Upwelling and ocean structures off Algoa Bay and the south-east coast of South Africa

As the Agulhas Current flows along the south-east coast of South Africa, a number of processes operate that bring cold, deep water up onto the narrow shelf. As a consequence, upwelling along the coastline is enhanced farther southward and downstream. This situation is investigated off Algoa Bay and along the south-east coast to Port Alfred, where measurements demonstrate that marked temperature variability occurs at the coastline, particularly in summer when temperature structures are more intense and easterly-component winds more common. There is no indication that upwelling is more prevalent at Port Alfred; increasing variability farther south is evident at Woody Cape/Cape Padrone, where the coastline veers westwards, forming the eastern boundary of Algoa Bay. Here it is found that, after a wind change to north-easterly, cold water is upwelled along the shoreline between 19 hours and 2.5 days later. Such upwelling progresses north-eastwards with the movement of the wind and weather systems, although colder water also moves south-westwards into Algoa Bay. Winds, currents, sea level and sea temperatures are highly correlated, with fluctuations in sea level measuring >50 cm being associated with coastal trapped waves (CTWs). Such barotropic wind-driven CTWs are frequently active during upwelling, although it is unclear whether there is any interaction between the two phenomena.     

夏季南黄海跨锋断面的生态环境特征及锋区生态系的提出

基于2006年夏季综合调查资料,分析和研究了南黄海陆架锋的分布以及跨锋断面的生态环境特征,并结合锋区的生物学和生态学现象,提出了锋区生态系的观点.结果表明:在南黄海西部冷水团边界附近海域存在因潮混合而形成的浅水陆架锋(潮汐锋),其中以长江口东北部至江苏北部外海、山东石岛外海和海州湾外侧的陆架锋最明显,而且与表层冷水区相...     

Observations and mechanisms of upwelling in the northern KwaZulu-Natal Bight,South Africa

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.     

Kinematics of the upwelling front off southern Africa

Mesoscale circulation features have been shown to play an important role in the cross-frontal mixing of upwelling cells, their frontal morphology and in their interaction with oceanic water masses. With three years of detailed thermal infra-red satellite information on the South-East Atlantic upwelling system available, it proved possible to present a preliminary study of four prevalent frontal features intrinsic to the short-term behaviour of upwelling in this area. Upwelling filaments are shown to extend between 50 and 600 km seawards of the main front and are found, as are upwelling plumes, predominantly off the recognized major upwelling cells. Frontal eddies have a range of diameters and are found distributed over the full area of upwelling and on both sides of the main upwelling front. Warm filaments of Agulhas Current origin are advected preferentially along the western border of the Agulhas Bank and follow closely the front of the southernmost upwelling cells, where they may play a catalytic role in the creation of frontal turbulence.     

Upward transport of iron at the west shelf edge–slope of the Okinawa Trough in the East China Sea

We studied the behavior of chemical substances in the upper 300 m of the water column across the continental shelf–slope interface in the East China Sea off the Okinawa Trough. The behaviors of iron, inorganic nutrients, and humic-like fluorescent dissolved organic matter were strongly influenced by the extensive water exchange between the East China Sea and the Kuroshio Current across the shelf break and slope via upwelling and frontal processes. We attributed the high humic-like fluorescent intensity at the subsurface of the shelf break and slope regions to the lateral supply of humic-like fluorescent dissolved organic matter from the shelf sediments to the outer shelf region due to the intrusion of shelf water into Kuroshio subsurface water. We found that the behavior of iron at the continental shelf–slope was remarkably different from the conservative mixing of inorganic nutrients and humic-like fluorescent dissolved organic matter. In deep and bottom waters at the shelf–slope, high total iron concentrations, which were closely related to water transmittance, possibly resulted from the swept transport of iron-rich resuspended sediments over the shelf floor from the slope by the invading Kuroshio Intermediate Water close to the bottom.     

On the occurrence of oxygen-depleted water south of Africa and its implications for Agulhas-Atlantic mixing

Data obtained during the Agulhas Retroflection cruise of the Knorr in November/December 1983 suggested the presence of an oxygen-depleted water layer at the boundary of the Agulhas Current/Return Current system. An analysis of historical data shows that such a layer is present off the edge of the Agulhas Bank during much of the year and may be found off the West Coast as far north as 32°S during summer. The origins, temporal and spatial variations and hydrography of this low-oxygen water mass are described, and its influence on the biology of the Western Cape upwelling region is discussed. It is considered that the presence of this low-oxygen water mass can be used to show penetration of Agulhas water into the South-East Atlantic.     

夏季台湾海峡水及黑潮次表层涌升水向东海近陆架区营养盐输送通量的估算

According to historical mean ocean current data through the field observations of the Taiwan Ocean Research Institute during 1991–2005 and survey data of nutrients on the continental shelf of the East China Sea(ECS) in the summer of 2006, nutrient fluxes from the Taiwan Strait and Kuroshio subsurface waters are estimated using a grid interpolation method, which both are the sources of the Taiwan Warm Current. The nutrient fluxes of the two water masses are also compared. The results show that phosphate(PO4-P), silicate(SiO3-Si) and nitrate(NO3-N) fluxes to the ECS continental shelf from the Kuroshio upwelling water are slightly higher than those from the Taiwan Strait water in the summer of 2006. In contrast, owing to its lower velocity, the nutrient flux density(i.e., nutrient fluxes divided by the area of the specific section) of the Kuroshio subsurface water is lower than that of the Taiwan Strait water. In addition, the Taiwan Warm Current deep water, which is mainly constituted by the Kuroshio subsurface water, might directly reach the areas of high-frequency harmful alga blooms in the ECS.     

Shelf currents,lee-trapped and transient eddies on the inshore boundary of the Agulhas Current,South Africa: their relevance to the KwaZulu-Natal sardine run

The existence and strength of the annual KwaZulu-Natal (KZN) sardine run has long been a conundrum to fishers and scientists alike ― particularly that the sardine Sardinops sagax migrate along the narrow Transkei shelf against the powerful, warm Agulhas Current. However, examination of ship-borne acoustic Doppler current profiler (S–ADCP) data collected during two research surveys in 2005 indicated that northward-flowing coastal countercurrents exist at times between the Agulhas Bank and the KZN Bight, near Port Alfred, East London, Port St Johns and Durban. The countercurrent near Port Alfred extended as far east as the Keiskamma River, within an upwelling zone known to exist there. An ADCP mooring at a depth of 32 m off Port Alfred indicated that the countercurrent typically lasted a few days, but at times remained in the same direction for as long as 10 days. Velocities ranged between 20 and 60 cm s^?1 with maximum values of ～80 cm s^?1. The S–ADCP data also highlighted the existence of cyclonic flow in the Port St Johns–Waterfall Bluff coastal inset, with a northward coastal current similarly ranging in velocity between 20 and 60 cm s^?1. CTD data indicated that this was associated with shelf-edge upwelling, with surface temperatures 2–4 °C cooler than the adjacent core temperature (24–26 °C) of the Agulhas Current. Vertical profiles of the S–ADCP data showed that the countercurrent, about 7 km wide, extends down the slope to at least 600 m, where it appeared to link with the deep Agulhas Undercurrent at 800 m. S–ADCP and sea surface temperature (SST) satellite data confirmed the existence of the semi-permanent, lee-trapped, cyclonic eddy off Durban, associated with a well-defined northward coastal current between Park Rynie and Balito Bay. Analysis of three months (May–July 2005) of satellite SST and ocean colour data showed the shoreward core-boundary of the Agulhas Current (24 °C isotherm) to commonly be close to the coast along the KZN south coast, as well as between the Kei and Mbhashe rivers on the Transkei shelf. The Port St Johns–Waterfall Bluff cyclonic eddy was also frequently visible in these satellite data. Transient cyclonic eddies, which spanned 150–200 km of shelf, appeared to move downstream in the shoreward boundary of the Agulhas Current at a frequency of about once a month. These seemed to be break-away Durban eddies. Data collected by ADCP moorings deployed off Port Edward in 2005 showed that these break-away eddies and the well-known Natal Pulse are associated with temporary northward countercurrents on the shelf, which can last up to six days. It is proposed that these countercurrents off Port Alfred, East London and Port St Johns assist sardine to swim northwards along the Transkei shelf against the Agulhas Current, but that their progress north of Waterfall Bluff is dependent on the arrival of a transient, southward-moving, break-away Durban cyclonic eddy, which apparently sheds every 4–6 weeks, or on the generation of a Natal Pulse. This passage control mechanism has been coined the ‘Waterfall Bluff gateway’ hypothesis. The sardine run survey in June–July 2005 was undertaken in the absence of a cyclonic eddy on the KZN south coast, i.e. when the ‘gate’ was closed.     

Structure and Transport of the Agulhas Current and Its Temporal Variability

Using a year-long moored array of current meters and well-sampled synoptic sections, we define the variability and mean structure and transport of the Agulhas current. Nineteen current meter records indicate that time scales for the temporal variability in the alongshore and offshore velocities are 10.2 and 5.4 days, respectively. Good vertical correlation exists between the alongshore or onshore velocity fluctuations, excluding the Agulhas Undercurrent. The lateral scale for the thermocline Agulhas current is about 60 km and the onshore velocity correlations are positive throughout the Agulhas Current system. Mean velocities from the array determine that the offshore edge of the Agulhas Current lies at 203 km and the penetration depth is 2200 m offshore of the Undercurrent. Hence, daily averaged velocity sections, determined by interpolation and extrapolation of current meter locations, for a 267-day period, from the surface to 2400 m depth and from the coast out to 203 km offshore encompass the main features of the Agulhas Current system. The Agulhas current is generally found close to the continental slope, within 31 km of the coast for 211 of 267 days. There are only five days when the core of the current is found offshore at 150 km. Total transport is always poleward, varying from −121 to −9 Sv, with maximum transport occurring when the core is 62 km from the coast. Average total transport for the 267 day period is −69.7 Sv; the standard deviation in daily transport values is 21.5 Sv; and the mean transport has an estimated standard error of 4.3 Sv. The Agulhas Undercurrent, which hugs the continental slope below the zero velocity isotach, has an average equatorward transport of 4.2 Sv, standard deviation of 2.9 Sv and an estimated standard error of 0.4 Sv. Transports from the moored array are in reasonable agreement with transport results from synoptic sections. Based on time series measurements at about 30° latitude in each ocean basin, the Agulhas Current is the largest western boundary current in the world ocean.     

Cyclonic eddies reveal Oegopsida squid egg balloon masses in the Agulhas Current,South Africa

In 2005, 2006, 2007 and 2011, distinct Oegopsida squid egg masses were observed by scuba divers on the narrow southern KwaZulu-Natal (KZN) shelf in depths of 35–50 m off the coastal resorts of Park Rynie, Pumula and Port Edward, South Africa. In 2006, larvae in the egg balloons were sampled. DNA barcoding (i.e. cytochrome c oxidase subunit 1 sequencing) linked the larvae to the genus Lycoteuthis, a group commonly found on the continental slope of the Agulhas Bank and the west coast of South Africa. In all cases, the sightings were concomitant with low water temperatures of 14–18 °C, indicative of shelf edge upwelling. Historical ship-collected CTD data show these cooler waters to originate from a depth of 100–180 m on the KZN continental slope. Complementary satellite imagery revealed the cooler water and discoveries of the egg balloons to be coincident with cold core cyclonic eddies embedded in the shoreward boundary of the Agulhas Current. The temperature data suggest that these egg balloons, in the absence of cyclonic eddy activity, would normally be found in the current on a density surface at a depth of ～130 m where velocities are typically around 100 cm s^–1.     

Circulation of shelf waters in the KwaZulu-Natal Bight,South Africa

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.     

Effects of deep-sea eddies on the northern KwaZulu-Natal shelf,South Africa

This paper describes the westward movement of a cyclonic eddy across the Mozambique Channel and the subsequent south-westward propagation of the eddy along the east coast of South Africa and its interaction with the shelf. A hydrographic survey on 13 September 2006 off Nine-mile Reef (NMR), Sodwana Bay, showed a well-developed Agulhas Current along the continental shelf inshore of a cyclonic eddy flanked by two anti-cyclonic eddies, further offshore. A satellite-tracked drifter and complementary altimetry data confirmed the dimensions of the eddy and tracked its movement towards the coast. Shelf-edge upwelling was measured at NMR by an underwater temperature recorder (UTR) when the cyclonic eddy first came into contact with the shelf and again when the cyclonic eddy interacted with the leading edge of the anti-cyclonic eddy moving onto the shelf. Further shelf–eddy interactions off Aliwal Shoal, south of Durban, and consequent upwelling were similarly caused by the same cyclonic eddy as it progressed south-westward along the east coast. Analysis of UTR data between 2004 and 2006 indicated that between two and five cyclonic eddies impact the shelf off NMR per year.     

2002年夏季广东陆架边缘上升流海域的内潮现象及其局地反馈初探

基于2002年夏季开展的"中国近海环流形成变异机理、数值预测方法及对环境的影响"观测项目获得的往复走航温盐流资料,结合同期的卫星观测(风、SST、海表动力高度)数据,初步探讨了粤东陆架边缘上升流区的内潮现象及其局地反馈特征.研究结果表明:粤东陆架边缘海域存在显著的、间歇性的相对低温海水的沿陆坡涌升现象,其中A航段的低温(18～23℃)海水涌升发生在约50～150m深度,B航段的低温(<18℃)海水涌升则主要位于150 m以深.由于观测期间海表风变化很小,而海洋涡旋在几周至几个月内相对稳定,并且研究海域涡旋对流场的影响似乎局限于75 m以浅的上层海洋,因而A、B航段的差异显然不能用海表风或海表动力高度变化来解释.进一步的分析表明粤东陆架边缘上升流区存在显著的内潮现象,尽管资料所限使得我们无法准确判定该内潮性质,往复走航海流剖面的确揭示了1阶和5阶内潮模的存在.内潮的不同模态极大地改变了上层海洋的热力和环流结构;低阶内潮模导致沿最大温度水平梯度处水温的剧烈垂向起伏(>30 m)以及跃层两侧的海流反向现象;高阶内潮模导致垂直陆坡方向水平流速的多次反向,强烈的流剪切可能与增强的混合联系在一起.导致低阶内潮模(A航段)向高阶内潮模(B航段)转变的原因可能是由于内潮特征线倾角与地形坡度比较接近而激发的"临界反射"效应.分析结果还表明,不同内潮模态导致的环流结构变异叠加在背景环流场之上,会显著影响粤东陆架边缘上升流的空间结构及强度.     

Hydrography of the Artabro Gulf in summer: western coastal limit of Cantabrian seawater and wind-induced upwelling at prior cape

The Artabro Gulf (1500 km² north-west of the Iberian Peninsula) has been studied based on data obtained in August 1993, under typical summer meteorological conditions (COPLA 893 cruise). This is the first study on the summer hydrography of the Artabro Gulf. During this season, the hydrography of this area is quite complex and must be understood in terms of three factors: 1) upwelling, 2) two different eastern North Atlantic Central Water (ENACW) water masses and 3) rias water exchange. A local wind-induced upwelling of ENACW_p (subpolar) occurs off Prior Cape. The plume is located off the gulf coastline, but inside the continental shelf. The frontal zone, rich in nutrients and chlorophyll, is parallel to the gulf line, starting in the Prior region and ending at the convergence of ENACW_p and ENACW_t (subtropical), giving it an arched shape. The Artabro Gulf is the place off the Galicia coast where ENACW_t and ENACW_p are observed and the gulf is the western limit of coastal Cantabrian Sea influence. The upwelling is strongly influenced by the topographic presence of the gulf which generates an ‘upwelling shadow’ where the rias water exchange is confined near the coast and the ENACW_p does not penetrate into the rias. The opposite occurs at the southernmost Rias Bajas, where ENACW_t upwells inside the rias.     

Stock separation and life history of Argyrozona argyrozona (Pisces: Sparidae) on the South African east coast

Fishery independent biomass surveys and commercial linefish catch returns were used to elucidate the spatial patterns of carpenter Argyrozona argyrozona distributed along the South African continental shelf. Two distinct areas of abundan ce ere determined, one on the central and the other on the eastern Agulhas Bank. Tagging studies revealed little exchange between them. Nurseries were identified in Algoa Bay on he eastern Agulhas Bank and on the central Agulhas Bank (CAB). Early juveniles (<100mm total length) on the CAB were found offshore in the vicinity of the Alphard Bank. They were found to move inshore with growth and then back offshore as they approached maturity. Juveniles in Algoa Bay dispersed both eastwards and westwards with growth. Otoith readability and growth rates varied between regions, with fish from the Eastern Cape (Port Elizabeth and Port Alfred combined) having the lowest average percentage error (4.82 vs 5.33 and 7.03) and the slowest growth rates. Size-at-50 % mturity (L₅₀) varied regionally, female fish in the Eastern Cape maturing at a smaller size (L₅₀ = 206mm fork length) than in the Tsitsikamma National Park (L₅₀ = 292mm) or the CAB (L₅₀ = 267mm). Mass-at-length varied between regions, with carpenter in the Park having the highest mass-at-length and those in the Eastern Cape having the lowest. Based on the distribution of carpenter, variability in otolith readability, mass-at-length, variation in growth and size-at-maturity, it is concluded that carpenter exist as two separate stocks and should be managed accordingly.     

Trace metal (Cd,Cu, Ni and Pb) cycling in the upper water column near the shelf edge of the European continental margin (Celtic Sea)

This study investigates the relative importance of processes that affect trace metal (TM) cycling in the upper water column at the shelf edge of the Celtic Sea on the western European continental margin. The examined processes include external inputs (by atmosphere and river), physical factors (upwelling, winter mixing and water mass advection) and biological processes (in situ uptake, regeneration and export to deep waters). The concentrations of dissolved Cd, Cu, Ni and Pb were measured with this aim in January 1994 and June 1995 at vertical stations across slope, including stations with upwelling, and in the surface waters along the Celtic Sea shelf. Additionally, deep sea (from sediment trap data) and atmospheric fluxes were estimated. The metal profiles over the slope off the Celtic Sea are quite similar to open ocean profiles already described in the northeast Atlantic, and the concentrations in surface waters are only slightly enriched compared to the nearby open ocean (1.2–1.3× for Cd and Ni). The external sources to the system appear to be of weak influence: the fluvial input is locally strong at the coast and then “diluted” along the large continental shelf; the atmospheric deposition is not significant at the annual scale in comparison to the metal content in the upper waters of the shelf edge (at least for Cd, Ni and Cu). In the upwelling zone, a significant increase in concentrations was observed in the summer surface mixed layer (×2 for nitrate and Cd and ×1.5 for Ni) in comparison to the non-upwelling zone. In winter, concentrations of bioreactive metals increased significantly in the surface waters in comparison to the low summer levels (×5 for nitrate and Cd). Our results suggest that upwelling and winter mixing act as regenerated sources that lead to the resupply of the bioreactive elements above the permanent thermocline with a low export to deeper waters. The tracing of the Mediterranean intermediate waters (MIW) from Gibraltar to the studied area shows indeed that its elemental content at the Celtic shelf edge is mainly due to the conservative mixing of the three “end-member” component waters which are thought to make up the MIW. The remineralization of organic matter within this water mass during its transport to the north would contribute only 20% of the nutrients and Cd concentrations recorded at the Celtic Sea shelf edge. According to the correlation found with nutrients in the 10–200-m layer, dissolved Pb would also be subjected to biological uptake and regeneration within the seasonal thermocline. Particulate scavenging removal of Pb would take place below the permanent thermocline throughout the water column.     

Physical processes contributing to harmful algal blooms in Saldanha Bay,South Africa

The study synthesises current understanding of the predominant physical processes responsible for the seasonality of harmful algal blooms, notably Alexandrium catenella and Dinophysis spp., in the nearshore environment of Saldanha Bay on the west coast of South Africa. Saldanha Bay is one of the few naturally sheltered areas on the South African coastline suitable for in situ shellfish farming and is the major site for the production of black mussel Mytilus galloprovincialis in South Africa. Mussel farming started there in 1985 and the present level of production is some 2 700 tons per annum. Since 1994, disruption of harvesting as a result of the presence of harmful algal species has been a regular late-summer phenomenon. Toxic blooms that are ultimately advected into the bay develop on the continental shelf to the north between 32°S and St Helena Bay, a region characterized by favourable conditions for dinoflagellate growth and circulation patterns that facilitate build-up of intense blooms during late summer. Offshore dinoflagellate populations are advected shorewards and polewards in response to relaxation of upwelling at the Namaqua cell to the north. Dinoflagellate blooms are advected south from the southern Namaqua shelf during upwelling relaxation. Under such conditions, the gyre south of Elands Bay moves offshore and a barotropic flow past Cape Columbine is established. Evidence suggests that the near surface component of the flow occurs as a sudden "flood" event. These dinoflagellate-containing shelf waters are in turn advected into Saldanha Bay when upwelling relaxes, when the density gradient between the bay and the shelf drives surface inflow and bottom water outflow. These flows are reversed with the resumption of upwelling over the shelf, resulting in intrusion and entrainment of bottom water and surface outflow. Entrainment dictates that the bay acts as a net importer of bottom water and net exporter of surface waters over a synoptic cycle. This system of exchange between Saldanha Bay and the shelf curtails the duration and severity of toxic episodes in the bay relative to the shelf.     

Dynamics and role of the Durban cyclonic eddy in the KwaZulu-Natal Bight ecosystem

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.     

Red Sea Intermediate Water at the Agulhas Current termination

The inter-ocean exchange of water masses at the Agulhas Current termination comes about through the shedding of rings, and this process plays an important role in the global thermohaline circulation. Using several hydrographic sections collected during the ARC (Agulhas Retroflection Cruise), MARE (Mixing of Agulhas Rings Experiment) and WOCE (World Ocean Circulation Experiment), this investigation aims to establish the degree to which Red Sea Intermediate Water (RSIW) is involved in this exchange and at what level of purity. To this end a wide range of hydrographic parameters were used. Upstream from the Agulhas Current retroflection water with clear RSIW origin is shown to move downstream on both the landward and seaward sides of the Agulhas Current with the highest water sample purity or water-mass content exceeding 15%. The least mixed water was found close to the continental shelf. At the retroflection the RSIW purity shows considerable variability that ranges between 5% and 20%. This suggests that RSIW moves down the current in patches of considerably varying degrees of previous mixing. This pattern was also observed in a ring sampled during the ARC experiment. The MARE sections in turn indicate that at times RSIW may be entirely absent in the Agulhas Current. RSIW is therefore shown to travel down the current as discontinuous filaments, and this intermittency is reflected in its presence in Agulhas Rings. From the sections investigated it is therefore clear that any calculation of RSIW fluxes involved in inter-ocean exchange can only be done on the basis of event scales. RSIW not trapped in Agulhas Rings flows east with the Agulhas Return Current.     

"Predation" of anchovy by an Agulhas ring: a possible contributory cause of the very poor year-class of 1989

There was a marked decrease in abundance of anchovy Engraulis capensis off South Africa between November 1988 and November 1990 caused by formation of poor year-classes in 1989 and 1990. The percentage of anchovy in the diet of Cape gannets Morus capensis indicated that the most marked decrease in anchovy biomass was between June and July 1989. A filament from the Benguela upwelling front was in the process of being entrained by a passing ring of Agulhas Current water at that time. The ring extracted a large volume of frontal water over a period of 2–3 months, prevented its return to the shelf region and possibly removed anchovy larvae and pre-recruits from the Benguela system. It may have played a role in depressing the 1989 anchovy year-class, and hence the 1989 recruit biomass.