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The Agulhas Current and its seasonality explained

By Katherine Hutchinson, PhD candidate*, SAEON Egagasini Node
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SAEON PhD student Katherine Hutchinson’s interest in the Agulhas Current has resulted in the first study ever to use a combination of in-situ measurements, satellite observations and idealised models to investigate the drivers of seasonal variations in the strength of the current

The Agulhas Current is what oceanographers term a “western boundary” current. This means that it flows along the western border of the southern Indian Ocean, which is the east coast of South Africa.

This may sound like a reverse way of seeing things, but it represents an ocean-centric viewpoint.

It is important to understand that the Agulhas is a western boundary current as this indicates that it forms part of a group of currents that are the fastest and strongest in the world. Others include the Gulf Stream, the Kuroshio, and the East Australian Current.

They all carry equatorial water towards the poles of their respective hemispheres. The Agulhas is, in fact, the strongest western boundary current in the world at its latitude, carrying over 75 million cubic metres per second - more than 350 times the flow rate of the Amazon River.

Role in global ocean circulation

The Agulhas Current transports warm and salty equatorial Indian Ocean water along the east coast of South Africa and eventually sheds some of this water at the tip of South Africa in the form of gigantic ocean vortexes called eddies. These eddies travel across the South Atlantic and join up with the equatorial current where they add to the source waters of the Gulf Stream.

The Gulf Stream plays an important role in modulating the climate conditions of North America and Western Europe. In other words, the Agulhas plays a very important role in global ocean circulation, which further plays a role in influencing climatic conditions across the world.

The Agulhas Current plays a very important role in global ocean circulation, which further plays a role in influencing climatic conditions across the world.

The Agulhas is also of local importance as it influences rainfall and climate over southern Africa and sets up the background environment for local fisheries. The variability of the Agulhas Current has thus been a topic of interest for many years.

Measuring versus simulating the Agulhas

Until very recently, observations of the Agulhas Current were limited to snapshots that oceanographers obtained by deploying instruments during research cruises. Data obtained in such a way is useful as it gives us a full depth view of the water column, but is limited as it only tells us about how the current is behaving at one point in time.

Satellite ocean measurements address this “snapshot” issue as they continuously provide us with measurements of the ocean surface at 10-day intervals. The restriction with satellite data, however, is that it only gives us information about the ocean surface as the satellites cannot see below into the depths of the water column.

Many oceanographers therefore turn to computer models that simulate the response of ocean currents to forcing mechanisms such as winds (which can be measured by satellites). These models are not perfect, but can be very useful in providing us with insight into connections between different mechanisms in the ocean.

For many years we did not understand how the Agulhas Current’s transport adjusted with the seasons. Some ocean model studies suggested a direct seasonal link between the flow in the Mozambique Channel and the Agulhas Current.

Measurements in the Mozambique Channel indicate that the flow is at a maximum in winter, and so one study suggests that the Agulhas experiences maxima in transport two months later in spring.

Models also suggest that the Agulhas responds very quickly to a change in Indian Ocean winds and therefore the current should be strongest in winter when the winds are faster. An array of current meter moorings was placed across the Agulhas Current in April 2010 and continuously measured the current speed and direction until February 2013.

These moorings were part of the Agulhas Current Time-series Experiment (ACT), which has now been replaced by the Agulhas System Climate Array (ASCA).

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The three years of ACT measurements showed that ocean models were incorrect in their prediction of a winter-spring maximum. The in-situ current meter measurements showed that the Agulhas has a stronger flow in summer!

When the ACT results were published in 2015, we were left with a big question mark as to what determines this observed seasonal cycle - and that was the motivation for my PhD thesis topic “Agulhas Current seasonality determined by near- and far-field winds”.

Seasonal variations in the Agulhas Current flow

Oceanographers hypothesise that the seasonal variability of western boundary currents is strongly influenced by winds. This hypothesis is grounded in ocean physics and the fact that the western boundary flows are set up by a required exodus of water from equatorial regions towards the poles, to balance the input of water driven into the basin by major wind systems.

As the winds vary in strength, the amount of water that they pump into the ocean basin varies, and so the amount of water the western boundary current carries out of the basin must likewise adjust. The lag time in this process and the coherence of signal communication between the winds and the western boundary currents, remains poorly understood.

I embarked on answering this question regarding the South Indian Ocean using a variety of tools: in-situ measurements, satellite observations, and idealised ocean models. I decided to use highly simplified idealised models as the realistic ocean models were incorrectly predicting the seasonal cycle of the Agulhas and so I needed to go back to basics.

Unexpected discoveries

Using these models, I found that the signals created by seasonal wind forcing across the entire southern Indian Ocean actually cancelled each other out. A signal that is kicked off by a wind event close to Australia is not likely to reach South Africa, as this signal experiences destructive interference with overlying wind forcing during its transit across the basin.

The only signals that communicate wind forcing to the Agulhas Current come from the near-field winds, west of 35E - only 644km away. The influence of mean wind forcing (no seasonal changes) was also investigated and found to play a crucial role in determining the speed of propagation of wind-driven signals.

My PhD is the first study to use a combination of in-situ measurements, satellite observations and idealised models to investigate the drivers of the seasonal variations in strength of the Agulhas Current. The discovery that near-field winds play a dominant role in determining this variability will help to inform oceanographers in the future about where to look for the source of observed changes in Agulhas Current transport.

In addition, we can now pay close attention to the changes happening in the near-field westerly winds in order to obtain a better idea of how the Agulhas Current may adjust to climate change.

* Katherine is a Professional Development Programme (PDP) student with Dr Juliet Hermes at SAEON’s Egagasini Node, working as part of the ASCA team. The Professional Development Programme of the Department of Science and Technology and the National Research Foundation aims to accelerate the development of scientists and research professionals in key research areas.

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