Spatio-temporal characteristics of Agulhas leakage: a model inter-comparison study

Abstract

Investigating the variability of Agulhas leakage, the volume transport of water from the Indian Ocean to the South Atlantic Ocean, is highly relevant due to its potential contribution to the Atlantic Meridional Overturning Circulation as well as the global circulation of heat and salt and hence global climate. Quantifying Agulhas leakage is challenging due to the non-linear nature of this process; current observations are insufficient to estimate its variability and ocean models all have biases in this region, even at high resolution . An Eulerian threshold integration method is developed to examine the mechanisms of Agulhas leakage variability in six ocean model simulations of varying resolution. This intercomparison, based on the circulation and thermohaline structure at the Good Hope line, a transect to the south west of the southern tip of Africa , is used to identify features that are robust regardless of the model used and takes into account the thermohaline biases of each model. When determined by a passive tracer method, 60 % of the magnitude of Agulhas leakage is captured and more than 80 % of its temporal fluctuations, suggesting that the method is appropriate for investigating the variability of Agulhas leakage. In all simulations but one, the major driver of variability is associated with mesoscale features passing through the section. High resolution (\({<} 1/10^{\circ }\)) hindcast models agree on the temporal (2–4 cycles per year) and spatial (300–500 km) scales of these features corresponding to observed Agulhas Rings. Coarser resolution models (\({<} 1/4^{\circ }\)) reproduce similar time scale of variability of Agulhas leakage in spite of their difficulties in representing the Agulhas rings properties. A coarser resolution climate model (\(2^{\circ }\)) does not resolve the spatio-temporal mechanism of variability of Agulhas leakage. Hence it is expected to underestimate the contribution of Agulhas Current System to climate variability.

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Acknowledgments

The authors are grateful to the DRAKKAR group for providing NEMO ORCA12 and ORCA025 outputs’ Claude Talandier (CNRS, LPO) and Juliette Mignot (IRD, LOCEAN) for making the outputs of ORCA025 and IPSL-CM5 simulations available. Dr Backeberg is jointly supported by the Nansen-Tutu Centre for Marine Environmental Research, the Nansen Environmental and Remote Sensing Center, Bergen, Norway, and the South African National Research Foundation through the Grants 87698 and 91426. The HYCOM simulation was developed with a grant for computer time from the Norwegian Program for supercomputing (NOTUR Project Number nn2993k). The financial assistance of the South African Environmental Observation Network (SAEON) towards this research is acknowledged. Opinions expressed and conclusions arrived at are those of the author and are not necessarily to be attributed to SAEON. The primary authors is grateful to UCT for assisting with funding the MSc research that this paper stems from.

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Holton, L., Deshayes, J., Backeberg, B.C. et al. Spatio-temporal characteristics of Agulhas leakage: a model inter-comparison study. Clim Dyn 48, 2107–2121 (2017). https://doi.org/10.1007/s00382-016-3193-5

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Keywords

  • Agulhas system
  • Ocean variability
  • Agulhas leakage
  • Climate
  • Model inter-comparison