Agulhas Region Research Articles

Impact of Agulhas Leakage on the Atlantic Overturning Circulation in the CCSM4

Abstract The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30–40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed.

Phylogeographic patterns and cryptic speciation across oceanographic barriers in South African intertidal fishes

Biogeographic boundaries are the meeting zone of broadly distributed faunas, or the actual cause of a faunal break. In the latter case, closely related sister species should be found across such a boundary. To achieve such a situation, preliminary stages are expected, where phylogeographic breaks followed by genetic cryptic speciation would be observed. Biogeographic boundaries, in the Cape Point/Cape Agulhas region of southern Africa, offer an ideal system to test such predictions. Here, we studied two intertidal clinid fish species that are endemic to southern Africa, Clinus superciliosus (n = 127) and Muraenoclinus dorsalis (n = 114). Using mitochondrial control region, 16S rRNA, 12S rRNA and NADH2 genes and the nuclear rhodopsin and the first intron of the S7 ribosomal protein gene, we show both phylogeographic breaks and likely cryptic speciation in each species. Pairwise Φ(st) results suggest population genetic structuring for both species, with higher levels for M. dorsalis (Φ(st) = 0.34-0.93) than for C. superciliosus (Φ(st) = 0.1-0.74). Further, we recover two and three distinct lineages within M. dorsalis and C. superciliosus, respectively. Phylogenetic topologies, concordance between nuclear and mitochondrial markers and levels of sequence divergence, which are consistent with closely related sister species pairs, suggest the presence of cryptic species. Our results therefore meet the expectation for reduced gene flow at a biogeographic barrier, which translates into significant genetic breaks and cryptic sister species.

Advective Time Scales of Agulhas Leakage to the North Atlantic in Surface Drifter Observations and the 3D OFES Model

Abstract The advective transit time of temperature–salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “supertrajectories” of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas the numerical supertrajectories can be used to study the full three-dimensional circulation. Results for the surface circulation indicate that the supertrajectories starting in the Agulhas Current and ending in the North Atlantic take at least 4 yr and most complete the journey in 30–40 yr. This time scale is, largely because of convergence and subduction in the subtropical gyres, longer than the 10–25 yr it takes the 3D numerical supertrajectories to complete the journey.

Advective Time Scales of Agulhas Leakage to the North Atlantic in Surface Drifter Observations and the 3D OFES Model

AbstractThe advective transit time of temperature–salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “supertrajectories” of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas th...

Flux comparison of Eulerian and Lagrangian estimates of Agulhas leakage: A case study using a numerical model

Estimating the magnitude of Agulhas leakage, the volume flux of water from the Indian to the Atlantic Ocean, is difficult because of the presence of other circulation systems in the Agulhas region. Indian Ocean water in the Atlantic Ocean is vigorously mixed and diluted in the Cape Basin. Eulerian integration methods, where the velocity field perpendicular to a section is integrated to yield a flux, have to be calibrated so that only the flux by Agulhas leakage is sampled. Two Eulerian methods for estimating the magnitude of Agulhas leakage are tested within a high-resolution two-way nested model with the goal to devise a mooring-based measurement strategy. At the GoodHope line, a section halfway through the Cape Basin, the integrated velocity perpendicular to that line is compared to the magnitude of Agulhas leakage as determined from the transport carried by numerical Lagrangian floats. In the first method, integration is limited to the flux of water warmer and more saline than specific threshold values. These threshold values are determined by maximizing the correlation with the float-determined time series. By using the threshold values, approximately half of the leakage can directly be measured. The total amount of Agulhas leakage can be estimated using a linear regression, within a 90% confidence band of 12 Sv. In the second method, a subregion of the GoodHope line is sought so that integration over that subregion yields an Eulerian flux as close to the float-determined leakage as possible. It appears that when integration is limited within the model to the upper 300 m of the water column within 900 km of the African coast the time series have the smallest root-mean-square difference. This method yields a root-mean-square error of only 5.2 Sv but the 90% confidence band of the estimate is 20 Sv. It is concluded that the optimum thermohaline threshold method leads to more accurate estimates even though the directly measured transport is a factor of two lower than the actual magnitude of Agulhas leakage in this model.

A weaker Agulhas Current leads to more Agulhas leakage

Time series of transports in the Agulhas region have been constructed by simulating Lagrangian drifter trajectories in a 1/10 degree two‐way nested ocean model. Using these 34 year long time series it is shown that smaller (larger) Agulhas Current transport leads to larger (smaller) Indian‐Atlantic inter‐ocean exchange. When transport is low, the Agulhas Current detaches farther downstream from the African continental slope. Moreover, the lower inertia suppresses generation of anti‐cyclonic vorticity. These two effects cause the Agulhas retroflection to move westward and enhance Agulhas leakage. In the model a 1 Sv decrease in Agulhas Current transport at 32°S results in a 0.7 ± 0.2 Sv increase in Agulhas leakage.

Lagrangian validation of numerical drifter trajectories using drifting buoys: Application to the Agulhas system

The skill of numerical Lagrangian drifter trajectories in three numerical models is assessed by comparing these numerically obtained paths to the trajectories of drifting buoys in the real ocean. The skill assessment is performed using the two-sample Kolmogorov–Smirnov statistical test. To demonstrate the assessment procedure, it is applied to three different models of the Agulhas region. The test can either be performed using crossing positions of one-dimensional sections in order to test model performance in specific locations, or using the total two-dimensional data set of trajectories. The test yields four quantities: a binary decision of model skill, a confidence level which can be used as a measure of goodness-of-fit of the model, a test statistic which can be used to determine the sensitivity of the confidence level, and cumulative distribution functions that aid in the qualitative analysis. The ordering of models by their confidence levels is the same as the ordering based on the qualitative analysis, which suggests that the method is suited for model validation. Only one of the three models, a 1/10° two-way nested regional ocean model, might have skill in the Agulhas region. The other two models, a 1/2° global model and a 1/8° assimilative model, might have skill only on some sections in the region.

Mesoscale perturbations control inter‐ocean exchange south of Africa

The quantification of inter‐ocean leakage from the South Indian to the South Atlantic Ocean is an important measure for the role of the Agulhas system in the global thermohaline circulation. To explore the specific role of mesoscale variability (such as Agulhas rings and Mozambique eddies) in this process a high‐resolution model (based on NEMO‐ORCA) for the Agulhas region has been set up. It is nested into a global coarse‐resolution model. The high‐resolution nest captures all salient features of the greater Agulhas region, including the upstream perturbations of the Agulhas Current and Natal Pulses along the African coast. A comparison of the inter‐ocean exchange in the high‐resolution nest with its coarse resolution counterpart reveals that the latter significantly over‐estimates the amount of water flowing into the Atlantic Ocean, demonstrating the need to explicitly simulate the mesoscale features. A sensitivity experiment that excludes the upstream perturbations revealed no difference in the amount of inter‐ocean exchange. However, the realistic representation of Agulhas rings and their drift path into the South Atlantic depends on the simulation of those upstream perturbations.

Cover Caption

Cover description: The Silver‐edge Pincushion (Leucospermum patersonii) occurs on limestone soils in the Agulhas region at the southern tip of Africa. It is one of 381 taxa of Proteaceae which occur in the species‐rich Fynbos Biome of the Cape Floral Kingdom. They were the focus of a 10‐year atlas project (http://www.sanbi.org/protea) involving over 500 amateurs on the subcontinent, who collected over 250 000 records from 65 000 localities. For over 15 years proteas have been used to guide systemic and objective conservation planning and management in the region. Photo: Justine Grantham.

Agulhas ring injection into the South Atlantic during glacials and interglacials

Abstract. Recent proxies suggest that, at the end of the last glacial, there was a significant increase in the injection of Agulhas rings into the South Atlantic (SA). This brought about a dramatic increase in the salt-influx (from the Indian Ocean) into the SA helping re-start the then-collapsed meridional overturning cell (MOC), leading to the termination of the Younger Dryas (YD). Here, we propose a mechanism through which large variations in ring production take place. Using nonlinear analytical solutions for eddy shedding, we show that there are restricted possibilities for ring detachment when the coast is oriented in the north-south direction. We define a critical coastline angle below which there is rings shedding and above which there is almost no shedding. In the case of the Agulhas region, the particular shape of the African continent implies that rings can be produced only when the retroflection occurs beyond a specific latitude where the angle is critical. During glaciation, the wind stress curl (WSC) vanished at a latitude lower than that of the critical angle, which prohibited the retroflection from producing rings. When the latitude at which the WSC vanishes migrated poleward towards its present day position, the corresponding coastline angle decreased below the critical angle and allowed for a vigorous production of rings. Simple process-oriented numerical simulations (using the Bleck and Boudra model) are in satisfactory agreement with our results and enable us to affirm that, during the glacials, the behavior of the Agulhas Current (AC) was similar to that of the modern East Australian Current (EAC), for which the coastline slant is supercritical.

Aquatic systems in and adjacent to Agulhas National Park with particular reference to the fish fauna

The study aimed to determine the distribution and relative abundance of freshwater fishes in and adjacent to Agulhas National Park. Fourteen fish species were collected during surveys carried out in the Heuningnes and Ratel river systems and a variety of endorheic pans from 2003 to 2005. Seven of the species are marine fishes tolerant of low salinity (Mugil cephalus, Liza richardsonii, Monodactylus falciformis, Caffrogobius gilchristi, Rhabdosargus holubi, Solea bleekeri, Gilchristella aestuaria) with the remaining seven species being primary freshwater fishes. Three are indigenous, including Pseudobarbus burchelli, Sandelia capensis, and Galaxias zebratus of which two morphological forms were recorded within Agulhas National Park. The remaining four species are invasive aliens (Cyprinus carpio, Lepomis macrochirus, Micropterus salmoides, Micropterus punctulatus). Classification using physico-chemical variables indicated four major wetland groups, with fishes occurring predominantly in the group comprising palustrine, lacustrine and riverine wetlands, and in one instance in a brackish endorheic pan. Management actions which should facilitate conservation of indigenous fishes in the Agulhas region are suggested.

Extreme wet years over southern Africa: Role of Indian Ocean sea surface temperatures

Southern Africa is a predominantly semiarid region with a high degree of interannual rainfall variability. Although much of the recent climate research has focused on the causes of drought events, the region has also experienced extremes of above average rainfall, the most recent examples being the major flooding episodes that devastated Mozambique during 2000 and 2001. This paper investigates extremely wet years over southern Africa during the twentieth century. Focusing on the two most extreme years, 1974 and 1976, we show that while ENSO serves as an important control on rainfall variability, a specific pattern of SSTs in the SW Indian Ocean, with warm anomalies in the subtropical SW Indian Ocean and cool anomalies in the northern SW Indian Ocean that is statistically independent of ENSO, plays a crucial role in generating extreme conditions. To do this, we use a series of multimodel experiments, to demonstrate first the importance of global sea surface temperatures. Through additional idealized experiments with HadAM3, we then isolate the role of SST anomalies in the Indian Ocean. The anomalies are based on the observed SSTs with the ENSO signal linearly removed. The critical influence is tied to cold SST anomalies in the Mascarene region which induce an anomalous anticyclonic circulation driving an anomalous low‐level easterly moisture flux along 10–20°S into eastern southern Africa. This results in enhanced moist convective uplift, conducive to enhanced rainfall, over a large part of southern Africa. Near surface humidity and 500‐hPa omega fields extend from eastern southern Africa into the Agulhas region in a tropical‐temperate cloud band like structure. The similarity between the reanalysis fields for the extreme years and the model experiments is striking.

Temporal Changes in Ocean Eddy Transports

Abstract New estimates from 11 yr of altimetric data are made of the global time-average variability kinetic energy and its decadal-scale variability. Making the approximation that the variability reflects primarily eddy motions, a time-mean, but spatially varying, eddy mixing coefficient is then estimated along with its changes over the last decade. With a record length more than 2 times that previously available, the time-mean variability kinetic energy KE is statistically more reliable and smoother in its spatial pattern. Minimum values of KE are present in the subpolar North Pacific Ocean and in the eastern South Pacific (both less than 100 cm2 s−2). In contrast to the North Pacific, the subpolar North Atlantic Ocean shows relatively enhanced KE. Eddy kinetic energy and eddy mixing appear to have declined during the last decade over large parts of the western Pacific Ocean, in some regions by as much as 50% of the time-mean value. Increased eddy variability can be found in the Kuroshio and Gulf Stream regions, as well as in the Agulhas region, east of Australia, and at several locations along the Antarctic Circumpolar Current. Somewhat enhanced eddy variability and eddy mixing are also apparent in the eastern tropical Pacific. A numerical simulation of the ocean circulation at 1° spatial resolution over a 10-yr period suggests that variations in eddy mixing of this order of magnitude measurably affect the deep temperature field in the vicinity of permanent frontal structures on a time scale of less than 4 yr. The meridional overturning circulation also reacts on these time scales. If persistent over longer periods in the ocean, these effects would be important for climate simulations.

Flow at intermediate depths around Madagascar based on ALACE float trajectories

During 1994–1996, 215 Autonomous Lagrangian Circulation Explorers (ALACE floats) were released at a nominal 900 m depth in the Indian Ocean as part of the World Ocean Circulation Experiment. Of these, 66 entered the region around Madagascar (2–30°S, 35–55°E), generally at a depth of 800–900 m. Floats approaching the island from the east were deflected either northward or southward depending on latitude, with the bifurcation point being near 20°S. Mean southward velocities in the western boundary current were 8.5 cm s-1 during each 25-day observation period, with mean northward velocities 7.7 cm s-1. Speeds past Cape Amber were about 11 cm s-1. These figures are comparable to those obtained from in situ current meter measurements. Floats rounding the island to the north frequently drifted north of the Comores for many months with no discernable pattern, before exiting the region either to the north in the East African Coastal Current and the equatorial current system or to the south via the Mozambique Channel. Flow rates in this region were highly skewed towards low (less than 5 cm s-1) velocities. Floats passing south of Madagascar showed little northward movement into the Mozambique Channel, but tended to move steadily westwards towards the African coast, becoming entrained in the Agulhas Current and its recirculation gyres near 28°S. Similarly, floats released within the southern portion of the Channel all tended to move to the south and west despite occasional entanglement with eddies. Mean flow in the Agulhas region was about 18 cm s-1, with maximum velocities over 25 days of up to about 35 cm s. All flows were extremely variable because of the ubiquity of eddies. In essence, the flow observed near 800 m resembled closely that seen at the surface from TOPEX/POSEIDON altimetry and at 845 m in the OCCAM global model. Temperature data collected by the floats were used to determine when a given float was being acted on by an anticyclonic eddy. Deviations, which were typically 0.5°C or more from the background temperature field, were found only in eddies south of 12°S and occurred in about 9% of float records within the Mozambique Channel. The results support the idea of a net southward flow of water through the Channel from the tropics to the Agulhas Current, with the eddies playing a major role in the transport in the southern part of the region. In the northern and eastern parts of the region, however, eddies were far less prevalent, and topographic steering by the Mascarene Ridge became important.

Eddies and variability in the Mozambique Channel

Abstract Between 1995 and 2000, on average 4 eddies per year are observed from satellite altimetry to propagate southward through the Mozambique Channel, into the upstream Agulhas region. Further south, these eddies have been found to control the timing and frequency of Agulhas ring shedding. Within the Mozambique Channel, anomalous SSH amplitudes rise to 30 cm , in agreement with in situ measured velocities. Comparison of an observed velocity section with GCM model results shows that the Mozambique Channel eddies in these models are too surface intensified. Also, the number of eddies formed in the models is in disagreement with our observational analysis. Moored current meter measurements observing the passage of three eddies in 2000 are extended to a 5-year time series by referencing the anomalous surface currents estimated from altimeter data to a synoptic LADCP velocity measurement. The results show intermittent eddy passage at the mooring location. A statistical analysis of SSH observations in different parts of the Mozambique Channel shows a southward decrease of the dominant frequency of the variability, going from 7 per year in the extension of the South Equatorial Current north of Madagascar to 4 per year south of Madagascar. The observations suggest that frequency reduction is related to the Rossby waves coming in from the east.

Agulhas eddy fluxes in a 1/6° Atlantic model

A 1/6° resolution primitive equation model of the Atlantic circulation is analyzed in the Agulhas region. The model has a realistic level of eddy kinetic energy, and produces anticyclonic Agulhas rings as well as cyclonic structures. In the model as well as in the data, ring trajectories undergo a transition between a turbulent character in the Cape Basin and a steady propagation in the rest of the South Atlantic. The topography of the Walvis Ridge does not seem to play a part in generating this contrast in the model. The model shows that cyclones are primarily generated from the negative shear vorticity side of the Agulhas Current as it leaves the coast, and they are most of the time paired with anticyclones in dipolar or tripolar structures. Contribution of Agulhas rings to the transports has been estimated by two methods, either focussing on the amount of water trapped inside the eddies and carried with them, or as a perturbation to the time-mean flow. The second estimate always produces smaller mass fluxes than the first. Even so, the transient eddy flux (2 Sv of warm water over the Agulhas Ridge) is very large when compared to parameterizations of eddy fluxes used in low-resolution climate models.

Climate variability in Malawi, part 1: dry summers, statistical associations and predictability

AbstractClimate variability and predictability over tropical southern Africa is studied using a Malawi summer rainfall index. Thermodynamic and circulation patterns are examined and statistical associations are analysed. Links with El Niño–southern oscillation (ENSO) and quasi‐biennial oscillation (QBO) phase are significant and consistent with those found for southern Africa. Correlations are mapped with respect to the Malawi rainfall index, and the regional expression of ENSO emerges quite clearly. Negative r values for sea‐surface temperature (SST) in the west Indian Ocean contrast with positive r values in the east Atlantic and Agulhas region. The thermodynamic pattern ‘anchors’ a standing Rossby wave, comprised of northwest–southeast‐oriented centres of action across the Mozambique Channel and southern Africa, and east of Madagascar to East Africa. A meandering of the sub‐tropical jet stream is an expression of the standing Rossby wave, leading to vertical motions over southern Africa. The pattern develops at least one season in advance, and predictive indices are extracted in the July–September season for use in a multivariate model to predict the Malawi summer rainfall. The three‐area SST index, formulated to capture the ENSO‐modulated Rossby wave pattern, is the most influential predictor, followed by air pressure over the east Indian Ocean and the stratospheric zonal wind anomaly (QBO). Skilful results are found, with a 55% hindcast fit and two‐thirds of tercile categories correctly forecast in independent tests. Copyright © 2002 Royal Meteorological Society.

How Topographic Smoothing Contributes to Differences between the Eddy Flows Simulated by Sigma- and Geopotential-Coordinate Models

The characteristics of the mesoscale turbulence simulated at a resolution of ⅓° by a sigma-coordinate model (SPEM) and a geopotential-coordinate model (OPA) of the South Atlantic differ significantly. These two types of models differ with respect to not only their numerical formulation, but also their topography (smoothed in SPEM, as in every sigma-coordinate application). In this paper, the authors examine how these topographic differences result in eddy flows that are different in the two models. When the topography of the Agulhas region is smoothed locally in OPA, as is done routinely in SPEM, the production mechanism of the Agulhas rings, their characteristics, and their subsequent drift in the subtropical gyre, are found to converge toward those in SPEM. Furthermore, the vertical distribution of eddy kinetic energy (EKE) everywhere in the basin interior becomes similar in SPEM and OPA and, according to some current meter data, becomes more realistic when mesoscale topographic roughness is removed from the OPA bathymetry (as in SPEM). As expected from previous process studies, this treatment also makes the sensitivity of the Agulhas rings to the Walvis Ridge become similar in SPEM and OPA. These findings demonstrate that many properties of the eddies produced by sigma- and geopotential-coordinate models are, to a significant extent, due to the use of different topographies, and are not intrinsic to the use of different vertical coordinates. Other dynamical differences, such as the separation of western boundary currents from the shelf or the interaction of the flow with the Zapiola Ridge, are attributed to intrinsic differences between both models. More generally, it is believed that, in the absence of a correct parameterization of current–topography interactions, a certain amount of topographic smoothing may have a beneficial impact on geopotential coordinate model solutions.

The Role of Mesoscale Eddies in the Source Regions of the Agulhas Current

A primitive equation model to study the dynamics of the Agulhas system has been developed. The model domain covers the South Atlantic and the south Indian Ocean with a resolution of ⅓° in the Agulhas region while coarser outside. It is driven by a climatology of the European Centre for Medium-Range Weather Forecasts. It is shown that the model simulates the Agulhas Current, its retroflection, and the ring shedding successfully. The model results show baroclinic anticyclonic eddies in the Mozambique Channel and east of Madagascar, which travel toward the northern Agulhas Current. After the eddies reach the current they are advected southward with the mean flow. Due to the limited numerical resolution only a few eddies reach the retroflection region without much modification. These eddies are responsible for drastic enhancement of the heat transfer from the Indian Ocean to the South Atlantic and lead to periodicities in the interoceanic heat transport of about 50 days superimposed on the seasonal variability. Combined satellite data from TOPEX/Poseidon and ERS-1 show that the observed vortices in the Mozambique Channel are comparable to those seen in the model. In contrast to this the simulated eddies east of Madagascar seem not to be well reproduced. Analyses of the energy conversion terms between the mean flow and the eddies suggest that barotropic instability plays an important role in the generation of Mozambique Channel eddies. For the generation of Agulhas rings and other eddy structures in the model the barotropic instability mechanism seems to be minor, and baroclinic instability mechanisms are more likely.

The time‐mean circulation in the Agulhas region determined with the ensemble smoother

The time‐mean circulation in the Agulhas Retroflection area is determined by combining TOPEX/POSEIDON data and a two‐layer quasi‐geostrophic model using the ensemble smoother. By taking the time‐mean circulation as the unknown in the data assimilation procedure, the time‐varying altimeter signal is used to constrain the time‐mean field. The quasi‐geostrophic model is applied as a strong constraint, with only the time‐mean circulation containing errors. Inspection of the posterior penalty function showed that the inversion was successful. The errors in the time‐mean sea surface topography reduced from about 10 to about 3 cm. A cyclonic recirculation cell over the Agulhas Plateau was found, related to the northward meander of the Agulhas Return Current. Another cyclonic recirculation cell was found west of Africa, probably related to the passage of anticyclonic Agulhas Rings south of it. The new field is compared with advanced very high resolution radiometer infrared satellite data, confirming the northward meander of the Agulhas Return Current.