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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Nov 2018

Research article | 12 Nov 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Ocean Science (OS).

Investigating the relationship between volume transport and sea surface height in a numerical ocean model

Estee Vermeulen1,2, Björn Backeberg2,3,4, Juliet Hermes1,5, and Shane Elipot6 Estee Vermeulen et al.
  • 1Department of Oceanography, University of Cape Town, Rondebosch, South Africa
  • 2Nansen-Tutu Centre for Marine Environmental Research, University of Cape Town, South Africa
  • 3CSIR, Coastal Systems Research Group, Stellenbosch, South Africa
  • 4Nansen Environmental and Remote Sensing Centre, Bergen, Norway
  • 5South African Environmental Observation Network, Egagasini Node, Cape Town, South Africa
  • 6Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149

Abstract. The Agulhas Current Time-series mooring array (ACT) measured transport of the Agulhas Current at 34° S for a period of 3 years. Using along-track satellite altimetry data directly above the array, a proxy of Agulhas Current transport was developed based on the relationship between cross-current sea surface height (SSH) gradients and the measured transports. In this study, the robustness of the proxy is tested within a numerical modelling framework, using a 34-year long regional-hindcast simulation from the Hybrid Coordinate Ocean Model (HYCOM). Two reference proxies were created using HYCOM data from 2010–2013, extracting model data at the mooring positions and along the satellite altimeter track for; (1) the box transport (Tbox) and (2) the jet (southwestward) transport (Tjet). Next, sensitivity tests were performed where the proxy was recalculated from HYCOM for (1) a period where the modelled vertical stratification was different compared to the reference proxy, and (2) different lengths of periods: 1, 3, 6, 12, 18 and 34 years. Compared to the simulated (native) transports, it was found that the HYCOM proxy was more capable of estimating the box transport of the Agulhas Current compared to the jet transport. The HYCOM configuration in this study contained exaggerated levels of offshore variability in the form of frequently-impinging baroclinic anticyclonic eddies. These eddies consequently broke down the linear relationship between SSH slope and vertically-integrated transport, resulting in stronger correlations for the inshore linear regression models compared to the ones offshore. Vertically-integrated transport estimates were therefore more accurate inshore than those offshore or when the current was in a meandering state. Results showed that calculating the proxy over shorter or longer time periods in the model did not significantly impact the skill of the Agulhas transport proxy, suggesting that 3-years was a sufficiently long time-period for the observation based transport proxy.

Estee Vermeulen et al.
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Estee Vermeulen et al.
Estee Vermeulen et al.
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Short summary
This modelling study aimed to recreate the Agulhas transport proxy within a regional HYCOM of the Agulhas Current System, attempting to test the validity of the underlying assumptions on which the satellite-altimeter proxy was based. Results showed that the proxy is sensitive to subsurface variability in the model but that the proxy remained robust regarding the time periods needed to build a sufficient linear relationship between transport and SSH slope.
This modelling study aimed to recreate the Agulhas transport proxy within a regional HYCOM of...