Description: Key Points: - Freshwater input has significantly contributed to the surface warming at the peak of the 1995 Benguela Niño - Anomalously high river discharge and precipitation increased stratification and reduced turbulent heat loss by creating barrier layers - Combination of high freshwater input and strong poleward surface current might play a role in temperature variability off Angola Benguela Nino events are characterized by strong warm sea surface temperature (SST) anomalies off the Angolan and Namibian coasts. In 1995, the strongest event in the satellite era took place, impacting fish availability in both Angolan and Namibian waters. In this study, we use direct observations, satellite data, and reanalysis products to investigate the impact that the up-until-now unnoticed mechanism of freshwater input from Congo River discharge (CRD) and precipitation had on the evolution of the 1995 Benguela Nino. In the onset phase of the event, anomalous rainfall in November/December 1994 at around 6 degrees S, combined with a high CRD, generated a low salinity plume. The plume was advected into the Angola-Namibia region in the following February/March 1995 by an anomalously strong poleward surface current generated by the relaxation of the southerly winds and shifts in the coastal wind stress curl. The presence of this low surface salinity anomaly of about -2 psu increased ocean stability by generating barrier layers, thereby reducing the turbulent heat loss, since turbulent mixing acted on a weak vertical temperature gradient. A mixed layer heat budget analysis demonstrates that southward advection of Angolan waters drove the warming at the onset, while reduced mixing played the main role at the event's peak. We conclude that a freshwater input contributed to the SST increase in this exceptionally strong event and suggest that this input can influence the SST variability in Angola-Namibia waters through a combination of high CRD, precipitation, and the presence of a strong poleward surface current. Benguela Nino events are characterized by excessive warming of the sea surface temperature off the Angolan and Namibian coasts. One of the strongest-ever recorded warm events dates back to 1995, impacting fish availability in both Angolan and Namibian waters. In our research, we investigate if freshwater from rain and from the Congo River could have impacted the evolution of this 1995 Benguela Nino. In the event's early stage, high precipitation and river discharge generated a low salinity pool at the Congo River mouth, which in February/March 1995 was taken to the south by an exceptionally strong surface current, generated by changes in wind strength and direction at the African coast. This low sea surface salinity in a shallow layer in the upper meters of the ocean increased the ocean's stability. As the stabilized waters diminished the usual mixing from the depths below which cools down the surface waters, it contributed to an increase in warming in the surface layer of the ocean. We conclude that the warming of the surface waters in the region was indeed influenced by the combination of high precipitation and high Congo River discharge with a strong surface current toward the south. Freshwater input has significantly contributed to the surface warming at the peak of the 1995 Benguela Nino Anomalously high river discharge and precipitation increased stratification and reduced turbulent heat loss by creating barrier layers Combination of high freshwater input and strong poleward surface current might play a role in temperature variability off Angola

Description: Eastern boundary upwelling systems are hotspots of marine life and primary production. The strength and seasonality of upwelling in these systems are usually related to local wind forcing. However, in some tropical upwelling systems, seasonal maxima of productivity occur when upwelling favorable winds are weak. Here, we show that in the tropical Angolan upwelling system (tAUS), the seasonal productivity maximum is due to the combined effect of coastal trapped waves (CTWs) and elevated tidal mixing on the shelf. During austral winter, the passage of an upwelling CTW displaces the nitracline upward by more than 50 m. Thereby, nitrate-rich waters spread onto the shelf, where elevated vertical mixing causes a nitrate flux into the surface mixed layer. Interannual variability of the productivity maximum is strongly correlated to the amplitude of the upwelling CTW as seen in sea level data. Given that CTWs are connected to equatorial forcing, a predictability of the strength of the productivity maximum is suggested.

Description: The eastern boundary regions of the Atlantic and Pacific oceans host highly productive ecosystems. These upwelling systems play a key role in supporting marine biodiversity, local and global fisheries, and biogeochemical cycles. Consequently, it is of high interest to understand the processes driving these systems. This thesis focuses on one of these highly productive ecosystems - the tropical Angolan upwelling system (tAUS). Conditions in the tAUS undergo strong seasonal modulations, where many characteristics exhibit variability on semi-annual and annual time scales. The lowest sea surface temperature (SST), highest primary productivity, and lowest along-shore wind are found in austral winter during the main upwelling season. Interestingly, and in contrast to other upwelling systems, the productivity signal cannot be explained by local wind-driven upwelling. Possible forcing mechanisms of the productivity signal are connected to equatorial dynamics via the propagation of coastal trapped waves (CTWs). The tAUS is thus an ecosystem influenced by both remote and local processes. This thesis focuses on understanding the physical drivers of the seasonal and interannual variability in the tAUS, particularly in SST and primary productivity. The analyses conducted within this thesis are mostly based on observational datasets. Additionally, the results are compared with output of a regional ocean model. The observational data includes shipboard measurements as well as satellite products. The shipboard measurements comprise an extensive ocean turbulence dataset. This dataset provides, for the first time, insights into turbulent heat and nitrate fluxes in the tAUS. In the tAUS, waters are colder directly at the coast than further offshore. Both SST and the crossshore SST gradient exhibit a seasonal cycle. A seasonal mixed layer heat budget is calculated to identify atmospheric and oceanic causes for heat content variability. The results 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 net surface heat flux warms the coastal ocean more strongly than the offshore region and thus acts to dampen cross-shore SST differences. Ocean turbulence data shows that turbulent mixing across the base of the mixed layer is an important cooling term. This turbulent cooling, being strongest in the shallow shelf regions, explains the observed negative cross-shore temperature gradient. The seasonal cycle of the cross-shore SST gradient exhibits semi-annual variability, likely linked to tidal mixing acting on changing background stratification associated with the passage of CTWs. The primary productivity in the tAUS peaks in late austral winter. Analyses of observational data and the output of a regional ocean model reveal that the seasonal productivity maximum is due to the combined effect of CTWs and elevated tidal mixing on the shelf. During austral winter, the passage of an upwelling CTW displaces the nitracline upward by more than 50 m. Thereby, nitrate-rich waters spread onto the shelf, where elevated vertical mixing causes a nitrate flux into the surface mixed layer. High-mode CTWs play an important role in the upward and onshore transport of nitrate-rich waters. The interannual variability of the productivity maximum in the tAUS is strongly correlated with the amplitude of the upwelling CTW. Consequently, it is of high interest to investigate dynamical factors controlling the characteristics of the CTW as their strength controls the amount of primary production in the tAUS. Regression analyses suggest that the timing and amplitude of the austral winter upwelling CTW in tAUS are influenced by variability in different regions. The timing of the CTW is mostly influenced by variability in the equatorial region and along the southern African coast. Here. weaker equatorial easterlies from April to July lead to a late arrival of the upwelling CTW. The amplitude of the CTW is influenced by variability in the eastern equatorial Atlantic and in the central South Atlantic, a region characterizing the strength of the South Atlantic anticyclone. A cooling in the eastern equatorial Atlantic four to three months before the arrival of the CTW causes stronger zonal winds, leading to a stronger austral winter upwelling CTW. In general, the results 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. A key result is the proposed mechanism explaining the austral winter productivity peak, based on the combined effect of CTWs and near-coastal mixing. This result not only advances process understanding in the tAUS but also suggests a potential predictability of productivity in the region.