Björn C. Backeberg

Modes of the southern extension of the East Madagascar Current

Data sets from satellite observations and a nested high-resolution model are used to study a source region of the Agulhas Current. Altimeter-derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long-term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross-correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1-month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21 degrees S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55 degrees E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC.

The greater Agulhas Current system: An integrated study of its mesoscale variability

For the purpose of developing an operational oceanography system for the greater Agulhas Current regime, a high resolution Hybrid Coordinate Ocean Model (HYCOM) has been set in a nested configuration. The intense and complex current regime poses a challenge in modelling. However, access to satellite and in-situ data with strong and persistent signals of the dynamics and mesoscale variability ensure that adequate model validation is feasible. The study concludes that HYCOM reproduces the general larger scale circulation of the greater Agulhas Current reasonably accurately in addition to the regionally specific characteristics and mesoscale variability. Furthermore, strong anticyclonic eddies occurring in the Mozambique Channel at a frequency of 5–6 per year, are found to drift southward and merge with the northern Agulhas Current. Evidence of these eddies can also be tracked further southwestwards into the southern Agulhas Current and sometimes all the way towards the Agulhas retroflection region. Operational forecasting of the greater Agulhas Current, and in particular the retroflection, must therefore adequately account for the presence and influence of the Mozambique Channel eddies, in order to forecast their evolution on time scales from days to weeks

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.

Observed eddy dissipation in the Agulhas Current

Analysing eddy characteristics from a global dataset of automatically tracked eddies for the Agulhas Current in combination with surface drifters as well as geostrophic currents from satellite altimeters, it is shown that eddies from the Mozambique Channel and south of Madagascar dissipate as they approach the Agulhas Current. By tracking the offshore position of the current core and its velocity at 30oS in relation to eddies, it is demonstrated that eddy dissipation occurs through a transfer of momentum, where anti-cyclones consistently induce positive velocity anomalies, and cyclones reduce the velocities and cause offshore meanders. Composite analyses of the anti-cyclonic (cyclonic) eddy-current interaction events demonstrate that the positive (negative) velocity anomalies propagate downstream in the Agulhas Current at 44 km/day (23 km/day). Many models are unable to represent these eddy dissipation processes, affecting our understanding of the Agulhas Current

Use of SAR data to Monitor the Greater Agulhas Current

The strong and dynamic greater Agulhas Current is known to have significant influence on the local marine environment and ecosystem. It is also considered to play an important role in the global thermohaline circulation, notably due to the transport of heat and salt from the Indian Ocean into the South Atlantic Ocean. Regular monitoring of the dominant processes and variability within the greater Agulhas Current is thus highly needed. Recently Doppler-derived ocean surface velocities from the European Space Agency’s (ESA) Envisat Advanced Synthetic Aperture Radar (ASAR) have demonstrated abilities to manifest the intensity of surface currents. In this paper the regular direct surface velocity estimates of the greater Agulhas Current from the ASAR since summer of 2007 are presented. These estimates are assessed by direct comparison to other independent estimation of the surface current in the greater Agulhas current obtained from surface drifter data, radar altimetry and an ocean circulation model. The results are promising and highlight that the Doppler-based ocean surface velocity retrieval offers a new innovative approach to monitor and advance the understanding of the dynamic processes of the greater Agulhas Current.

Using an eddy-tracking algorithm to understand the impact of assimilating altimetry data on the eddy characteristics of the Agulhas system

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.

Agulhas Current Meanders Facilitate Shelf-Slope Exchange on the Eastern Agulhas Bank: MEANDERS ON THE AGULHAS SHELF

Large solitary meanders are arguably the dominant mode of variability in the Agulhas Current. Observational studies have shown that these large meanders are associated with strong upwelling velocities and affect the shelf circulation for over 100 days per year. Here 10-year time series from two ocean general circulation models are used to create a composite picture of the Agulhas Current and its interactions with the shelf circulation in meandering and nonmeandering modes. Both models show good agreement with the size, propagation speed, and frequency of observed meanders. These composite meanders are then used to examine the response of shelf waters to the onset of large meanders, with the use of model output enabling the dynamics at depth to be explored. Results show a composite mean warming of up to 3°C of depth-averaged temperature along the shelf edge associated with an intrusion of the current jet onto the shelf driven by an intensification of the flow along the leading edge of large meanders. However, this intensification of flow results in cooling of bottom waters, driving cold events at the shelf break of <10°C at 100 m. Thus, the intensification of the current jet associated with large meander events appears to drive strong up and downwelling events across the inshore front of the Agulhas Current, facilitating shelf-slope exchange.