Published Research

d’Hotman, J., Malan, N., Collins, C., de Vos, M., Lumpkin, R., Morris, T. and Hermes, J. (2019). J. Geophys. Res. Oceans, 124.

Abstract: Studies based on drifters which are deployed using fixed geographical locations can alias the variability in the Agulhas Current. Numerical model simulations have shown that tracking particles using jet coordinate systems will improve our understanding of the variability in Western Boundary currents. In this study we use in situ observations to show the potential of quasi‐Lagrangian measurements with an investigation into the relationship of the upstream surface velocity configuration and the trajectories surface drifters follow. Additionally, we use these drifters, along with ship based measurements, to expose biases in satellite derived geostrophic velocities in the Agulhas Current. Between September 1992 and October 2017, 49 surface drifters crossed the altimeter track #096 in a non‐meandering state. Of the 49 surface drifters, 16 crossed inshore of the surface velocity maxima, 3 of which leaked into the South Atlantic Ocean. Biases between altimetry derived geostrophic velocities and absolute velocities from S‐ADCP and drifters measurements, have pointed towards surface drifters leaking from inshore of the Agulhas Current core in a region of high shear. However, the bias between these velocities are inconsistent, with the highest range in bias found inshore of the Agulhas Current core. Due to the lack of data in the Agulhas Current and various sources of error, much work remains to be done and results presented here may provide motivation for further targeted drifter deployments in the future.

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De Vos, M., Backeberg, B. C. and Counillon, F . (2018). Ocean Dynamics. 68 (1071).

Abstract: A complex and highly dynamical ocean region, the Agulhas Current System plays an important role in the transfer of energy, nutrients and organic material from the Indian to the Atlantic Ocean. Its dynamics are not only important locally, but affect the global ocean-atmosphere system. In working towards improved ocean reanalysis and forecasting capabilities, it is important that numerical models simulate mesoscale variability accurately—especially given the scarcity of coherent observational platforms in the region. Data assimilation makes use of scarce observations, a dynamical model and their respective error statistics to estimate a new, improved model state that minimises the distance to the observations whilst preserving dynamical consistency. Qualitatively, it is unclear whether this minimisation directly translates to an improved representation of mesoscale dynamics. In this study, the impact of assimilating along-track sea-level anomaly (SLA) data into a regional Hybrid Coordinate Ocean Model (HYCOM) is investigated with regard to the simulation of mesoscale eddy characteristics. We use an eddy-tracking algorithm and compare the derived eddy characteristics of an assimilated (ASSIM) and an unassimilated (FREE) simulation experiment in HYCOM with gridded satellite altimetry-derived SLA data. Using an eddy tracking algorithm, we are able to quantitatively evaluate whether assimilation updates the model state estimate such that simulated mesoscale eddy characteristics are improved. Additionally, the analysis revealed limitations in the dynamical model and the data assimilation scheme, as well as artefacts introduced from the eddy tracking scheme. With some exceptions, ASSIM yields improvements over FREE in eddy density distribution and dynamics. Notably, it was found that FREE significantly underestimates the number of eddies south of Madagascar compared to gridded altimetry, with only slight improvements introduced through assimilation, highlighting the models’ limitation in sustaining mesoscale activity in this region. Interestingly, it was found that the threshold for the maximum eddy propagation velocity in the eddy detection scheme is often exceeded when data assimilation relocates an eddy, causing the algorithm to interpret the discontinuity as eddy genesis, which directly influences the eddy count, lifetime and propagation velocity, and indirectly influences other metrics such as non-linearity. Finally, the analysis allowed us to separate eddy kinetic energy into contributions from detected mesoscale eddies and meandering currents, revealing that the assimilation of SLA has a greater impact on mesoscale eddies than on meandering currents.

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De Vos, M. and Rautenbach, C. (2019). Safety Science, 117, pp. 217–228.

Abstract: South Africa hosts a multitude of interests along its coastline and in its coastal waters. These range from recreational and small-scale commercial activities to those related to tourism and large-scale industry. The associated need for robust coastal risk management is key in securing both economic interests and safety of life. Despite widespread intuitive appreciation of the relationship between weather and coastal safety, objective analysis in this regard is lacking. This study strives to address this gap to assist, for example, coastal management, the efficient deployment of search and rescue (SAR) resources and investment in safety infrastructure. Further, we used statistical relationships between weather and incident-occurrence to develop a basic risk characterisation framework for different coastal areas and activities. Results from our investigation revealed varying sensitivities to coastal marine meteorological parameters. For activities in which individuals are more inherently vulnerable (e.g. swimming), incidents were most frequent during Good conditions. For activities involving small personal water craft (e.g. kayaks), incidents were most frequent during Marginal conditions. Incidents involving small vessels (e.g. rigid-inflatable boats) were most numerous during Bad conditions, with no clear pattern in respect of larger, commercial vessels (e.g. fishing trawlers). Finally, we present empirically derived risk coefficients, showing the relationship between risk, user vulnerability and user exposure for given weather severity scenarios.

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Morris, T. and Lamont, T. (2019). S Afr J Sci; 115 (1/2).

Morris, T., Rautenbach, C. and Stander, J. (2019). S Afr J Sci; 115(5/6).

Rautenbach, C., Barnes, M.A., and De Vos, M. (2019). Journal for Deep Sea Research 1. (In Press)

Abstract: The tidal characteristics of South Africa are explored in the present study by means of a calibrated and validated regional numerical model. The coastal tidal characteristics and semi-diurnal resonance of the South African coastline have yet to be accurately quantified. Model development was conducted in the numerical code Delft 3D for a two year simulation period. A horizontal model resolution of 1/16th geographical degree was employed. The results were calibrated against long-term measured water levels provided by the South African Navy Hydrographic Office. Model validation was performed for each major constituent's amplitude and phase lag at nine coastal locations around the South African coastline. Regional, two-dimensional comparisons were also made between this study's model results and those of the data assimilative TPXO 8 African regional model. The tide was characterized in terms of constituent amplitude and phase lag, Form Factor and tidal ellipse eccentricity regional by means of map plots. The model was particularly sensitive to bathymetry-related friction and model resolution. Accurate model results were obtained, providing the first identification and quantification of the semi-diurnal coastal resonance around South Africa. The phase lag associated with the shallower shelf areas is also clearly observed with Form Factor calculations confirming the semi-diurnal dominance of the South African coastline. The intermediate and shallow water friction-induced phase lag of the mixed progressive and standing tidal wave is also mapped, together with the tidal current phase lags.
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Rautenbach, C., and Theron, A. K. (2018). Journal of the South African Institution of Civil Engineering, 60 (4).

Roemmich, D,.... Morris, T. (2019). Frontiers in Marine Science. doi: 10.3389/fmars.2019.00439.

Abstract: The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo’s global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al., 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities.

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Coppin, R., Rautenbach, C., and Smit. A. J. (2018). Submission pending.

Pfaff, M. C., Logston, R. C., … Rautenbach, C., et al. (2019). Elementa – Ocean. In press 03/04/2019.

Rautenbach, C., Barnes, M.A., and De Vos, M. (2019). Submitted to Journal for Deep Sea Research 1.

Veitch, J., Rautenbach, C., Hermes, J., Reason, C. (2019). Journal of Marine Systems – In press 03/04/2019.

Conference Papers

De Vos, M. Backeberg, B. C. and Counillion, F. (2016). 32nd Annual conference of South African Society for Atmospheric Sciences (SASAS Cape Town October 2016). Peer reviewed proceedings

De Vos, M. and Rautenbach, C. (2018). 34th Annual conference of South African Society for Atmospheric Sciences (SASAS September 2018). Peer reviewed proceedings

De Vos, M. and Rautenbach, C. (2018). 4th GEO Blue Planet Symposium, Toulouse, France.

d'Hotman, J., De Vos, M. Malan, N., Morris, T. Stander, J. and Hermes, J., (2016) Data Buoy Cooperation Panel (DBCP 2016).

d'Hotman, J., Morris, T., Krug, M., and Hermes, J., (2019) 11th WIOMSA Conference, 1-6 July 2019, Mauritius.

Rautenbach, C. and De Vos, M. (2018). 4th GEO Blue Planet Symposium, Toulouse, France.

Rautenbach, C. and Veitch, J. (2018). 34th Annual conference of South African Society for Atmospheric Sciences (SASAS September 2018). Peer reviewed proceedings

Williams, T. and Rautenbach, C. (2018). 34th Annual conference of South African Society for Atmospheric Sciences (SASAS September 2018). Peer reviewed proceedings