Agulhas Current System Research Articles

Role of El Niño Southern Oscillation (ENSO) Events on Temperature and Salinity Variability in the Agulhas Leakage Region

This study explores the relationship between the Agulhas Current system and El Niño Southern Oscillation (ENSO) events. Specifically, it addresses monthly to yearly variations in Agulhas leakage where the Agulhas Current sheds waters into the Atlantic Ocean, in turn affecting meridional overturning circulation (MOC). Sea surface temperature (SST) data from the National Oceanic and Atmospheric Administration’s (NOAA) Advanced Very High Resolution Radiometer (AVHRR) combined with sea surface salinity (SSS) from Soil Moisture Ocean Salinity (SMOS) and Simple Ocean Data Assimilation (SODA) reanalysis are used to explore changes in Agulhas leakage dynamics. Agulhas leakage is anomalously warm in response to El Niño and anomalously cool in response to La Niña. The corresponding SSS signal shows both a primary and secondary signal response. At first, the SSS signal of Agulhas leakage is anomalously fresh in response to El Niño, but this primary signal is replaced by a secondary anomalously saline signal. In response to La Niña, the primary SSS signal of Agulhas leakage is anomalously saline, while the secondary SSS signal is anomalously fresh. The lag between the peak of ENSO and the response in SST and the corresponding primary SSS signal of Agulhas leakage is about 20 months, followed by the secondary SSS signal at a lag of about 26 months. In general, increasing ENSO strength increases the extremes of the resulting anomalous SST and SSS signal and impacts the Agulhas leakage region earlier during El Niño and slightly later during La Niña.

Modulation of the Agulhas Current Retroflection and Leakage by Oceanic Current Interaction with the Atmosphere in Coupled Simulations

AbstractCoupled ocean–atmosphere simulations are carried out for the Mozambique Channel, the Agulhas Current system, and the Benguela upwelling system to assess the ocean surface current feedback to the atmosphere and its impact on the Agulhas Current (AC) retroflection and leakage. Consistent with previous studies, the authors show that the current feedback slows down the oceanic mean circulation and acts as an oceanic eddy killer by modulating the energy transfer between the atmosphere and the ocean, reducing by 25% the mesoscale energy and inducing a pathway of energy transfer from the ocean to the atmosphere. The current feedback, by dampening the eddy kinetic energy (EKE), shifts westward the distribution of the AC retroflection location, reducing the presence of eastern retroflections in the simulations and improving the realism of the AC simulation. By modulating the EKE, the AC retroflection and the Good Hope jet intensity, the current feedback allows a larger AC leakage (by 21%), altering the water masses of the Benguela system. Additionally, the eddy shedding is shifted northward and the Agulhas rings propagate less far north in the Atlantic. The current–wind coupling coefficient sw is not spatially constant: a deeper marine boundary layer induces a weaker sw. Finally the results indicate that the submesoscale currents may also be weakened by the current feedback.

Importance of monitoring the Greater Agulhas Current and its inter-ocean exchanges using large mooring arrays


 
 
 The 2013 Intergovernmental Panel on Climate Change report, using CMIP5 and EMIC model outputs suggests that the Atlantic Meridional Overturning Circulation (MOC) is very likely to weaken by 11–34% over the next century, with consequences for global rainfall and temperature patterns. However, these coupled, global climate models cannot resolve important oceanic features such as the Agulhas Current and its leakage around South Africa, which a number of studies have suggested may act to balance MOC weakening in the future. To properly understand oceanic changes and feedbacks on anthropogenic climate change we need to substantially improve global ocean observations, particularly within boundary current regions such as the Agulhas Current, which represent the fastest warming regions across the world’s oceans. The South African science community, in collaboration with governing bodies and international partners, has recently established one of the world’s most comprehensive observational networks of a western boundary current system, measuring the Greater Agulhas Current System and its inter-ocean exchanges south of Africa. This observational network, through its design for long-term monitoring, collaborative coordination of resources and skills sharing, represents a model for the international community. We highlight progress of the new Agulhas System Climate Array, as well as the South African Meridional Overturning Circulation programme, which includes the Crossroads and GoodHope hydrographic transects, and the South Atlantic MOC Basin-wide Array. We also highlight some of the ongoing challenges that the programmes still face.
 
 
 
 
 Significance: 
 
 
 
 Large mooring arrays have been successfully deployed to monitor the Greater Agulhas Current system.
 Capacity development is ongoing, although established, in marine science around South Africa.
 Challenges exist with regard to retention of skilled staff, resources and funding.
 
 
 

Observations of the East Madagascar Current system: Dynamics and volume transports

The South Equatorial Current (SEC) in the Indian Ocean bifurcates when it reaches Madagascar leading, respectively, to the North East Madagascar Current (NEMC), which contours the island flowing northwestward, and to the South East Madagascar Current (SEMC), which flows southwestward. Both branches eventually contribute to the greater Agulhas Current system and thus play a part in the global ocean circulation. In addition, these currents have important effects on the local conditions for marine life. In this study, the NEMC and the SEMC are investigated based on a comprehensive, multidisciplinary survey of the east coast of Madagascar in 2008. Results from conductivity-temperaturedepth stations, underway measurements with thermosalinograph and fluorometer, and ship-mounted acoustic Doppler current profiler are discussed along with concomitant remotely sensed data. Maximum core velocities of >150 cm s–1 were observed in both the NEMC and the SEMC. The SEMC appeared as a nearshore southward jet, which extended at its widest nearly 200 km offshore. Near the southern tip of Madagascar, the SEMC was flanked by reverse velocities: a northeastward current about 30 km wide along the coast and a northeastward offshore current greater than 140 km wide suggesting the presence of the South Indian Ocean Countercurrent. The NEMC had the structure of a narrow nearshore jet, with weak currents beyond 100 km offshore. Volume transport for the upper 1,100 m was estimated to be 22 Sverdrup (Sv) for the SEMC and 48 Sv for the NEMC. The high NEMC transport compared with earlier estimates might be because of anomalously high influx of the SEC during the measurement period. Off the south coast, an undercurrent below 900 m carried an equatorward transport of 3.1 Sv. No undercurrent was observed in the north, but a geostrophic velocity minimum and a similar vertical velocity shear between surface and subsurface currents were found at the level where such a countercurrent has been previously observed from direct measurements.

Unconsolidated sediment distribution patterns in the KwaZulu-Natal Bight, South Africa: the role of wave ravinement in separating relict versus active sediment populations

This paper examines the distribution of unconsolidated sediment in the KwaZulu-Natal Bight located along the east coast of South Africa. Results show that there is a general shelf-wide sediment distribution of coarser grain sizes between depths of 60 and 100 m, punctuated by a broad swathe of mud offshore of the Thukela River. Seasonal changes in sediment distribution patterns are small, being restricted to seaward fining on the inner shelf off the fluvial sources. Sediment distribution reflects a partitioning between sediment populations that are current- influenced and relict (palimpsest) populations associated with submerged shorelines. Wave ravinement during the deglacial transgression, the reworking of submerged shorelines during sea-level stillstands and, to a lesser extent, the Agulhas Current system, are the dominant controls on sediment distribution.

Spatio-temporal characteristics of Agulhas leakage: a model inter-comparison study

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.

Tracing the Agulhas leakage with lead isotopes

The transport of warm and salty waters from the Indian Ocean to the South Atlantic by the Agulhas Current constitutes a key return route of the meridional overturning circulation. Despite the importance of the Agulhas Leakage on interoceanic exchange, its role on biogeochemical cycles is poorly documented. Here we present the first lead (Pb) concentration and isotope data for surface seawater collected during the GEOTRACES cruise D357 in the Agulhas current system. Lead in surface waters of the Cape Basin is described by three distinct end-members: the South African coast, open South Atlantic seawater, and Indian Ocean seawater. The latter stands out in its Pb isotopic composition and can be tracked within two distinct Agulhas rings. High Pb concentrations in the Agulhas rings further corroborate an Indian Ocean provenance of waters and suggest that the Agulhas Leakage represents a major conduit not only for heat but also for trace metals.

Salt exchange in the Indian‐Atlantic Ocean Gateway since the Last Glacial Maximum: A compensating effect between Agulhas Current changes and salinity variations?

AbstractThe import of relatively salty water masses from the Indian Ocean to the Atlantic is considered to be important for the operational mode of the Atlantic Meridional Overturning Circulation (AMOC). However, the occurrence and the origin of changes in this import behavior on millennial and glacial/interglacial timescales remains equivocal. Here we reconstruct multiproxy paleosalinity changes in the Agulhas Current since the Last Glacial Maximum and compare the salinity pattern with records from the Indian‐Atlantic Ocean Gateway (I‐AOG) and model simulations using a fully coupled atmosphere‐ocean general circulation model. The reconstructed paleosalinity pattern in the Agulhas Current displays coherent variability with changes recorded in the wider I‐AOG region over the last glacial termination. We infer that salinities simultaneously increased in both areas consistent with a quasi interhemispheric salt‐seesaw response, analogous to the thermal bipolar seesaw in response to a reduced cross‐hemispheric heat and salt exchange during times of weakened AMOC. Interestingly, these hydrographic shifts can also be recognized in the wider Southern Hemisphere, which indicates that salinity anomalies are not purely restricted to the Agulhas Current System itself. More saline upstream Agulhas waters were propagated to the I‐AOG during Heinrich Stadial 1 (HS1). However, the salt flux into the South Atlantic might have been reduced due to a decreased volume transport through the I‐AOG during the AMOC slowdown associated with HS1. Hence, our combined data‐model interpretation suggests that intervals with higher salinity in the Agulhas Current source region are not necessarily an indicator for an increased salt import via the I‐AOG into the South Atlantic.

Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation

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.

Eddy surface properties and propagation at Southern Hemisphere western boundary current systems

Abstract. Oceanic eddies exist throughout the world oceans, but are more energetic when associated with western boundary currents (WBC) systems. In these regions, eddies play an important role in mixing and energy exchange. Therefore, it is important to quantify and qualify eddies associated with these systems. This is particularly true for the Southern Hemisphere WBC system where only few eddy censuses have been performed to date. In these systems, important aspects of the local eddy population are still unknown, like their spatial distribution and propagation patterns. Moreover, the understanding of these patterns helps to establish monitoring programs and to gain insight in how eddies would affect local mixing. Here, we use a global eddy data set to qualify eddies based on their surface characteristics in the Agulhas Current (AC), the Brazil Current (BC) and the East Australian Current (EAC) systems. The analyses reveal that eddy propagation within each system is highly forced by the local mean flow and bathymetry. Large values of eddy amplitude and temporal variability are associated with the BC and EAC retroflections, while small values occur in the centre of the Argentine Basin and in the Tasman Sea. In the AC system, eddy polarity dictates the propagation distance. BC system eddies do not propagate beyond the Argentine Basin, and are advected by the local ocean circulation. EAC system eddies from both polarities cross south of Tasmania but only the anticyclonic ones reach the Great Australian Bight. For all three WBC systems, both cyclonic and anticyclonic eddies present a geographical segregation according to radius size and amplitude. Regions of high eddy kinetic energy are associated with the eddies' mean amplitudes, and not with their densities.

Assimilating along-track SLA data using the EnOI in an eddy resolving model of the Agulhas system

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.

Madagascar corals track sea surface temperature variability in the Agulhas Current core region over the past 334 years

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.

The water mass variability and southward shift of the Southern Hemisphere mid-depth supergyre

The Southern Hemisphere subtropical supergyre at intermediate depths connects all three ocean basins and plays a significant role in responding and conveying the climate-change-related variations in the global ocean. On the basis of the Simple Ocean Data Assimilation (SODA) ocean reanalysis, the thermohaline variability and southward shift of the mid-depth supergyre are demonstrated. The steric height of the subsurface relative to 1 500 m (400-1 500 m) from the SODA depicts exactly the flow patterns and variability of the oceanic supergyre. During 1958-2007 the water masses in the gyre interiors become cooler/fresher, with the significant exceptions of the Agulhas Current system and Agulhas leakage. The results also exhibit a pronounced strengthening of the inter-basin connection of the supergyre, and the strongest southward shift, by about 2.5A degrees over the whole period, occurs in the central-south Pacific, which is associated with the changes in the basin-scale wind forcing.

Prevalence of strong bottom currents in the greater Agulhas system

Deep current meter data and output from two high‐resolution global ocean circulation models are used to determine the prevalence and location of strong bottom currents in the greater Agulhas Current system. The two models and current meter data are remarkably consistent, showing that benthic storms, with bottom currents greater than 0.2 m s−1, occur throughout the Agulhas retroflection region south of Africa more than 20% of the time. Furthermore, beneath the mean Agulhas Current core and the retroflection front, bottom currents exceed 0.2 m s−1 more than 50% of the time, while away from strong surface currents, bottom currents rarely exceed 0.2 m s−1. Implications for sediment transport are discussed and the results are compared to atmospheric storms. Benthic storms of this strength (0.2 m s−1) are comparable to a 9 m s−1 (Beaufort 5) windstorm, but scaling shows that benthic storms may be less effective at lifting and transporting sediment than dust storms.

The Role of the Agulhas System in Regional and Global Climate

The AGU Chapman Conference on the Agulhas system was the first held on the African continent. There was a feeling of excitement among participants about the great diversity of ongoing research related to the Agulhas Current system, including its role in global and regional climate, its possible influence on human origins in southern Africa, its link to the Madagascar phytoplankton bloom, and its influence on South Atlantic hurricane development (Catarina) through warming related to Agulhas leakage over the past decades.

High latitudes control on the Agulhas current system and possible influence on South Africa vegetation changes

High latitudes control on the Agulhas current system and possible influence on South Africa vegetation changes

Temperature changes in the mid‐ and high‐ latitudes of the Southern Hemisphere

AbstractA Hierarchical Ascending Classification is used to regionalize monthly temperature anomalies measured at 24 weather stations in Antarctica and the Sub‐Antarctic and mid‐latitude southern islands from 1973 to 2002. Three principal regions are identified that are geographically coherent: Eastern Antarctica, the Antarctic Peninsula and the Sub‐Antarctic and mid‐latitude islands. Within each region, consistent trends are observed: namely, stationary temperatures in ‘East‐Antarctica’; a robust warming in the ‘Sub‐Antarctic and mid‐latitude islands’, most pronounced in austral summer (nearly 0.5 °C per decade); and a strong but more recent warming in the ‘Antarctic Peninsula’. Austral summer temperature anomalies are related to (1) the Southern Annular Mode (SAM) indexes computed using two reanalysis products (20th Century Reanalyses and ERA40) over two periods (1958–2002 and 1973–2002), (2) the seasonal frequencies of four recurrent daily weather regimes identified with a k‐means algorithm applied on the 500hPa geopotential height (DJF 1958–2002) and (3) HadSST2 sea surface temperature (SST) anomalies (DJF 1958–2002). East‐Antarctica interannual temperature anomalies are associated with the SAM variability. In the Antarctic Peninsula, only the long‐term trend is common with the SAM. The SAM does impact significantly the temperature anomalies of the Sub‐Antarctic and mid‐latitude islands. Trend and interannual variability of the islands' temperatures are associated with the nearby SST. For the Indian Ocean stations, warming in the Agulhas Current system could also have led to these changes. Copyright © 2012 Royal Meteorological Society

Impacts of SST Anomalies in the Agulhas Current System on the Regional Climate Variability

Abstract The potential impact of the variability in the Agulhas Current system on the large-scale atmospheric state in the Southern Hemisphere is examined, using the monthly near-surface baroclinicity as the key parameter, for the period between September 1980 and August 2002. Dominant patterns of anomalous near-surface baroclinicity found from empirical orthogonal function (EOF) analyses in the region that includes most of the Agulhas Current system show a wide variety of anomaly patterns: some of which indicate spatial shifts in the position of the Agulhas Retroflection and/or Agulhas Return Current. Composited anomalies in various atmospheric fields, sea surface temperature, and the net surface heat flux at the air–sea boundary based on the signals in the EOFs suggest that sea surface temperature anomalies in the Agulhas Current system thermally force the atmosphere on the synoptic scale via modification of the near-surface baroclinicity in March and April and possibly in January and February as well.

High-latitude obliquity as a dominant forcing in the Agulhas current system

Abstract. The Agulhas Current transport of heat and salt from the Indian Ocean into the South Atlantic around South Africa (Agulhas leakage), can affect the Atlantic meridional overturning circulation (AMOC) and, thus, influence global climate. However, efforts to elucidate forcing mechanisms connecting the Agulhas leakage with the upstream dynamics of the current have been hampered by a lack of climate records extracted from the area where the Agulhas current originates. We determine 800-kyr sea surface temperature (SST) and salinity (SSS) records from the "precursor" region of the Agulhas current and show that these records contain strong 100-kyr and 41-kyr cycles. This latter obliquity-driven cycle is nearly in phase with changes in the annual mean insolation and air temperature at high southern latitudes. In contrast, our SST and SSS records did not reveal precession-driven cycles, which is surprising given the low-latitude location of the upstream Agulhas current. Together, this indicates that the dynamics of the Agulhas current system is mainly controlled by high latitude obliquity through its influence on the position of the Southern Hemisphere subtropical front (STF) and its associated westerlies. Our study demonstrates that obliquity may drive an important part of the 100 kyr cycles observed in the system rather than precession. Our results also suggest that a stronger Agulhas current, associated with a northward shift of the wind system during glacial periods, leads to reduced leakage, in accordance with the theory. We argue that during terminations, stronger Agulhas leakage of heat and salt was triggered by increased obliquity exerting a positive feedback on the global climate system through modulating long-term AMOC variations.

The Role of Interocean Exchanges on Decadal Variations of the Meridional Heat Transport in the South Atlantic

Abstract The interocean exchange of water from the South Atlantic with the Pacific and Indian Oceans is examined using the output from the ocean general circulation model for the Earth Simulator (OFES) during the period 1980–2006. The main objective of this paper is to investigate the role of the interocean exchanges in the variability of the Atlantic meridional overturning circulation (AMOC) and its associated meridional heat transport (MHT) in the South Atlantic. The meridional heat transport from OFES shows a similar response to AMOC variations to that derived from observations: a 1 Sv (1 Sv ≡ 106 m3 s−1) increase in the AMOC strength would cause a 0.054 ± 0.003 PW increase in MHT at approximately 34°S. The main feature in the AMOC and MHT across 34°S is their increasing trends during the period 1980–93. Separating the transports into boundary currents and ocean interior regions indicates that the increase in transport comes from the ocean interior region, suggesting that it is important to monitor the ocean interior region to capture changes in the AMOC and MHT on decadal to longer time scales. The linear increase in the MHT from 1980 to 1993 is due to the increase in advective heat converged into the South Atlantic from the Pacific and Indian Oceans. Of the total increase in the heat convergence, about two-thirds is contributed by the Indian Ocean through the Agulhas Current system, suggesting that the warm-water route from the Indian Ocean plays a more important role in the northward-flowing water in the upper branch of the AMOC at 34°S during the study period.