Environmental variability in the Gulf of Guinea Large Marine Ecosystem: Physical features, forcing and fisheries
This thesis examines the forcing and behaviour of oceanographic physical features, relevant to recruitment in fish populations, in the Gulf of Guinea Large Marine Ecosystem, on seasonal and interannual time scales. Remotely sensed sea-surface temperature (SST) data covering the period 1981– 1991 was used to identify and describe a number of oceanographic features, including the Senegalese Upwelling influence, the Ghana and Côte d’Ivoire coastal upwelling, river run-off, fronts and the previously unrecorded observation of shelf-break cooling along the coast of Liberia and Sierra Leone during the boreal winter. Interannual variability in SST was observed on an approximate three year scale and an extended warm phase was noted between 1987 and 1991. Principal components analysis (PCA) was used to further investigate the variance structure of these SST data and this technique was shown to be able to accurately define boundaries of the Gulf of Guinea system and its constituent subsystems. River discharge data from throughout the Gulf of Guinea was also investigated using PCA, confirming the hydroclimatic regions identified by Mahé and Olivry (1999). The boundaries between these regions correspond closely to those identified between subsystems in the SST data, suggesting a degree of coupling between oceanographic and meteorological variability in the Gulf of Guinea. To further investigate this coupling, local climate data and global/basin scale indices were compared qualitatively and statistically with remotely sensed and in situ SST data and indices of interannual variability in oceanographic features. A new basin scale index was proposed as a measure of zonal atmospheric variability in the subtropical North Atlantic (SNAZI) and this was shown to be the dominant mode of climate variability forcing SST in the Gulf of Guinea. The implications of these results for fisheries recruitment dynamics are discussed.