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Understanding the Agulhas Current's complex relationship with our shores
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Barrelling down the east coast of southern Africa, the Agulhas Current is one of the natural world’s behemoths.
Transporting some 80 million cubic metres of sea water per second at speeds approaching two metres a second and driven by the aggregate effect of winds over the entire Indian Ocean, the amount of energy involved in this system is somewhat inconceivable.
With this great power comes great complexity, and the collision of this great ocean current with the continental shelf of southern Africa results in extremely strong, and rapidly evolving, gradients of ocean temperature, salinity and currents. These strong dynamics are both a blessing and a curse for oceanographers seeking to understand the Agulhas System, with ocean models struggling to consistently simulate the currents, and the observational system being too limited to capture the vigorous dynamics of the Current.
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The goal of my PhD was to understand the way in which the Agulhas Current interacts with the shelf waters that lie between it and the southern African coastline. The power of the Current has a large effect on these coastal waters, but due to an incomplete observing system, we lack a coherent understanding of how the Agulhas might drive these processes along the continental shelf. These shelf waters are always of interest, as they act as the interface between the deep ocean and the coast, where the human population interacts with the sea.
Due to the lack of long-term observations at depth, we chose to use an ocean modelling approach, which allowed us to try and coherently understand what the various historical observations have told us about the dynamics of the Agulhas Current’s interaction with coastal waters. Unfortunately, the Agulhas Current is somewhat notorious as one of the more difficult places in the world’s oceans to accurately model, and with no obvious solution to these products, we chose to make use of two very different ocean models. The idea behind this being that, if these two different systems agree, the results are likely to be robust, and not due to individual model bias.
First stop was to try and understand the effect of the large meander events which propagate down the east coast a couple of times a year.
What did the models tell us?
Well, the good news is that they agreed with observations and with each other, showing that large current meanders are a robust intrinsic property of the current. They also showed that the change in the shape of the continental shelf which happens to the west of Port Elizabeth has a strong effect on how meanders drive cold, nutrient-rich water onto the shelf.
Next stop was a glance into the past, using the long-term (1948–2008) INALT01 model to simulate multi-decadal variability in the temperature of continental shelf water between East London and Cape Agulhas. Good news? The model recreates the mid-nineties shifts which have been related to changes in small pelagic fish populations.
The interesting part? The model suggests that these shifts in temperature are in fact part of a multi-decadal cycle which is controlled by changes in the large-scale wind regime.
What the satellite-tracked ocean drifters told us
Lastly, we put the fancy computer models down for a bit, to peer into the fine scales of 10 km or less, where our understanding of the ocean is very limited. Here a pair of satellite-tracked ocean drifters showed how the strong shear between the main Agulhas Current jet and the calmer shelf waters can spin up small coherent vortices, which carried our pair of drifters hundreds of kilometres.
The interesting part? After being caught in this submesoscale vortex, both these drifters ended up in the Benguela Current and disappeared off into the South Atlantic Ocean as part of the much written about Agulhas Leakage. The next generation of Agulhas Current ocean models will resolve these finer scales and allow us to further understand the important linkages and biological consequences of these small, strong eddies.
I suppose as with all PhDs I have ended up with more questions than I began with. But one thing’s for sure, we still have much to learn about the ocean behemoth which goes tearing down our eastern shores.
