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Roberts M.J.,Oceans and Coasts Research | Roberts M.J.,Rhodes University | Ternon J.-F.,Institute Of Recherche Pour Le Developpement | Morris T.,Bayworld Center for Research and Education
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Sea Level Anomaly (SLA) data were used to track a southward propagating eddy dipole along the western slope of the Mozambique Channel over some 6 months. In April 2005, this dipole (with the cyclone to the south) was close to the continental slope off southern Mozambique. The contact zone between the contra-rotating vortices and the slope was surveyed by ship using onboard (S-)ADCP and CTD lines. The data showed strong (>1.4ms-1) southward (geostrophic) currents over the slope adjacent to the anticyclone with horizontal divergence over the shelf edge. Significant slope upwelling between the dipole and the shelf was evident, concomitant with enhanced nutrient and chlorophyll levels enriching shelf near-surface waters. Satellite observations depicted a 300km long surface chlorophyll filament extending offshore in the frontal zone between the contra-rotating vortices. A satellite-tracked drifter deployed at the coastal base of this filament confirmed the offshore advection of chlorophyll-enriched shelf water, which ultimately wrapped around the cyclone and filling its centre. The slope upwelling was also clearly evident in hourly temperature data collected by a recorder deployed on a nearby reef (Zambia Reef) in a depth of 18m. According to the SLA data, the dipole took several weeks to pass Zambia Reef causing prolonged bouts of upwelling that finally ceased when it left the continental slope and moved southwards into the open ocean. Further analysis showed that lone anticyclones and cyclones against the Mozambique continental shelf also induce slope upwelling as a result of horizontal divergence created by the radial circulation of the vortex. In the case of cyclones, the divergence occurs north of the contact zone. Overall, this case study confirms that eddies moving southwards along the western side of the Mozambique Channel are the main mechanism for pumping nutrients into the otherwise oligotrophic surface waters, and moreover, provide a vigorous mechanism for shelf-open ocean exchange. © 2013 Elsevier Ltd.

Lamont T.,Oceans and Coasts Research | Lamont T.,University of Cape Town | Barlow R.G.,Bayworld Center for Research and Education | Barlow R.G.,University of Cape Town | And 2 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Variability of hydrographic characteristics and phytoplankton distribution associated with mesoscale eddies in the Mozambique Channel was investigated during four cruises in September 2007, December 2008, November 2009, and April/May 2010. Satellite altimetry was used to map the location of mesoscale features, and together with ETOPO1 bathymetry, was used in a Discriminant Function Analysis to classify in situ sampling stations into five categories, namely cyclonic (C), anti-cyclonic (A), frontal (F), divergence (D), and shelf (S). Fluorescence profiles were integrated through four depth ranges in the upper 200. m and used to determine the depth of the maximum chlorophyll a concentration and to model the euphotic zone depth. At a depth of 100. m, distinctly different hydrographic characteristics were observed between mesoscale features with cyclonic eddies consisting of Subtropical Surface Water and anti-cyclonic eddies containing Tropical Surface Water from the Indian Ocean. Hydrographic properties at divergence and frontal stations reflected a mixture of these water masses, while shelf stations showed considerable variability as a result of the interaction of eddies with the continental slope. Chlorophyll a concentrations in the surface waters were found to be low, with subsurface levels being significantly greater. Phytoplankton biomass in cyclonic and anti-cyclonic eddies was relatively low and not significantly different. The interaction of mesoscale eddies with the continental slope on the western side of the Channel caused upwelling of cooler, nutrient-rich water, which resulted in elevated phytoplankton biomass in the shelf regions. Strong currents at the perimeters of these eddies produced offshore advection of the high biomass into the frontal regions. © 2013 Elsevier Ltd.

Ternon J.F.,Institute Of Recherche Pour Le Developpement | Roberts M.J.,Oceans and Coasts Research | Roberts M.J.,Rhodes University | Morris T.,Bayworld Center for Research and Education | And 3 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Circulation and the related biological production have been studied during five cruises conducted in the Mozambique Channel (MZC) between 2005 and 2010. The circulation in the MZC is known to be highly turbulent, favouring enhanced primary production as a result of mesoscale eddy dynamics, and connectivity throughout the Channel due to the variable currents associated with migrating eddies. This paper presents the results of in situ measurements that characterize the horizontal and vertical currents in the surface and subsurface layers (0-500. m). The in situ data were analysed together with the geostrophic eddy field observed from satellite altimeter measurements. Different circulation regimes were investigated, including the "classical" anticyclonic eddy generated at the Channel narrows (16°S), the enhancement of southward migrating eddies by merging with structures (both cyclonic and anticyclonic) formed in the east of the Channel, and the presence of a fully developed cyclonic eddy at the Channel narrows. Comparison between in situ measurements (S-ADCP and velocities derived from surface drifters) and the geostrophic current derived from sea surface height measurements indicated that the latter can provide a reliable, quantitative description of eddy driven circulation in the MZC, with the exception that these currents are weaker by as much 30%. It is also suggested from in situ observation (drifters) that the departure from geostrophy of the surface circulation might be linked to strong wind conditions. Finally, our observations highlight that a-geostrophic currents need to be considered in future research to facilitate a more comprehensive description of the circulation in this area. © 2013 Elsevier Ltd.

Hancke L.,Bayworld Center for Research and Education | Roberts M.J.,Oceans and Coasts Research | Roberts M.J.,Rhodes University | Ternon J.F.,Institute Of Recherche Pour Le Developpement
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

The pattern of surface circulation in the Mozambique Channel was elucidated from the trajectories of 82 satellite-tracked drifters over the period 2000-2010 and complementary satellite-derived altimetry. Overall, the trajectories indicated that anticyclonic activity was mostly observed on the western side of the Channel, with cyclonic activity being more prevalent in the east. A lack of eddy activity was noted in the southeast corner of the Channel (i.e. SW of Madagascar). Drifter behaviour illustrated that surface water from the Comoros Basin, entrained into anticyclonic eddies during formation, can be retained and isolated for months whilst being transported southwards through the Channel. During a tropical cyclone weather event, a drifter was observed to switch between counter-rotating eddies indicating that horizontal mixing of the Ekman layer does occur. The drifters also illustrated and emphasised the flow field and transport between eddies (i.e. the interstitial flow) in the Mozambique Channel. Despite the dominance of southward propagating anticyclones, drifters were able to move north and south through the Channel in the frontal flow field between eddies within periods of 51-207 days. Cross-channel transport in both directions between the Madagascan and Mozambique shelf regions was similarly observed, with time spans of 19-30 days. Surprisingly, drifters from the southern limb of the East Madagascar Current were transported westward across the channel to the Mozambique shelf. This transport was similarly facilitated by the frontal flow field between eddies. It is hypothesised that the frontal zones between eddies and interstitial waters play an important role in distributing biota in the Mozambique Channel. © 2013 Elsevier Ltd.

Henderiks J.,Uppsala University | Winter A.,University of Puerto Rico at Mayaguez | Elbrachter M.,Alfred Wegener Institute for Polar and Marine Research | Feistel R.,Leibniz Institute for Baltic Sea Research | And 3 more authors.
Marine Ecology Progress Series | Year: 2012

Two distinct morphotypes of the coccolithophore Emiliania huxleyi were observed as part of the phytoplankton succession offshore of Namibia, where coastal upwelling created strong gradients in sea surface temperature (SST), salinity, and nutrient conditions. The sampled surface waters hosted a characteristic succession of phytoplankton communities: diatoms bloomed in newly upwelled waters above the shelf, whereas dense coccolithophore communities dominated by E. huxleyi were found farther offshore, in progressively aging upwelled waters. A substantially calcified E. huxleyi morphotype (labeled Type A*) dominated plankton assemblages at stations influenced by upwelling, that immediately succeeded coastal diatom blooms. This morphotype caused a chlorophyll and 19'-hexanoyloxyfucoxanthin (19'-HF) maximum with >1 × 10 6 cells l -1, straddling a pycnocline at 17 m depth where the in situ N:P ratio was ≅13. Farther offshore, within <20 nautical miles distance, populations of Type A* drastically declined, and a more delicate morphotype with thin distal shield elements and open central area (Type B/C) was found. This morphotype was most abundant (~0.2 × 10 6 cells l -1) in high-phosphate, nitrogen-depleted surface waters (N:P ≅ 8), where it co-existed with other coccolithophores, most notably Syracosphaera spp. Extensive surface blooms of coccolithophores observed by satellites in the same region in the past were identified by microscopy as being produced by E. huxleyi and S. pulchra. However, blooms of E. huxleyi at greater depths in the euphotic zone, such as those observed in this study, will go undetected by satellites and thus underestimate coccolithophore biomass and calcification within upwelling regions. © Inter-Research 2012.

Roberts M.J.,Nelson Mandela Metropolitan University | Roberts M.J.,UK National Oceanography Center | Roberts M.J.,Rhodes University | Nieuwenhuys C.,Bayworld Center for Research and Education | And 2 more authors.
African Journal of Marine Science | Year: 2016

Ship-based acoustic Doppler current profiler (S-ADCP) technology, used in survey mode, has enabled near- synoptic views of the in situ 3-D current field in the KwaZulu-Natal (KZN) Bight to be elucidated for the first time. Data acquired by the research vessels RS Africana and RS Algoa in June 2005, September 2007, March 2009 and July 2010 are presented. Each S-ADCP dataset showed similar circulation characteristics whereby the continental slope and outer shelf of the KZN Bight were strongly influenced by the south-westward flowing Agulhas Current. This was particularly evident in the extreme north between Cape St Lucia and Richards Bay where the shelf is narrowest and velocities exceeded 200 cm s−1. The widening of the bight to the south moves the Agulhas Current further from the coast, resulting in a diminishing velocity gradient on the outer shelf which terminates around the midshelf axis. The southern region of the bight was mostly influenced by the Durban cyclonic eddy (Durban Eddy), and in June 2005 and September 2007, by a cyclonic ‘swirl’ that occupied the entire southern half of the KZN Bight, the latter identified by a combination of S-ADCP-, satellite-derived SST- and ocean colour data. Satellite data showed low-chlorophyll offshore water to move into this swirl and northwards along the inner- and midshelf, reaching the Thukela River. Inner-shelf circulation north of the Thukela River was weak (<20 cm s−1) and highly variable. Satellite-tracked surface drogues deployed in the Durban Eddy found their way into the northward coastal current in the KZN Bight, with velocities exceeding 90 cm s−1 at times. The drogues also highlighted the strong influence of wind, especially in the northern bight between Durnford Point and Cape St Lucia, with residence times on the shelf exceeding 14 days, suggesting this region to be of biological importance particularly for recruitment. © 2016 NISC (Pty) Ltd.

Roberts M.J.,Nelson Mandela Metropolitan University | Roberts M.J.,UK National Oceanography Center | Roberts M.J.,Rhodes University | Nieuwenhuys C.,Bayworld Center for Research and Education
African Journal of Marine Science | Year: 2016

New in situ time-series data were acquired by two ADCP moorings placed on the shelf off Richards Bay on the east coast of South Africa at depths of 25 m and 582 m between October 2009 and August 2010. The 11-month inshore bottom-temperature record revealed five substantial upwelling events lasting 5–10 days each where temperatures decreased by about 7 °C to 17–18 °C. Satellite sea surface temperature data showed these events to coincide with cold-water plumes occupying the northern wedge of the KwaZulu-Natal (KZN) Bight. Numerous shorter duration (1–2 days) upwelling events with less vivid surface expressions were also observed throughout the entire record where bottom temperature dropped by 2–3 °C. The last four months of the record were characterised by a protracted cool period lacking a seasonal trend but punctuated with oscillations of warm and cooler bottom water. In contrast to earlier studies that suggested upwelling was topographically and dynamically driven by the juxtaposition of the Cape St Lucia offset and the Agulhas Current (a solitary mechanism), our analysis showed almost all major and minor cold-water intrusions to coincide with upwelling-favourable north-easterly winds that simultaneously force a south-westerly coastal current. Ekman veering in the bottom boundary layer of the Agulhas Current, and the concomitant movement of cold water up the slope, was found to coexist at times with coastal upwelling, but its absence did not impede inshore cold-water intrusions, calling into question its role as a primary driver of upwelling. Both major and minor upwelling events were observed to promote phytoplankton blooms in the northern KZN Bight which commonly extended to the Thukela River. Wind-driven upwelling was also observed in the inner bight between Richards Bay and Port Durnford, explaining the ribbon of coastal chlorophyll continuously observed on ocean colour images between Cape St Lucia and the Thukela River. Similarities in upwelling character and mechanisms are observed between the northern KZN Bight and the Florida Current shelf systems. © 2016 NISC (Pty) Ltd.

Guastella L.A.,Bayworld Center for Research and Education | Guastella L.A.,University of Cape Town | Roberts M.J.,Nelson Mandela Metropolitan University | Roberts M.J.,UK National Oceanography Center | Roberts M.J.,Rhodes University
African Journal of Marine Science | Year: 2016

The semi-permanent Durban Eddy is a mesoscale, lee-trapped, cold-core cyclonic circulation that occurs off the east coast of South Africa between Durban in the north and Sezela, some 70 km to the south. When present, strong north-eastward countercurrents reaching 100 cm s–1 are found inshore. It is hypothesised that the cyclone is driven by the strong south-westward flowing Agulhas Current offshore of the regressing shelf edge near Durban. Analysis of ADCP data and satellite imagery shows the eddy to be present off Durban approximately 55% of the time, with an average lifespan of 8.6 days, and inter-eddy periods of 4 to 8 days. After spin-up the eddy breaks loose from its lee position and propagates downstream on the inshore boundary of the Agulhas Current. The eddy is highly variable in occurrence, strength and downstream propagation speeds. There is no detectable seasonal cycle in eddy occurrence, with the Natal Pulse causing more variability than any seasonal signal. A thermistor array deployed in the eddy centre, together with ship CTD data, indicates upward doming of the thermal structure in the eddy core associated with cooler water and nutrients being moved higher in the water column, stimulating primary production. Together with the use of satellite imagery, our findings indicate a second mechanism of upwelling, viz. divergent upwelling in the northern limb of the eddy. Satellite-tracked surface drifters released in the eddy demonstrated the potential for nutrient-rich eddy water to be transported northwards along the inshore regions of the KwaZulu-Natal (KZN) Bight, thus contributing to the functioning of the bight ecosystem, as well as southwards along the KZN and Transkei coasts – both by the eddy migrating downstream and by eddy water being recirculated into the inshore boundary of the Agulhas Current itself. © 2016 NISC (Pty) Ltd.

Guastella L.A.,University of Cape Town | Guastella L.A.,Bayworld Center for Research and Education | Smith A.M.,University of KwaZulu - Natal
Estuarine, Coastal and Shelf Science | Year: 2014

Webcams have become popular means of showcasing beach conditions for a wide variety of beach users. However, webcams can also be a useful tool in assessing changes in coastal morphology and coastal processes. This information can be used by managers to assist in planning. A number of fixed-position beach webcams are freely available to the South African public via various tourism, surfing, weather and aviation websites, individual clubs and a cell-phone network provider. The advantages of these public networks are that the information is free and as the webcams are fixed, afford a consistent and comparable view of the beach. The disadvantage is that you are at the mercy of the provider: resolution is generally poor, downtime and communication are out of your control, and you have no influence over the positioning of the webcam or the discontinuity of service. Notwithstanding the above, the existing webcams can still provide valuable information. From the network of beach webcams available in South Africa we analyse imagery from three beach webcams located in the province of KwaZulu-Natal, at Umhlanga, Margate beach and lagoon, and Amanzimtoti beach and lagoon to examine the coastal dynamics. From these case studies we illustrate seasonal beach rotation and lagoon mouth dynamics, specifically why outlets migrate southwards in opposition to regional longshore drift. © 2014 Elsevier Ltd.

Roberts M.J.,Oceans and Coasts | Downey N.J.,Bayworld Center for Research and Education | Sauer W.H.,Rhodes University
ICES Journal of Marine Science | Year: 2012

It is well known that the spawning grounds of chokka squid Loligo reynaudii lie along the shallow inshore regions of South Africas south coast. However, egg masses have been found in deeper water on the Agulhas Bank, and hydroacoustic targets deemed to be large aggregations of spawning squid have been identified. The aim of this study was to investigate the extent, depth range, and importance of deep spawning. Trawl data collected during demersal research surveys between Port Nolloth on the west and Port Alfred on the south coast were examined for egg capsules. No spawning was found on the west coast. Data showed that chokka squid preferred the eastern Agulhas Bank for spawning. Spawning occurred not only inshore but also on the mid-shelf extending to depths of 270 m near the shelf edge. Squid egg biomass markedly decreased beyond 70 m, suggesting delineation between the inshore and offshore spawning grounds. Total egg biomass calculations for depths shallower and deeper than 70 m indicated the coastal area to be strongly favoured, i.e. 82 vs. 18. These results contest the commonly accepted notion that L. reynaudii is an inshore spawner and redefine the spawning grounds to extend across the shelf. © 2012 International Council for the Exploration of the Sea. Published by Oxford University Press. All rights reserved.

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