South Atlantic Anticyclone Research Articles

Variability in the annual cycle of the Río Atuel streamflows and its relationship with tropospheric circulation

ABSTRACTClimate‐induced changes in the annual regime of snow‐fed rivers have serious implications for water resource management. In the Central Andes (CA, 28°–36°S) of Argentina–Chile, the snow accumulated in high‐elevation mountains in winter is the dominant component of streamflows during the spring–summer melting season. Although topography introduces complexity in snowpack responses to the annual temperature cycle, streamflow series over a century in length make the CA particularly suitable for identification of long‐term hydrological changes. Principal component (PC) analysis of Río Atuel annual hydrographs from 1906 to 2012 discriminates between precipitation‐ and temperature‐related components associated with variations in snow accumulation (49% of variance) and advances/delays of the streamflow annual peak (21% of variance), respectively. The temporal evolution of PC1 loadings reveals a predominant negative period from 1917 to 1976 and from 1988 to present, suggesting the propensity to undergo long periods with reduced flows. In turn, the PC2 pattern is predominantly positive from 1948 to the present, revealing a tendency to more frequent peak flows in late spring since the mid‐20th century. Above‐average streamflows related to abundant snowfalls in the CA are associated with northward shifts in stormtracks that are remotely induced by above‐average sea surface temperatures in the equatorial Pacific. On the other hand, earlier streamflow peaks in November‐December are concurrent with above‐average temperatures across the Atuel basin induced by enhanced meridional circulation from the Tropics due to the strengthening of the South Atlantic anticyclone. These circulation anomalies are linked to the persistent positive phase of the Southern Annular Mode during the last decades. Additionally, years with reduced streamflows in January, and proportionally larger flow contributions in November‐December, are associated with anomalous air cooling at high levels induced by low pressure centres over the region as part of a quasi‐zonal stationary Rossby wave train that extends from Australia to the South American–South Atlantic sector.

Variability in the South Atlantic Anticyclone and the Atlantic Niño Mode*

Abstract Previous studies have argued that the strength of the South Atlantic subtropical high pressure system, referred to as the South Atlantic anticyclone (SAA), modulates sea surface temperature (SST) anomalies in the eastern equatorial Atlantic. Using ocean and atmosphere reanalysis products, it is shown here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic via anomalous tropical wind power. This modulation in the timing and amplitude of seasonal cold tongue development manifests itself via SST anomalies peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events, the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA-induced wind power variations south of the equator influence eastern equatorial Atlantic SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variations.

Eolian sand sheet deposition in the San Luis paleodune field, western Argentina as an indicator of a semi-arid environment through the Holocene

Eolian deposits are common in the western Pampas of Argentina, and most are assumed to be associated with glacial conditions. Stratigraphic and sedimentologic studies coupled with OSL dating in San Luis Province document for the first time a nearly continuous sequence of eolian sand sheet deposits that span most of the Holocene. Petrology and geochemical analyses indicate that the source of the sand is from pre-existing Pleistocene eolian sediments. Sand sheet deposition between ca. 12 and 1ka is associated with sparse, Monte-type vegetation that occurs with drier conditions (MAP 450–100mm) than the late 20th century (~700mm). This paleoenvironmental inference is consistent with nearby pollen and lake level records. A persistent semi-arid environment in western Argentina during the Holocene may reflect sustained warm SSTs in the western equatorial Atlantic Ocean, which may have suppressed the pressure gradient between the South Atlantic Anticyclone and Chaco Low and thus, the flux of summer moisture to western Argentina. There appears to be a paleoclimatic “dipole” response between a dry western Argentina and a wet southeastern Brazil, which is consistent with the increasing strength of the South American Monsoon through the Holocene. Sand sheet accretion appears to cease by 800 to 200years ago with wetter conditions and succession to Espinal vegetation prior to European contact.

Dynamics of late Cenozoic aeolian deposition along the South African coast: a record of evolving climate and ecosystems

Abstract Dune systems along the South African coast are sensitive barometers of fluctuations in palaeoenvironments, as archived in their orientation, geometry, internal architecture, composition, granulometry, diagenesis, palaeontology and archaeological content. Presently, the pronounced climatic/oceanographic gradients around the southern African coastline, including the west coast Mediterranean climate type, with cold upwelling to progressively warmer in terms of climate and sea temperatures eastwards, are mirrored by variations in these parameters. Here, we review and contribute new information concerning their fluctuations from the Miocene to the present to track changes in the bio-, hydro- and geospheres through time. West coast dunes take the form of dune plumes, which have an orientation since the Miocene that mirrors the southerlies of the South Atlantic Anticyclone (SAA), muted during the warm Pliocene, as reflected by intense bioturbation. Shoreline-parallel, vertically aggraded dune cordons dominate along the southern and eastern coasts, formed by (winter) polar westerlies since the Miocene. The contrasting dunefield morphologies relate to seasonality of wind strength and precipitation. Subtropical east coast dunes are profoundly weathered – on the shelf, glacial-period dunes indicate different atmospheric circulations. The long-term stability of the warm Agulhas Current contrasts with variability in the Benguela. The aeolianites host a rich human and faunal archive, including human ichnofossils. Supplementary material: Tables S1 and S2, which provide details of the data and data sources for Figure 9 in the text, are available at http://www.geolsoc.org.uk/SUP18711 .

A Note on Pacific Decadal Oscillation, El Nino Southern Oscillation, Atlantic Multidecadal Oscillation and the Intertropical Front in Sahel, Africa

The advance and retreat of the Sahara Desert are repetitive in decadal time scale. This climate variability, unique in the world, results from the irregular motion of the Intertropical Front (ITF), a discontinuity originated from the convergence of two air masses over the African continent. It results from the convergence of a dry air mass flowing off the North African anticyclone and a southwestern warm and humid air mass that originates off the South Atlantic anticyclone. As it crosses the equator, this air stream is shifted rightwards, becoming southwesterly winds and transporting moisture into the continent. The equatorial thermal low associated with ITF over the continent is deeper and more intense than the Intertropical Convergence Zone (ITCZ) one and its thermodynamic properties differ remarkably, moisture concentration in particular. The ITF annual latitudinal displacement is larger than that of the Atlantic ITCZ, reaching 20?N commonly during the North Hemisphere summer. In this manuscript one discuss about the possibility that Pacific Decadal Oscillation (PDO) is not a regular physical oscillation and its phases duration may not be predictable and be dependent of large magnitude seismic events, which are rare and random. An important point, however, is that the Pacific sea surface temperature (SST) pattern, once established and recognized, may remain for a relative long (decades) period. Its SST pattern, due to its large area extension, modulates the global climate forcing the large scale general circulation features, ENSO in particular, to produce distinct regional impacts, depending on the signal and strength of the SST anomalies. This happened with the ITF in the Sahel and may possibly have occurred with other large scale atmospheric structures, such as the Asian Monsoon, convergence zones and subtropical anticyclones centers positioning.

Middle- to late-Holocene environmental changes in Bajo de la Quinta, NE Patagonia, inferred by palynological records and their relation to human occupation

The north Patagonian region between 38° and 42°S extends along the proximity of two major atmospheric circulation features in South America: the northern border of the southern westerly circulation belt, and the northeast trade wind circulation associated with the subtropical South Atlantic Anticyclone. The influence of this westerly wind system determines the annual and seasonal precipitation regimen in north Patagonia, while the influence of the South Atlantic Anticyclone is more pronounced towards the east. The changing strength of either of these circulation systems has had a strong influence on Holocene environmental change in the region. Palaeoenvironmental changes are inferred from a 500 cm long core from Bajo de la Quinta, (40°55'42"S, 64°20'19"W) by means of pollen, sediment and geomorphological analyses. A dry phase with westerly winds during the middle Holocene, at around 7000–6600 cal. yr BP, is indicated by xerophytic/psammophytic vegetation and Patagonian steppe components. Between 6600 and 5200 cal. yr BP the pollen record reflects shrub steppe vegetation and low values of long distance transported pollen indicates little influence of the westerly winds. Between 5200 and 2900 cal. yr BP very arid conditions are indicated by high pollen values of Ephedra sp. and Nassauvia sp. and westerlies influenced by long distance pollen types such as Nothofagus sp. In the late Holocene (2900 cal. yr BP) the pollen record shows an increase in typical shrub taxa of the Monte and Espinal vegetation and low influence of long distance taxa. This may suggest a change to semi-arid conditions that persisted until the present, where the shrub steppe is the dominant vegetation. The environmental and geomorphological changes that occurred during the middle and late Holocene had influenced the space and resources used by hunter-gatherers of the area.

Central-West Argentina Summer Precipitation Variability and Atmospheric Teleconnections

The interannual-to-multidecadal variability of central-west Argentina (CWA) summer (October–March) precipitation and associated tropospheric circulation are studied in the period 1900–2010. Precipitation shows significant quasi cycles with periods of about 2, 4–5, 6–8, and 16–22 yr. The quasi-bidecadal oscillation is significant from the early 1910s until the mid-1970s and is present in pressure time series over the southwestern South Atlantic. According to the lower-frequency spectral variation, a prolonged wet spell is observed from 1973 to the early 2000s. The precipitation variability shows a reversal trend since then. In that wet epoch, the regionally averaged precipitation has been increased about 24%. The lower-frequency spectral variation is attributed to the climate shift of 1976/77. From the early twentieth century until the mid-1970s, the precipitation variability is associated with barotropic quasi-stationary wave (QSW) propagation from the tropical southern Indian Ocean and the South Pacific, generating vertical motion and moisture anomalies at middle-to-subtropical latitudes east of the Andes over southern South America. The QSW propagation could be related to anomalous convection partly induced by tropical anomalous SSTs in the western Indian Ocean (WIO). It could also be linked to another midlatitude source along the storm tracks, to the east of New Zealand. After 1976/77, the precipitation variability is associated with equatorial symmetric circulation anomalies linked to El Niño–Southern Oscillation (ENSO)-like warmer conditions. Positive moisture anomalies are consistently observed at lower latitudes in association with inflation of the western flank of the South Atlantic anticyclone. Outside of this, the precipitation variability is unrelated to ENSO.

A preliminary synoptic assessment of soil frost on Marion Island and the possible consequences of climate change in a maritime sub-Antarctic environment

Located in the sub-Antarctic, Marion Island (46˚ 54' S, 37˚ 45’ E) has a distinct periglacial environment that is sensitive to climate change. Diurnal soil frost is the most important geomorphic process occurring on the island and this paper aims to understand the synoptic weather circulation pattern associated with summer soil frost occurrence in a sub-Antarctic environment. Preliminary results from automated microclimate measurements in the interior of Marion Island show that summer soil frost is dependent on Antarctic air mass circulation. This occurs exclusively during post-cyclonic airflow after the passage of a cold front connected to a mid-latitudinal cyclone and subsequent ridging in of the South Atlantic Anticyclone behind the cold front, or when a series of low pressure systems passes over the island. The duration and intensity of soil frost cycles are dependent on the duration of post-cyclonic Antarctic air mass circulation. Summer soil frost on Marion Island is driven by a complex interaction between the latitudinal position of the passing cyclone, the latitudinal position of the ridging anticyclone as well as the trajectory of the air mass circulation. The data suggest that predicted trends in synoptic climate change in the sub-Antarctic may lead to non-linear responses in soil frost dynamics. Keywords: Marion Island; climate change; synoptic weather; soil frost; sub-Antarctic; air mass circulation (Published: 22 March 2012) Citation: Polar Research 2012, 31 , 17626, DOI: 10.3402/polar.v31i0.17626

On the connection between Benguela and equatorial Atlantic Niños and the role of the South Atlantic Anticyclone

For the eastern tropical Atlantic two recurring El Niño‐like phenomena with high interannual SST anomalies have been described, one centered in the equatorial region as part of the Atlantic zonal mode and one off Angola referred to as Benguela Niño. Both events are supposed to be generated not locally but by a relaxation of the trade winds in the western equatorial Atlantic. Here the connection between SST variations in the two regions is investigated with observational data sets as well as ocean model simulations. They are correlated to such an extent that joint events should rather be viewed as one Atlantic Niño. An intriguing feature, counterintuitive in view of the remote forcing mechanism, is that SST anomalies off Angola precede those in the equatorial Atlantic. We show this behavior to be related to the difference in thermocline depths and a different seasonality of interannual SST variability in the two regions. While Benguela Niños peak in austral fall due to the Angola Benguela Front being located furthest to the south and high interannual variability in coastal Kelvin wave activity, warm events at the equator are phase‐locked to austral winter when the thermocline is shallow. Perturbation experiments confirm the importance of remote forcing from the equator for SST variability off Angola and demonstrate the leading role of wind stress in the generation of SST anomalies in the eastern tropical Atlantic. These wind stress variations are shown to be linked to fluctuations in the strength of the South Atlantic Anticyclone, a connection that might be important with respect to the predictability of Atlantic Niños.

CMIP3 projected changes in the annual cycle of the South American Monsoon

Nine models from the Coupled Model Intercomparison Project version 3 dataset are employed to examine projected changes in the South American Monsoon System annual cycle by comparing the 20th Century and SRES A2 scenarios. The following hypotheses are examined: (1) the warm season climate responses in the Southeast, Continental South Atlantic Convergence Zone (CSACZ) and Monsoon regions are related by regional circulation and moisture transport changes which, in turn, must be consistent with robust large-scale changes in the climate system, and (2) an increased threshold for convection in a warmer world may affect the timing of warm season rains. The present analysis reaffirms that the Southeast region is likely to experience increased precipitation through the warm season. Additional results exhibit more uncertainty due to large inter-model variance and disagreement in the A2 scenarios. Nevertheless several statistically significant results are found. In the Monsoon and to a lesser extent in the CSACZ region, the multi-model median suggests reduced precipitation during spring (Sep–Nov). These continental precipitation changes are accompanied by a southward shift of the maximum precipitation in the South Atlantic Convergence Zone. Changes in circulation include a poleward displaced South Atlantic Anticyclone (SAAC) and enhanced moisture transport associated with a strengthened northerly low level flow east of the Andes during spring. Moisture transport divergence calculations indicate unchanged divergence in the Monsoon region during spring and increased convergence in the Southeast throughout the warm season. The circulation and moisture transport changes suggest the increased precipitation in the Southeast during spring may be related to changes in the SALLJ and SAAC, which both enhance moisture transport to the Southeast. The seasonally dry Monsoon region is further affected by an increased threshold for convection in the warmer, more humid and stable climate of the 21st century, which combined with the circulation changes may weaken the onset of the rainy season. Although there is substantial variability among the models, and the results are represented by small changes compared with the multi-model variance, their statistical significance combined with their consistency with expected robust large scale changes provides a measure of confidence in otherwise tentative results. Further testing of the relationships presented here will be required to fully understand projected changes in the South American Monsoon.

Sea surface height variability in the Rio de la Plata estuary from synoptic to inter-annual scales: Results of numerical simulations

A long term simulation of the barotropic circulation in the Río de la Plata estuary was performed with the aim of identifying the characteristic patterns of sea surface height (SSH) variability from synoptic to inter-annual time scales and their forcing mechanisms. Hamburg Shelf Ocean Model (HamSOM), forced by tides, monthly mean runoffs and 4-daily 10 m winds and surface atmospheric pressure from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis was run. The solution was analyzed for the period 1965–2004. Inter-annual variability accounts for almost 10% of the variance. The first EOF mode of SSH variability on this time scale is associated with a mean anomaly of approximately 0.25 m at the upper estuary forced by both runoff and winds, which seems to be strongly associated with the ENSO cycles. Other two modes, with periodicities around 2.5 and 10 years were also found. Even though they are linked to weaker SSH anomalies, they are consistent with inter-annual modes of wind variability reported by other authors. Those modes are important, particularly if they act in phase, because they can provide a background for stronger surges. In contrast with the salinity field, SSH variability on seasonal time scales accounts for a very small percentage of variance and it is the combination of an annual and a semi-annual signal forced by winds and runoff, respectively. Approximately 90% of the variance is due to wind driven variability on sub-annual time scales. The most significant SSH anomalies in this band are associated with cyclogenetic events in the atmosphere, occurring either over Uruguay or over the Patagonian Shelf, whereas the strengthening or weakening of the semi-permanent South Atlantic anticyclone displays a relatively smaller influence. In agreement with previous publications, the estuary's spatial patterns in response to short-scale wind variability seem to be determined by wind direction more than by wind speed.

West coast dune plumes: Climate driven contrasts in dunefield morphogenesis along the western and southern South African coasts

This study examines two major late Quaternary coastal dune systems situated in the southwestern-most extremity of Africa. The False Bay and Duinefontyn dune plumes formed in close proximity, but under contrasting oceanographic regimes (warm Agulhas and cold Benguela oceanic current systems respectively). The False Bay and Duinefontyn dune plumes have hitherto lacked the objective, numerical chronology required to realize their full potential in unraveling the complex palaeo-oceanographic and palaeoclimatic history of the region. Here we present a dual amino acid racemizationn (AAR) and optically stimulated luminescence (OSL) chronological framework from the extensive three dimensional exposures in sea cliffs. At False Bay, three generations of dune deposition, corresponding with orbitally forced climatic/sea level cycles (MIS 7, MIS 5 and Holocene) were resolved, whereas the Duinefontyn Plume is dominated by MIS 5 and Holocene sedimentation. At False Bay and Duinefontyn, glacial periods are marked by the development of mature palaeosols, confirming the link between glacio-eustacy and dune deposition determined previously on the west and southern coasts of South Africa. Along the west coast dune plumes typify dunefield morphology, whereas shoreline-parallel dune cordons are commonly developed on the southern coast. These regional variations in dune system morphogenesis are largely controlled by contrasting wind and rainfall patterns. The attenuated dimensions of the MIS 5 plumes at False Bay and Duinefontyn may indicate a weakening of the South Atlantic Anticyclone during the Last Interglacial. At False Bay, stone artifacts illustrate a human presence in the region during MIS 7.

The 1976/77 Austral Summer Climate Transition Effects on the Atmospheric Circulation and Climate in Southern South America

Abstract Previous works suggest that more El Niño–like conditions can be expected over the South American (SA) climate and atmospheric circulation because of the similarity of the predominately warm conditions in the sea surface temperature (SST) over the central-equatorial Pacific after the 1976/77 summer with those of the SSTs during El Niño events. Here, the summer (October to March) low-level atmospheric circulation over southern SA is studied in order to determine the specific changes that can be related with the global climate transition 1976/77. The rotated principal component analysis is applied to the daily 850-hPa geopotential height fields from the NCEP–NCAR reanalysis I for the periods before and after 1976/77. The second and third principal patterns reveal changes both in the order of explained variances and in some of their spatial features. They can be associated with an expansion of the subtropical South Atlantic anticyclone over SA and lower midlatitude cyclone activity after the 1976/77 summer. The latter is partly associated with the actual tendency toward the positive phase of the southern annular mode. The main patterns can even explain some changes in the observed precipitation over subtropical central-west Argentina as well as for other subtropical regions. Different inhomogeneity tests applied to the atmospheric circulation climatology support the changes. Results suggest that the atmospheric circulation change could be somewhat unique (not observed in the twentieth century) and, thus, it could not be thoroughly ascribed to the El Niño–like variability.

Moisture transport between the South Atlantic Ocean and southern Africa: relationships with summer rainfall and associated dynamics

Moisture exchange between the South Atlantic and southern Africa is examined in this study through zonal moisture transport. Along the west coast of southern Africa, a multivariate analysis of the zonal flow of moisture computed from NCEP-DOE AMIP II Re-analyses reveals a primary mode of variability typical of variations in intensity and of the latitudinal migration of the circulation associated with the midlatitude westerlies and the South Atlantic anticyclone. In austral summer (January–February), this mode, referred to as the South Atlantic midlatitude mode, is found to be well correlated with rainfall over southern Africa (i.e. to the south of the upper lands surrounding the Congo basin). Its positive/negative phases are found to correspond with surface pressures changes over the South Atlantic region in austral summer when the South Atlantic anticyclone is shifted northward/southward respectively. Such changes are accompanied by dipole-like SST anomalies in the midlatitude South Atlantic Ocean, while simultaneous SST anomalies with a similar structure are also found over South Indian Ocean regions. In January–February, positive/negative events linked to the South Atlantic midlatitude mode are marked by meridional shifts (northward/southward) and weakening/strengthening of the ITCZ over the southern tropics, together with modulations in intensity (weakened/sustained) of the Angola low, which could act as a tropical source of moisture for Tropical Temperate Troughs (TTTs). In association with a strengthened/weakened zonal component of the southern extension of the African Easterly Jet (AEJ), this could modulate the meridional transfer of moisture south of 15°S to the advantage/detriment of Angolan coastal regions, where above/below rainfall are expected. Variations in the latitudinal position (northward/southward) of the South Atlantic anticyclone, and thus of the midlatitude westerlies, are also found to reduce/favour moisture advection towards southern Africa subtropics allowing the southern Indian trades to penetrate less/more over the subcontinent south of 25°S. This would create a situation where convection processes are inhibited/supported within the SICZ/TTTs region resulting in drier/wetter conditions locally for positive/negative events respectively.

What Determines the Position and Intensity of the South Atlantic Anticyclone in Austral Winter?—An AGCM Study

Abstract The South Atlantic anticyclone is a major feature of the austral winter climatology. An atmospheric general circulation model (AGCM) is used to study the dynamics of the South Atlantic anticyclone by means of control simulations and experiments to investigate sensitivity to prescribed orography, sea surface temperatures, and soil wetness. The South Atlantic anticyclone in the first control simulation is unrealistically zonally elongated and centered too far west—errors typical of coupled ocean–atmosphere GCMs. Results of the sensitivity experiments suggest that these deficiencies are associated with another family of systematic model errors: the overprediction of convection over the tropical land surfaces, particularly over eastern tropical Africa and India, and the concurrent large-scale westward shift in the divergence center at upper levels and the convergence center at lower levels. The results also confirm the important role of South American and African orography in localizing the South Atlantic anticyclone over the ocean. Other factors, however, like the regional zonal gradients of sea surface temperatures, are found to have only a minor impact on the anticyclone. To further substantiate these findings, the wintertime anticyclone is examined using a revised version of the atmospheric GCM. Improvements are found in both the anticyclone as well as the Asia–African summer monsoon circulations. The results demonstrate the existence of links between intensity and structure of the wintertime South Atlantic anticyclone and the major summer monsoons in the Northern Hemisphere.

Water vapour transport from the tropical Atlantic and summer rainfall in tropical southern Africa

An empirical orthogonal functions analysis of the onshore flow of moisture along the west coast of southern Africa using NCEP-DOE AMIP II Re-analyses suggests two dominant modes of variability that are linked to (a) variations in the circulation linked with the South Atlantic anticyclone (b) the intensity of the flow that penetrates from the tropical Atlantic. The second mode, referred as the Equatorial Westerly mode, contributes the most to moisture input from the Atlantic onto the subcontinent at tropical latitudes. Substantial correlations in austral summer between the Atlantic moisture flux in the tropics and rainfall over the upper lands surrounding the Congo basin suggest the potential role played by this zonal mode of water vapour transport. Composites for austral summer months when this Equatorial Westerly mode had a particularly strong expression, show an enhanced moisture input at tropical latitudes that feeds into the deep convection occurring over the Congo basin. Sustained meridional energy fluxes result in above normal rainfall east and south of the Congo belt. During years of reduced equatorial westerly moisture flux, a deficit of available humidity occurs in the southern tropics. A concomitant eastward shift of deep convection to the southwest Indian ocean and southeastern Africa, leads to below normal rainfall over the uplands surrounding the Congo basin.

Tropical South-East Atlantic response to ENSO as an ecosystem indicator for the southern Benguela

The response of the tropical South-East Atlantic Ocean to the El Niño Southern Oscillation (ENSO) was studied for the period 1950–1999. Interaction between the ocean thermocline 'see-saw' and the overlying zonal wind circulation was examined. The two are closely linked through equatorial upwelling dynamics, with the thermocline leading during most of the period. Composite differences in climatic structure in respect of ENSO phase were analysed. Cases were selected based on indices of Pacific sea surface temperature and South-East African rainfall. Prior to an El Niño-induced drought, anomalous easterly flow over the Atlantic strengthens the cold tongue extending from Angola. How these climatic conditions affect fisheries abundance in the southern Benguela was investigated. Higher anchovy Engraulis encrasicolus catches tend to follow an El Niño event, and higher sardine Sardinops sagax catches tend to follow a La Niña event, through the northward and southward shift respectively of the South Atlantic anticyclone and attendant coastal upwelling. In the southern Benguela, enhanced westerlies and warmer water favour anchovy catches over sardine catches. Recognition of the climatic patterns that impact fisheries abundance represents a potential management tool.

BARCHAN DUNES ON THE KUISEB RIVER DELTA, NAMIBIA

ABSTRACT The shape and behaviour of the barchan dunes on the Kuiseb River delta, south of the town of Walvis Bay, and the temporal and spatial variability of those characteristics are the focus of this study. Data from a field survey undertaken in August 1999 are used to investigate a number of salient aspects of dune morphology. The Kuiseb dunes are found to exhibit many of the same morphometric characteristics as those found in other dune fields around the world, such as the strong interrelationship between dune height and horn width (r = 0.853). Abnormalities in the transverse and longitudinal profiles of the dunes are noted however, and the causative factors investigated. Grain size analysis provides an insight into the intra-dune processes that may cause horn asymmetry and the absence of a slip face. Aerial photographs taken in 1961, 1976, 1988 and 1997, in conjunction with the results of the August 1999 field survey, are used to calculate rates and directions of barchan dune movement for 58 dunes between 1961 and 1997, and for 10 dunes from 1961 through to 1999. Between 1961 and 1997 the dunes moved at an average rate of 13.15m a′1, in an approximately northwards direction ([xbar]= 0.67°N). Over this period there has however been a substantial fluctuation in the rate of movement, from a minimum mean value of 4.24 m a″′ to a maximum of 18.9m a″1. The direction of dune movement has also fluctuated considerably between mean values of 12.58°N and 338.2°N. In the past two years rates of movement appear to have been very high, at 56.1m a″1, with the dunes moving in an average direction of 355.1 °N, although the data available for this stage has considerable limitations. Composite meteorological records derived from the National Centre for Environmental Prediction (NCEP) climate model suggest that the probable cause of this fluctuation in rates of dune movement (1961–97) is a variability in the regional wind regime. Comparative plots of mean wind speed during the three subsections of this 36 year period—1961 to 1976, 1976 to 1988 and 1988 to 1997—show significant changes in conditions. The first and last periods are characterised by markedly stronger mean winds, which can explain the faster rates of dune movement. This change in mean wind speed is related to fluctuations in the strength of the South Atlantic anticyclone, also illustrated by the NCEP data.

Atmospheric effects in the Angola‐Benguela frontal zone

The Angola‐Benguela Front is the generic border between, on the one hand, the Benguela Current and the coastal upwelling regime along the southwest African coastline and, on the other hand, the warm, southward flowing Angola Current. This frontal zone has a variable morphology, geographic location, and thermal characteristics. It has been suggested that this variability may be related to changing wind patterns. We have studied the thermal characteristics of the surface expression of the frontal zone for a 74‐day period using daily thermal infrared images and compared these to wind patterns from synoptic weather maps. There is little correlation between the location of the northern and the southern front that form the envelope for this frontal zone. The location of the southern front largely determines the thermal gradient across the entire frontal zone. Large short‐term displacements in the geographic location of the frontal zone as well as short‐term variations in its thermal characteristics are shown to be strongly correlated to the atmospheric pressure gradient of the South Atlantic anticyclone with a 1‐day lag period. This is true irrespective of the latitude changes of the center of the South Atlantic anticyclone.

Vulnerability of oceanic dune systems under wind pattern change scenarios in Uruguay

The dynamics of the large dune systems adjacent to lowlands and to the oceanic coastline in Uruguay is likely to be affected both by sea level rise and wind circulation patterns. A simple eolian sand transport model, which relates wind friction, frequency and direction to sand transport, was used to estimate annual sand transport as a measure of the impact of potential climate changes on the stability of dune systems and beaches. The model was adjusted to local conditions using real wind-matrix and eolian-transport data from the past 5 decades. The climate change scenarios considered assume a positive or negative 10% change in the frequency of the South Atlantic Anticyclone winds, since its position determines the near-surface wind circulation pattern. Results obtained at the Cabo Polonio study area indicate that, in the medium and long term, coastal recession is likely to occur due to a decrease in coastal dunefield activity induced by changes in wind pattern. It was also observed that, should current anthropogenic impact in the area remain unchanged, the situation would be equally serious even under a non-climate-change scenario. This is due to the fact that the forestation process that has taken place over the past 20 yr has caused the immobilization of a significant portion of the dune system, preventing natural sand transfer to the coast. In recent years a still insufficient deforestation effort, intended to stop coastline recession, has taken place.