Browsing by Author "Ansorge, I J"

Now showing 1 - 6 of 6
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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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    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 first oceanographic survey of the Conrad Rise
    (2008) Ansorge, I J; Roman, R; Durgadoo, J V; Ryan, P G; Dlamini, L; Gebhardt, Z; Rainier, S; Smith, M; Mtonsti, T; Lutjeharmsa, J R E
    This article presents data collected during the first hydrographic survey of the Conrad Rise, South-West Indian Ocean. Past investigations have shown that the Conrad Rise acts as an obstacle to the flow of the Antarctic Circumpolar Current (ACC). Numerical modelled data suggest that two eastward-flowing jets are formed on the northern and southern extremities of the rise as a result of the bifurcation of the ACC. Hydrographic data collected during the research cruise corroborate the model findings and provide a wealth of empirical data for further investigation of this dynamic ocean region. Counts of seabirds conducted during the cruise revealed unusually large numbers of penguins and diving petrels associated with the frontal jets, suggesting that the area is important for the large populations of penguins breeding at the Prince Edward and Crozet islands farther north.