Coastal Upwelling indices related parameters extracted from satellite SST fields over the West Africa and the Benguela ecosystem region

摘要

The comparative dynamic of the coastal upwellings of the West-African coasts and the Benguela ecosystem is characterised from thermal infrared data of the European satellites of the METEOSAT series from 1990. A specific data processing is used to restore the sea surface temperature (SST) at a spatial resolution of 5 km and a time step of 5 days (Demarcq, Citeau, 1995). A normalised intensity index is routinely calculated from SST differences and the characterisation of the spatial structure of the upwellings as the automatic localisation of the local SST minima. An original index of the potential retention capabilities is also calculated from the same data. The strong interannual variability of the indexes is quantified, mainly in term of intensity and seasonal lag. The regular remote sensing sampling allows a precise description of the spatio-temporal dynamics of the upwellings. An objective method of space partitioning is proposed according to the mean observed spatial variability of the proposed indexes. The seasonal dynamic of some characteristics area in West Africa and in the Benguela upwelling ecosystem are compared in term of upwelling intensity and retention capabilities. The spawning area of the main pelagic species are clearly evidenced via totally objectives methods, which shows the importance of the retention processes for spawning capabilities and surviving. According the theoretical concept of the Bakun's fundamental triade in upwelling areas, a precise determination of the coastal upwellings intensity and their associated spatial processes are on the first importance to evaluate the balance between enrichment and optimal use of this enrichment in an ecological point of view. Long series of such environmental parameters are now accessible throw several remote sensing data sets. A specific JAVA-based software is implemented to quickly visualise long time series of pre-processed SST images and to extract time series of coastal upwelling intensity indexes and some linked parameters such as a retention index and the offshore upwelling extension.