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The influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied using satellite altimeter data and a 1.5-layer ocean model. The satellite observations suggest the northward shift of the STF leads to the AC leaping across the gap with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding path, while there are three flow patterns of AC for moderate latitude of the STF. The ocean model results suggest no hysteresis (associated with multiple equilibrium states) exists in the AC system. The model reproduces similar AC regimes depending on different gap widths as in the observations, and model results can be used to explain the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggests that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current paths, while the β and instantaneous vorticity terms are mainly balanced by the advection and viscosity terms for the AC in the eddy-shedding path. These findings help explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north–south shift of the STF in the leaping regime or penetrating regime. [ABSTRACT FROM AUTHOR]

Terrestrial‐marine dust fluxes, pedogenic carbonate δ13C values, and various paleovegetation proxies suggest that Africa experienced gradual cooling and drying across the Pliocene‐Pleistocene (Plio‐Pleistocene) boundary (2.58 million years ago [Ma]). However, the timing, magnitude, resolution, and relative influences of orbitally‐driven changes in high latitude glaciations and low latitude insolation differ by region and proxy. To disentangle these forcings and investigate equatorial eastern African climate across the Plio‐Pleistocene boundary, we generated a high‐resolution (∼3,000‐year) data set of compound‐specific n‐alkane leaf wax δ2H values—a robust proxy for atmospheric circulation and precipitation amount—from the HSPDP‐BTB13‐1A core, which spans a ∼3.3–2.6 Ma sequence in the Baringo‐Tugen Hills‐Barsemoi Basin of central Kenya. In combination with the physical sedimentology, our data indicate that precipitation varied strongly with orbital obliquity, not precession, during the late Pliocene, perhaps imparted by variations in the cross‐equatorial insolation gradient. We also observe a marked shift toward wetter conditions beginning ∼3 Ma that corresponds with global cooling, drying in western Australia, and a steepening of the west‐east zonal Indian Ocean (IO) sea surface temperature (SST) gradient. We propose that northward migration of the Subtropical Front reduced Agulhas current leakage, warming the western IO and causing changes in the IO zonal SST gradient at 3 Ma, a process that has been observed in the latest Pleistocene‐Holocene but not over longer timescales. Thus, the late Cenozoic moisture history of eastern Africa is driven by a complex mixture of low‐latitude insolation, the IO SST gradient, and teleconnections to distal high‐latitude cooling. Key Points: We present new high resolution (3,000‐year) leaf wax δ2H data from central Kenya spanning 3.3–2.6 million years before present (Ma)Pronounced 41,000‐year variability throughout is potentially attributable to the cross‐equatorial insolation gradientLong‐term (>200,000‐year) variability in eastern African hydroclimate tracks the zonal Indian Ocean surface temperature gradient [ABSTRACT FROM AUTHOR]

A high-resolution climate archive was reconstructed based on carbon isotope analysis and radiocarbon dating of the Chapman baobab in northeastern Botswana. The Chapman baobab, which exhibited an open ring-shaped structure composed of six stems, collapsed in January 2016 during an intense El Niño event. Two samples belonging to the oldest stems were investigated in order to obtain a proxy rainfall record, which provides insight into the precipitation regime over the last millennium, evincing centennial and decadal scale variability. The results indicate that the Medieval Warm Period was marked by relatively stable precipitation, whereas rainfall variability and drought frequency increased during the Little Ice Age. The investigated area has experienced both wetter and drier conditions in the past. The wettest conditions of the last millennium were registered before 1450 while the driest period occurred in 1835. For southern Africa, inter-annual rainfall variability is mainly associated with sea surface temperatures in the Agulhas Current core region, which determine the east–west displacement of tropical temperate troughs. Previous studies suggested that positive sea surface temperature anomalies in the Mozambique Channel led to an eastward movement of the troughs but the Chapman record demonstrates a westward displacement in the past, causing drought in northeastern South Africa and wetter conditions in the central part of southern Africa. The positive rainfall correlation with SST anomalies reversed after 1900, causing a gradual decrease in precipitation and confirming the current aridity trend for Botswana. The results contribute to a better understanding of the past climate of southern Africa for which paleoclimate reconstructions remain scarce. [ABSTRACT FROM AUTHOR]

The Pongola-Mtamvuna (PM) water management area in southeastern Africa is a highly biodiverse and sensitive region whose rainfall variability is not well understood. It is prone to drought and the occasional devastating flood events which are studied here for the early (October–December) and late summer rainy seasons (January–March) from 1984 to 2019 using CHIRPS daily rainfall data. Over 60% of the top 50 extreme rainfall events occurred during late summer (January–March) with tropical lows being by far the main contributor. The remainder are more or less equally distributed between mesoscale convective systems, tropical-extratropical cloud bands and cut-off lows, the latter contribute in October and November. There is considerable interannual variability in the numbers of these events during the record some of which is related to ENSO. Two prominent exceptions are the wet El Niño of OND 2006 and the dry La Niña of OND 2011, SST anomalies in the greater Agulhas Current region seem to have played an important role in both cases. Far fewer extreme events occurred from 2002 onwards than in the first two decades. However, 7 of the top 50 events occurred during the multi-year drought of 2007–2018 when almost all the late summers as well as many of the early summers experienced below average rainfall. The dry conditions during this period would have been much worse had 7 of the top 50 extreme events not occurred then, highlighting the importance of extreme rainfall event analysis for the region. [ABSTRACT FROM AUTHOR]

The Agulhas Current (AC) creates a sharp temperature gradient with the surrounding ocean, leading to a large turbulent flux of moisture from ocean to atmosphere. We use two simulations of the Weather Research and Forecasting (WRF) Model to show the seasonal impact of the warm core of the AC on southern Africa precipitation. In one simulation the sea surface temperature (SST) of the AC is similar to satellite observations, while the second uses satellite SST observations spatially smoothed to reduce the temperature of the core of the AC by ~1.5°C. We show that decreasing the SST of the AC reduces the precipitation of the wettest seasons (austral summer and autumn) inland. Over the ocean, reducing the SST reduces precipitation, low-level wind convergence, SST, and SLP Laplacians above the AC in all seasons, consistent with the pressure adjustment mechanism. Moreover, winter precipitation above the AC may also be related to increased latent flux. In summer and autumn, the AC SST reduction is also associated with decreased precipitation farther inland (more than 1.5 mm day−1), caused by an atmospheric circulation that decreases the horizontal moisture flux from the AC to South Africa. The reduction is also associated with higher geopotential height extending from the surface east and over the AC to the midtroposphere over southeastern Africa. The westward tilted geopotential height is consistent with the linear response to shallow diabatic heating in midlatitudes. An identical mechanism occurs in spring but is weaker. Winter rainfall response is confined above the AC. [ABSTRACT FROM AUTHOR]

High-resolution mapping, sampling and analysis of upper Quaternary southern African continental margin sediments recovered from beyond the Last Glacial Maximum shoreline (>130 m water depth) have expanded our understanding of how marine and terrestrial records are linked over glacial-interglacial climatic cycles. This paper synthesises data currently available from the deep seafloor around southern Africa and, specifically, core sites that demonstrate terrestrial sedimentological connectivity. Several proxies and case studies reveal the evolution of depositional systems, palaeoceanography and palaeoclimate over the last 191 kyr. Hydroacoustic mapping and investigations of submarine canyons have been carried out primarily on the eastern and southwestern margins, while palaeoceanographic productivity and microfossil assemblages have been applied most extensively on the western marine and southern Agulhas Bank. Studies on the western margin indicate that enhanced productivity, less oxygenated bottom waters and reduced marine faunal diversity in the transition to glacial periods, while glacial terminations are associated with reduced productivity and more oxygenated bottom waters. These changes, linked to palaeoceanography and late Quaternary sea-level fluctuations, influence the sedimentary record and sedimentation rates. On the eastern margin, sediment fluxes applied as proxies for rainfall offshore of the Great Kei, Umzimvubu, Limpopo and Zambezi rivers indicate that the southern African climate responds to changes in orbitally-modulated insolation and in particular, to the ~23 kyr precessional cycle, where the proxy records keep pace with this and then diverge at ~80 to 70 kyr. Since the penultimate glacial (Marine Isotope Stage/MIS 6), more humid conditions observed in southern Africa, as the Northern Hemisphere entered phases of rapid cooling, were potentially driven by a combination of warming in the Agulhas Current and shifts of the subtropical anticyclones. Broadly, the sedimentary records reviewed suggest fluctuations in climate and oceanographic circulation that are strongly correlated with the global benthic δ18O record, suggesting sensitivity to high-latitude forcing, and a strong influence of late Quaternary glacial-interglacial cycles despite these marine sites being far-removed from terrestrial environments. [ABSTRACT FROM AUTHOR]

During strong El Niño events, below-average rainfall is expected in large parts of southern Africa. The 1992 El Niño season was associated with one of the worst drought episodes in large parts of South Africa. Using reanalysis data set from NCEP-NCAR, this study examined circulation types (CTs) in Africa south of the equator that are statistically related to the El Niño signal in the southwest Indian Ocean and the implication of this relationship during the 1992 drought episode in South Africa. A statistically significant correlation was found between the above-average Nino 3.4 index and a CT that features widespread cyclonic activity in the tropical southwest Indian Ocean, coupled with a weaker state of the south Indian Ocean high-pressure. During the analysis period, it was found that the El Niño signal enhanced the amplitude of the aforementioned CT. The impacts of the El Niño signal on CTs in southern Africa, which could have contributed to the 1992 severe drought episode in South Africa, were reflected in (i) robust decrease in the frequency of occurrence of the austral summer climatology pattern of atmospheric circulation that favors southeasterly moisture fluxes, advected by the South Indian Ocean high-pressure; (ii) modulation of easterly moisture fluxes, advected by the South Atlantic Ocean high-pressure, ridging south of South Africa; (iii) and enhancement of the amplitude of CTs that both enhances subsidence over South Africa, and associated with the dominance of westerlies across the Agulhas current. Under the ssp585 scenario, the analyzed climate models suggested that the impact of radiative heating on the CT significantly related to El Niño might result in an anomalous increase in surface pressure at the eastern parts of South Africa. [ABSTRACT FROM AUTHOR]

This paper explores the Holocene climatic dynamics of South Africa's southern Cape, a region that supports a large proportion of the Greater Cape Floristic Region and contains an array of important archaeological sites. While South African climates are generally characterised by marked rainfall seasonality, the southern Cape is currently situated at the interface between tropical and temperate climate systems, resulting in a largely aseasonal rainfall regime. This regime, however, is thought to have been particularly sensitive to past changes in late Quaternary boundary conditions, meaning that variability in either tropical or temperate systems could have significant environmental impacts. Evidence of past climate change, however, remains limited. We present a 9000-year record of hydroclimatic variability obtained from rock hyrax midden stable nitrogen records, from Papkuilsfontein, on the southern slope of the Anysberg Mountains. Resolved to an average 6-year resolution and spanning the period c. 9050 cal yr BP to 1990 CE, this is the highest resolution Holocene record from southern Africa and presents a unique opportunity for the detailed study of the primary drivers and spatial gradients of Holocene climate change in the southern Cape. The data indicate a long-term decrease in aridity across the Holocene and a pattern of variability that reveals remarkable similarities with records from the South African tropics and El Niño–Southern Oscillation proxies, highlighting the significance of tropical systems as drivers of Holocene climate change in the region. This substantially expands what has been previously considered to be the zone of tropical influence, extending from a coastal phenomenon associated with heat transport via the Agulhas Current to encompass much, if not all, of the Agulhas Plain south of the Cape Fold Mountains. These findings provide a valuable new climatic framework for contextualizing changes in ecological and archaeological records in the southern Cape, and contribute to a more comprehensive understanding of the spatio-temporal dynamics of climate systems in southern Africa. • Sub-decadal 9000-year record of aridity from South Africa's southern Cape. • Reveals dominance of tropical forcing across the southern Cape and Agulhas Plain. • ENSO variability indicated to be important driver of regional climate change. • Presents high-resolution framework for study of human-environment interactions. [ABSTRACT FROM AUTHOR]

The oceanic circulation south of Africa is characterised by a complex dynamics with a strong variability due to the presence of the Agulhas current and numerous eddies. This area of interest is also the location of several natural gas fields under seafloor which are targeted for drilling and exploitation. The complex and powerful ocean currents induces significant issues for ship operations at the surface as well as under the surface for deep sea operations. Therefore, the knowledge of the state of the currents and the ability to forecast them in a realistic manners could greatly enforce the safety of various marine operation. Following this objective, an array of HF radar systems were deployed to allow a detailed knowledge of the Agulhas currents and its associated eddy activity. It is shown in this study that assimilation of HF radar allow to represent the surface circulation more realistically. Two kind of experiments have been performed, a one month analysis and nine consecutive forecast of two days each. The one month 4DVAR experiment have been compared to geostrophic currents issued from altimeters and highlight an important improvement of the geostrophic currents. Furthermore despite the restricted size of the area covered with HF radar, we show that the solution is improved almost in the whole domain, mainly upstream and downstream of the HF radar's covered area. We also show that while benefits of the assimilation on the surface current intensity is significantly reduced during the second day of forecast, the correction in direction persists after 48 h. [ABSTRACT FROM AUTHOR]

Moisture sources and pathways over the Limpopo River Basin (LRB) in southern Africa were identified using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model applied to NCEP II (2.5° × 2.5°) reanalysis data for 1981–2016. The 10-day air parcel backward trajectories were produced for the extended wet season (October–April) as well as the early and late summer. Analysis of a 36-year climatology of air parcel trajectories indicated seven moisture source regions for the LRB; namely, local continental, tropical southeast Atlantic Ocean, midlatitude South Atlantic Ocean, tropical northwest Indian Ocean, tropical southwest Indian Ocean, subtropical southwest Indian Ocean, and the Agulhas Current. The results have shown that important differences in moisture source regions and pathways exist between early (October–December) and late (January–April) summers, with the tropical northwestern Indian Ocean and the northern Agulhas Current sources more prominent during JFMA than OND. On interannual time scales, there are notable differences in moisture source regions between anomalously wet and dry summers, with the South Indian Ocean moisture contribution and transport over the LRB smaller during dry summers than wet summers. Changes in specific humidity for trajectory linked to heavy daily rainfall events (defined from CHIRPS data) showed that subtropical South Indian Ocean source is more extensive for the heavy rainfall than for the moderate case. Generally, moisture source regions and transport pathways for LRB tend to be influenced by both the regional summer season circulation and the synoptic setting. [ABSTRACT FROM AUTHOR]

The distribution of marine debris both with the open oceans and along coastlines is largely governed by surface currents and thereby dependent upon seasonal circulation patterns and annual variability inherent in the major current systems. The recovery of a satellite-tracked current drifter on a South African beach some 22 months after release off Western Australia was an indicator of the ocean-wide processes operating in the southern Indian Ocean gyre. Using this single drift trajectory in combination with data from other current drifters released along the Western Australian coast between 2008 and 2013, from earlier satellite-tracked drifters and a selection of historical drift bottle returns, drift patterns across the southern Indian Ocean between Western Australia and east Africa were analysed to quantify inter-continental debris transport rates via the South Equatorial Current (northern east-west pathway) and the South Indian Ocean Current (southern west-east pathway). The northern east-west trans-ocean pathway can be considered in three segments: the complex and highly variable Leeuwin Current eddy system in the east, which can retain drifting debris for up to a year, the comparatively uniform westward travel in the South Equatorial Current, and the eddying components in the western boundary current system off southern Africa. A simple Eddy Activity Index is derived here to clearly distinguish the Leeuwin Current eddy region from the South Equatorial Current. The southern route to Australia is simpler via the Agulhas Return Current and the South Indian Ocean Current. [ABSTRACT FROM AUTHOR]

[Display omitted] • A large-scale study investigating the complex genetic structure, species boundaries, evolutionary relationships, and diversification among all the species in the subfamily Acontinae. • The subfamily Acontinae is an African group of legless skinks which consists of multiple species complexes. • This study resolved many long-standing phylogenetic enigmas and clarified many blurry species boundaries within the subfamily Acontinae in general and within its species complexes. • Lineage diversification is associated with Miocene climatic oscillations, epeirogenic uplift, expansion of open habitats, variable rainfall patterns, and the presence of the warm Agulhas Current since the early Miocene, as well as their co-effects. • Possible "ghost introgression" and interspecies gene flow was detected in the subfamily Acontinae. Cladogenic diversification is often explained by referring to climatic oscillations and geomorphic shifts that cause allopatric speciation. In this regard, southern Africa retains a high level of landscape heterogeneity in vegetation, geology, and rainfall patterns. The legless skink subfamily Acontinae occurs broadly across the southern African subcontinent and therefore provides an ideal model group for investigating biogeographic patterns associated with the region. A robust phylogenetic study of the Acontinae with comprehensive coverage and adequate sampling of each taxon has been lacking up until now, resulting in unresolved questions regarding the subfamily's biogeography and evolution. In this study, we used multi-locus genetic markers (three mitochondrial and two nuclear) with comprehensive taxon coverage (all currently recognized Acontinae species) and adequate sampling (multiple specimens for most taxa) of each taxon to infer a phylogeny for the subfamily. The phylogeny retrieved four well-supported clades in Acontias and supported the monophyly of Typhlosaurus. Following the General Lineage Concept (GLC), many long-standing phylogenetic enigmas within Acontias occidentalis and the A. kgalagadi , A. lineatus and A. meleagris species complexes, and within Typhlosaurus were resolved. Our species delimitation analyses suggest the existence of hidden taxa in the A. occidentalis , A. cregoi and A. meleagris species groups, but also suggest that some currently recognized species in the A. lineatus and A. meleagris species groups, and within Typhlosaurus , should be synonymised. We also possibly encountered "ghost introgression" in A. occidentalis. Our inferred species tree revealed a signal of gene flow, which implies possible cross-over in some groups. Fossil evidence calibration dating results showed that the divergence between Typhlosaurus and Acontias was likely influenced by cooling and increasing aridity along the southwest coast in the mid-Oligocene caused by the opening of the Drake Passage. Further cladogenesis observed in Typhlosaurus and Acontias was likely influenced by Miocene cooling, expansion of open habitat, uplifting of the eastern Great Escarpment (GE), and variation in rainfall patterns, together with the effect of the warm Agulhas Current since the early Miocene, the development of the cold Benguela Current since the late Miocene, and their co-effects. The biogeographic pattern of the Acontinae bears close resemblance to that of other herpetofauna (e.g., rain frogs and African vipers) in southern Africa. [ABSTRACT FROM AUTHOR]

Aim To examine patterns of marine hydrozoan richness around southern Africa and to test the hypothesis that patterns of biogeography become weaker with increasing dispersal ability. Location The coastline of southern Africa from 21° S, 14° E to 28° S, 33° E, extending from the intertidal zone seawards a distance of 200 nautical miles. Methods Published and unpublished information on the distribution of marine Hydrozoa was entered as presence/absence data onto a gridded coastline of the region. A similarity matrix between the species composition of grid squares was constructed using the Bray–Curtis index and visualized using non-metric multidimensional scaling ordinations. Separate analyses were conducted, and compared, on the three major life cycle groupings: holoplanktic, meroplanktic and benthic. Results Over 450 species of marine Hydrozoa have been reported from the region, and species richness increases eastwards, in a manner at odds with the distribution of sampling effort. There was a significant correlation between the geographic structures of the resemblance matrices generated for the three life cycle groupings. In other words, all three groups showed similar patterns of biogeography around the region, and these were broadly similar to those reported by others. However, there were differences between them that reflect the resolution at which the data were examined. At a level of 40% similarity, there was no biogeographic structure to the holoplanktic fauna, the meroplanktic taxa were simply sub-divided into cool- and warm-temperate/subtropical elements, and in the case of benthic taxa, the cool-water fauna was further split into a southern Namaqua and a depauperate northern Namib component. Even at a resolution of 70% similarity, the holopelagic taxa could be separated only into cool-temperate and warm-temperate/subtropical faunas. Main conclusions Holoplanktic taxa show comparatively less biogeographic structure than meroplanktic taxa, which in turn show less clearly defined biogeographic structure than benthic taxa. It is suggested that this is related to the interaction between oceanography and dispersive-stage duration. The role that the Agulhas Current plays in influencing the Benguela Current fauna is highlighted. This study has implications for conservation planning exercises based on protecting representative biotopes in different biogeographic regions. [ABSTRACT FROM AUTHOR]

The Agulhas Bank on the tip of southern Africa, like other shelf seas, is a relatively productive environment which plays a crucial role in the biology and success of many commercially valuable fish species. Fish and their larvae depend on zooplankton to feed on but, despite their importance, little is known about zooplankton distribution and production on the Agulhas Bank. Here we present results from a survey conducted in March 2019 on the East and Central Agulhas Bank, investigating mesozooplankton abundance, biovolume, taxonomic composition, size distribution (normalised biovolume size spectrum (NBSS) approach) and secondary production. A clear cross-shore gradient was observed with the inner-shelf having higher abundance and biovolume of mesozooplankton dominated by small-size organisms, most likely mirroring higher overall productivity of the coastal waters, while the outer-shelf showed the opposite trend (i.e. , low abundance and biovolume; shallow NBSS slopes). This general pattern on the outer-shelf was, however, disturbed in one location (between 24 and 25°E) with a distinguishable mesozooplankton community, most likely linked to the passage of a meander on the inshore side of the Agulhas Current. The Central Agulhas Bank was typified by high mesozooplankton biomass (∼4 g C m-2), comparable to upwelling areas and dominated by copepods and doliolids. High copepod biomass was observed in this region before and was linked to a feature called the "Cold Ridge". However, during our survey, no such ridge was observed. The mixed layer depth was relatively deep (>20 m) and high Chlorophyll a concentration was measured at depth, despite low net primary production rates. We suggest that this region is prone to other mechanisms such as retention due to cyclonic circulation or processes injecting nutrients in the upper mixed layer that require further research. Secondary production on the Agulhas Bank was in the same range as other shelf seas (0.03–1.55 g C m-2 d-1) and was correlated with mesozooplankton biomass. The comparison of primary and secondary production, measured simultaneously, suggested that mesozooplankton exert a significant control on net primary production, and can, in some areas, be food-limited (e.g. , Central Agulhas Bank and inshore waters). • Mesozooplankton biomass on the Agulhas Bank in March was 0.1–4.27 g C m-2. • The inner-shelf reveals higher zooplankton biomass and steeper NBSS slopes. • The Central Agulhas Bank has high zooplankton biomass, comparable to upwelling areas. • Secondary production in March ranged from 0.03 to 1.55 g C m-2 d-1. [ABSTRACT FROM AUTHOR]

Abstract: The southern coasts of Africa are influenced by two major oceanic currents, leading to biogeographic patterns in inshore and offshore species assemblages, and in the stable isotope signatures of suspended particulate matter and filter-feeding mussels. We used the stable isotope ratios of carbon (13C/12C) and nitrogen (15N/14N) from the blood and feathers of adult and chick Cape gannets (Morus capensis) to investigate whether the geographic differences observed at the lower levels in the marine communities are deep penetrating effects that reach top predators. Additionally, we evaluated whether trophic segregation occurs between adult and reared chick gannets, and whether a shift to wintering habitat occurs in adults. The study was conducted during the 2006 breeding season on Bird Island in the Agulhas system, and on Malgas and Ichaboe Islands, in the south and north Benguela respectively. Our results showed significant differences in the isotope ratios of members of different colonies, but no intra-colony differences between tissues or age groups. These results indicate that there is neither age-related nor temporal segregation in the diet of members of the same colony. Feather isotopic values suggest that adults remain all year round in the same habitats, and do not undertake long migration after reproduction. Since all gannets tend to target similar prey, we attributed among-colony differences in isotope signatures mostly to the oceanic conditions experienced by the main prey of birds rather than substantial differences in diet composition. Overall, isotopic signatures segregate the two current systems, with depleted carbon values in the Agulhas and enriched nitrogen values in the upwelled waters of the Benguela. Within the Benguela birds from Ichaboe in the north had higher δ15N values than those from Malgas in the south, which we attributed to differences in the functioning of the upwelling cells in the vicinity of the two colonies. Finally, slight variation in the proportion of main prey and discards from fisheries may contribute to the variation in the stable-isotope signatures between colonies in the Benguela. [Copyright &y& Elsevier]

The inter-ocean leakage of heat and salt from the South Indian Ocean to the South Atlantic has important consequences for the global thermohaline circulation and in particular for the strength of overturning of the Atlantic Ocean as a whole. This leakage between these two subtropical gyres takes place south of Africa. The main mechanisms are the intermittent shedding of Agulhas rings from the retroflection of the Agulhas Current and the advection of Agulhas filaments from the border of the Agulhas Current, both of which move northwestward into the South Atlantic. The subsequent behaviour and mixing of Agulhas rings has been much studied over the past years, that of Agulhas filaments not at all.We report here on fortuitous hydrographic observations of the behaviour of an Agulhas filament that interacted with a number of mesoscale features shortly after formation. This suggests that Agulhas filaments may be involved in many other circulation elements and not only the Benguela upwelling front, as was surmised previously, and may mix out in a very site-specific region. [ABSTRACT FROM AUTHOR]

Several satellite-derived Sea Surface Temperature (SST) products were compared to determine their potential for research and monitoring applications around the southern African marine region. This study provides the first detailed comparison for the region, demonstrating good overall agreement (variance < 0.4 °C2) between merged SST products for most of the South African marine region. However, strong disagreement in absolute SST values (variance of 0.4–1.2 °C2 and differences of up to 6 °C) was observed at well-known oceanographic features characterized by complex temperature structures and strong SST gradients. Strong seasonal bias in the discrepancy between SST was observed and shown to follow seasonal increases in cloud cover or local oceanographic dynamics. Disagreement across the L4 products showed little dependence on their spatial resolutions. The periods of disagreement were characterized by large deviations among all products, which resulted mainly from the lack of input observations and reliance on interpolation schemes. This study demonstrates that additional methods such as the ingestion of additional in situ observations or daytime satellite acquisitions, especially along the west coast of southern Africa, might be required in regions of strong SST gradient, to improve their representations in merged SST products. The use of ensemble means may be more appropriate when conducting research and monitoring in these regions of high SST variance. [ABSTRACT FROM AUTHOR]

The geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments. International Ocean Discovery Program Expedition 361 drilled sites along the eastern margin of southern Africa that potentially provide archives of rainfall on the continent as well as dispersal in the Agulhas Current. We used standard methods to remove carbonate and ferromanganese oxides and Stokes settling to isolate the clay fractions. In comparison to most previous studies that aimed to extract the detrital signal from marine sediments, we additionally applied a cation exchange wash using CsCl as a final step in the sample preparation. The motivation behind the extra step, not frequently applied, is to remove ions that are gained on the clay surface due to adsorption of authigenic trace metals in the ocean or during the leaching procedure. Either would alter the composition of the detrital fraction if no cation exchange was applied. Moreover, using CsCl will provide an additional measure of the cation exchange capacity (CEC) of the samples. However, no study so far has evaluated the potential and the limitations of such a targeted protocol for marine sediments. Here, we explore the effects of removing and replacing adsorbed cations on the clay surfaces with Cs+, conducting measurements of the chemical compositions, and radiogenic isotopes on a set of eight clay sample pairs. Both sets of samples underwent the same full leaching procedure except that one batch was treated with a final CsCl wash step. In this study, organic matter was not leached because sediments at IODP Site U1478 have relatively low organic content. However, in general, we recommend including that step in the leaching procedure. As expected, significant portions of elements with high concentrations in seawater were replaced by Cs+ (2SD 2.8%.) from the wash, including 75% of the sodium and approximately 25% of the calcium, 10% of the magnesium, and 8% of the potassium. Trace metals such as Sr and Nd, whose isotopes are used for provenance studies, are also found to be in lower concentrations in the samples after the exchange wash. The exchange wash affected the radiogenic isotope compositions of the samples. Neodymium isotope ratios are slightly less radiogenic in all the washed samples. Strontium and Pb isotopes showed significant deviations to either more or less radiogenic values in different samples. The radiogenic isotopes from the CsCl-treated fractions gave more consistent correlations with each other, and we suggest this treatment offers a superior measure of provenance. Although we observed changes in the isotope ratios, the general trend in the data and hence the overall provenance interpretations remained the same. However, the chemical compositions are significantly different. We conclude that a leaching protocol including a cation exchange wash (e.g. CsCl) is useful for revealing the terrestrial fingerprint. CEC could, with further calibration efforts, be useful as a terrestrial chemical weathering proxy. • Application of CsCl-wash leads to significant change in geochemical signature of the sediment. • Elements in high concentrations in seawater are replaced by Cs+ from the wash. • Cation exchange wash reveals a truer terrigenous signal. • The cation exchange capacity of the sediment may provide additional environmental information. [ABSTRACT FROM AUTHOR]

The Agulhas Current, the largest Western Boundary Current in the Southern Hemisphere, strongly influences the oceanographic conditions of its adjacent shelf system. As a result of limited societal drivers, such as large-scale fisheries and marine mining, the South African southeast shelf, between Port Elizabeth and Port Shepstone, is one of the least studied and under-sampled shelf systems in Southern Africa. This study describes hydrographic conditions from the first two shelf-wide high-resolution surveys (January/February and July/August 2017) of this region. These data provide a baseline of environmental conditions and hydrographic processes across the shelf. The Agulhas Current, as well as cyclonic mesoscale eddies along the inshore edge of the Current, were observed to strongly drive conditions on the shelf. Substantial differences in the impacts of these cyclonic eddies were evident, with the smaller less intense January/February eddy inducing much stronger upwelling than the larger more intense July/August eddy. Indications of seasonality in temperature, salinity and chlorophyll-a were observed in the surface layers. Surface waters were 4 °C warmer in January/February than in July/August. Considerably lower salinities (33.77 g kg−1) were observed in January/February as a result of high rainfall and subsequent increased river outflow. Upwelling mechanisms such as divergence-induced upwelling, Ekman veering in the bottom boundary layer, Ekman pumping associated with cyclonic eddies, were observed to uplift colder, nutrient-rich water from deeper layers onto the mesotrophic shelf. These colder waters ultimately reached the surface as a result of wind-driven Ekman transport and vertical mixing. Tropical Surface Water, Subtropical Surface Water and South Indian Central Water were observed on the shelf. Red Sea Water as well as Antarctic Intermediate Water generally occupied the slope regions except where cyclonic eddies uplifted intermediate waters onto the shelf. The shelf-wide dissolved oxygen levels (>3 ml l−1) suggested that oxygen availability adequately facilitated the survival of the existing shelf biology. • The Agulhas Current and its meanders strongly impacted shelf hydrography. • Divergence, Ekman veering, and Ekman pumping upwelled deeper waters onto the shelf. • Wind-driven Ekman transport and vertical mixing uplifted deeper waters to the surface. • Seasonality in temperature, salinity and chlorophyll-a were observed. • Substantial differences in impacts of Agulhas Current meanders were evident. [ABSTRACT FROM AUTHOR]

Southern east Africa is prone to some extreme weather events and interannual variability of the hydrological cycle, including tropical cyclones and heavy rainfall events. Most of these events occur during austral summer and are linked to shifts in the intertropical convergence zone, changes in El Niño Southern Oscillation signatures, sea surface temperature and sea level pressure. A typical example include mesoscale convective systems (MCSs) that occur between October and March along the eastern part, adjacent to the warm waters of Mozambique Channel and Agulhas Current. In this study we discuss a heavy rainfall event over southern Africa, focusing particularly on the period 15–20 January 2013, the period during which MCSs were significant over the subcontinent. This event recorded one of the historic rainfalls due to extreme flooding and overflows, loss of lives and destruction of economic and social infrastructure. An active South Indian Convergence Zone was associated with the rainfall event sustained by a low-level trough linked to a Southern Hemisphere planetary wave pattern and an upper-level ridge over land. In addition, also noteworthy is a seemingly strong connection to the strength of the African Easterly Jet stream. Using rainfall data, satellite imagery and re-analysis (model processed data combined with observations) data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs. The low-level trough and upper-level ridge contributed to moisture convergence, particularly from tropical South East Atlantic Ocean, which in turn contributed to the prolonged life span of the rainfall event. Positive temperature anomalies favored the substantial contribution of moisture fluxes from the Atlantic Ocean. This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa. [ABSTRACT FROM AUTHOR]