PREFACE International Conference on Ocean, Climate and Ecosystems

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 under-explored extensive in-situ dataset that was analysed in the framework of a capacity strengthening effort. The dataset was acquired within the Nansen Programme executed by the Food and Agricultural Organization of the United Nations and funded by the Norwegian government. It consists of hydrographic and velocity data from the Angolan continental margin acquired bi-annually during the main downwelling and upwelling seasons over more than 20 years. The mean seasonal changes of the Angola Current from 6°S 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 timescales, the hydrographic data reveals remarkable variability in subsurface upper ocean heat content. In particular, the 2011 Benguela Niño was preceded by a strong subsurface warming of about 2 year duration. PREFACE International conference on Ocean, Climate and Ecosystems, 17 – 19 Apr. 2018, Lanzarote (ES): Presentation Titles, Authors and Abstracts Oral presentation – Session 1 Page 4 of 86 Title: Role of Equatorial Basin-Mode Resonance for the Seasonal Variability of the Angola Current at 11°S Authors and affiliations: Robert Kopte [1], Peter Brandt [1,2], Martin Claus [1,2], Richard J. Greatbatch [1,2] and Marcus Dengler [1] [1] GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, Germany [2] Christian-Albrechts-Universität zu Kiel, Kiel, Germany Abstract: 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. 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. PREFACE International conference on Ocean, Climate and Ecosystems, 17 – 19 Apr. 2018, Lanzarote (ES): Presentation Titles, Authors and Abstracts Oral presentation – Session 1 Page 5 of 86 Title: Sources and Propagation Pathways of Water Masses to the Northern Benguela Upwelling System Authors and affiliations: Martin Schmidt [1], Lydia Siegfried [1], Volker Mohrholz [1], Tim Junker [1], Anja van der Plas [2], Pascal Nardini [3], Gerik Scheuermann [3] and Michael Böttinger [4] [1] Leibniz-Institute for Baltic Sea Research Warnemünde, Germany [2] National Marine Information and Research Centre, Namibia [3] Universität Leipzig, Institute for Computer Science, Germany [4] Deutsches Klimarechenzentrum GmbH, Germany Abstract: We investigate sources and propagation pathways of water masses determining the hydrographic conditions in the Northern Benguela Upwelling System (NBUS). Our results challenge the common view on the water mass variability in the NBUS as derived from a classical water mass analysis and limited field data. From in-situ temperature and salinity measurements two major source water masses are well known, South Atlantic Central Water (SACW) and Eastern South Atlantic Central Water (ESACW). Both have different oxygen and nutrient characteristics. Mooring data have shown the direct relation between poleward SACW transport with a coastally trapped undercurrent and the oxygen conditions in the NBUS. Hence, the coastal wave guide is considered as the major pathway and direct link for poleward spreading SACW from the equatorial area into the NBUS. Based on a numerical tracer release experiment, we present a more comprehensive view on the origin of the water mass variability in the NBUS. We could identify three major source areas for water masses determining the hydrographic, oxygen and nutrient variability in the NBUS. In addition to the coastal wave guide, the South Equatorial Counter Current (SECC) and the South Equatorial Under Current (SEUC) reveal as major pathways of tropical SACW into the NBUS. Near the northern rim of the NBUS in the Kunene upwelling cell, south-eastward extensions of the SECC merge with the coastal flow (Angola current) and feed the poleward undercurrent. Away from the coast and on a decadal scale the simulated ocean current field partly resembles a Sverdrup balanced flow, indicating the importance of the large scale wind stress curl over the south-eastern Atlantic for the variability of the water mass composition, oxygen and nutrient conditions in the NBUS. We discuss seasonal cycle and interannual variability of conservative model tracers, oxygen and nutrient conditions in the NBUS in response to the wind field variability. PREFACE International conference on Ocean, Climate and Ecosystems, 17 – 19 Apr. 2018, Lanzarote (ES): Presentation Titles, Authors and Abstracts Oral presentation – Session 1 Page 6 of 86 Title: Coastal trapped wave propagation along the southwest African shelf as revealed by moored observations Authors and affiliations: Tim Junker [1], Volker Mohrholz [1], Martin Schmidt [1], Lydia Siegfried [1] and Anja van der Plas [1] Leibniz-Institut fur Ostseeforschung Warnemünde (IOW), Germany [2] National Marine Information and Research Centre (NatMIRC), Swakopmund, Namibia Abstract: Coastal trapped waves (CTWs) that propagate poleward along the south west African shelf potentially leak energy from lower latitudes into the Benguela Upwelling System (BUS). Thus, in addition to the local winds, these waves provide an important remote forcing mechanism for the upwelling region. Coastal trapped waves (CTWs) that propagate poleward along the south west African shelf potentially leak energy from lower latitudes into the Benguela Upwelling System (BUS). Thus, in addition to the local winds, these waves provide an important remote forcing mechanism for the upwelling region. The present study aims at elucidating the basic nature of CTWs in the northern BUS. To this end we make use of unique, multi-site velocity observations from the Namibian shelf