Temporal trends and spatial structures of the sea surface temperature in the Bay of Biscay

A time series spatial grid of sea surface temperature (SST) data provided by the archives of Meteo-France and covering the period 1972-1993 was analysed in order to define both the long-term and periodic (mainly seasonal) components at different locations in the Bay of Biscay. The study confirmed the existence of a long-term increasing trend in the SST, but showed that this trend was not homogeneous over the entire area. It revealed also that the amplitude of the cyclic components (summer-winter differences) is spatially heterogeneous. The south-eastern part of the Bay of Biscay, close to the French-Spanish border, shows the stronger warming trend (a mean increase of 1.4 degrees C for the period 1972-1993). This trend decreases in the adjacent regions and is not statistically significant in the northern part of the bay and the Celtic Sea. The increasing trend was recorded both in winter and summer periods with the winter slopes being slightly higher. The amplitude of the SST seasonal changes (summer-winter differences) is stronger close to the southern French Atlantic coast in the area off the Gironde estuary. The lowest values were observed in the SW of the bay in the region of intense upwelling processes. Finally, the analysis of the main components of this series provides a simple empirical model describing the time-related changes in the SST. This model could be useful for studies in the field of biological oceanography and in the context of comparative studies. The possible linkage of these observations with changes in the structure of ecosystem in this area is also discussed. (C) Elsevier, Paris.