Description: Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind-stress-curl. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine ecosystem.

Description: The southeastern tropical Atlantic hosts a coastal upwelling system characterized by high biological productivity. Three subregions can be distinguished based on differences in the physical climate: the tropical Angolan and the northern and southern Benguela upwelling systems (tAUS, nBUS, sBUS). The tAUS, which is remotely forced via equatorial and coastal trapped waves, can be characterized as a mixing-driven system, where the wind forcing plays only a secondary role. The nBUS and sBUS are both forced by alongshore winds and offshore cyclonic wind stress curl. While the nBUS is a permanent upwelling system, the sBUS is impacted by the seasonal cycle of alongshore winds. Interannual variability in the region is dominated by Benguela Niños and Niñas that are warm and cold events observed every few years in the tAUS and nBUS. Decadal and multidecadal variations are reported for sea surface temperature and salinity, stratification and subsurface oxygen. Future climate warming is likely associated with a southward shift of the South Atlantic wind system. While the mixing-driven tAUS will most likely be affected by warming and increasing stratification, the nBUS and sBUS will be mostly affected by wind changes with increasing winds in the sBUS and weakening winds in the northern nBUS.

Description: Anthropogenic global warming, with both short- and far-reaching consequences, is a fact which has been supported by multiple independent studies. However, observed regional and global climatic changes are due to both human-induced changes and those generated naturally in the climate system over a broad range of timescales. Differentiating anthropogenic climate change from background climate noise is a big challenge for the climate science community. This thesis provides an improved understanding of the impacts of long-term internal climate variability on 20th and 21st century climate simulations, which is of great importance for socio-economic planners. Here, a quantitative assessment of the uncertainty associated with internally generated variability in 21st century climate projections of dynamic sea level (DSL), defined as the local departure from globally averaged sea level, is presented. Furthermore, the contribution of internal climate variability to the observed multidecadal changes in the tropical Pacific, a key component of the global climate system, is investigated. To this end, a series of unforced and forced climate model simulations, including a unique set of initial-condition experiments with the Kiel Climate Model (KCM), as well as several observational data sets, are analysed. Special emphasis is placed on the role of climate variability in designing and analysing a novel experimental setup, in which all realizations undergo identical external forcing but differ in their initial conditions covering a wide range of climate states. It is shown in chapter 2 of the thesis that the uncertainty in the DSL centennial trend over large parts of the globe, in particular the mid- and high-latitudes, is of the same order of magnitude as globally averaged steric sea level rise. Thus, the impacts of long-term internal variability cannot be neglected in centennial projections of the DSL. However, the uncertainty is substantially reduced when perfect knowledge of oceanic initial condition is assumed and only the atmospheric component is perturbed. This suggests that if climate model integrations start with an initial state which is reasonably consistent with the current oceanic state and its past radiative forcing, the uncertainty in centennial climate projections can be significantly reduced. Analysis of model results shows that despite the presence of considerable uncertainty, a large signal-to-noise ratio over some oceanic sectors is simulated, indicating the robustness of the anthropogenic global warming signal in these regions. These results are further confirmed by several multi-model ensembles of global warming provided in the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive. The contribution of long-term internal variability to the observed multidecadal changes in the tropical Pacific climate, taken as a showcase, is the focus of chapter 3 of the thesis. In this context, model unforced simulations indicate the high level of internal variability over the tropical Pacific sector, which can potentially obscure anthropogenic climate change. It is shown that the observed decadal and multidecadal changes over this region are still within the range of unforced internal variability simulated by models. Moreover, it is found that the spatial structure of extreme decadal trends in Sea Surface Temperature (SST), Sea Level Pressure (SLP) and wind stress obtained from control runs are in a good agreement with the observed trend patterns. Findings of this chapter suggest that the most recent decadal changes in the tropical Pacific surface climate can be largely attributed to internal variability. Using multiple large ensembles of global warming experiments conducted with different models, the uncertainty in tropical Pacific climate projections due to internal variability is investigated in the fourth chapter. Results of this assessment indicate large irreducible uncertainty associated with internal variability. It is found that the anthropogenic climate changes in decadal timescale can be readily obscured by internal variability. Further, each ensemble provides some members displaying decadal timescale trends in SST, SLP and wind stress which are fairly similar to the observed decadal trends. This feature is poorly represented in CMIP5 experiments forced by historical radiative forcing. The findings of this chapter suggest using the probabilistic approach to study tropical Pacific climate trends which would provide new insights for the environmental planners.