F. A. Shillington

Climatology and Variability of Sea Surface Temperature and Surface Chlorophyll in the Benguela and Agulhas Ecosystems As Observed by Satellite Imagery

A climatology of satellite-derived sea surface semperature (SST) and surface chlorophyll a concentration (Chl), and their associated variability at time-scales from weeks to years, was constructed for the Benguela and Agulhas ecosystems. Global area coverage data at 4.5 km spatial resolution from both AVHRR and SeaWiFS sensors were used to assemble the climatology, from weekly and 5-day time-series respectively. The SST data series spanned 18 years (1982–1999), the Chl data the period September 1997–April 2002. The dominant pattern in the annual SST is the cold upwelled water on the western continental shelf of South Africa and Namibia. SST was high at the Angola-Benguela Front (15–17°S) and on the Agulhas Bank, the northern and southern extremities of the upwelling region. Monthly SST variability was high in both regions, except for the Lüderitz and Cape Columbine-Cape Peninsula upwelling zones, where variability was low. The western Agulhas Bank exhibited a clear seasonal pattern of warm surface water in summer and cool surface water in winter, with an amplitude of 2.5°C. A band of high Chl (>5–10 mg m−3) was apparent close to the coast from the Angola-Benguela Front to Cape Town, but there was a well-defined relative minimum at Lüderitz. On the South Coast, highest Chl (>3 mg m−3) was between Cape Agulhas and Port Elizabeth, in the form of a plume moving offshore. In contrast to SST, Chl variability was mainly at intramonthly intervals, although variability was particularly high north of the Angola-Benguela Front in summer, and at various upwelling sites in winter.

An agulhas ring in the South Atlantic ocean and its interaction with the Benguela upwelling frontal system

Abstract An Agulhas ring was detected using XBT probes on a cruise between Cape Town and Vema Seamount in the south-east Atlantic Ocean in April 1989. CTD and nutrient data, collected on a second cruise in May 1989, GEOSAT altimeter data for February–April 1989 and cloud-free NOAA-11 satellite imagery from June 1989 were used to characterize the ring. The ring was elliptical (330 km E-W and 165 km N-S, relative to the 16°C isotherm at 200 m depth), evident to at least 1200 m, and centred on 30.5°S, 9.2°E in May, about 700 km west of the Orange River. Its drift velocity was 6.4 ± 1 cm s −1 to the NW. Maximum anticyclonic geostrophic surface currents near its edge were 55 cm s −1 relative to 1150 db. The available potential energy was estimated to be 38.8 × 10 15 J and the kinetic energy 2.3 × 10 15 J using a two layer model of the ring. A cool filament extending 450 km offshore from the Benguela upwelling front was identified in the hydrography and the NOAA-11 imagery. Entrainment velocities (maximum of 75 cm s −1 ) of mature upwelled water from the Benguela frontal region were inferred from feature tracking.

Modeling Equilibrium Dynamics of the Benguela Current System

The Regional Ocean Modeling System (ROMS) is used to systematically investigate equilibrium conditions and seasonal variations of the Benguela system at a resolution of 9 km, including both the large-scale offshore flow regime and the economically and ecologically important coastal upwelling regime. A shelf-edge poleward flow exists in the northern Benguela region (i.e., north of 28°S) and is driven primarily by the wind stress curl via the Sverdrup relation. As such, it is strongly seasonal and is most intense during spring and summer, when the wind stress curl is most negative. The poleward flow deepens as it moves southward; between 25° and 27°S, much of it veers offshore because of the nature of the wind stress curl and its interaction with the northwestward path of the Benguela Current, which is influenced by alongshore topographical variations. The Benguela Current is driven by nonlinear interactions of passing Agulhas rings and eddies and does not have a striking seasonal signal. In the mean state, it is characterized by two streams. The more inshore stream is topographically controlled and follows the run of the shelf edge. The meandering nature of the offshore stream is a result of the preferential path of Agulhas rings. The model simulates all seven of the major upwelling cells within its domain. The three southernmost cells have the strongest seasonal signal and experience their greatest upwelling during spring and summer months, whereas the two northernmost cells have less seasonal variability but nevertheless have increased upwelling from autumn to spring (and least upwelling in summer), and the central Benguela upwelling cells experience year-round upwelling. The effect of topography on coastal upwelling was investigated by smoothing alongshore coastline and topography variations, which showed that, in all of the seven major upwelling cells, upwelling is enhanced on the downstream side of capes.

Interannual scales of variation of pigment concentrations from coastal zone color scanner data in the Benguela Upwelling system and the Subtropical Convergence zone south of Africa

South African oceanographers were engaged in collecting hydrographic and biological sea truth data in order to calibrate the coastal zone color scanner (CZCS) measurements from the Nimbus 7 satellite over the Benguela Upwelling region near Cape Town, South Africa, during the period 1978 to 1981 (Shannon, 1985). We give a brief overview of this research and then concentrate on a more recent analysis of level III CZCS data obtained from NASA for the region 10-60-degrees-S, 10-100-degrees-E. This area includes the Benguela Upwelling system on the continental shelf and the southern ocean with the Subtropical Convergence zone south of Africa. High annual concentrations of chlorophyll (5 mg m-3) typical of other upwelling systems in the world ocean occurred in the Benguela shelf region, and the data show a strong interannual signal in the 7 years of composited data from 1978 to 1985. Two distinct regimes were found in the Benguela Upwelling system, the seasonal variations of pigment concentration in the northern and southern Benguela regions being out of phase. In the southern ocean, levels of chlorophyll were generally low (0.15 mg m-3), with the strongest signal (1.5 mg m-3) found at the southern border of the Agulhas retroflection region and its frontal boundary with the colder Subantarctic water to the south. The high levels of chlorophyll found in this region are 10 times those of the typical open southern ocean. There is a very clear interannual signal in the CZCS data for this Subtropical Convergence region, which has a low value in 1979, rises to a maximum in 1981, and then decreases to another low value in 1985. There appears to be no clear seasonal variation in the Subtropical Convergence data. Reasons for the strong signal in the surface chlorophyll concentrations at the front between the Agulhas Return Current and the southern ocean are discussed, and it is shown that the Agulhas Plateau sets up a topographic Rossby ave in the Agulhas Return Current, which can be clearly identified in the CZCS signal.

Remotely sensed variability of temperature and chlorophyll in the southern Benguela: upwelling frequency and phytoplankton response

High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones.

Phytoplankton pigment distribution and frontal structure in the subtropical convergence region south of Africa

Satellite data from the Coastal Zone Colour Scanner (CZCS) and Advanced Very High Resolution Radiometer were examined to determine the relationship between the phytoplankton pigment distribution and sea surface temperature (SST) in the Subtropical Convergence (STC) region south of Africa. The structure of the temperature fronts and their role in phytoplankton pigment distribution was explored. The multichannel sea surface temperature (MCSST) data for 1981-1986 showed considerable interannual variability in both the position the Agulhas retroflection and the extent of the Rossby wave in the Agulhas Return Current. Pronounced interannual variation in pigment levels was found in the CZCS data for early 1979, 1982 and 1983, with highest pigment levels in early 1982. These greater pigment levels were shown to be a function of increased frontal intensity of the STC front rather than of the Agulhas front, or of SST per se: The principal interannual differences were found in the region where a strong Agulhas front occurred to the north of the STC front. The Agulhas front appeared to play a role in limiting the spatial distribution of phytoplankton pigment. Time series of level-2 CZCS data supported the scenario of upstream Agulhas retroflection resulting in a pronounced Rossby wave in the Agulhas Return Current, and increased phytoplankton pigment levels in the STC region. Copyright (C) 1996 Elsevier Science Ltd

Identification and classification of vertical chlorophyll patterns in the Benguela upwelling system and Angola-Benguela front using an artificial neural network

Information on the vertical chlorophyll structure in the ocean is important for estimating integrated chlorophyll a and primary production from satellite. For this study, vertical chlorophyll profiles from the Benguela upwelling system and the Angola-Benguela front were collected in winter to identify characteristic profiles. A shifted Gaussian model was fitted to each profile to estimate four parameters that defined the shape of the curve: the background chlorophyll concentration (B 0), the height parameter of the peak (h), the width of the peak (σ) and the depth of the chlorophyll peak (zm ). A type of artificial neural network called a self-organizing map (SOM) was then used on these four parameters to identify characteristic profiles. The analysis identified a continuum of chlorophyll patterns, from those with large surface peaks (>10 mg m−3) to those with smaller near-surface peaks (<2 mg m−3). The frequency of occurrence of each chlorophyll pattern identified by the SOM showed that the most frequent pattern (∼12%) had a near-surface peak and the least frequent pattern (∼2%) had a large surface peak. These characteristic profile shapes were then related to pertinent environmental variables such as sea surface temperature, surface chlorophyll, mixed layer depth and euphotic depth. Partitioning the SOM output map into environmental categories showed large peaks of surface chlorophyll dominating in water with cool temperature, high surface chlorophyll concentration and shallow mixed layer and euphotic depth. By contrast, smaller peaks of subsurface chlorophyll were in water with warmer temperature, lower surface chlorophyll concentration, intermediate mixed layer and deep euphotic depth. These relationships can be used semi-quantitatively to predict profile shape under different environmental conditions. The SOM analysis highlighted the large variability in shape of vertical chlorophyll profiles in the Benguela. This suggests that an ideal typical chlorophyll profile, as used in the framework of biogeochemical provinces, may not be applicable to this dynamic upwelling system.

Mesoscale eddy variability in the southern extension of the East Madagascar Current: Seasonal cycle, energy conversion terms, and eddy mean properties

In this study, we used more than 17 years of satellite altimetry observations and output from an ocean model to investigate the mesoscale eddy variability and forcing mechanisms to the south of Madagascar. Analysis of energy conversion terms in the model has shown seasonality on eddy formation, both by barotropic and baroclinic instabilities: maximum in winter (JJA) and minimum in summer (DJF). The eddies were mainly formed in the upper ocean (0–300 m) and at intermediate depths (800–2000 m) by barotropic and baroclinic instabilities, respectively. The former dominated in the southeastern margin of Madagascar, and the latter to the southwest, where the South-East Madagascar Current (SEMC) separates from the continental shelf. Seasonality of the eddy formation appeared linked with the seasonal intensification of the SEMC. The energy conversion terms indicated that the eddies have a significant contribution to the large-scale circulation, but not being persistent throughout the year, occurring mainly during the fall season (MAM). Eddy demography from altimetry and model provided information on eddy preferential sites for birth, annual occurrence (6–13 per year), eddy mean diameter (124–178 km), mean amplitude (9–28 cm), life-time (90–183 days), and maximum traveling distances (325–1052 km). Eddies formed to the southwest of Madagascar exhibited distinct characteristics from those formed in the southeast. Nevertheless, all eddies were highly nonlinear, suggesting that they are potential vectors of connectivity between Madagascar and Africa. This may have a significant impact on the ecology of this region.

Reviewing evidence of marine ecosystem change off South Africa

Recent changes have been observed in South African marine ecosystems. The main pressures on these ecosystems are fishing, climate change, pollution, ocean acidification and mining. The best long-term datasets are for trends in fishing pressures but there are many gaps, especially for non-commercial species. Fishing pressures have varied over time, depending on the species being caught. Little information exists for trends in other anthropogenic pressures. Field observations of environmental variables are limited in time and space. Remotely sensed satellite data have improved spatial and temporal coverage but the time-series are still too short to distinguish long-term trends from interannual and decadal variability. There are indications of recent cooling on the West and South coasts and warming on the East Coast over a period of 20–30 years. Oxygen concentrations on the West Coast have decreased over this period. Observed changes in offshore marine communities include southward and eastward changes in species distributions, changes in abundance of species, and probable alterations in foodweb dynamics. Causes of observed changes are difficult to attribute. Full understanding of marine ecosystem change requires ongoing and effective data collection, management and archiving, and coordination in carrying out ecosystem research.

Satellite comparison of the seasonal circulation in the Benguela and California current systems

Satellite surface height and surface temperature fields are used to examine the seasonal surface circulation in the Benguela and California Current systems. In the California Current system, an equatorward jet develops in spring and summer near to the coast, with a latitudinal structure that responds to the equatorward longshore winds. This jet moves offshore from spring to autumn and contributes eddy kinetic energy to the deep ocean. In the Benguela system north of 32°S, winds are upwelling-favourable and currents are equatorward all year, but stronger in summer. The current strengthens in summer, when water parcels with high steric heights move into the region offshore of the jet from the Agulhas Retroflection area at the same time that steric heights next to the coast drop as a result of coastal upwelling. Off the Cape (32–34°S), winds and currents are more seasonal. The Geosat altimeter fields do not resolve the equatorward flow along the SST front next to the coast in spring and summer, but pick up s...