Physical Drivers of Seasonal Variability in the tropical Angolan Upwelling System
The eastern boundary regions of the Atlantic and Pacific oceans host highly productive ecosystems. This thesis focuses on the tropical Angolan upwelling system (tAUS). Conditions in the tAUS undergo strong seasonal modulations. In contrast to other upwelling systems, the productivity signal cannot be explained by local wind-driven upwelling. Possible forcing mechanisms of the seasonal cycle in productivity signal are connected to equatorial dynamics via the propagation of coastal trapped waves (CTWs). This thesis focuses on understanding the physical drivers of the seasonal variability in the tAUS, particularly in SST and primary productivity. In the tAUS, sea surface temperature (SST) exhibit a distinct seasonal cycle. A seasonal mixed layer heat budget show that the seasonal cycle in SST is mainly controlled by surface heat fluxes and turbulent heat loss at the base of the mixed layer. The primary productivity in the tAUS peaks in late austral winter. Analyses reveal that the seasonal productivity maximum is due to the combined effect of CTWs and elevated tidal mixing on the shelf. The interannual variability of the productivity maximum in the tAUS is strongly correlated with the amplitude of the upwelling CTW. This suggests a potential predictability of productivity in the region. Consequently, it is of high interest to investigate dynamical factors controlling the characteristics of the CTW. Results found in this thesis suggest that the timing and amplitude of the upwelling CTW in the tAUS during austral winter are predictable on seasonal time scales. Overall, this thesis enhances our understanding of the seasonal to interannaul dynamics in the tAUS. The results of this thesis show that CTWs, near-coastal mixing, and surface heat fluxes are essential processes to explain the seasonal variability of SST and productivity in the tAUS.
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