Mesoscale variability study of the Agulhas Current from satellite radar altimetry and a high resolution model
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Towards understanding the dynamics and eddy evolving processes in the greater Agulhas Current system, a Hybrid Coordinate Ocean Model (HYCOM) system has been set up to simulation the ocean circulation around Southern Africa in a hindcast experiment. In a region characterised by a general paucity of hydrographic observations, model validation from satellite remote sensing data products play an important role. In particular, radar altimetry observations from satellites are of beneficial use in the Agulhas Current regime, since it is known to exhibit some of the strongest mesoscale variability in the global ocean. Model validation from available in-situ data is presented with the focus on evaluating and implementing satellite altimetry data products for the purpose of validating the model mesoscale variability. An initial comparison of along-track sea level anomaly data from the Jason-1 satellite to merged gridded multi−altimeter data for the Agulhas Current suggested no significant loss of information in the generation of the merged maps. Thus allowing the use of the gridded data product in further model evaluation. It was found that the altimeter data provides good temporal and spatial comparisons, and as a by-product to the validation process, a frequency and variability study of the Agulhas Current regime was undertaken. Additionally to confirming strong seasonal signals in the region, they revealed that mesoscale eddies in the Mozambique Channel frequently interact at the Agulhas Retroflection. These observations are quantified statistically with the application of a Fourier analysis, which additionally provided a further method of validating the model simulation. The model simulation was able to represent the general characteristics of the circulation in the Agulhas regime, and supports many of the findings in the altimetry variability study. However, with the development of a large anticyclonic circulation feature upstream of the Agulhas Plateau in the current proper, the model simulation deteriorates in the later years. Available hydrographic data suggests a salinity deficit in the model fields, resulting in reduced baroclinic gradients, which in addition to other numerical instabilities may provide an explanation for the development of this feature. With continuously improving numerical schemes, increased computing power, and improved data assimilation techniques these models become increasingly realistic. As an oceanographic tool, the development of numerical models is vital, the first steps towards setting up and validating a regional HYCOM model for the oceans around Southern Africa are presented.