Browsing by Author "Lutjeharms, J R E"

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    A review of migratory behaviour of sea turtles off southerneastern Africa
    (2006) Luschi, P; Lutjeharms, J R E; Lambardi, P; Mencacci, R; Hughes, G R; Hays, G C
    The survival of sea turtles is threatened by modern fishing methods, exploitation of eggs and habitat destruction. Forming keystone species in the ocean, their extinction would disrupt the marine food chain in ways as yet unknown. The Indian Ocean has many breeding areas for sea turtles, the southernmost ones being on the Maputaland coast of KwaZulu-Natal, where loggerhead and leatherback turtles nest in large numbers thanks to long-lasting protection programmes. For the leatherback this is the only known nesting site in the entire western Indian Ocean. At the end of the reproductive season, both loggerheads and leatherbacks undertake migrations towards disparate feeding areas. To contribute to their conservation, the migratory behaviour of these animals needs to be understood. Here we review 10 years studying this behaviour using transmitters that telemeter data via satellite. It emerges that these species frequent widely dispersed areas ranging from the Atlantic Ocean to the Mozambique Channel. The migratory behaviour of leatherback and loggerhead turtles is, however, very different, probably due to their differing food requirements. While loggerhead postnesting movements have a truly migratory nature, the large-scale wanderings of leatherbacks are better described as prolonged sojourns in extended feeding areas.
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    A review of South African research in atmospheric science and physical oceanography during 2000-2005
    (2006) Reason, C J C; Engelbrecht, F; Landman, W; Lutjeharms, J R E; Piketh, S; Rautenbach, C J W; Hewitson, B C
    The purpose of this article is to review progress in the fields of atmospheric science and physical oceanography made by workers based at South African institutions over approximately the last 5 years. Research published by South African scientists working abroad is not included. Most published research in these fields falls within the broad areas of climate variability, climate change, aerosols and atmospheric pollution, seasonal forecasting, numerical modelling (both atmospheric and oceanic), and the physical oceanography of the Agulhas and Benguela current systems. Most but not all of the atmospheric science papers relate to South Africa or southern Africa; however, some work pertaining to the southern hemisphere as a whole or to other regions has been done. We note that funding and institutional support for atmospheric science and physical oceanography research in South Africa remains poor and this situation significantly hampers local efforts.
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    Abrupt environmental shift associated with changes in the distribution of Cape anchovy Engraulis encrasicolus spawners in the southern Benguela
    (2007) Roy, C; van der Lingen, C D; Coetzee, J C; Lutjeharms, J R E
    Cape anchovy Engraulis encrasicolus spawners in the southern Benguela showed an eastward shift in their distribution on the Agulhas Bank that occurred abruptly in 1996 and has since persisted. We assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf of the Agulhas Bank to the east of Cape Agulhas abruptly became 0.5°C colder than in previous years and has since remained that way. In addition, signals, coherent with the 1996 shift recorded in sea surface temperatures, were also found in atmospheric surface pressure and zonal wind data for that region; interannual coastal SST variability is also shown to be correlated with zonal wind-stress forcing. As a result, increased wind-induced coastal upwelling east of Cape Agulhas is proposed as the main driver of the observed cooling in the coastal region. The synchrony between the environmental and biological signals suggests that the eastward shift in anchovy spawner distribution was environmentally mediated and arose from a change in environmental forcing that altered the relative favourability for spawning between regions to the west and east of Cape Agulhas. The results highlight how a relatively minor change in environmental conditions can lead to a drastic spatial reorganisation of the life history of one species in an ecosystem.
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    An interdisciplinary cruise dedicated to understanding ocean eddies upstream of the Prince Edward Islands
    (2004) Ansorge, I J; Froneman, P W; Lutjeharms, J R E; Bernard, K; Bernard, A; Lange, L; Lukác, D; Backeburg, B; Blake, J; Bland, S; Burls, N; Davies-Coleman, M; Gerber, R; Gildenhuys, S; Hayes-Foley, P; Ludford, A; Manzoni, T; Robertson, E; Southey, D; Swart, S; Van Rensburg, D; Wynne, S
    A DETAILED HYDROGRAPHIC AND BIOLOGical survey was carried out in the region of the South-West Indian Ridge during April 2004. Altimetry and hydrographic data have identified this region as an area of high flow variability. Hydrographic data revealed that here the Subantarctic Polar Front (SAF) and Antarctic Polar Front (APF) converge toform a highly intense frontal system. Water masses identified during the survey showed a distinct separation in properties between the northwestern and southeastern corners. In the north-west, water masses were distinctly Subantarctic (>8.5°C, salinity >34.2), suggesting that the SAF lay extremely far to the south. In the southeast corner water masses were typical of the Antarctic zone, showing a distinct subsurface temperature minimum of <2.5°C. Total integrated chl-a concentration during the survey ranged from 4.15 to 22.81mgchl-am–2,with the highest concentrations recorded at stations occupied in the frontal region. These data suggest that the region of the South-West Indian Ridge represents not only an area of elevated biological activity but also acts as a strong biogeographic barrier to the spatial distribution of zooplankton.
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    Estimation of sea-surface temperature around southern Africa from satellite-derived microwave observations
    (2003) Rouault, M; Lutjeharms, J R E
    Sea-surface temperatures may give strong indications of the location of fronts, currents, eddies and other components of ocean circulation. This has been recognized in particular for the seas around southern Africa. Almost all studies using this property have employed measurements of thermal infrared radiation from orbiting satellites. This has distinct disadvantages due to the shading effect of persistent cloud cover. Another option is to use microwave observations, which are not affected by cloud cover. Until recently, however, the spatial resolution of microwave data was far too coarse for the purpose of studying ocean circulation in detail. We describe here a new set of microwave data that does not have this disadvantage, and show how useful it is by describing examples of local applications.
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    Interaction of Agulhas filaments with mesoscale turbulence: a case study
    (2008) Whittle, C P; Lutjeharms, J R E; Duncombe Rae, C M; Shillington, F A
    The inter-ocean leakage of heat and salt from the South Indian Ocean to the South Atlantic has important consequences for the global thermohaline circulation and in particular for the strength of overturning of the Atlantic Ocean as a whole. This leakage between these two subtropical gyres takes place south of Africa. The main mechanisms are the intermittent shedding of Agulhas rings from the retroflection of the Agulhas Current and the advection of Agulhas filaments from the border of the Agulhas Current, both of which move northwestward into the South Atlantic. The subsequent behaviour and mixing of Agulhas rings has been much studied over the past years, that of Agulhas filaments not at all. We report here on fortuitous hydrographic observations of the behaviour of an Agulhas filament that interacted with a number of mesoscale features shortly after formation. This suggests that Agulhas filaments may be involved in many other circulation elements and not only the Benguela upwelling front, as was surmised previously, and may mix out in a very site-specific region.
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    Intrusions of sub-Antarctic surface water across the subtropical convergence southwest of Africa
    (2003) Fillis, C S; Lutjeharms, J R E
    The terminal region of the Agulhas Current south of Africa is characterized by the complete retroflection of the Current. The region has been shown to be populated by a range of eddies and rings. It has been observed that the spawning of an Agulhas Current ring at the retroflection is preceded by the northward wedging of the Subtropical Convergence (STC) through the retroflection loop to effectively "pinch" off these rings. The resultant entrainment of cold Sub-Antarctic Water Surface (SAASW) behind the displaced STC is of climatic and oceanographic interest in light of the concurrent interruption and eastward retreat of the warm Agulhas Current. The leakage of Agulhas Current water into the South-eastern Atlantic Ocean in the form of filaments may also be temporarily terminated during these SAASW intrusion episodes. In order to investigate intrusions of SAASW into the Agulhas Retroflection region, all available data of any kind have been accessed and analyzed. A serial satellite study, using both METEOSAT and NOAA images, suggests that approximately four intrusions of SAASW are observed per year. These intrusions generally occur between 11° E and 19° E; the westerly intrusions being more prevalent during extreme episodes of SAASW intrusions. The mean temperature and salinity distribution at the retroflection shows that the longitudinal location of SAASW intrusions seems to be geographically invariant suggesting a possible topographic influence by prominent geographical features. Hydrographic analysis of sub-Antarctic water intrusions leads one to believe that they are not just shallow, short-term phenomena but may reach to depths of approximately 800 m to 1000 m, persisting for about 28 days on average. These intrusions introduce low temperature, low salinity (< 35) water into the retroflection region with an average areal geographical coverage of 158 000 ± 118 256 km2. This suggests that these sub-Antarctic water intrusions may have important oceanographic and biologic implications to the dynamics of the Agulhas Retroflection and the oceanic region to the west of it in light of the sheer magnitude of the amount of water involved.
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    Intrusions of sub-Antarctic water across the Subtropical Convergence south of Africa
    (2003) Lutjeharms, J R E; Fillis, C
    The contents of the Cape Basin of the South Atlantic Ocean, southwest of Cape Town, is a melange of water types from a number of different sources. One of the least studied of these water types comes from intrusions of sub-Antarctic water that are associated with the spawning of Agulhas rings. An analysis of a variety of data on the region shows that these intrusions originate along a latitude of 40°S, but only between longitudes of 8° and 22°E. In extreme cases they can extend equatorward beyond the southern tip of Africa. Intrusions take place at least five times per year. Their distinct surface expressions are shown to be but outcrops of water masses that usually are found at greater depths. These vertical perturbations may extend to depths exceeding 1500 m.
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    Monitoring the oceanic flow between Africa and Antarctica: Report of the first GoodHope cruise
    (2005) Ansorge, I J; Speich, S; Lutjeharms, J R E; Goni, G J; Rautenbach, C J de W; Froneman, P W; Rouault, M; Garzoli, S L
    The southern ocean plays a major role in the global oceanic circulation role in the global oceanic circulation, as a component of the Meridional Overturning Circulation, and it is postulated that it has a great influence on present-day climate. However, our understanding of its complex three-dimensional dynamics and of the impact of its variability on the climate system is rudimentary. The newly constituted, international GoodHope research venture aims to address this knowledge gap by establishing a programme of regular observations across the Southern Ocean between the African and Antarctic continents. The objectives of this programme are fivefold: (1) to improve understanding of Indo-Atlantic inter-ocean exchanges and their impact on the global thermohaline circulation and thus on global climate change; (2) to understand in more detail the influence these exchanges have on the climate variability of the southern African subcontinent; (3) to monitor the variability of the main Southern Ocean frontal systems associated with the Antarctic Circumpolar Current; (4) to study air–sea exchanges and their role on the global heat budget, with particular emphasis on the intense exchanges occurring within the Agulhas Retroflection region south of South Africa, and (5) to examine the role of major frontal systems as areas of elevated biological activity and as biogeographical barriers to the distribution of plankton. We present here preliminary results on the physical and biological structure of the frontal systems using the first GoodHope transect that was completed during February–March 2004.
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    Ocean currents south of Africa from drifters
    (2008) Durgadoo, J V; Ansorge, I J; Lutjeharms, J R E
    The ocean currents and their variability south of Africa are known to cover a wide spectrum, spatially and possibly temporally. Hydrographic observations in this vast ocean region are logistically demanding and expensive. In recent years the prevalence of drifting buoys has allowed one to infer certain current characteristics for the region that otherwise would be difficult. Observations from satellite-tracked drifters drogued at a depth of 15 m, collected between 1988 and 2005, were used to infer the mean surface circulation and kinetic energy distributions of the surface flow in the African sector of the Southern Ocean between 30 and 60°S. Regions of intensified flow and of higher levels of eddy kinetic energy were identified and agree fairly well with those established from remote sensing products. These results confirm the value of these observations and indicate the increasing usefulness of this data set as the number of drifter tracks increases.
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    Physical and biological processes at the Subtropical Convergence in the South-west Indian Ocean
    (Academy of Science of South Africa, 2007) Ansorge, I J; Froneman, P W; Richoux, N; Blake, J; Daly, R; Sterley, J; Hart, N; Mostert, B; Heyns, E; Sheppard, J; Kuyper, B; George, C; Howard, J; Mustafa, E; Pey, F; Lutjeharms, J R E
    A detailed hydrographic and biological survey was conducted in the region of the Subtropical Convergence in the Indian sector of the Southern Ocean in April 2007. Hydrographic data revealed that the subsurface expression of the Subtropical Convergence (at 200 m), marked by the 10°C isotherm, appeared to meander considerably between 41°S and 42°15'S. Total surface chlorophyll-a concentration was low and ranged from 0.03 to 0.42 μg l-1 and was always dominated by the pico- (<2 μm) and nano-(2-120 μm) size classes, which contributed between 81% and 93% of the total pigment. The total chlorophyll-a integrated over the top 150 m of the water column showed no distinct spatial trends, and ranged from 12.8 to 40.1 mg chl-a m2. There were no significant correlations between the total integrated chlorophyll-a concentration and temperature and salinity (P > 0.05). The Zooplankton community was dominated, numerically and by biomass, by mesozooplankton comprising mainly copepods of the genera, Oithona, Paraeuchaeta, Pleuromamma, Calanus and Clausocalanus. An exception was recorded at those stations in the region of the front where the tunicate, Salpa thompsoni, dominated the total Zooplankton biomass.
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    Sea-surface temperatures at the sub-Antarctic islands Marion and Gough during the past 50 years
    (2003) Mélice, J-L; Lutjeharms, J R E; Rouault, M; Ansorge, I J
    Sea-surface temperatures (SSTs) have been measured at Marion and Gough islands for nearly 50 years. These are some of the longest records of their kind in the sub-Antarctic. We present the trend, the seasonal cycle, and the time-frequency characteristics of the SST for both islands, which rose by 1.4°C at Marion Island and by 0.5°C at Gough Island over the 50-year period. Intermittent temperature oscillations, with periods of between 1 and 5 years, were observed throughout the record. A 5-year periodicity, compatible with an Antarctic CircumpolarWave signature, dominated after 1990 in both records. We also observed a strong low-frequency component with a period varying from 9.3 to 11.4 years at Marion Island, and a much weaker component with a period varying from 9.9 to 11.8 years at Gough Island.
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    The ocean environment off southeastern Africa: A review
    (2006) Lutjeharms, J R E
    The ocean environment off southeastern Africa consists of a continental shelf region of diverse widths and an off-shelf circulation that is similarly multifaceted. These factors lead to a complex and as yet imperfectly understood coastal and shelf habitat for living organisms. To complicate this further, the coverage of the region by hydrographic and other investigations differs markedly. Some regions have been relatively thoroughly studied whereas in others no hydrographic or current observations of any kind have been made to date. The only attribute that connects the coastal oceans of this region is that they all belong to what may be considered to be the greater Agulhas Current system. This consists of the East Madagascar Current, the Mozambique Channel eddies and the Agulhas Current proper. In this paper I review current knowledge on the region from both a physical as well as a biological oceanographic point of view. Some of the most glaring gaps in our knowledge of the region are identified.