Observed eddy dissipation in the Agulhas Current

Braby, Laura; Backeberg, Björn; Ansorge, Isabelle; Roberts, Michael J.; Krug, Marjolaine; Reason, C. J. C.

doi:10.1002/2016gl069480

Cited by 20 publications

(27 citation statements)

References 32 publications

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“…These findings are in agreement with previous investigations which have shown that the characteristics of eddies within the region are well described by altimetry data [8,9,33], and the properties of the MB eddy, as well as the interactions with surrounding eddies, are similar to those defined in existing literature [3,[6][7][8][9][10][11]. Our observations also agree with a more recent study that demonstrated the dissipation of eddies upon reaching the Agulhas Current due to eddy-current interactions [34].…”

Section: Discussionsupporting

confidence: 93%

Variations in Remotely-Sensed Phytoplankton Size Structure of a Cyclonic Eddy in the Southwest Indian Ocean

2018

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Phytoplankton size classes were derived from weekly-averaged MODIS Aqua chlorophyll a data over the southwest Indian Ocean in order to assess changes in surface phytoplankton community structure within a cyclonic eddy as it propagated across the Mozambique Basin in 2013. Satellite altimetry was used to identify and track the southwesterly movement of the eddy from its origin off Madagascar in mid-June until mid-October, when it eventually merged with the Agulhas Current along the east coast of South Africa. Nano-and picophytoplankton comprised most of the community in the early phase of the eddy development in June, but nanophytoplankton then dominated in austral winter (July and August). Microphytoplankton was entrained into the eddy by horizontal advection from the southern Madagascar shelf, increasing the proportion of microphytoplankton to 23% when the chlorophyll a levels reached a peak of 0.36 mg·m −3 in the third week of July. Chlorophyll a levels declined to <0.2 mg·m −3 in austral spring (September and October) as the eddy propagated further to the southwest. Picophytoplankton dominated the community during the spring period, accounting for >50% of the population. As far as is known, this is the first study to investigate temporal changes in chlorophyll a and community structure in a cyclonic eddy propagating across an ocean basin in the southwest Indian Ocean.

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Section: Discussionsupporting

confidence: 93%

Variations in Remotely-Sensed Phytoplankton Size Structure of a Cyclonic Eddy in the Southwest Indian Ocean

2018

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“…The distribution of eddy birth points is similar to a previous study (Figure b of Braby et al, ), and the majority of the observed and simulated eddies passing through north of the Natal Bight is from the SEMC (Figure ). It is found that in the AVISO (OFES), 33 (40) eddies originate from the SEMC, while 5 (15) eddies are from the MC (Table ).…”

Section: Sources Of the Natal Pulsesupporting

confidence: 85%

“…In‐situ observations support the idea that barotropic instability due to the horizontal shear of the Agulhas Current plays a major role in the formation of the Natal Pulse (Elipot & Beal, ). Also, several studies reported the periodicity and evolution of the Natal Pulse (Rouault & Penven, ), and the role of cyclonic eddies on the strength of meanders (Braby et al, ). Although many efforts have been devoted to modeling of the Agulhas Current, recent sophisticated ocean models still exaggerate eddy activities offshore of the Agulhas Current core (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Although many efforts have been devoted to modeling of the Agulhas Current, recent sophisticated ocean models still exaggerate eddy activities offshore of the Agulhas Current core (e.g. Backeberg et al, 2014;Durgadoo et al, 2013;Loveday et al, 2014;Siedler et al, 2009), and are unable to reproduce observed eddy dissipations (Braby et al, 2016). Tournadre (2012) reported an increase in the annual number of Natal Pulses detected near 34°S after 2001.…”

Section: Introductionmentioning

confidence: 99%

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Interannual Variability of the Natal Pulse

2019

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Satellite altimetry data and high-resolution model output are used to investigate the mechanism of interannual variations of the solitary meander in the Agulhas Current, called the Natal Pulse. It is shown that most of the Natal Pulses are triggered by anticyclonic eddies originating from the Southeast Madagascar Current (SEMC) and anticyclonic eddies of the Mozambique Channel are the secondary source. In addition to the barotropic conversion suggested by previous studies, the advection of eddy kinetic energy (EKE) plays a statistically significant role in the growth of the Natal Pulse. The number of anticyclonic eddies arriving at the Natal Bight largely explains the interannual variations in the Natal Pulse. The barotropic conversion from the SEMC, whose interannual variations are likely related to the remote wind forcing, is the main EKE source and corresponds well with the interannual variations of anticyclonic eddies generated off the southern coast of Madagascar. It is also shown that the interannual variations of the Natal Pulse are linked to large-scale wind variability in the southern Indian Ocean.Plain Language Summary Natal Pulse is the large meander of the Agulhas Current originating in the Natal Bight and it affects both the local marine environment and global ocean circulation. Previous observational studies suggested that the occurrence of the Natal Pulse varies from year-to-year. Since oceanic eddies tend to trigger the Natal Pulse, we investigate the relationship between the Natal Pulse and incoming eddies in this study. We show that variations of the Natal Pulse are mainly explained by variations of eddies propagating from the south of Madagascar and the Mozambique Channel. We also show that variations in the Natal Pulse are linked to the large-scale wind variability over the southern Indian Ocean. This study is the first to show the mechanism of the year-to-year variations in the Natal Pulse.

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“…The first simulation experiment is forced by absolute winds; the second is forced by relative winds (the wind speed relative to the surface current speed). It is anticipated that through the process of “mechanical damping,” a reduced EKE would enable a more accurate representation of cyclonic and anticyclonic eddy dynamics in the source region of the Agulhas Current (a different region to the study by Renault, Molemaker, Mcwilliams, et al, , which focuses on the simulation of eddies in the California Upwelling System) and that the eddy‐current dissipation processes shown by Braby et al () would be better simulated in the regional HYCOM. In addition to this, a detailed investigation of the eddy field in the source region of the Agulhas Current is undertaken.…”

Section: Introductionmentioning

confidence: 99%

Quantifying the Impact of Wind‐Current Feedback on Mesoscale Variability in Forced Simulation Experiments of the Agulhas Current Using an Eddy‐Tracking Algorithm

et al. 2020

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The variability in the source regions of the Agulhas Current is influenced by mesoscale eddies.While existing numerical models are successfully able to capture many aspects of the Agulhas Current, many models are unable to accurately represent the observed eddy dissipation and interaction processes, affecting our understanding of mesoscale variability within the current. In this study, we compare two forced simulation experiments in a regional Hybrid Coordinate Ocean Model (HYCOM), where we change the wind forcing, and using an eddy-tracking algorithm quantify the local effect of the changed wind stress on source region eddies and their interaction with the northern Agulhas Current. There is an overall reduction in eddy kinetic energy (EKE) over the domain, and the representation of Agulhas Current velocities are improved. This study shows that the contribution of meanders and small eddies to the total EKE of the region is greater than the EKE contribution from tracked mesoscale eddies. Changes in eddy properties resulting from the change in forcing from absolute to relative winds (the wind speed relative to the current speed) have resulted in significantly different mesoscale eddy properties. Finally, the evolution of meanders in the Agulhas Current, including the frequency and dissipation of smaller meanders and Natal Pulse type meanders, was assessed using HYCOM experiments and compared to satellite observations. The simulated frequency of smaller meanders compares more favorably to the observations with the change in wind forcing. However, larger Agulhas Current meanders are now too infrequent in the regional HYCOM. Plain Language Summary Variability in the Agulhas Current is influenced by eddies forming inthe Mozambique Channel and south of Madagascar. Many numerical models are not able to accurately simulate the interaction of these eddies with the Agulhas Current. Recently, coupled ocean-atmosphere models have shown that the inclusion of current feedback to the atmosphere has a damping effect on mesoscale eddies. Here we compare two forced Hybrid Coordinate Ocean Model (HYCOM) simulations to determine the effect of a change in wind forcing on source region eddies and their interaction with the northern Agulhas Current. The first simulation is forced with absolute winds, while the second simulation is forced with relative winds (the wind speed relative to the current speed). In this study we show that the representation of the Agulhas Current is improved with a relative wind forcing. There is a reduction in eddy kinetic energy over the domain, and it is demonstrated that beyond the large mesoscale eddies, other processes such as meanders, small eddies, and short-lived eddies contribute extensively to the total eddy kinetic energy budget. The change in wind forcing in the regional HYCOM has reduced the strength of mesoscale eddies and also resulted in fewer Agulhas Current meanders.

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Observed eddy dissipation in the Agulhas Current

Cited by 20 publications

References 32 publications

Variations in Remotely-Sensed Phytoplankton Size Structure of a Cyclonic Eddy in the Southwest Indian Ocean

Variations in Remotely-Sensed Phytoplankton Size Structure of a Cyclonic Eddy in the Southwest Indian Ocean

Interannual Variability of the Natal Pulse

Quantifying the Impact of Wind‐Current Feedback on Mesoscale Variability in Forced Simulation Experiments of the Agulhas Current Using an Eddy‐Tracking Algorithm

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