Published Research

Barnes, M.A. and Rautenbach, C. (2020). Journal of Geophysical Research: Oceans

Barnes, M.A., Turner, K., Ndarana, T. and Landman, W.A. (2020). Atmospheric Research

Abstract: An intense surface low pressure system and associated cut-off low (COL) that affected the south-western Cape of South Africa on 6–7 June 2017 are analysed. The storm, locally named Cape Storm, was associated with heavy rains, strong winds, runaway fires, storm surge and extremely large waves. These extreme meteorological conditions resulted in a loss of life and damage to infrastructure in various forms around the province. The societal impacts that occurred have been collected and summarized in this study. Cape Storm was tracked back to its initial development as a COL extension to a surface low pressure system to the south-west of South America. ERA-Interim reanalysis is analysed from a potential vorticity (PV) perspective to assess the factors that influenced the development of this impactful storm. The analysis found that the system was associated with a Rossby wave breaking (RWB) event and an intrusion of high-potential vorticity (PV) from the stratosphere, assisting the development of the surface low pressure system. The COL migrated across the South American continent and made its way across the South Atlantic Ocean. As it migrated towards South Africa, two more COLs associated with RWB events and stratospheric intrusions maintained and deepened the surface low pressure system, increasing its eventual impact on South Africa. The surface cyclone deepening events were both associated with high-PV in both the upper to mid-levels and lower levels of the troposphere. One of these events was associated with a deep, tightly wrapped, cyclonic intrusion of high-PV air into the troposphere and higher PV values in the lower tropospheric, resulting in rapid and intense surface cyclogenesis.
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d’Hotman, J., Malan, N., Collins, C., de Vos, M., Lumpkin, R., Morris, T. and Hermes, J. (2019). Journal of Geophysical Research: Oceans.

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. , Aguiar-González, B., Ansorge, I., and Hermes, J. (2019). Journal of Geophysical Research: Oceans, 124.

Abstract: We investigate the 3D Lagrangian evolution of two Madagascar cyclonic eddies based on ad hoc Argo experiments undertaken in April and July 2013. Eight Argo floats were configured to measure temperature and salinity at high temporal resolutions (daily and five‐daily experiments) and varying park depths (300, 500, 650, and 1,000 m) to test their performance with regard to retention within the eddies described. Near‐surface eddy properties are derived from an eddy detection and tracking algorithm applied to satellite altimetry data and a quasi eddy‐resolving (1/4°) ocean general circulation model (GLORYS2v4). Both eddies propagated southwestward from southwest Madagascar (26°S, 40°E), where the South East Madagascar Current separates from the continental shelf. During a travel of about 130 days at an average speed of 11 km day−1, the eddies experienced well‐defined growth, mature, and decay phases, interacting with the Agulhas Current at the KwaZulu Natal Bight (28°S, 34°E). Model‐based estimates indicate the April (July) eddy showed mean trapping water depths of 595 ± 294 m (914 m ± 107 m), volume transport about 13.4 ± 5.2 Sv (21.2 ± 9.1 Sv), heat flux of −0.07 ± 0.06 PW (‐0.2 ± 0.09 PW), and freshwater flux of 0.04 ± 0.04 Sv (0.09 ± 0.05 Sv). Peak estimates were found for both eddies during the mature eddy phase. These results highlight the role of Madagascar cyclonic eddies as transporters of cooled and freshened source waters into the Agulhas Current and illustrate the benefits of ad hoc Argo configurations for the study of 3D Lagrangian eddy dynamics in combination with a “state‐of‐the‐art” ocean model and remotely sensed data.

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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 Part 1

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., Barnes, M.A., Wang, D.W. and Dykes, J. (2020). Journal of Marine Science and Engineering.

Abstract: Numerous studies have identified the complexities of the wave climatology around the South African coast, but limited studies have investigated these complex dynamics in the available literature. Several freely available parameterized wave boundary conditions are produced around southern Africa. However, none of these are fully spectral outputs from global or larger regional spectral wave models. This constraint results in local engineering and scientific organizations, reconstructing their own spectral boundary conditions. For coastal models, this is a reasonable assumption, assuming that the single parameterization is accurate and a representation of a non-multimodal sea state. The South African Weather Service (SAWS) Marine unit recently launched their coupled, operational wave and storm surge forecasting model. The aim of the SAWS Wave and Storm Surge (SWaSS) platform was to provide accurate, high-resolution coastal forecasts for the entire South African coastline. The present investigation thus presents the validation of the spectral wave component of the coupled system, developed in Delft3D. Various wave boundary reconstructions are investigated together with the two most used and well-known whitecapping formulations. Validation is performed with both in situ wave-rider buoy data (at nine locations along the coastline) and regional remotely sensed, along track, altimetry data. Full model performance statistics are provided, and the accuracy of the model is discussed.
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Rautenbach, C., Daniels, T., De Vos, M. and Barnes, M.A. (2020). Natural Hazards

Abstract: Regional storm tidal levels of the South African coastline are investigated by means of a calibrated and validated numerical model. The model was developed utilizing the shallow water hydrodynamic model, Delft3D. This model was coupled (online) with a non-stationary spectral wave model (developed in the Simulating WAves in the Nearshore (SWAN) numerical code). A local, 4.4 km version of the Unified Model was applied as atmospheric forcing for the coupled system. The models presented in this study form part of the operational marine forecasts of the South African Weather Service, Wave and Storm Surge model. The operational protocol and model calibration and validation are presented via statistical correlations with measured water levels at six South African coastal locations. The main calibration parameters and thus physical drivers were winds, atmospheric pressure and waves. The validated numerical model is used to provide an experimental physical description of South African storm surge characteristics, per coastline. The dominant driver of South African storm surge is winds associated with mid-latitude cyclones. Further novelty in the present study is the quantification of the relative contribution of extreme storm wave set-up to the South African storm surge signal. This wave set-up contributes approximately 20% of the total surge signal in the southwest, with winds contributing approximately 55%. The importance of the continental shelves is also elucidated concerning the frictional shoaling effects of the long surge wave propagation.
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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..

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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Veitch, J., Rautenbach, C., Hermes, J., Reason, C. (2019). Journal of Marine Systems.

Abstract: Situated on the southwestern coast of South Africa, Cape Point is directly impacted by ocean surface waves generated by midlatitude cyclones in the westerly wind belt. Severe swell events have a huge impact on coastal communities, infrastructure and maritime operations. We use the long term Council for Industrial and Scientific Research (CSIR) Cape Point Waverider buoy data and WaveWatch III model data together with atmospheric reanalysis data to investigate the role of the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) and their associated adjustments of cyclone tracks and intensities on the variability and extreme events of the Cape Point wave record. During austral summer (DJF), when the waves are generally smaller and more southerly, SAM and ENSO tend to result in robust responses: a negative (positive) SAM tends to result in bigger (smaller) more westerly (southerly) waves, while a positive (negative) ENSO results in bigger (smaller) waves from a more westerly (southerly direction). The mean austral (JJA) winter waves do not respond coherently to either SAM or ENSO. A shift in the wave characteristics between periods 1980–1992 and 1994–2006 is clear. The waves become smaller and more southerly during the latter period. This shift is associated with the shift to a more coherently out of phase relationship between SAM and ENSO, dominated by a positive SAM and negative ENSO, that amplifies the wave response, giving rise to smaller waves from a more southerly direction. During summer months there is a significant reduction in not only the mean wave heights, but also the extreme wave heights between the two periods, while during winter months there is only a significant change in the mean wave direction that becomes more southerly during the latter period. During a positive ENSO and negative SAM combination, the summer extreme wave heights tend to be more westerly while the inverse combination does not produce a robust result. Finally, we show that during summer months a positive (negative) ENSO leads to more frequent extreme wave events. On the other hand, a positive (negative) SAM leads to less (more) frequent extreme wave events during winter months.

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Xie, J., De Vos, M., Bertinoc, L., Zhud,J. and Counillon, F . (2020). Ocean Modelling.

Abstract: From satellite altimetry, it is known the benefit of assimilating sea level anomalies (SLA) has been shown in the context of operational ocean forecast systems. However, how much data assimilation (DA) of altimetry data improves the representation of mesoscale eddies has still not been investigated in previous studies. Especially in the South China Sea (SCS), no estimation for that has been done in a long time. In this study, a nested SCS model system uses the Ensemble Optimal Interpolation method to assimilate along-track SLA data from 1993 to 2011. We assess the representation of eddy characteristics in two hindcast simulations - one with DA and the other without - and compare them to satellite-derived eddy characteristics. In the whole SCS, the assimilation improves the number of simulated cyclonic (anticyclonic) eddies by 10.3% (13.6%). The corresponding improvement in the eddy-rich northern SCS is 17.9% (19.6%). Assimilation improves the seasonality of eddy occurrence, with cyclones and anticyclones showing an obviously asymmetric seasonality. However, diagnosed assimilation effects and associated residual errors show large spatial and temporal dependencies. The radii of anticyclonic eddies smaller than 70 km are not changed with DA. The results show that deficiencies of cyclonic eddies in winter and anticyclonic eddies in summer around 13N in the southeastern SCS are not well corrected by DA, where one of the shortcomings is resulted from the used wind forcing. Although this study does not conduct one-to-one forecasting experiments for each eddy track, improved eddy reproduction is the first step towards detailed validation metrics for eddy forecasting systems.

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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