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Abstract
The summer coastal winds of 1982-3 were anomalous in two respects: 1) the effect of cyclonic perturbation was stronger in the south, resulting in weaker seasonally integrated longshore winds and a larger than normal eastward (onshore) component; 2) in the north, the cumulative longshore wind was slightly stronger than normal, with more upwelling, colder water and lower air temperatures at the coast. The cause of this may perhaps be found in steeper cross-coastal pressure gradients with a stronger component of geostrophic wind aloft.-from Authors English
... A more constant wind field would provide for more sustained upwelling and, thus, cooler temperatures. Wind data collected at Hondeklip Bay, 400 km south of Uideritz, suggest that coastal lows were infrequent in the central Benguela at least during the 1982/83 austral summer (Nelson and Walker 1984). ...
Principal components analysis was used to investigate interannual sea surface temperature (SST) variability in the South-East Atlantic from 14 to 34°S over 23 years. More than 75 per cent of the non-seasonal SST variability was accounted for by the first three principal components. Principal component 1 (PCI) demonstrated region-wide warming and cooling trends, centred within the main shipping lane. Major "events" revealed by this component included a strong but fairly short-lived warm event in 1963, a cold period from 1969 through 1971 and a warm period from 1972 to 1975. PC2 isolated SST behaviour at the extreme southern sites which contrasted with that farther north. PC3 was dominated by events in the upwelling regions north of 30°S where warm and cool events differed somewhat in timing and amplitude in comparison with those offshore. Furthermore, this component revealed a warm event in 1967/68 and a pronounced and sustained cool event from 1981 to 1983. Major warm events were accompanied by reduction of average equatorward wind speed and cool events by its augmentation. Heat flux anomalies contributed to the generation and maintenance of SST anomalies offshore. Although local changes in wind forcing accompanied warm and cool events, more remote forcing from the tropical Atlantic was crucial to the evolution of nearshore warm and cool events north of 30°S.
... spond more to Figures 2c and2d, and to Figure 3, though obviously only including summer. Figure 6 shows that there is a tendency for weaker southerly winds at Cape Town during ENSO events; Nelson and Walker [1984] also found that the southeasterly winds were much weaker during 1983 compared with 1981. The increasing wind stress trend reported by Bakun [1990] does not appear, though after an initial decrease in wind stress until 1960, there then appears to be a slow increase. ...
An analysis of 38 years of wind data from three sites along the South African east and south coasts is made to determine interannual variability. Different period bands are investigated, and it is found that the northernmost site (Durban, at about 30°S) differs markedly from the two southern sites (Port Elizabeth and Cape Town, at about 34°S). At the latter sites interannual trends exist in the yearly averaged wind directions, while for Port Elizabeth the principal axis orientations including winds lying within the ``weather band'' also show such a trend. It appears that a major readjustment occurred at the two southern sites during the very strong El Niño Southern Oscillation (ENSO) event in 1982/1983, with abrupt changes of up to 30° in the wind directions being registered. On the other hand, there are no clearly identifiable trends in the wind speeds, and longer time series will be needed to establish correlations with weaker ENSO events.
The atmosphere and the ocean may, from a functional point of view, be regarded as forming a coupled entity. They interact on a wide spatial range, from the molecular to the global. The necessity of considering the ocean and the atmosphere as a coupled system is illustrated most effectively if the interaction between them is studied insofar as it affects climate. This is done here by discussing the problem of an increase in atmospheric carbon dioxide, the influence of sea surface temperatures on the weather and the global El Nino/Southern Oscillation phenomenon. In conclusion mention is made of South African research in this connection.
The concept of relaxation time (or maturation time, or lag) ought to concern many prehistorians, particularly those interested in relating climate change to prehistoric settlement. The reason is fairly obvious but often not faced. People in the past presumably responded not directly to the changes in basic climatic parameters such as temperature, precipitation, and wind strength, but indirectly to changes in landscape and resource distributions that were initiated by them. Interactions between different components of the ecosystem, including people, are complex responses with variable sensitivities to the direction and magnitude of change.
The 1982-1984 El Niño and associated events affected seabirds in the Pacific and Atlantic oceans. Effects ranged from extralimital dispersal to nest desertion and adult mortality, and appeared most severe in the eastern Pacific upwellings off Peru and California, and in the central Pacific. Up to 85% of seabirds died in Peru. In contrast, effects were mild in the Caribbean Sea. Other areas such as the North Atlantic and Indian Oceans were apparently unaffected. El Niño appears to be an important force shaping life-histories only for seabirds In the Peruvian upwelling, where El Niño events are both frequent and severe. In other areas, with milder or less frequent events, responses to normal environmental variability, such as deferred breeding, readiness to abandon nesting efforts, and low reproductive effort, may be sufficient to buffer seabirds against all but the most severe El Niño events. On an evolutionary scale, El Niño, by generating massive extralimital dispersals or by exterminating colonies, may have caused disjunct ranges or even speciation in Pacific seabirds. Our ability to study the effects of El Niño on seabirds is limited by the lack of long-term, intensive studies of seabird populations. Without such studies, our knowledge is likely to remain anecdotal and superficial. We also lack information on the environmental features to which seabirds respond during El Niño. These may include sea and air temperature, rainfall, food abundance or parasite loads.
Interannual variability in the environment in the South-East Atlantic Ocean has been shown by various authors to influence significantly fish resources in the Benguela system. In this paper a large-scale perspective on the environmental variability in the area is provided. Monthly sea surface temperature anomaly distributions in the Atlantic and Indian oceans are shown to be useful in the context of the Benguela system, and a time-series of anomaly maps between mid 1983 and mid 1988 demonstrates the origin and evolution of the most important environmental perturbations during the 1980s.
Long time-series of sea surface temperature (SST) and pseudo wind stress () in six areas in the South-East Atlantic are analysed as possible inputs into fisheries models. The areas encompass oceanic and coastal regimes. A clear seasonal signal is evident in all areas, with an amplitude in SST of 3–4°C in the upwelling areas and 5°C farther offshore and on the Agulhas Bank. Warming lags in the north and offshore by 1–2 months. Monthly variability is highest in the upwelling areas. An increasing trend in SST is suggested in all areas, the post-World War II era being about 1,0°C warmer than earlier periods. Some coherence occurs between areas at times, although at other times the anomalies are distinctly out of phase. There is a suggestion of an upward trend in the equatorward wind stress in some offshore areas after 1964, with a sharp change in 1975. Benguela Niños are evident in the environmental record and have a periodicity of around 10 years.
A spatially explicit individual-based model (IBM) forced by 3D temperature and current fields simulated by a hydrodynamic model of the southern Benguela upwelling region was used to test two hypotheses concerning the role of diel vertical migration (DVM) by Cape anchovy Engraulis encrasicolus larvae and pre-recruits. These hypotheses were that: (1) DVM enhances alongshore transport of anchovy eggs and larvae from the spawning grounds to the nursery area while avoiding the lethal effect of low water temperatures in the upwelling system, and/or (2) DVM enhances the transport of larvae and pre-recruits from the offshore to the onshore domain of the nursery area, and then counteracts offshore advection by favouring retention. We tracked the trajectories of virtual particles in the model and calculated a pre-recruitment index as a proxy for transport success to the nursery area (onshore and offshore) and found that the index increased from 10% to 20% after the incorporation of larval vertical migration into the IBM, with virtual individuals held at depths of around 60 m showing maximal pre-recruitment index values. Hence, DVM does appear to enhance transport to the nursery area (offshore) for early and late larvae. Model outputs showed coarse-scale horizontal distribution patterns of larvae by age/size class that are similar to field observations for early, small larvae but not for large larvae and pre-recruits. Observations show that early/small larvae are located offshore whereas older/larger larvae and pre-recruits are found closer to the continental shelf and the inner nursery grounds. This disparity between model results and field observations does not support the hypothesis that DVM is one of the mechanisms involved in the onshore movement of early life-history stages, especially for large larvae.
Sea surface temperature was monitored off the Namaqualand coast of South Africa (29–32°S, 16–18°E) during the spring of 1980 by aerial radiation thermometry. The aircraft flew a grid pattern extending 30 km inland and about 100 km offshore. Wind data were obtained from aircraft drift measurements at 152 m above sea or local ground level. A time series of wind information from a coastal station in the centre of the study area showed a longshore wind direction (170°) with a slight onshore component due to the prevalence of seabreezes. The South Atlantic high-pressure cell and equatorward wind flow became established during October 1980. However, in November a persistent upper-air trough caused wind reversals affecting the distribution of sea surface temperature. Upwelling off Namaqualand is typically confined to the coastal strip, but with a broad plume of cold water extending offshore near 30°S. The location of the minima in sea surface temperature is attributed to the proximity of a southerly wind maximum and to an equatorward broadening of the continental shelf. Three cases are presented to describe the pattern of upwelling off Namaqualand under a varying history of winds.
Coccolithophores, together with other phytoplankton, were sampled at ten stations along a transect following the general orientation of the Cape Peninsula upwelling plume in March, 1983. Thirteen coccolithophore species were identified with Emiliania huxleyi dominating the assemblages. Coccolithophores and other microflagellates were more abundant than diatoms along the transect except at one station in mature upwelled water. Very high populations (2 340103 cells l-1) of coccolithophores were found in mature upwelled water adjacent to the oceanic front. This count is one of the highest counts ever recorded in an upwelling area. The anomalous warm event prior to sampling may have had a residual effect on seeding, contributing towards the predominance of small-celled phytoplankton over chain-forming diatoms.
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