Browsing by Author "Backeberg, Bjorn Christoph"
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- ItemOpen AccessAssessing the impacts of assimilating satellite SST in addition to along-track SLA into a HYCOM of the Agulhas System(2017) Rapeti, Tharone; Backeberg, Bjorn Christoph; Rouault, MathieuThe greater Agulhas Current System, is considered to be the largest western boundary current in the Southern Hemisphere, with only the Gulf Stream, and possibly the Kuroshio, considered to be larger globally (Bryden et al., 2005). The Current System is a crucial factor for determining the mean state and variability of the regional marine environment, resources and ecosystems in the region, regional weather, as well as the global climate on a broad range of temporal and spatial scales. Due to an absence of a coherent in situ and satellite-based observing system in the area, modelling and data assimilation techniques are utilised. These both further the quantitative understanding of the ocean dynamics as well as providing better forecasts of this complicated western boundary current system. In this study, we compare two assimilation experiments using the Ensemble Optimal Interpolation (EnOI) data assimilation scheme in a regional implementation of the Hybrid Coordinate Ocean Model (HYCOM). In the first experiment, we assimilate along-track satellite sea level anomaly (SLA) data only, and in the second experiment we assimilate both along-track sea level anomaly (SLA) as well as satellite sea surface temperature (SST) data. The objectives of the study are to investigate the impacts of assimilating SST along with SLA into the regional HYCOM model, with the hopes of improving the model performance. The long term aim of this experiment is to develop a regional ocean prediction system. The additional assimilation of SST along with SLA into the HYCOM model, has improved upon the representation of the SST field across the region by reducing the error. However, with regards to velocity, surface eddy kinetic energies (EKE), as well as subsurface velocities, the updated SST model shows less improvement. A velocity bias can be seen as the reason for underperformance in these aspects. The model still struggles to recreate subsurface water masses, underestimating salinity in the upper 500 m; assimilating T/S profiles in the future could improve on this. The assimilation of SST has improved upon the SST-SSH correlation in the model, as well as the spatial distribution and accuracy. The assimilation of SST along with SLA has had many positive impacts, with unfortunately, a few negatives. The shortcomings of the numerical model will have to be improved upon and additional assimilation variables should be tested in further studies, to provide a solid forecasting system.
- ItemOpen AccessDynamics, interactions and ecosystem implications of mesoscale eddies formed in the southern region of Madagascar(2014) Braby, Laura; Ansorge, Isabelle Jane; Backeberg, Bjorn Christoph; Roberts, Michael JSeveral species of marine organisms occurring off the southern African coast have been found to be identical to those occurring in the Madagascan coastal water although the reason for this is unknown. It has been proposed that eddies act as a vector of transport for planktonic larvae from the Madagascar island to the southern African east coast. In this study it is shown that eddies spawned off southern Madagascar entrain chlorophyll-a rich coastal waters into their periphery. This is indicative of the mechanism whereby organisms could become entrained in eddies. Approximately one eddy per year, usually cyclonic, interacts with the southern Madagascan coast, then from its origin crosses the southern Mozambique Channel and arrives at the African coast where it dissipates. By tracking eddies and combining their trajectories with drifter data and satellite remote sensing observations of ocean colour, it is shown that chlorophyll-a rich waters are entrained within the eddies, and these waters are mostly conserved during their passage across the channel. This study suggests that biota may be transported from Madagascar to Africa in eddies, providing further evidence that eddies are potentially a viable mechanism for the transport of organisms across the southern Mozambique Channel.
- ItemOpen AccessEvaluating global ocean reanalysis systems for the greater Agulhas Current System(2014) Cooper, Kyle Francis; Backeberg, Bjorn Christoph; Hermes, Juliet C; Deshayes, Julie; Veitch, Jennifer AnneOperational oceanography aims to accurately hindcast and forecast the state of the ocean. An international initiative, the Global Ocean Data Assimilation Experiment (GODAE), developed and increased the capacity for global operational oceanography. However, the products from the global initiatives were regionally inapplicable due to low spatial resolutions, and have recently improved through GODAE OceanView. A number of local operational oceanographic initiatives have been setup over the southern African regional ocean, but proved to be unsustainable and ended. Recently, the aim to develop a regional ocean prediction system has arisen, and initial steps have been taken. This thesis aims to address the lack of local capacity in operational oceanography, and contribute to a crucial process in developing a regional ocean prediction system. Here, we validate and investigate the differences between three global reanalysis products, namely MyOcean (GLORYS2V1), HYCOM (U.S Naval Research Laboratory) and BlueLINK (OFAM3). These reanalysis products are validated and investigated over the greater Agulhas Current System, which is a crucial system in Southern African regional ocean. The salient oceanographic features represented in the reanalysis products are initially compared to historical literature of the region and followed by available unassimilated observations (i.e. independent). The results show that the reanalysis products from MyOcean, and the U.S Naval Research Laboratory satisfactorily simulate the major salient oceanographic features of the Agulhas Current System. Bluelink does not correctly portray the structure of the source and retroflection regions, and therefore has limited use over the Agulhas Current System. The differences between the three products indicates that the data assimilate does not sufficiently constrain the models in order for their solutions over the Agulhas System to converge. The evaluation of these global ocean reanalysis products is a critical step toward a regional ocean prediction system over Southern Africa, and building toward the local capacity to accomplish this goal.
- ItemOpen AccessEvaluation of an eddy resolving HYCOM simulation in the South Atlantic Ocean(2014) Giddy, Isabelle Sindiswa; Backeberg, Bjorn Christoph; Ansorge, Isabelle Jane; Reason, Chris; Campos, EdmoThe South Atlantic Ocean acts as a key region of ocean teleconnections and water mass transformation, directly transporting waters from the Southern, Pacific and Indian Oceans towards the North Atlantic and feeding the upper branch of the Atlantic Meridional Overturning Circulation. To date, this region remains under sampled over long time periods. Models form useful platforms for the wholistic study of such regions where long term datasets are not available. However, it is important to first gauge to what extent the model deviates from available observations. In a detailed analysis, this study evaluates an eddy resolving 1/12° HYbrid Coordinate Ocean Model simulation in the South Atlantic and Agulhas regions with reference to available observations and literature. The outcomes of this research contribute to improving the configurations of future ocean models by identifying the limitations of the model analysed. In addition, analysis of the 50 year (1960-2009) simulation reveals that, in this model simulation, mesoscale activity and sea surface temperature trends in the Cape Basin are not related to Agulhas Current transport changes. These findings highlights the need for long term observational databases and further research into climate related trends and interannual variability of the ocean, in particular around Southern Africa.
- ItemOpen AccessModelling the mesoscale variability in the greater Agulhas Current system using hybrid coordinate Ocean model(2009) Backeberg, Bjorn Christoph; Reason, Chris; Johannessen, y Johnny A; Bertino, LaurentThe ocean circulation dynamics in the greater Agulhas Current system are dominated by mesoscale variability, which is highly non-linear, and therefore difficult to measure and simulate accurately. Moreover, the shedding of Agulhas rings from the retroflection south of Africa, which is the dominant mechanism by which warm and saline water flows from the Indian into the Atlantic Ocean, is thought to be a crucial component of the thermohaline circulation. With the goal of providing an accurate simulation of the greater Agulhas Current system, and in particular its mesoscale variability, a high resolution Hybrid Coordinate Ocean Model is set up in a nested configuration. In two 11 year simulation experiments, the effect of a higher order momentum advection scheme on the simulated ocean dynamics is tested and evaluated against available satellite observations and in-situ measurements. Quantitative analyses and model validation methods are developed to objectively evaluate the simulation experiments. The resultant skewness analyses and spatial variograms are objective measures for assessing the model simulation and additionally provide new insights on the mesoscale dynamics of the greater Agulhas Current system. A 4th order momentum advection scheme is shown to significantly improve the simulation of the region, in particular the dynamics of the southern Agulhas Current and the retroflection are greatly improved. From the analyses of the two model simulations in conjunction with satellite observations and in-situ measurements, it is found that the Indo-Atlantic inter-ocean exchange, and the shedding of Agulhas rings from the retroflection, is sensitive to the strength of the Agulhas Current, which in turn is influenced by the flow dynamics in the Mozambique Channel and south of Madagascar. Mesoscale eddies drifting from these source regions to the Agulhas Current play an important role, and the connection between the Agulhas Current and the respective source regions provides a link to large-scale variability in the Indian Ocean, which in turn is related to interannual modes of variability such as the Indian Ocean Dipole and El NiÑo Southern Oscillation.