Submesoscale cyclones in the Agulhas current

Krug, Marjolaine; Swart, Sebastiaan; Gula, Jonathan

doi:10.1002/2016gl071006

Cited by 44 publications

(53 citation statements)

References 30 publications

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“…The triggering of the barotropic instability along the Agulhas inshore front is compared between the model and the SAGE data. With the 1‐month long glider dataset, a partial

\overset{H R S}{true}

term can be computed along a cross‐front section at 25.75° E (Krug et al, ). This partial term accounts for the cross‐front derivative subterms:

- \overset{u^{'} v^{'}}{true} \partial_{y} \overset{u}{true} - \overset{v^{'} v^{'}}{true} \partial_{y} \overset{v}{true}

, the overbar denotes a monthly time average, which corresponds to the full SAGE period, and the prime denote fluctuations relative to this time mean.…”

Section: Submesocale Eddies Along the Agulhas Cyclonic Frontmentioning

confidence: 99%

Generation of Submesoscale Frontal Eddies in the Agulhas Current

et al. 2019

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This study addresses the dynamics of the Agulhas inshore front in the submesoscale range upstream of 26° E. Submesoscale frontal eddies are observed in the vicinity of Port Elizabeth (26° E) from satellite images and in observations collected from underwater gliders. Using a submesoscale‐resolving numerical model (dx ∼ 0.75 km), we are able to simulate similar submesoscale eddies. Barotropic instability is confirmed as the generation mechanism by a one‐dimensional linear stability analysis and an eddy kinetic energy budget. Kinetic energy is transferred from the mean flow to the eddies through the mean horizontal shear, which is a signature of barotropic instability. When the Agulhas Current is in a nonmeandering state, submesoscale eddy generation is a recurrent process which locally drives the front's variability. Along the front, the spatial variability of barotropic instability is shaped by the background strain. A large strain aligned with the frontal axis intensifies the frontal shear upstream of 28° E while a weakening of the strain allows for barotropic instability to be triggered downstream. Although an intermittent process, the barotropic instability shows a dominant period of variability comparable with the variability of the Agulhas Current and Undercurrent.

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“…The triggering of the barotropic instability along the Agulhas inshore front is compared between the model and the SAGE data. With the 1‐month long glider dataset, a partial

\overset{H R S}{true}

term can be computed along a cross‐front section at 25.75° E (Krug et al, ). This partial term accounts for the cross‐front derivative subterms:

- \overset{u^{'} v^{'}}{true} \partial_{y} \overset{u}{true} - \overset{v^{'} v^{'}}{true} \partial_{y} \overset{v}{true}

, the overbar denotes a monthly time average, which corresponds to the full SAGE period, and the prime denote fluctuations relative to this time mean.…”

Section: Submesocale Eddies Along the Agulhas Cyclonic Frontmentioning

confidence: 99%

Generation of Submesoscale Frontal Eddies in the Agulhas Current

et al. 2019

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“…Using this jet reference frame has shown that three of the five drifters that leaked into the South Atlantic were in the same location relative to the core. These drifters were in a region of high shear that has been linked to the generation of submesoscale cyclones (Krug et al, ). This supports the idea that smaller cyclonic features play a significant role in Agulhas Leakage; this has been shown by Doglioli et al () who found that cyclonic looping trajectories are responsible for 17.4% of Agulhas Leakage.…”

Section: Discussionmentioning

confidence: 99%

The Use of a Jet Reference Frame to Analyze Drifter Trajectories in the Agulhas Current

et al. 2019

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Studies based on drifters that 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 nonmeandering 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 ship acoustic Doppler current profiler and drifters measurements have pointed toward surface drifters leaking from inshore of the Agulhas Current core in a region of high shear. However, the bias between these velocities is 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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“…The region south of 34°S is a key region for the AC system where small‐scale dynamics can have a large impact on the Agulhas leakage and thus on the global ocean. At the northern boundary of the AC, shear‐edge instabilities lead to the formation of meanders, shear‐edge eddies, filaments, plumes, and submesoscale vorticies (Krug et al, ; Lutjeharms et al, ; Lutjeharms, Boebel, et al, ; Lutjeharms, Penven, et al, ). The warm and salty plumes advected from the northern flank of the AC move either onto the Agulhas Bank or drift northwestward into the South Atlantic Ocean as “Agulhas filaments” (Lutjeharms & Cooper, ).…”

Section: Introductionmentioning

confidence: 99%

Submesoscale Impacts on Mesoscale Agulhas Dynamics

Schubert

Schwarzkopf

Baschek

et al. 2019

J Adv Model Earth Syst

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Mesoscale dynamics of the Agulhas Current system determine the exchange between the Indian and Atlantic oceans, thereby influencing the global overturning circulation. Using a series of ocean model experiments compared to observations, we show that the representation of mesoscale eddies in the Agulhas ring path improves with increasing resolution of submesoscale flows. Simulated submesoscale dynamics are validated with time‐mean horizontal‐wavenumber spectra from satellite sea surface temperature measurements and mesoscale dynamics with spectra from sea surface height. While the Agulhas ring path in a nonsubmesoscale‐resolving (1/20)° configuration is associated with too less power spectral densities on all scales and too steep spectral slopes, the representation of the mesoscale dynamics improves when the diffusion and the dissipation of the model are reduced and some small‐scale features are resolved. Realistic power spectral densities over all scales are achieved when additionally the horizontal resolution is increased to (1/60)° and a larger portion of the submesoscale spectrum is resolved. Results of an eddy detection algorithm applied to the model outputs as well as to a gridded sea surface height satellite product show that in particular strong cyclones are much better represented when submesoscale flows are resolved by the model. The validation of the submesoscale dynamics with sea surface temperature spectra provides guidance for the choice of advection schemes and explicit diffusion and dissipation as well as for further subgrid‐scale parameterizations. For the Agulhas ring path, the use of upstream biased advection schemes without explicit diffusion and dissipation is found to be associated with realistically simulated submesoscales.

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Submesoscale cyclones in the Agulhas current

Cited by 44 publications

References 30 publications

Generation of Submesoscale Frontal Eddies in the Agulhas Current

Generation of Submesoscale Frontal Eddies in the Agulhas Current

The Use of a Jet Reference Frame to Analyze Drifter Trajectories in the Agulhas Current

Submesoscale Impacts on Mesoscale Agulhas Dynamics

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