around 4000 m, occupies the central part of the bay. This area is influenced by the general circulation in the North Atlantic following an anticyclonic gyre. In the rest of the bay, bathymetric conditions together with local forcings (wind pattern, river discharges…) give as result a circulation pattern that changes depending on the season. So, the scientific interest of the Bay of Biscay resides in that it is a semi-enclosed sea representing a complex merger between ocean features and local processes that show their influence at different temporal and spatial scales. The main aim of this thesis is to analyze oceanic physical changes occurred in the Bay of Biscay during the last decades. To carry out this purpose the newest databases were selected in order to ensure appropriate spatial and temporal resolutions. Thus, insitu databases (Argo floats), satellite data (MODIS sensor) and reanalysis databases (SODA, OISST1/4 and CFSR) were combined to analyze changes in salinity and temperature, as well as, variations in other physical variables such as mixed layer depth, for the whole Bay of Biscay. Thermohaline variability of the two main upper water masses, Eastern North Atlantic Central Water (ENACW) and Mediterranean Water (MW), were studied over the periods 1975-2010 and 2004-2013. ENACW has been observed to warm and salinificate over both periods, whilst a cooling and a freshening were detected for MW over the period 2004-2013. These variations were mainly due to changes in the región were both water masses originated. This fact was evidenced applying the methodology developed by Bindoff and McDougall (1994) and also analyzing air temperature trends and precipitation less evaporation balance trend in the Northeastern Atlantic. Wintertime MLD trends in the Bay of Biscay over the period 1975–2010 were calculated using a potential temperature criterion to define the isothermal layer depth and a potential density criterion to define the isopycnal layer depth. MLD tended to deepen at the southeastern corner and shallow at the rest of the bay. It was observed that air temperature trends play a key role to explain the different MLD trend pattern. Apart from changes in the mean values of thermohaline variables, variations in the seasonality and in the frequency of extreme hot SST days were also studied for the whole Bay of Biscay since 1982. Overall, a warming was detected in all months for the whole bay. This warming was higher during spring and autumn months. This fact led to an increase in the duration of the warm season in more than one month for most of the bay. In addition, an increase in the number of extreme hot SST days was detected during spring and autumn months, while no clear trend was observed during august, when the highest mean SST values occur. Finally, the influence of Loire and Gironde Rivers discharges over SST trends was nalyzed using reanalysis and satellite data. The oceanic area under the influence of the turbid plume is the only area of the bay where a cooling was detected during Winter months. In addition, it was concluded that river discharges and southwestern winds over their mean values favors the maintenance of the turbid plume and also the cooling in this area. So, river discharges in the Bay of Biscay can modulate the warming pattern. In conclusion, significant changes in physical properties and processes were detected in the Bay of Biscay during the last decades. The combination between ocean features and local processes, which show their influence at different temporal and spatial scales, explain these variations. This thesis evidences the importance of carrying out regional studies to understand physical changes in oceans, as well as to understand its influence on marine ecosystems in order to determine scenarios and policies to mitigate those changes.