Browsing by Author "Krug, Marjolaine"

Now showing 1 - 8 of 8
  • Thumbnail Image
    Item
    Open Access
    A study of Mesoscale Eddies, the Agulhas current and the evolution of its meanders using satellite observations and numerical modelling experiments
    (2019) Braby, Laura; Backeberg Björn; Krug, Marjolaine; Reason, Christopher
    The Agulhas Current is the strongest western boundary current in the Southern Hemisphere and plays an important role in the exchange of heat and salt between the Indian and South Atlantic Ocean basins, thereby affecting global climate. The variability in the northern Agulhas Current is influenced by both cyclonic and anticyclonic mesoscale eddies, originating from the Mozambique Channel and south of Madagascar (known as source region eddies) and which propagate toward the offshore edge of the Agulhas Current. Using a combination of an eddy-tracking data set with in-situ surface drifter observations and altimetry-derived geostrophic currents, it is shown that source region eddies dissipate upon approaching the Agulhas Current. Their entrainment into the Agulhas Current affects its mean velocity and offshore position through a transfer of momentum, with anti-cyclonic eddies consistently increasing the Agulhas Current’s velocity by 0.16 ± 0.17 m.s -1 . In contrast, entrainment of cyclonic eddies results in a decrease in velocity by 0.13 ± 0.16 m.s-1 and shifting the current up to 144 ± 85 km offshore. These velocity anomalies propagate downstream at rates of 44 km.d-1 (anti-cyclonic eddies) and 23 km.d-1 (cyclonic eddies). Whilst 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 in the current. In this study, we compare two simulation experiments in a regional Hybrid Coordinate Ocean Model (HYCOM), where we change the wind forcing, and using an eddy tracking algorithm assess 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) of 33% over the Agulhas Current domain. Changes in eddy pathways, properties and energy conversion terms, 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 eddies in the regional HYCOM. The effects of the change in wind forcing on the variability within the Agulhas Current were examined and the differences between the two simulations were found to be very small. Finally, the evolution of meanders in the Agulhas Current, including the properties and dissipation of smaller meanders as well as mesoscale Natal Pulses type meanders, were assessed using both HYCOM experiments and compared to satellite observations. The representation of smaller meanders (under 50km in size) improved with the changed in wind forcing. However, larger Agulhas Current meanders (greater than or equal to 50km) which previously occurred too frequently in the regional HYCOM, are now too infrequent in the regional HYCOM, with an average of 1.1 meanders occurring each year. A decrease in the frequency of larger meanders was observed from the location offshore of Port Edward (30.22° E, 31.05° S) to the region of the ACT array (27.48° E, 33.35° S), in the satellite data as well as both model experiments, indicating that some of the meanders have dissipated and that both regional HYCOM models are able to resolve this.
  • Thumbnail Image
    Item
    Open Access
    Annual cycle of the Benguela Jet
    (2014) Kamwi, Blessing K; Veitch, Jennifer Anne; Hermes, Juliet C; Krug, Marjolaine
    The Benguela Jet is a north-westward flowing current in the southern Benguela region. It is known to have an important influence on the fish recruitment yet little is known about the physical properties of the jet. In this study the ability of satellite data (SST and altimetry) to resolve the Jet was investigated. Following this, the annual cycle of the Benguela Jet was investigated using monthly climatological means computed from both remotely sensed and model output data (from the Regional Ocean Modelling System). Two altimeter tracks were identified as best suited to study the Benguela Jet: the Topex-A / Jason-1A Track number 209 which crosses the Benguela Jet current in its northern region off the Cape Columbine and the Topex-B / Jason-1B track number 31, which crosses the Benguela Jet current in the southern region. The month of January and July were chosen due to the fact that they represent the peaks of summer and winter. The surface geostrophic currents derived from both the model and satellite data reproduced the existence of the jet current off Cape Columbine and the Cape Peninsula. The jet was narrow and strong in January off Cape Columbine and off the Cape Peninsula and was situated farther offshore based on the model, relative to the altimeter data. Outputs from the numerical model showed that in July the jet was confined to the coast and was stronger off the Cape Peninsula (0.5 m.s-1) compared to Cape Columbine (0.4 m.s-1). A comparison between the regions of strong velocity gradient and the position of the upwelling front were in agreement in depicting the position of the jet. Altimetry, which suffers from imitation in coastal regions, could not reveal the jet in July due to its proximity to the shore at this season. The offshore boundary of the jet is resolved by altimetry in January. The interannual variability of the Benguela Jet has been identified.
  • Thumbnail Image
    Item
    Open Access
    Circulation through the mouth of Langebaan Lagoon and implications
    (1999) Krug, Marjolaine; Largier, John
    In March 1997 a two-weeks field survey was conducted in Langebaan Lagoon and Saldanha Bay. The aim of this survey was to farther our understanding of the processes driving the mixing and the exchange at the Langebaan Lagoon-Saldanha Bay interface. The parameters measured included currents, water-levels, temperature, salinity, density and wind. The nature of the flow at the Langebaan Lagoon inlets was ascertained by combining statistical analysis of the measurements to a theoretical understanding of the system hydrodynamics. The flow in the vicinity of the straight was predominantly driven by the tide. It was found that during high tidal range periods, there existed an asymmetry between the ebb and the flood flows at both of the lagoon's inlets. When tidal forcing was strong, water particles released at the lagoon inlets during the ebb were subject to long drifts. The outflow from the east inlet appeared to take the form of a turbulent jet. At the west inlet strong frictional interactions between the flow and land boundaries occurred, causing the flow to rapidly expand and lose momentum and therefore impeding the formation of a jet. It was established that, generally, buoyancy forcing on the Langebaan Lagoon outflow would be small and that water issuing from the lagoon during the ebb would remained attached to the sea-bed as it propagated into Saldanha Bay. However, when Saldanha Bay was strongly stratified, the east inlet ebb jet would lift off from the bottom as it reached the 8m depth contour. The large drifts resulting from the sink-like nature of the inflow and the jet-like nature of the outflow induced a very rapid and strong exchange between Langebaan Lagoon and Saldanha Bay. The propagation of the lagoon effluent also contributed extensively to vertically stir the water-column in Big Bay. As the tidal range weakened, the regions of influence of the ebb and the flood overlapped to a greater extent and the exchange between the lagoon and the bay decreased significantly. The asymmetry between the ebb and the flood flows at the Langebaan Lagoon inlets generated a Lagrangian residual circulation, with the east inlet constituting the entrance for Saldanha Bay water, while the west inlet would be the exit route for Langebaan Lagoon water. Southerly winds, contributed to the overall residual circulation by driving water out of the Lagoon. Bibliography: 126-133 pages.
  • Thumbnail Image
    Item
    Open Access
    The effect of the Agulhas Current on synthetic aperture radar derived wind fields
    (2016) Schilperoort, Daniel E; Krug, Marjolaine; Rouault, Mathieu; Hansen, Morten
    In this study, 5 years (987 swaths) of high resolution wind speeds, derived from Advanced Synthetic Aperture Radar data collected over the Agulhas Current region, are studied to investigate the effect of warm, high intensity currents on the ocean's surface roughness and resulting derived wind fields. The wind data are derived using the CMOD5.n GMF with CFS reanalysis wind data as direction input. The CFS direction data are validated using ASCAT derived wind observations Globcurrent ocean current velocity data is used to investigate the difference between the satellite derived wind speeds compared to surface velocities of the current and the true wind speed. The, so called, current-relative effect is investigated for different wind direction regimes, namely: upcurrent, downcurrent, crosscurrent west and crosscurrent east. Our analyses are conducted for 6 locations of interest, evenly spaced along the Northern Agulhas Current. MODIS, SEVIRI and OSTIA SST data are used as proxy for locating the core of the Agulhas and it's temperature fronts, as well as to investigate wind speed modifications as a result of ocean-atmosphere energy transfer. It is found that higher resolution SAR derived winds have a greater ability to represent higher intensity and smaller scale wind features in comparison to winds derived from Scatterometers. A combination of the current relative effect and SST-atmospheric heating for upcurrent wind directions results in a sharp increase in mean wind speeds over the inshore boundary of the current of between 5m/s and 7m/s (50−60%). Individual events can reach as high as 15m/s (100%) over 10′s of kilometres. For downcurrent winds, the expected current relative effect is overridden by increased wind speeds of up to 5m/s (40%) across the entire current due to the influence of SSTs. The mean effect of SSTs on wind speeds has a stronger effect than the current relative effect on wind speed changes over the current. The wind speed differences are best represented under moderate wind speeds, between 5−15m/s. This investigation will contribute to future satellite wind speed derivations to identifying new wind speed and surface roughness altering effects. It will also serve to increase understanding of high resolution wind features and sharp changes over ocean features.
  • Thumbnail Image
    Item
    Open Access
    Impact of Wind Driven Variability on Sea Surface Temperature and Ocean Colour in False Bay
    (2019) Seymour, Sian; Krug, Marjolaine; Smith, Marie; Mouche, Alexis; Rouault, Mathieu
    False Bay is the largest true bay in South Africa and is an important area for conservation, the local fishing industry and marine based recreational activities. A large amount of studies, both recent and historical, have been carried out on the biology of the bay, but studies on the physics of the bay are very few in comparison. In this study high resolution satellite imagery is used to investigate wind variability and its impact on sea surface temperature (SST) and chlorophyll concentration (Chl-a) variability within False Bay and the Cape Peninsula region. High resolution (1 km) coastal winds derived from the Sentinel-1 satellite Synthetic Aperture Radar (SAR) show that winds are strongly influenced by topography under the predominantly south-easterly wind regime. The Hottentots Holland mountain range and Cape Peninsula mountain range create wind shadows as well as areas of increased wind speed within False Bay and west of the Cape Peninsula. Our observations also show that global atmospheric models, such as ECMWF, are not able to capture the spatial variability in the wind fields driven by the orography. Analyses of the SST and ocean colour imagery show that wind shadows are generally associated with warmer surface waters and higher Chl-a. In contrast, regions of enhanced wind speeds show colder surface waters and decreased chlorophyll concentration. Our results suggest that spatial variation in the horizontal wind fields have direct and significant impact on the water properties within False Bay. This study highlights the need for high resolution wind observations and simulations to force regional oceanic models of False Bay and the Cape Peninsula region.
  • Thumbnail Image
    Item
    Open Access
    Resolving cross-shelf dynamics in the Agulhas Current from GlobCurrent and glider observations
    (2019) Maja, Tumelo; Krug, Marjolaine; Rouault, Mathieu; Johannessen, Johnny A
    The Agulhas Current is the strongest Western Boundary Current of the Southern Hemisphere and it plays a significant role in the circulation of the shelf and coastal waters, whereby mesoscale (50- 500 km) and submesoscale (1 -10 km) instabilities in the Agulhas Current impact the local oceanography of the shelf region. The main objective of this study is to evaluate the ability of a gap-free and merged gridded satellite ocean current dataset, GlobCurrent, to resolve and monitor the variability of the Agulhas Current’s cross-shelf dynamics. In this study, GlobCurrent is compared to in-situ observations collected from underwater gliders through mapping and correlation analysis to assess the product’s accuracy in different subdomains and water depths of the Agulhas Current’s main area domain. We also investigate the value of using a higher resolution satellite and gap-free Sea Surface Temperature (SST) dataset to complement the GlobCurrent dataset in observing the Agulhas Current’s flow processes and features. The results show that GlobCurrent is adequate for describing large mesoscale features and deep water flows but the product has limitations in capturing fast-evolving and small mesoscale features, particularly the Durban Eddy in the KZN bight region. GlobCurrent also exhibits, at times, directional errors in addition to the current speed discrepancies. This research study demonstrates the limitation of the GlobCurrent product for monitoring ocean current variability in shallow, coastal waters and regions dominated by small mesoscale variability. This study also provides new insights on the joint use of other merged satellite products i.e. merged ODYSSEA SST, which may compensate for some of the GlobCurrent product’s shortfalls. Future studies should consider complementing altimetry-based satellite products like GlobCurrent with other merged satellite observation products such as ODYSSEA SST for better imaging of small mesoscale processes and features in shallow coastal waters.
  • Thumbnail Image
    Item
    Open Access
    Surface circulation in the KwaZulu-Natal Bight and its impact on the connectivity of marine protected areas
    (2022) Heye, Sonia; Krug, Marjolaine; Hart-Davis, Michael; Penven, Pierrick; Rouault, Mathieu
    The KwaZulu-Natal Bight is a small, coastal region along South Africa's north-east coast. It stretches from Richards Bay to Durban and has a wide shelf compared to the surrounding coastline. As a result, the Agulhas Current is forced offshore, allowing the formation of complex circulation features on the KwaZulu-Natal Bight's shelf that assist with recruitment and retention of marine organisms in this region. This study aims to gain a deeper understanding of the surface circulation within the KwaZulu-Natal Bight and its impact on the connectivity between several surrounding Marine Protected Areas. These include iSimangaliso, uThukela Banks, Aliwal Shoal and Protea Banks and the information about their connectivity contributes to the CAPTOR (Connectivity And disPersal beTween prOtected aReas) project. The aim of this study is met by using high-resolution CROCO model output over a 10-year period, in combination with particle tracking tools, wind and surface drifter data. According to the model's mean circulation, the KwaZulu-Natal Bight's surface currents have a strong south-westward flow on the continental shelf slope where the effects of the Agulhas Current are strongly felt, butare weak and variable on the shelf. Observed variabilities of the mean flow have no distinct seasonal pattern and include a north- eastward current that repeatedly dominates the shelf. It is referred to as the Natal Bight Coastal Counter Current, which originates within the semi-permanent Durban Eddy in the southern KwaZulu- Natal Bight, where it extends throughout the water column. The Natal Bight Coastal Counter Current stretches along the mid-shelf into the northern KwaZulu-Natal Bight, gradually becoming shallower, weaker and narrower. When anticyclonic eddies offshore of the Agulhas Current pass this region, they occasionally replace the Durban Eddy and its associated Natal Bight Coastal Counter Current with a southward flow on the KwaZulu-Natal Bight's shelf. Therefore, the circulation in the KwaZulu-Natal Bight appears to be primarily driven by perturbations at the Agulhas Current front. However, there is also some indication of a direct wind-driven influence in coastal waters inshore of the 50 m isobath. To investigate the impact of the KwaZulu-Natal Bight's circulation on the connectivity between the above-mentioned Marine Protected Areas, particle tracking tools are used. Virtual particles are released in each Marine Protected Area within the model, during multiple northward and southward KwaZulu-Natal Bight surface circulation events. Their pathways are tracked for 30 days and reveal an overall strong southward Marine Protected Area connectivity, which is driven by the Agulhas Current, while a northward connection is less commonly observed. The northward flow of the Natal Bight Coastal Counter Current increases the water retention within uThukela Banks, but it does not extend into iSimangaliso to establish a northward Marine Protected Area connection. However, when the Natal Bight Coastal Counter Current originates within Aliwal Shoal, it may result in a northward Marine Protected Area connection between Aliwal Shoal and uThukela Banks. In this study, the virtual particles represent passively drifting larvae that are buoyant. To make these simulations more realistic, the virtual particles should be able to sink and appropriate swimming behaviours could be considered. However, swimming abilities will likely be overpowered by the surrounding circulation and observations on these behaviours are difficult to make. Therefore, the passive dispersion used in this study to mimic their trajectories may be sufficient and provides valuable insight into the impact of the KwaZulu-Natal Bight's surface circulation on Marine Protected Area connectivity and larval dispersion. The virtual particle tracking tools used in this study are not limited to biological applications. Future studies could use them to investigate the path and accumulation regions of virtual pollutants, such as microplastics, to determine the regions in which clean-ups would be most effective.
  • Thumbnail Image
    Item
    Open Access
    Variability of coastal upwelling south of Madagascar
    (2018) Ramanantsoa, Heriniaina Juliano Dani; Krug, Marjolaine; Rouault, Mathieu; Penven, Pierrick
    Madagascar’s southern coastal marine zone is a region of high biological productivity which supports a wide range of marine ecosystems, including fisheries. This high biological productivity is attributed to coastal upwelling. The thesis seeks to characterise the variability of the coastal upwelling south of Madagascar. The first part of the thesis provides new insights on the structure, variability and drivers of the coastal upwelling south of Madagascar. Satellite remote sensing is used to characterize the spatial extent and strength of the coastal upwelling. A front detection algorithm is applied to thirteen years of Multi-scale Ultra-high Resolution (MUR) Sea Surface Temperatures (SST) and an upwelling index is calculated. The influence of winds and ocean currents as drivers of the upwelling are investigated using satellite, in-situ observations, and a numerical model. Results reveal the presence of two well-defined upwelling cells. The first cell (Core 1) is located in the southeastern corner of Madagascar, and the second cell (Core 2) is west of the southern tip of Madagascar. These two cores are characterized by different seasonal variability, different intensities, different upwelled water mass origins, and distinct forcing mechanisms. Core 1 is associated with a dynamical upwelling forced by the detachment of the East Madagascar Current (EMC), which is reinforced by upwelling favourable winds. Core 2 which appears to be primarily forced by upwelling favourable winds, is also influenced by a poleward eastern boundary flow coming from the Mozambique Channel. This intrusion of Mozambique Channel warm waters could result in an asynchronicity in seasonality between upwelling surface signature and upwelling favourables winds. The second part of the thesis focuses on the interaction between the intrusion of warm water from Mozambique channel and the upwelling cell in Core 2. Cruise datasets, satellite remote sensing observations and model data analyses are combined to highlight the existence of a coastal surface poleward flow in the south-west of Madagascar: the South-west MAdagascar iv Coastal Current (SMACC). The SMACC is a relatively shallow (Coastal Current (SMACC). The SMACC is a relatively shallow (<300 m) and narrow (<100km wide) warm and salty coastal surface current, which flows along the south western coast of Madagascar toward the south, opposite to the dominant winds. The warm water surface signature of the SMACC extends from 22◦S (upstream) to 26.4◦S (downstream). The SMACC exhibits a seasonal variability: more intense in summer and reduced in winter. The average volume transport of its core is about 1.3 Sv with a mean summer maximum of 2.1 Sv. It is forced by a strong cyclonic wind stress curl associated with the bending of the trade winds along the southern tip of Madagascar. The SMACC directly influences the coastal upwelling regions south of Madagascar. Its existence is likely to influence local fisheries and larval transportpatterns, as well as the connectivity with the Agulhas Current, affecting the returning branch of the global overturning circulation. The last part of the thesis provides a holistic understanding of the inter-annual variability of the upwelling cells associated with the multiple forcing mechanisms defined in the first two parts of this work. Results reveal that the upwelling cells, Core 1 and Core 2, have different inter-annual variabilities. Inter-annual variability of Core 1 is associated with the East Madagascar Current (EMC) while Core 2 is linked with the South-west MAdagascar Coastal Current (SMACC). Inter-annual changes in the EMC occur as a result of oscillations in the South Equatorial Current (SEC) bifurcation off Madagascar, while the inter-annual variability in the SMACC is influenced by the cyclonic wind stress curl inter-annual variability. The upwelling is also linked with global/regional climate modes. Both Cores are highly correlated with the Subtropical Indian Ocean Dipole (SIOD). Core 2 is also correlated to the Indian Ocean Dipole (IOD). Both cores are significantly correlated with the El Ni˜no-Southern Oscillation (ENSO) after 12 months lag.