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The hydrodynamics of coral reefs strongly influences their biological functioning, impacting processes such as nutrient availability and uptake, recruitment success and bleaching. For example, coral reefs located in oligotrophic regions depend on upwelling for nutrient supply. Coral reefs at Sodwana Bay, located on the east coast of South Africa, are an example of high latitude marginal reefs. These reefs are subjected to complex hydrodynamic forcings due to the interaction between the strong Agulhas current and the highly variable topography of the region. In this study, we explore the reef scale hydrodynamics resulting from the bathymetry for two steady current scenarios at Two-Mile Reef (TMR) using a combination of field data and numerical simulations. The influence of tides or waves was not considered for this study as well as reef-scale roughness. Tilt current meters with onboard temperature sensors were deployed at selected locations within TMR. We used field observations to identify the dominant flow conditions on the reef for numerical simulations that focused on the hydrodynamics driven by mean currents. During the field campaign, southerly currents were the predominant flow feature with occasional flow reversals to the north. Northerly currents were associated with greater variability towards the southern end of TMR. Numerical simulations showed that Jesser Point was central to the development of flow features for both the northerly and southerly current scenarios. High current variability in the south of TMR during reverse currents is related to the formation of Kelvin-Helmholtz type shear instabilities along the outer edge of an eddy formed north of Jesser Point. Furthermore, downward vertical velocities were computed along the offshore shelf at TMR during southerly currents. Current reversals caused a change in vertical velocities to an upward direction due to the orientation of the bathymetry relative to flow directions. Highlights: A predominant southerly current was measured at Two-Mile Reef with occasional reversals towards the north. Field observations indicated that northerly currents are spatially varied along Two-Mile Reef. Simulation of reverse currents show the formation of a separated flow due to interaction with Jesser Point with Kelvin–Helmholtz type shear instabilities along the seaward edge. [ABSTRACT FROM AUTHOR]

Direct observations of turbulent mixing in energetic ocean currents are fundamentally important to improving the understanding of stratified turbulence and the predictive capability of ocean models. In this study, we have selected an energetic region of the Western Indian Ocean to make simultaneous measurements of temperature and velocity to directly infer the geography of turbulent mixing on the east coast of South Africa. Direct measurements of the rate of turbulent kinetic energy dissipation, ϵ , were recorded by an autonomous underwater vehicle glider platform and vertical microstructure profiler (VMP). The glider was deployed in the Agulhas current in 2015 over a 200 km transect and measured approximately 250 vertical profiles of turbulent microstructure. Additional vertical profiles near the shelf were obtained from approximately 85 VMP casts during 2018 in a region where the powerful Agulhas current is formed and interacts with the shelf. VMP and glider data collected at Sodwana Bay and the Agulhas current showed evidence of elevated dissipation rates 10 - 8 < ϵ < 10 - 6 Wkg - 1 . Our results indicate that in general turbulent mixing in the Agulhas is concentrated spatially in pancake-shaped patches. Ordinary Kriging was used to interpolate between consecutive profiles and showed that the horizontal scale of the turbulent patches was typically O(1 km) while the vertical scales of the patches ranged between 0.01 and 10 m. Article Highlights: Direct measurements of turbulent dissipation in the strong persistent Agulhas current are presented for the first time using sensors fitted on a glider and vertical microstructure profiler. Ordinary Kriging revealed the geography of mixing and that turbulent mixing in the Agulhas is concentrated spatially in pancake-shaped patches. VMP data collected at Sodwana showed evidence of elevated dissipation rates 10 - 8 < ϵ < 10 - 6 Wkg - 1 . [ABSTRACT FROM AUTHOR]

In this paper, we study the region of the Agulhas leakage origin. The study region is limited by 20–46° S, 0–24° E; it includes the Cape Basin and is crossed by Walvis Ridge. This region is characterized by high dynamic activity of mesoscale eddies and variability of hydrographic parameters in a wide range of spatial and temporal scales. The main element of large-scale circulation is represented by a phenomenon called the Agulhas leakage which is influenced by the South Atlantic Gyre, Benguela Current, and Benguela upwelling. Water particles of various origins are mixed and affect the thermohaline properties of mesoscale eddies, which form a source of the Agulhas leakage. We divided the region into three zones and showed that eddies and their properties differ in each of them. The Agulhas leakage is formed only by long-lived eddies, mainly anticyclones, while cyclones have smaller trajectories and shorter lifespans. In this research, we apply statistical analysis using AMEDA (Angular Momentum Eddy Detection Algorithm), Lagrangian analysis, and the study of vertical thermohaline cross-sections. We establish that in the Cape Basin, the waters of the Agulhas leakage mix with waters of the South Atlantic Gyre and the Benguela Current, and the thermohaline properties of the Agulhas eddies change since the warm and salty waters of the Indian Ocean mix with fresher and colder ones of the Atlantic Ocean. The water particles of the South Atlantic Gyre cross the western border of the region and travel a distance exceeding 500 km in the eastern direction mixing with other particles. We demonstrate that the eddies generated in the Cape Basin are also significantly influenced by the waters of the South Atlantic Gyre. These waters are transported to the Cape Basin region from the west and southwest. This explains the existence of a two-mode water structure noted by other researchers. [ABSTRACT FROM AUTHOR]

Using a spatiotemporal dataset of dissolved lead (dPb) from the subtropical oceans surrounding South Africa, this study quantifies the exchange of dPb between the Indian and Atlantic Oceans. Despite the absence of a major Pb source within the South Atlantic sector and the complete phase-out of leaded petroleum in Southern Africa, the ecologically important southeast Cape Basin shows an elevated surface dPb concentration (21–30 pmol kg−1). We estimated up to 90% of the measured dPb in surface waters of the Cape Basin was delivered from the Indian Ocean via the Agulhas Current (AC). Eddy dynamics and leakage at Agulhas retroflection result in an increased Pb flux from winter to summer, while a long-term (2008–2019) temporal change in dPb in the AC-derived water of Cape Basin was contemporaneous to a change in atmospheric Pb emissions from South Africa. The South African-origin atmospheric Pb, however, contributes first to the Agulhas waters in the West Indian Ocean, which is then transported to the South Atlantic, thereby regulating the dPb inventory of the Cape Basin. This indirect mechanism of Pb transfer emphasizes the importance of regulating Pb emissions from Southern Africa to protect rich fishing grounds associated with the Benguela marine ecosystem. [ABSTRACT FROM AUTHOR]

This paper is a continuation of the authors' research [5]. We apply some aspects of the theory about strong horizontal elongation of large-scale mesoscale vortices in the World Ocean based on the theory ellipsoidal vortices to oceanic conditions. Two areas are considered, characterized by increased eddy manifestation: the Lofoten Basin in the Norwegian Sea and the Agulhas Current area. We also discuss the condition of the Atlantic Ocean and the World Ocean. The main objective of this study is to check the feasibility of the physical conditions for unbounded vortex elongation in the real ocean and estimate the fraction of mesoscale ocean vortices that are elongated into filaments with energy redistributed from the mesoscale to the submesoscale. We estimate the share of vortices in the World Ocean that are strongly elongated. We propose maps of the geographical location of unlimited vortex elongation zones and interpret the results. We also describe the effect of a decrease in the self-energy of vortices due to the tensile action of the background flow. We propose that it is a possible mechanism for energy transfer from the vortex to the flow and a manifestation of the negative viscosity effect. [ABSTRACT FROM AUTHOR]

In this paper we study the seasonal and interannual variability of the morphometric characteristics of the Benguela upwelling—shape, area, and length—using satellite measurements of the ocean surface temperature (sear surface temperature (SST)) for 1985–2016. The northern part of the Benguela Upwelling (NBU) (18°–26° S) is characterized by an increase in area from January to July and a minimum in August–December. In the northernmost part of the upwelling (18°–21° S), the largest area is observed in April–August, while it takes on minimal values in January–February and October–November. These changes are associated with the intensification of the southern Angola current, which occurs in February and October, the effect of which can be traced up to 24° S. The maximum in the southern part (28–32°C) of the Benguela upwelling (SBU) is observed for 1–2 months later in May–June, and SBU practically disappears in September–November. In the autumn months, the jet of the northern current of the Agulhas presses against the coast of Africa, contributing to a sharp decrease in the SBU area. On interannual scales, there is a pronounced variability in the upwelling area and the time of its peak. In this case, there is a trend towards an earlier onset of upwelling after 2010. In the northernmost part of the studied area, high values of the upwelling width (150 km) were observed at latitudes of 19°–22° S up to 2000. Then they sharply decrease and the average width decreases almost three times to 50 km. At the same time, there was a sharp increase in the area by the same amount of ~100 km2 at latitudes 23°–28° S. Thus, there was a sharp change in the upwelling shape, while the total upwelling area remained constant. The weakening and decrease in upwelling in its northernmost part is probably caused by the long-term strengthening of the Angola current, which is observed by altimetry data. [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 South African coast is known to harbor four different species of intertidal oribatid mites and their distribution strongly correlates with marine ecoregions. Relatively little is known about the dispersal of these organisms and how populations of different locations are connected. To test dispersal abilities and connectivity of these South African species, we performed a morphometric and molecular genetic study. COI gene sequences of two of the widely distributed South African intertidal oribatid mite species revealed clearly contrasting patterns. Halozetes capensis, which occurs in the Agulhas Ecoregion, shows distinct genetic structuring, whereas Fortuynia elamellata micromorpha, which is distributed in the Natal Ecoregion, exhibits gene flow between all populations. The paleoenvironmental history and specific ocean current pattern are suggested to be responsible for these patterns. During the last glacial maximum, the colder climate and the weakening of the Agulhas Current possibly resulted in a bottleneck in the warm-adapted F. e. micromorpha populations, but the subsequent global warming allowed the populations to expand again. The cold-adapted H. capensis populations, on the other hand, experienced no dramatic changes during this period and thus could persist in the Agulhas Ecoregion. Considering transport on ocean currents, the Agulhas Current could be further responsible for the connectivity between the Fortuynia populations. But the deflection of this current in the Agulhas Ecoregion could support the isolation of Halozetes populations. The concomitant morphometric study demonstrated morphological homogeneity among populations of Fortuynia and thus confirms strong connectivity. The Halozetes populations, on the other hand, form two different morphological groups not reflecting geography. [ABSTRACT FROM AUTHOR]

Climate change impacts on maize production in South Africa, i.e., interannual yield variabilities, are still not well understood. This study is based on a recently released reanalysis of climate observations (AgERA5), i.e., temperature, precipitation, solar radiation, and wind speed data. The study assesses climate change effects by quantifying the trend of agrometeorological indicators, their correlation with maize yield, and analyzing their spatiotemporal patterns using Empirical Orthogonal Function. Thereby, the main agrometeorological factors that affected yield variability for the last 31 years (1990/91–2020/21 growing season) in major maize production provinces, namely Free State, KwaZulu-Natal, Mpumalanga, and North West are identified. Results show that there was a significant positive trend in temperature that averages 0.03–0.04 °C per year and 0.02–0.04 °C per growing season. There was a decreasing trend in precipitation in Free State with 0.01 mm per year. Solar radiation did not show a significant trend. Wind speed in Free State increased at a rate of 0.01 ms−1 per growing season. Yield variabilities in Free State, Mpumalanga, and North West show a significant positive correlation (r > 0.43) with agrometeorological variables. Yield in KwaZulu-Natal is not influenced by climate factors. The leading mode (50–80% of total variance) of each agrometeorological variable indicates spatially homogenous pattern across the regions. The dipole patterns of the second and the third mode suggest the variabilities of agrometeorological indicators are linked to South Indian high pressure and the warm Agulhas current. The corresponding principal components were mainly associated with strong climate anomalies which are identified as El Niño and La Niña events. [ABSTRACT FROM AUTHOR]

When compared to tide-dominated shelves, shelves swept by geostrophic flows are relatively understudied, yet geostrophic currents have the potential to construct substantial current-generated shelf bedforms. This paper examines the evolution of a series of bedforms encountered along the narrow Agulhas Current-swept northern KwaZulu-Natal shelf. Their evolution is placed in the context of progressive current impingement and variation in flows associated with the postglacial transgression and culminating in the present-day highstand. Ultra-high resolution seismic reflection, multibeam, side-scan sonar and single beam data sets reveal several bedform scales and morphologies; wave ripples and shoreface-connected ridges are associated only with the inner shelf, and 2D and 3D very large dunes are more cosmopolitan and span the entire shelf sector. The inner shelf is marked by rock outcrop (aeolianite), surrounded by sandy sediment, grading seaward into sediment starved bedforms associated with bioclastic gravels. Where sufficient sediment exists in local depocenters, very large dunes form discontinuous fields along the outer shelf. The seismic stratigraphy of the outer shelf reflects the effects of postglacial flooding by rising sea levels and increasing current impingement by the Agulhas Current. The Holocene wave ravinement surface is overlain by flat-lying strata (early dune development and dune amalgamation with first current exposure), in turn covered by hummocky, sub-horizontal aggrading beds (amalgamation), overlain by inclined cross-bedded packages (lee faces of the bedforms formed during migration and full current interaction). Morphometric analyses show that for both the inner and outer shelf, no relationships exist between water depth, wavelength and spacing. Height to spacing (H/L) relationships are weak but nevertheless show a broadly positive trend. Bedform heights are lower on the inner shelf compared to the outer shelf, but bedform spacing is greater on the inner shelf, with a 40% overlap in H/L indices observed between the two areas. The departures in overlap can be linked to the competing offshore Agulhas Current and the inshore wave-dominated processes. Bedforms of the inner shelf plot below the global H/L mean and can be related to the more infrequent incursions of the Agulhas Current core. At the time of survey, the Agulhas Current was likely situated well offshore, resulting in reduced current activity in the survey area resulting in rounding of the dune crests, degradation of the dune crests and trough infilling. On the outer shelf, the H/L values plot above the mean global trend, suggesting vertical accretion due to the faster currents. Crest rounding and downlapping of the upstream lee faces onto the downstream stoss faces indicate dune degradation, which is related to a seaward location of the Agulhas Current at the time of survey. [ABSTRACT FROM AUTHOR]

This study investigates the spatial scale dependence of relationship between turbulent surface heat flux (SHF) and sea surface temperature (SST) variations in the mid-latitude frontal zones, subtropical gyres, and tropical Indo-western Pacific region in winter and summer with daily observational data. A comparison of the SHF and SST/SST tendency correlation between 1° and 4° spatial scale displays a decrease of the positive SHF–SST correlation and an increase of the negative SHF–SST tendency correlation as the spatial scale increases in all the above regions. The lead–lag SHF and SST/SST tendency correlation at different spatial scales illustrates an obvious transition from the oceanic forcing to the atmospheric forcing in the western boundary currents (WBCs) and the Agulhas Return Current (ARC) in both winter and summer. The transition length scale is smaller in summer than in winter, around 2.6°–4.5° in winter and around 0.8°–1.3° in summer based on the OAFlux data. In the subtropical gyres and tropical Indo-western Pacific region, atmospheric forcing dominates up to 10° spatial scale with the magnitude of forcing increasing with the spatial scale in both winter and summer except for the Arabian Sea in summer. The Arabian Sea distinguishes from the other tropical regions in that the SST forcing dominates up to more than 10° spatial scale in summer with the magnitude of forcing decreasing slowly with the spatial scale increase. [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]

The present study examined the relationship between turbulent surface heat flux (SHF) and sea surface temperature (SST) variations using daily observational data. The SHF and SST relationship displays notable differences between winter and summer and prominent time-scale dependence in both seasons. In the mid-latitude SST frontal regions, SST has a larger role in driving SHF in winter than in summer. In the subtropical gyre regions, SHF plays a larger role in the SST change in summer than in winter. In winter, SHF has a larger effect on the SST change in the South China Sea than in the Arabian Sea and Bay of Bengal. In summer, the SST effect on SHF is dominant in the Arabian Sea, whereas the SHF impact on SST is dominant in the Philippine Sea. In the Gulf Stream, Kuroshio Extension and Agulhas Return Current, the SST effect extends up to 90-day time scales in winter, the SHF impact is limited to time scales below 20 days and the SST effect is dominant on time scales above 20 days in summer. In winter, the SHF effect extends up to 90-day time scales in the Bay of Bengal, South China Sea, and Philippine Sea, but is limited to time scales below 40 days in the Arabian Sea. In summer, the SST effect extends up to 90-day time scales in the Arabian Sea, whereas the SHF and SST effect is large on time scales shorter and longer than 40 days, respectively, in the Philippine Sea. [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]

A shallow mixed layer depth bias in Austral winter in the Subantarctic Zone is a common feature of Coupled Model Intercomparison Project (CMIP5) models, including the Community Earth System Model (CESM). The bias is related to other deficiencies in the model solution, including too-weak Subantarctic Mode water formation and excessive leakage of Agulhas waters into the Atlantic instead of into the Indian Ocean and Subantarctic Frontal Zone. This work investigates the hypothesis that the shallow bias is due to errors in the simulated ocean circulation. Results from a model with low resolution ocean component (1° grid) are compared against: (i) an experiment with an eddy resolving (0.1°) grid and (ii) experiments using the 1° grid and employing an adiabatic nudging of the ocean pressure field to observations that is referred to as a semi-prognostic method. For both the higher resolution and semi-prognostic experiments, improved horizontal advection of warm and salty water near the surface leads to a more realistic sea surface temperature (SST) and salinity front associated with the Agulhas Return Current and Antarctic Circumpolar Current. The warmer surface waters in the high-resolution model and semi-prognostic model lead to stronger air–sea heat loss, with a response of about 40 Wm−2 °C−1 in winter in the area of deep mixed layers. Budgets of the temperature and salinity stratification show that the deeper mixed layers in the high-resolution experiment are primarily a result of the increased surface heat loss, whereas in the semi-prognostic case salinity advection is the main factor leading to destabilization of the water column. Both results indicate that ocean circulation is a key factor in wintertime deep Southern Ocean mixing, associated with advection of water masses and air–sea feedbacks. [ABSTRACT FROM AUTHOR]

The study of mesoscale eddies provides an understanding of entire systems of interconnected oceanic characteristics. Mesoscale eddies have their own dynamics, which are dominated by nonlinear effects. Unlike waves, they can transfer heat, mass, kinetic energy, and biochemical characteristics from the region of their formation over vast distances, influencing climate fluctuations. "Agulhas leakage" refers to the waters exported from the Indian Ocean to the Atlantic Ocean by the Agulhas system of currents. These waters consist mainly of upper and intermediate waters of Indo-Oceanic origin. Mesoscale eddies formed by the Agulhas Current are the dominant structures that transport the waters of the Indian Ocean to the Atlantic. This work studies the Agulhas leakage via a comprehensive analysis of altimetry maps and Argo float data. Argo floats captured by the flow allow the vertical structure of Agulhas leakage eddies to be studied. It is found that the mesoscale eddies of the Agulhas leakage travel thousands of kilometers across the Atlantic while retaining their structure. It is shown that the mean Agulhas leakage transport by one mesoscale eddy is 8.5 Sv. The transport of heat and salt by an individual Agulhas leakage eddy is 2.25 × 109 W and 5.36 × 105 kg s−1, respectively. The heat and salt anomalies inside the eddy are 2.03 × 1015 J and 4.83 × 1011 kg, respectively. [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]

Despite the efforts of the modelling community to improve the representation of the sea surface temperature (SST) over the South Eastern Tropical Atlantic, warm biases still persist. In this work we use four different configurations of the fully-coupled AWI Climate Model (AWI-CM) which allow us to gain physics-based insight into the role of the oceanic and atmospheric resolutions of the model in the regional distribution of the SST. Our results show that a sole refinement of the oceanic resolution reduces warm biases further than a single increase of the atmospheric component. An increased oceanic resolution is required (i) to simulate properly the Agulhas Current and its associated rings; (ii) to reinforce the northward-flowing Benguela Current and (iii) to intensify coastal upwelling. The best results are obtained when both resolutions are refined. However, even in that case, warm biases persist, reflecting that some processes and feedbacks are still not optimally resolved. Our results indicate that overheating is not due to insufficient upwelling, but rather due to upwelling of waters which are warmer than observations as a result of an erroneous representation of the vertical distribution of temperature. Errors in the representation of the vertical temperature profile are the consequence of a warm bias in the simulated climate state. [ABSTRACT FROM AUTHOR]

The Western Indian Ocean results from a complex geological history, implying both plate motion since about 180 million years and volcanic hot-spot activity since about 68 million years. This region exhibits a myriad of islands different in nature and origin, including continental remnants (Madagascar, the granitic Seychelles), high volcanic islands (Reunion, Mauritius, Rodrigues, the Comoros), high- and low-lying carbonate reef platforms (the carbonate Seychelles, the French Scattered Island Group, Agalega Islands) and a few submerged reef banks (Geyser, Nazareth, Saya de Malha). This region contains all known types of reefs—fringing, barrier, atoll, platform and bank. The data about reef development history over the Quaternary were gained mainly from two distinct sources: boreholes and emerged outcrops. Reef drilling indicates that modern reefs started regionally to grow from about 10,000–9600 years BP. Rates of vertical reef accretion ranges from 0.2 to about 10 mm/year according to exposure and shape of coral builders. The sea-level curves, reconstructed from vertical reef-accretion and lagoon-deposition curves, express a rapid rise in sea level at rates averaging 6 mm/year between 10,000 and 7500–7000 years BP, with a stabilization at the present position around 2500–3000 years BP. There are significant variations in the composition and distribution of dominant reef-building coral assemblages and facies (framework versus detritus) according to exposure to wave energy. Frameworks composed of robust coral branching assemblages—mainly Acropora robusta, A. humilis groups; Isopora palifera—dominate in higher energy settings, while those with domal and tabular branching forms—mainly Porites, various faviids, Acropora hyacinthus group—are best represented in lower-energy to protected environments. Detritus-reefs are best developed in sheltered sites. The recent contribution of some dominant coral species in modern WIO reefs seem to have resulted from their delayed arrival in the region as controlled by the regional prevailing surface circulation—South Equatorial Current and Agulhas Current—and/or by the competency period of their larvae. Coral-based, geochemical records suggest that, during the mid Holocene, the seasonal variability of sea surface temperatures was lower than today. The reconstruction of the WIO Pleistocene reef history is chiefly based on a limited number of exposed reef remains deposited during the last interglacial MIS 5.5 stage (129,000–125,000 years BP), at present culminating at elevations ranging from − 20 to + 9 m with respect to present mean sea level. Such differences in elevation can be explained by a differential tectonic behaviour of reef-bearing substrates, from stable continental areas to subsiding volcanic islands. The existence of older reef generations (Plio-Pleistocene) was identified from coral-rich inclusions trapped into pyroclastites of Comoro volcanoes. [ABSTRACT FROM AUTHOR]

The article provides a summary of the first National Oceanography Scholars' Meet, called Samudra Manthan 2023, which was organized by the National Institute of Oceanography in Goa, India. The event aimed to bring together young oceanographers and experts in the field to discuss research findings, provide training on instruments and techniques, and share the latest discoveries and insights in marine biology, oceanography, and climate science. The conference featured oral and poster presentations, invited lectures, and a field excursion for participating students. Overall, the event was successful in promoting oceanic knowledge, sustainability, and conservation, highlighting India's dedication to marine science and the importance of protecting marine ecosystems. [Extracted from the article]

Frontolysis mechanisms by which surface heat flux relaxes the sea surface temperature (SST) front in the eastern Japan Sea (JS) are investigated in detail using observational datasets. On the warm southern side of the front, larger air–sea specific humidity and temperature differences induce stronger turbulent heat release compared to the cool northern side. As a result, stronger wintertime cooling and weaker summertime warming occur south of the front, and the meridional gradient in the surface net heat flux (NHF) tends to relax the SST front throughout the year. In the mixed-layer deepening phase (September–January), a higher entrainment velocity occurs on the warm southern side because of weaker stratification. Since the resulting thicker mixed layer on the southern side is less sensitive to surface cooling, the mixed layer depth (MLD) gradient damps the frontolysis by the NHF gradient. In the shoaling phase (April–June), a deeper mixed layer south of the front is caused by the weaker warming and reduced sensitivity of the thicker mixed layer to a shoaling effect by shortwave radiation. Owing to weaker sensitivity of the thicker mixed layer on the southern side to surface warming, the MLD gradient enhances the frontolysis by the NHF gradient. Therefore, it is shown that the mixed layer processes cause seasonality of weaker (stronger) frontolysis by surface heat fluxes, damping (enhancing) the frontolysis by the NHF gradient in winter (summer). This study reveals unique features of the frontolysis in the eastern JS compared with the Agulhas Return Current and Kuroshio Extension regions. [ABSTRACT FROM AUTHOR]

Since the end of the twentieth century, the global mean surface temperature (GMST) exhibited a shift from a rapid warming to an unexpected deceleration. An anomalous heat was transported from the Pacific Ocean into the Indian Ocean through a strengthened Indonesian Throughflow during the same period. Within this background, it is essential to continue tracking the fate of the anomalous heat arriving in the Indian Ocean to form a comprehensive picture of the global ocean energy redistribution. The anomalous heat may continue flowing westward into the Atlantic Ocean along the main pathway of the regional ocean currents via the South Equatorial Current (SEC) and Agulhas Current. However, here we examine an alternate pathway: a southward heat transport in conjunction with a weakened SEC, diverting the canonical westward transport. This additional transport pathway causes an increase in heat content in the South Indian Ocean mid-latitudes (15–30°S, 95–110°E), may contribute to the Southern Ocean warming, and intensifies hemispheric asymmetry of oceanic heat content. The heat increase has important climate impacts such as changes to rainfall and increased coral bleaching over the western coast of Australia. The new path discovered here may be an essential route of heat transport linking the tropical Indo-Pacific Ocean and the Southern Ocean in 2003–2012. [ABSTRACT FROM AUTHOR]

A complex and highly dynamical ocean region, the Agulhas Current System plays an important role in the transfer of energy, nutrients and organic material from the Indian to the Atlantic Ocean. Its dynamics are not only important locally, but affect the global ocean-atmosphere system. In working towards improved ocean reanalysis and forecasting capabilities, it is important that numerical models simulate mesoscale variability accurately—especially given the scarcity of coherent observational platforms in the region. Data assimilation makes use of scarce observations, a dynamical model and their respective error statistics to estimate a new, improved model state that minimises the distance to the observations whilst preserving dynamical consistency. Qualitatively, it is unclear whether this minimisation directly translates to an improved representation of mesoscale dynamics. In this study, the impact of assimilating along-track sea-level anomaly (SLA) data into a regional Hybrid Coordinate Ocean Model (HYCOM) is investigated with regard to the simulation of mesoscale eddy characteristics. We use an eddy-tracking algorithm and compare the derived eddy characteristics of an assimilated (ASSIM) and an unassimilated (FREE) simulation experiment in HYCOM with gridded satellite altimetry-derived SLA data. Using an eddy tracking algorithm, we are able to quantitatively evaluate whether assimilation updates the model state estimate such that simulated mesoscale eddy characteristics are improved. Additionally, the analysis revealed limitations in the dynamical model and the data assimilation scheme, as well as artefacts introduced from the eddy tracking scheme. With some exceptions, ASSIM yields improvements over FREE in eddy density distribution and dynamics. Notably, it was found that FREE significantly underestimates the number of eddies south of Madagascar compared to gridded altimetry, with only slight improvements introduced through assimilation, highlighting the models’ limitation in sustaining mesoscale activity in this region. Interestingly, it was found that the threshold for the maximum eddy propagation velocity in the eddy detection scheme is often exceeded when data assimilation relocates an eddy, causing the algorithm to interpret the discontinuity as eddy genesis, which directly influences the eddy count, lifetime and propagation velocity, and indirectly influences other metrics such as non-linearity. Finally, the analysis allowed us to separate eddy kinetic energy into contributions from detected mesoscale eddies and meandering currents, revealing that the assimilation of SLA has a greater impact on mesoscale eddies than on meandering currents. [ABSTRACT FROM AUTHOR]

Investigating the variability of Agulhas leakage, the volume transport of water from the Indian Ocean to the South Atlantic Ocean, is highly relevant due to its potential contribution to the Atlantic Meridional Overturning Circulation as well as the global circulation of heat and salt and hence global climate. Quantifying Agulhas leakage is challenging due to the non-linear nature of this process; current observations are insufficient to estimate its variability and ocean models all have biases in this region, even at high resolution . An Eulerian threshold integration method is developed to examine the mechanisms of Agulhas leakage variability in six ocean model simulations of varying resolution. This intercomparison, based on the circulation and thermohaline structure at the Good Hope line, a transect to the south west of the southern tip of Africa , is used to identify features that are robust regardless of the model used and takes into account the thermohaline biases of each model. When determined by a passive tracer method, 60 % of the magnitude of Agulhas leakage is captured and more than 80 % of its temporal fluctuations, suggesting that the method is appropriate for investigating the variability of Agulhas leakage. In all simulations but one, the major driver of variability is associated with mesoscale features passing through the section. High resolution ( $${<} 1/10^{\circ }$$ ) hindcast models agree on the temporal (2-4 cycles per year) and spatial (300-500 km) scales of these features corresponding to observed Agulhas Rings. Coarser resolution models ( $${<} 1/4^{\circ }$$ ) reproduce similar time scale of variability of Agulhas leakage in spite of their difficulties in representing the Agulhas rings properties. A coarser resolution climate model ( $$2^{\circ }$$ ) does not resolve the spatio-temporal mechanism of variability of Agulhas leakage. Hence it is expected to underestimate the contribution of Agulhas Current System to climate variability. [ABSTRACT FROM AUTHOR]

Detailed mechanisms for frontogenesis/frontolysis of the Agulhas Return Current (ARC) Front, defined as the maximum of the meridional sea surface temperature (SST) gradient at each longitude within the ARC region (40°-50°E, 55°-35°S), are investigated using observational datasets. Due to larger (smaller) latent heat release to the atmosphere on the northern (southern) side of the front, the meridional gradient of surface net heat flux (NHF) is found throughout the year. In austral summer, surface warming is weaker (stronger) on the northern (southern) side, and thus the NHF tends to relax the SST front. The weaker (stronger) surface warming, at the same time, leads to the deeper (shallower) mixed layer on the northern (southern) side. This enhances the frontolysis, because deeper (shallower) mixed layer is less (more) sensitive to surface warming. In austral winter, stronger (weaker) surface cooling on the northern (southern) side contributes to the frontolysis. However, deeper (shallower) mixed layer is induced by stronger (weaker) surface cooling on the northern (southern) side and suppresses the frontolysis, because the deeper (shallower) mixed layer is less (more) sensitive to surface cooling. Therefore, the frontolysis by the NHF becomes stronger (weaker) through the mixed layer processes in austral summer (winter). The cause of the meridional gradient of mixed layer depth is estimated using diagnostic entrainment velocity and the Monin-Obukhov depth. Furthermore, the above mechanisms obtained from the observation are confirmed using outputs from a high-resolution coupled general circulation model. Causes of model biases are also discussed. [ABSTRACT FROM AUTHOR]

The Agulhas current system transports warm and salty water masses from the Indian Ocean into the Southern Ocean and into the Atlantic. The transports impact past, present, and future climate on local and global scales. The size and variability, however, of the respective transports are still much debated. In this study, an idealized model based twin experiment is used to study whether sea surface height (SSH) anomalies estimated from reflected signals of the Global Navigation Satellite System reflectometry (GNSS-R) can be used to determine the internal water mass properties and transports of the Agulhas region. A space-borne GNSS-R detector on the International Space Station (ISS) is assumed and simulated. The detector is able to observe daily SSH fields with a spatial resolution of 1-5. Depending on reflection geometry, the precision of a single SSH observation is estimated to reach 3 cm (20 cm) when the carrier phase (code delay) information of the reflected GNSS signal is used. The average precision over the Agulhas region is 7 cm (42 cm). The proposed GNSS-R measurements surpass the radar-based satellite altimetry missions in temporal and spatial resolution but are less precise. Using the estimated GNSS-R characteristics, measurements of SSH are generated by sampling a regional nested general circulation model of the South African oceans. The artificial observations are subsequently assimilated with a 4DVAR adjoint data assimilation method into the same ocean model but with a different initial state and forcing. The assimilated and the original, i.e., the sampled model state, are compared to systematically identify improvements and degradations in the model variables that arise due to the assimilation of GNSS-R based SSH observations. We show that SSH and the independent, i.e., not assimilated model variables velocity, temperature, and salinity improve by the assimilation of GNSS-R based SSH observations. After the assimilation of 90 days of SSH observations, improvements in the independent variables cover the whole water column. Locally, up to 39 % of the original model state are recovered. Shorter assimilation windows result in enhanced reproduction of the observed and assimilated SSH but are accompanied by an insufficient or wrong recovery of sub-surface water properties. The assimilation of real GNSS-R observations, when available, and consequently the estimation of Agulhas water mass properties and the leakage of heat and salt into the Atlantic will benefit from this model-based study. [ABSTRACT FROM AUTHOR]

The greater Agulhas Current is one of the most energetic current systems in the global ocean. It plays a fundamental role in determining the mean state and variability of the regional marine environment, affecting its resources and ecosystem, the regional weather and the global climate on a broad range of temporal and spatial scales. In the absence of a coherent in-situ and satellite-based observing system in the region, modelling and data assimilation techniques play a crucial role in both furthering the quantitative understanding and providing better forecasts of this complicated western boundary current system. In this study, we use a regional implementation of the Hybrid Coordinate Ocean Model and assimilate along-track satellite sea level anomaly (SLA) data using the Ensemble Optimal Interpolation (EnOI) data assimilation scheme. This study lays the foundation towards the development of a regional prediction system for the greater Agulhas Current system. Comparisons to independent in-situ drifter observations show that data assimilation reduces the error compared to a free model run over a 2-year period. Mesoscale features are placed in more consistent agreement with the drifter trajectories and surface velocity errors are reduced. While the model-based forecasts of surface velocities are not as accurate as persistence forecasts derived from satellite altimeter observations, the error calculated from the drifter measurements for eddy kinetic energy is significantly lower in the assimilation system compared to the persistence forecast. While the assimilation of along-track SLA data introduces a small bias in sea surface temperatures, the representation of water mass properties and deep current velocities in the Agulhas system is improved. [ABSTRACT FROM AUTHOR]

The frontal structure in the region south of Africa is investigated on the basis of CTD and SADCP measurements along the SR02 hydrophysical section carried by the R/V Akademik Ioffe in December of 2009 from the Cape of Good Hope to 57° S at the Prime Meridian. Eleven jets of the Antarctic Circumpolar Current (ACC) were revealed along the section. These were six jets of the Subantarctic Current (SAC), three jets of the South Polar Current (SPC), and two jets of the Southern Antarctic Current (SthAC). The jet combining the Weddell Front and the Southern Boundary of the ACC was also revealed. All the jets of the SPC based on the data of direct measurements were joined into a single 'superjet.' The others were manifested by the local velocity maxima in the surface layer of the ocean. The subtropical water along the section from the Southern Subtropical Front to the Shelf-Slope Front near the African shore was almost completely represented by the Indian Ocean (Agulhas Retroflection) water modified by mixing with the fresher water of the southeastern periphery of the Subtropical Atlantic. [ABSTRACT FROM AUTHOR]

The Agulhas Current (AC) is the strongest western boundary current in the Southern Hemisphere and is key for weather and climate patterns, both regionally and globally. Its heat transfer into both the midlatitude South Indian Ocean and South Atlantic is of global significance. A new composite coral record (Ifaty and Tulear massive Porites corals), is linked to historical AC sea surface temperature (SST) instrumental data, showing robust correlations. The composite coral SST data start in 1660 and comprise 200 years more than the AC instrumental record. Numerical modelling exhibits that this new coral derived SST record is representative for the wider core region of the AC. AC SSTs variabilities show distinct cooling through the Little Ice Age and warming during the late 18th, 19th and 20th century, with significant decadal variability superimposed. Furthermore, the AC SSTs are teleconnected with the broad southern Indian and Atlantic Oceans, showing that the AC system is pivotal for inter-ocean heat exchange south of Africa. [ABSTRACT FROM AUTHOR]

ARGO hydrographic profiles, two hydrographic transects and satellite measurements of air-sea exchange parameters were used to characterize the properties and seasonal heat budget variations of the Surface Mixed Layer (SML) south of Africa. The analysis distinguishes the Subtropical domain (STZ) and the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) of the Antarctic Circumpolar Current. While no Subantarctic Mode Water forms in that region, occurrences of deep SML (up to ∼450 m) are observed in the SAZ in anticyclones detached from the Agulhas Current retroflection or Agulhas Return Current. These are present latitudinally throughout the SAZ, but preferentially at longitudes 10-20° E where, according to previous results, the Subtropical Front is interrupted. Likely owing to this exchange window and to transfers at the Subantarctic Front also enhanced by the anticyclones, the SAZ shows a wide range of properties largely encroaching upon those of the neighbouring domains. Heat budget computations in each zone reveal significant meridional changes of regime. While air-sea heat fluxes dictate the heat budget seasonal variability everywhere, heat is mostly brought through lateral geostrophic advection by the Agulhas Current in the STZ, through lateral diffusion in the SAZ and through air-sea fluxes in the PFZ and AZ. The cooling contributions are by Ekman advection everywhere, lateral diffusion in the STZ (also favoured by the ∼10° breach in the Subtropical Front) and geostrophic advection in the SAZ. The latter likely reflects an eastward draining of water warmed through mixing of the subtropical eddies. [ABSTRACT FROM AUTHOR]

The transport of warm and salty Indian Ocean waters into the Atlantic Ocean—the Agulhas leakage—has a crucial role in the global oceanic circulation and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC). There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages and model studies that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies. A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing, but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a high-resolution ocean general circulation model to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contributed to the observed salinification of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC. [ABSTRACT FROM AUTHOR]

Ice cores extracted from the Antarctic ice sheet suggest that glacial conditions, and the relationship between isotopically derived temperatures and atmospheric have been constant over the last 800,000 years of the Late Pleistocene epoch. But independent lines of evidence, such as the extent of Northern Hemisphere ice sheets, sea level and other temperature records, point towards a fluctuating severity of glacial periods, particularly during the more extreme glacial stadials centred around 340,000 and 420,000 years ago (marine isotope stages 10 and 12). Previously unidentified mechanisms therefore appear to have mediated the relationship between insolation, CO2 and climate. Here we test whether northward migration of the subtropical front (STF) off the southeastern coast of South Africa acts as a gatekeeper for the Agulhas current, which controls the transport of heat and salt from the Indo-Pacific Ocean to the Atlantic Ocean. Using a new 800,000-year record of sea surface temperature and ocean productivity from ocean sediment core MD962077, we demonstrate that during cold stadials (particularly marine isotope stages 10 and 12), productivity peaked and sea surface temperature was up to 6 °C cooler than modern temperatures. This suggests that during these cooler stadials, the STF moved northward by up to 7° latitude, nearly shutting off the Agulhas current. Our results, combined with faunal assemblages from the south Atlantic show that variable northwards migration of the Southern Hemisphere STF can modulate the severity of each glacial period by altering the strength of the Agulhas current carrying heat and salt to the Atlantic meridional overturning circulation. We show hence that the degree of northwards migration of the STF can partially decouple global climate from atmospheric partial pressure of carbon dioxide, , and help to resolve the long-standing puzzle of differing glacial amplitudes within a consistent range of atmospheric . [ABSTRACT FROM AUTHOR]

Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC). Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic (RAPID, at latitude 26.5° N, and MOVE, at latitude 16° N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer-term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades. Inferences based on long-term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of ±1.5–3 Sv (1 Sv = 106 m3 s-1) on decadal timescales in the subtropics. Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water. Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south. [ABSTRACT FROM AUTHOR]

Abstract  Using a combination of Quick Scatterometer (QuikSCAT), Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E), and Lagrangian drifter measurements, we demonstrate that wind data alone are not sufficient to derive ocean surface stress (momentum flux) over mid-latitude ocean fronts, specifically the Kuroshio Extension. There was no continuous and large-scale stress measurement over ocean until the launch of the scatterometers. Stress had been derived from winds through a drag coefficient, and our concept of stress distribution may be largely influenced by our knowledge of wind distribution. QuikSCAT reveals that the variability of stress could be very different from wind. The spatial coherence between the magnitude of stress and sea surface temperature (SST), between the divergence of surface stress and the downwind SST gradient, and between the vorticity of stress and crosswind SST gradient, are the inherent characteristics of stress (turbulence production by buoyancy) that would exist even under a uniform wind field. The coherence between stress vorticity and SST gradient is masked by the rotation of ocean currents over the Kuroshio meanders. Surface stress rotates in the opposite direction to surface currents because stress is the vector difference between wind and current. The results are in agreement with a previous study of the Agulhas Extension and confirm the unique stress measuring capability of the scatterometer. [ABSTRACT FROM AUTHOR]

The interocean transfer of thermocline water between the Indian and the Atlantic Oceans known as 'Agulhas leakage' is of global significance as it influences the Atlantic Meridional Overturning Circulation (AMOC) on different time scales. Variability in the Agulhas Current regime is key in shaping hydroclimate on the adjacent coastal areas of the African continent today as well as during past climates. However, the lack of long, continuous records from the proximal Agulhas Current region dating beyond the last glacial cycle prevents elucidation of its role in regional and wider global climate changes. This is the first continuous record of hydrographic variability (SST; δ18Osw) from the Agulhas Current core region spanning the past 270,000 years. The data set is analytical sound and provides a solid age model. As such, it can be used by paleoclimate scientists, archaeologists, and climate modelers to evaluate, for example, linkages between the Agulhas Current system and AMOC dynamics, as well as connections between ocean heat transport and Southern African climate change in the past and its impact on human evolution. Measurement(s) temperature of sea surface • salinity Technology Type(s) gas chromatography of ketone fractions • gas chromatography • Magnesium-to-calcium ratio measurements of planktonic foraminifera • stable isotope analysis • glycerol dialkyl glycerol tetraether (GDGT) analysis • computational modeling technique Factor Type(s) temporal measurement Sample Characteristic - Environment hydroclimate • ocean Sample Characteristic - Location Agulhas Current Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13019678 [ABSTRACT FROM AUTHOR]

The spiny lobster Panulirus homarus rubellus is endemic to the Southwest Indian Ocean, where it inhabits a narrow continental shelf between an exposed coast and the upper reaches of the strong western-boundary Agulhas Current. Long-lived phyllosoma larvae released in this dynamic ocean environment have an uncertain fate—they can be retained over the shelf by sub-mesoscale processes, dispersed downstream along the coast or across the Mozambique Channel, or become entrained in the Agulhas Current, and presumably lost. To assess gene flow and population genetic structure, we analyzed mitochondrial cytochrome oxidase subunit 1 and hypervariable control region sequences, and 19 nuclear microsatellite loci obtained from lobsters collected at nine sites in eastern South Africa, Mozambique and southeast Madagascar. Clustering analyses confirmed genetic connectivity among all populations, and gene flow patterns supported the hypothesis that nearshore processes, such as lee eddies and counter currents, retain some phyllosomas over the shelf; whereas, net gene flow direction was moderate towards the southwest. The Mozambique Channel did not impede contemporary gene flow from Madagascar to the African shelf, but return gene flow was rare. Different marker types showed contrasting gene flow patterns during contemporary and evolutionary periods, when Pleistocene glacial/interglacial cycles would have affected sea level, ocean currents and dispersal patterns. Despite genetic connectivity and the importance of local recruitment, recent gene flow suggests an ancillary source/sink dynamic concordant with the prevailing southwesterly direction of boundary currents at the shelf-edge—a factor to consider in regional fisheries and conservation strategies. [ABSTRACT FROM AUTHOR]

The case of a freak wave collision with the ship in the Agulhas current is described. The explanation of the appearance of the freak waveas a result of wind-wave transformation in the Agulhas current is given. Swell is captured and intensified by the counter-current and is located in the neighbourhood of the maximum value of the current velocity, as a result of which there is a great concentration of wave-energy density. The superposition of wind and sea with swell transformedby the current promotes the formation of the freak waves. Using a simple mathematical analysis, an optimal ship track is proposed which could reduce the risk of collision with a freak wave. [ABSTRACT FROM AUTHOR]