Going with the flow

01 December 2017 | Story Sarah Wild and Katherine Hutchinson
Cape Town Waterfront Harbour
Cape Town Waterfront Harbour

The Agulhas Current, a warm and salty current, carries Indian Ocean water along the east coast of South Africa, passing Durban and Port Elizabeth, in the direction of Cape Town, and ultimately towards the comparatively cooler and fresher South Atlantic Ocean.

The American vessel RV Knorr bobs on the surface of the Southern Indian Ocean. Beneath it, the Agulhas Current, the ocean current which hugs the east coast of South Africa, roars towards the South Atlantic. “It’s a kick-ass current,” shouts the third mate, as she adjusts the thrusters that keep the ship in place. The water is gushing below us at over 75 million cubic metres per second – that’s about 350 times the flow rate of the Amazon River.

In the past, we thought that westerly winds over the Southern Indian Ocean were largely responsible for the variability of the Agulhas Current. These winds are strongest in winter so scientists thought, for many years, that the current would be strongest in winter too. But it’s actually stronger in summer. We recently realised that there is a serious gap in our understanding, which thwarts our ability to accurately model the Agulhas Current and the impact it has on local and global climate.

Seasonality is the most basic way the earth’s system changes. If ocean models cannot correctly capture this seasonal adjustment in the Agulhas Current, we cannot adequately simulate how the current will respond to climate change and what role it will play in global ocean circulation and climate regulation in the future.

The Agulhas Current is vital in fuelling rainfall over water-scarce South Africa. Wind systems blowing over the current and towards South Africa pick up moisture and carry it onto the land. Globally, the current connects the Indian and Atlantic Oceans and facilitates an exchange of water between them – a crucial link in global ocean circulation. The Agulhas Current leaks warm salty water into the South Atlantic, which eventually joins the Gulf Stream in the north. So South Africa’s coastal Agulhas ultimately moderates the icy winters of North America and Western Europe.

But it is important at home too: our fisheries depend on it. Friction between the Agulhas Current and the continental shelf edge, which extends to about 50km south of Port Elizabeth, draws nutrient-rich bottom waters towards the surface. Surface water usually has few nutrients in it because tiny ocean plants, known as phytoplankton, have already absorbed them. But when the bottom water is brought closer to the surface, phytoplankton now have both plenty of light and nutrients; their blooms sustain the aquatic food web of the East Coast.

In 2015, scientists finally had access to the treasure trove of information they needed. Through the deployment of sensor-laden buoys across the current, the Agulhas Current Time-series experiment (ACT, now ASCA – the Agulhas System Climate Array) was able to observe how the current varied with seasons. The data showed that the Agulhas Current transported 25% more water in summer than in winter – the opposite seasonality to what was previously thought.

My research looked at how winds over the Southern Indian Ocean determine the Agulhas Current seasonality and revealed where models may have been going wrong. By combining data from the ACT ocean instruments and satellites, as well as idealised ocean models, I found that winds are important in determining the Agulhas Current’s variability, but not in the way that we originally thought.

When winds blow over the ocean they transfer some of their energy to the surface layer of the ocean. The ocean then responds by changing its sea surface height. Small bumps and troughs form. These disturbances in sea surface height are called planetary waves. They are different from the waves we see at the beach – one planetary wave can extend for tens of kilometres, communicating signals across whole ocean basins.

These waves carry information to the coasts, detailing how the interior ocean responded to the wind. They are like the ocean’s nervous system, telling one part of the ocean what happened at another spot on its surface. Planetary waves balance the ocean system so that there is not a continuous buildup of water in any one place. Winds over the Southern Indian Ocean influence the volume of water transported by the Agulhas Current. If the winds are stronger, then more water is carried by the current and vice versa. But it takes time for the planetary waves to cross the ocean basin. That is why there is a delay between when the winds are strongest and the current’s volume is greatest.

If ocean models do not correctly simulate the speed of these planetary waves, then they will not be able to correctly represent the response time and they will likely get the seasonal cycle of the Agulhas Current wrong. For example, if you want to tell whether or not a plate is hot, you hover your hand over it. Your nervous system then carries the heat message to your brain and your brain tells you how quickly you need to remove your hand. If your nervous system is impaired, you may leave your hand too long, or not long enough to gauge the temperature of the plate. In much the same way, ocean models that do not have the correct speed of planetary waves end up being too fast or too slow in adjusting to winds.

Turns out, the dominant winds creating the planetary waves that are important for the seasonality of the Agulhas Current are the winds in our neighbourhood. We used to think that remote winds and waves were equally important, but they aren’t. Planetary waves from Australia die out before they make it to South Africa and have very little influence on the seasonal cycle of the Agulhas Current. Only planetary waves created by near-field winds, up to 644km from the South African coast – about the distance from Pretoria to Durban – actually make it to the current and consequently moderate its flow.

The discovery that near-field winds play a dominant role in determining the seasonal variability of the Agulhas Current is novel and very useful. Scientists now know where to target further research efforts to better understand the current. My findings will inform numerical models and aid them in better simulating the Agulhas Current. This means that the models will be able to better predict how the current will respond to climate change. Researchers can now pay close attention to how the near-field winds are responding to alterations in our climate and gauge how this may affect the current and the adjacent coastal regions of South Africa.

As climates around the world change, this piece of knowledge helps us to better understand how the Southern Indian Ocean responds to seasonally changing winds and highlights how this response may be spedup with a warming of the ocean due to climate change.

This article was first published on ScienceToday, a source of publications for South Africa's best post-graduate research and science writing. Science Today is open to any postgraduate science student at a South African university. Contact Sarah Wild via sarah@wildonsciece.com or on twitter @SarahEmilyWIld for more on Science Today.

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