The Angola Current in a Tropical Seasonal Upwelling System: Seasonal Variability in Response to Remote Equatorial and Local Forcing

In this thesis, the flow and hydrographic characteristics of the boundary circulation off Angola are investigated, with particular emphasis on the mean properties and the seasonal cycle of the Angola Current. Moored velocity observations acquired at the Angolan shelf at 11°S between 2013 and 2016 reveal a highly variable alongshore flow with velocities in the range of ±40 cm/s superimposed on a weak poleward mean current with core velocities not exceeding 8 cm/s. These measurements question the former view of a permanently poleward flowing Angola Current, which was based on results from few ship campaigns. During the observational period a mean Angola Current southward transport of 0.32±0.05 Sv is determined. Pronounced annual and semiannual oscillations are found in the alongshore current characterized by distinct baroclinic structures. In the equatorial Atlantic, annual and semiannual oscillations are associated with resonant equatorial basin-modes of the fourth baroclinic mode for the annual cycle and the second baroclinic mode for the semiannual cycle. Equatorial basin-modes represent standing modes in a zonally bounded basin being composed of equatorial Kelvin and Rossby waves. A series of shallow water model experiments differing in the basin geometry and/or the applied forcing show a structural robustness of the corresponding horizontal patterns associated with the annual and semiannual cycles. The off-equatorial lobes of the basin-modes also impact alongshore velocity at the eastern boundary off Angola. In a suite of shallow water model simulations the remote equatorial forcing related to basin-modes is isolated from the effects of local forcing. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. For the semiannual cycle, the inclusion of local forcing improves the agreement between observed and simulated velocity oscillations at 11°S

Der Angolastrom in einem tropischen, saisonalen Auftriebsgebiet: Saisonale Variabilität hervorgerufen durch äquatorialen und lokalen Antrieb

In this thesis, the flow and hydrographic characteristics of the boundary circulation off Angola are investigated, with particular emphasis on the mean properties and the seasonal cycle of the Angola Current. Moored velocity observations acquired at the Angolan shelf at 11°S between 2013 and 2016 reveal a highly variable alongshore flow with velocities in the range of ±40 cm/s superimposed on a weak poleward mean current with core velocities not exceeding 8 cm/s. These measurements question the former view of a permanently poleward flowing Angola Current, which was based on results from few ship campaigns. During the observational period a mean Angola Current southward transport of 0.32±0.05 Sv is determined. Pronounced annual and semiannual oscillations are found in the alongshore current characterized by distinct baroclinic structures. In the equatorial Atlantic, annual and semiannual oscillations are associated with resonant equatorial basin-modes of the fourth baroclinic mode for the annual cycle and the second baroclinic mode for the semiannual cycle. Equatorial basin-modes represent standing modes in a zonally bounded basin being composed of equatorial Kelvin and Rossby waves. A series of shallow water model experiments differing in the basin geometry and/or the applied forcing show a structural robustness of the corresponding horizontal patterns associated with the annual and semiannual cycles. The off-equatorial lobes of the basin-modes also impact alongshore velocity at the eastern boundary off Angola. In a suite of shallow water model simulations the remote equatorial forcing related to basin-modes is isolated from the effects of local forcing. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. For the semiannual cycle, the inclusion of local forcing improves the agreement between observed and simulated velocity oscillations at 11°S

Tides, Stratification, and Counter Rotation: The German Bight ROFI in Comparison to Other Regions of Freshwater Influence

The characteristics of tidal velocity profiles and their relation to stratification are investigated based on high-resolution field data collected at four locations in the German Bight Region of Freshwater Influence (ROFI) in the North Sea. The deployments each include two to three tidal cycles and were conducted during field campaigns in August 2016 and May 2018. The depth-averaged semidiurnal tidal motion is dominated by a standing wave directed toward the coast, but modified by a smaller, coast-parallel progressive wave contribution. The time series of the tidal velocity profiles consistently show tidal asymmetries with higher flood than ebb velocities near the surface and counter-clockwise rotation of the velocity trajectories at depth. Near the surface, phase-locked periodic changes in the sense of rotation within the tidal cycle are evident for three deployments, resulting in periodic counter-rotation of the upper and lower layer. During these episodes, stratification of the water column is observed. Counter-rotation is initiated after a sudden decoupling developing from the surface downward, with subsequent rapid development of stratification and velocity shear. The observed decoupling is most likely triggered by advection of the plume-induced lateral surface density gradient by weakly sheared ebb currents toward the study site. Due to the dominance of the standing wave in the German Bight ROFI, the observed intra-tidal variations of stratification are more similar to the Liverpool Bay and differ significantly from the Rhine ROFI, where the tidal dynamics are controlled by a progressive Kelvin wave

Role of Equatorial Basin-Mode Resonance for the Seasonal Variability of the Angola Current at 11°S

Multi-year moored velocity observations of the Angola Current near 11°S reveal a weak southward mean flow superimposed by substantial intraseasonal to seasonal variability, including annual and semiannual cycles with distinct baroclinic structures. In the equatorial Atlantic these oscillations are associated with basin-mode resonances of the fourth and second baroclinic modes, respectively. Here, the role of basin-mode resonance and local forcing for the Angola Current seasonality are investigated. A suite of linear shallow-water models for the tropical Atlantic is employed, each model representing a single baroclinic mode forced at a specific period. The annually and semiannually oscillating forcing is given by 1) an idealized zonally uniform zonal forcing restricted to the equatorial band corresponding to a remote equatorial forcing or 2) realistic, spatially-varying Fourier components of wind stress data that include local forcing off Angola, particularly alongshore winds. Model-computed modal amplitudes are scaled to match moored velocity observations from the equatorial Atlantic. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. Including local forcing slightly improves the agreement between observed and simulated semiannual oscillations at 11°S compared to the purely equatorial forcing. However, the model-computed semiannual cycle lacks amplitude at mid-depth. This could be the result of either underestimating the strength of the second equatorial basin-mode of the fourth baroclinic mode or other processes not accounted for in the shallow-water models. Overall, our findings underline the importance of large-scale linear equatorial wave dynamics for the seasonal variability of the boundary circulation off Angola

Eastern Boundary Circulation and Hydrography Off Angola: Building Angolan Oceanographic Capacities: Building Angolan Oceanographic Capacities

The eastern boundary region off Angola encompasses a highly productive ecosystem important for the food security of the coastal population. The fish-stock distribution, however, undergoes large variability on intraseasonal, interannual, and longer time scales. These fluctuations are partly associated with large-scale warm anomalies that are often forced remotely from the equatorial Atlantic and propagate southward, reaching the Benguela upwelling off Namibia. Such warm events, named Benguela Niños, occurred in 1995 and in 2011. Here we present results from an underexplored extensive in situ dataset that was analyzed in the framework of a capacity-strengthening effort. The dataset was acquired within the Nansen Programme executed by the Food and Agriculture Organization of the United Nations and funded by the Norwegian government. It consists of hydrographic and velocity data from the Angolan continental margin acquired biannually during the main downwelling and upwelling seasons over more than 20 years. The mean seasonal changes of the Angola Current from 6° to 17°S are presented. During austral summer the southward Angola Current is concentrated in the upper 150 m. It strengthens from north to south, reaching a velocity maximum just north of the Angola Benguela Front. During austral winter the Angola Current is weaker, but deeper reaching. While the southward strengthening of the Angola Current can be related to the wind forcing, its seasonal variability is most likely explained by coastally trapped waves. On interannual time scales, the hydrographic data reveal remarkable variability in subsurface upper-ocean heat content. In particular, the 2011 Benguela Niño was preceded by a strong subsurface warming of about 2 years’ duration

Atlantic Equatorial Undercurrent intensification counteracts warming-induced deoxygenation

The tropical Atlantic upper-ocean circulation experiences multiannual to decadal changes associated with different climate modes and is simultaneously adjusting to climate warming. The most energetic current in the tropical Atlantic is the Equatorial Undercurrent (EUC), which flows eastwards along the Equator. On the basis of long-term moored observations, we show that the EUC strengthened by more than 20% from 2008 to 2018. The intensification of the EUC is associated with increasing subsurface oxygen concentrations and a thickening of the upper-ocean oxygenated layer in the equatorial Atlantic. These changes counteract climate-warming-induced deoxygenation in the region. The EUC strengthening is found to be mainly forced by trade wind changes in the western tropical North Atlantic. A 60-yr dataset reveals that the recent oxygen increase in the upper equatorial Atlantic is associated with multidecadal variability. This variability is characterized by low oxygen concentrations in the 1990s and early 2000s, and high oxygen concentrations in the 1960s and 1970s. The observed oxygen variability seems to be linked to a compression and expansion of the habitat of tropical pelagic fish, and must be accounted for when evaluating the possible consequences of deoxygenation for marine ecosystems and fisheries