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The geography of near-shelf mixing on the east coast of South Africa

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Abstract

Direct observations of turbulent mixing in energetic ocean currents are fundamentally important to improving the understanding of stratified turbulence and the predictive capability of ocean models. In this study, we have selected an energetic region of the Western Indian Ocean to make simultaneous measurements of temperature and velocity to directly infer the geography of turbulent mixing on the east coast of South Africa. Direct measurements of the rate of turbulent kinetic energy dissipation, \(\epsilon\), were recorded by an autonomous underwater vehicle glider platform and vertical microstructure profiler (VMP). The glider was deployed in the Agulhas current in 2015 over a 200 km transect and measured approximately 250 vertical profiles of turbulent microstructure. Additional vertical profiles near the shelf were obtained from approximately 85 VMP casts during 2018 in a region where the powerful Agulhas current is formed and interacts with the shelf. VMP and glider data collected at Sodwana Bay and the Agulhas current showed evidence of elevated dissipation rates \(10^{-8}<\epsilon <10^{-6}\) Wkg\(^{-1}\). Our results indicate that in general turbulent mixing in the Agulhas is concentrated spatially in pancake-shaped patches. Ordinary Kriging was used to interpolate between consecutive profiles and showed that the horizontal scale of the turbulent patches was typically O(1 km) while the vertical scales of the patches ranged between 0.01 and 10 m.

Article Highlights

  • Direct measurements of turbulent dissipation in the strong persistent Agulhas current are presented for the first time using sensors fitted on a glider and vertical microstructure profiler.

  • Ordinary Kriging revealed the geography of mixing and that turbulent mixing in the Agulhas is concentrated spatially in pancake-shaped patches.

  • VMP data collected at Sodwana showed evidence of elevated dissipation rates \(10^{-8}<\epsilon <10^{-6}\) Wkg\(^{-1}\).

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Acknowledgements

Other: DDS thanks the iSimangaliso Wetland Park Authority for facilitating this research as part of our multi-disciplinary project entitled “Biodiversity & Hydrodynamics of the the iSimangaliso Wetland Park & adjacent Marine Environment” We further scknowledge the following for their assistance with field work: (1) The Slocum glider was supplied/supported by WHOI with on site support from Lou St Laurent, Sean Patrick Whelan & Karan Venayagamoorthy; (2) VMP 250 supplied by Rockland Scientific with on site support from Rolf Lueck & Evan Cervelli; (3) Other UKZN helpers include Dr Stef Corbella, Dr Robynne Chrystal, Calvin Wells, Bright Chifamba, Atish Deoraj, Chris Muledy, Dr M. Kumarasamy; (4) Boat operations provided by SeaXplore Dive Centre—Petro and Christo van Jaarsveld.

Funding

DDS acknowledges funding for this collaborative research project from ONR GLOBAL, NICOP Research Grant N62909-14-1-N249: He thanks ONR project managers Terri Paluszkiewicz and Augustus Vogel for their support. Collaborators on the project were Prof Karan Venayagamoorthy (Colorado State Univ), and Dr Louis St Laurent (WHOI). DDS also thanks eThekwini Municipality for supporting a sponsored Chair at UKZN which provided PG student funding. JJP is supported by the National Research Foundation UID: 127361.

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  1. Department of Civil Engineering, University of KwaZulu Natal, Durban, South Africa

    J. Pringle, D. Stretch & K. Tirok

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Pringle, J., Stretch, D. & Tirok, K. The geography of near-shelf mixing on the east coast of South Africa. Environ Fluid Mech 23, 1145–1165 (2023). https://doi.org/10.1007/s10652-022-09866-1

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