Natural environment

International experts to take a relook at the powerful Agulhas Current

Dr Marjolaine Krug

Christo Whittle

CSIR physical oceanographers and remote sensing specialists, Dr Marjolaine Krug and Christo Whittle, will share the floor with some of the world’s leading experts in ocean systems and global change when they participate in a special Chapman Conference on the Agulhas current taking place in South Africa later this year.

Under the auspices of the American Geophysical Union (AGU), Chapman Conferences bring together a small group of experts to permit in-depth exploration of specialised subjects. In this instance, the purpose of the five-day conference in October will be to identify the most pressing questions related to the Agulhas Current and its role in changing ocean circulation, climate and marine ecosystems.

As the major current in the Southern Hemisphere, the Agulhas Current system transports on average 70 million cubic metres per second. About 100 kilometres across and up to 2.3 kilometres deep, one can easily detect the current from space as it forms a ‘bulge’ of about 1.5 metres above average sea level.

The Agulhas Current has been described as the most intense western boundary current of the world, and some say even stronger or equivalent to the Gulf Stream, explains Dr Krug, a physical oceanographer with the CSIR’s earth observation research group in Cape Town.

“If you want to know how these powerful currents influence climate and coastal regions, you need to understand how the current is changing with time. Because there are so few ship or mooring based measurements of the Agulhas Current in its entirety, a large part of our understanding thus far has relied on theoretical studies and computer simulations,” she says.

The conference will focus on four themes, each related to a leading scientific question. These questions include understanding the main characteristics of the greater Agulhas system; the effects of changes in this system on regional weather, climate, bio-physical interactions, marine ecosystems and fisheries; and the influence of the Agulhas system on large-scale ocean circulation and climate. Dr Krug will participate in a session focusing on the past, present and future characteristics of the Agulhas current.

Working towards understanding the Agulhas Current
Dr Krug recently completed a doctoral thesis on the dynamics and variability of the Agulhas Current using multi-sensor satellite observations including close to 20 years of altimetry data, high frequency sea surface temperature imagery and observations from the Envisat satellite’s Advanced Synthetic Aperture Radar. Her findings provide new perspectives on the dynamics of the Natal Pulse (see Figure 1). Natal Pulses drive a lot of the variability in the Agulhas Current and have a profound impact on the coastal and shelf regions. Another major finding is her detection of an annual cycle in the Agulhas Current, with a stronger flow detected during summer (January to March). This is the first time that an annual cycle in the Agulhas Current has been shown using observations. A paper on this work is soon to be published in the prestigious journal Geophysical Research Letters.

This kind of work is important to monitor global and regional changes. It has an impact for the fishery and shipping industry, as well as the metocean industry which needs a good understanding of meteorological and oceanographic conditions to which offshore installations such as drifting buoys and other environmental platforms may be exposed.

The height of the Agulhas Current above average sea surface (sea surface height) and its thermal signature make it easily detectable from space using altimeters and temperature measuring sensors. In this picture, the Agulhas Current shows in shades of yellow and red (warm). The arrows on top show geostrophic currents derived using the sea surface height of the Agulhas Current – the longer the arrow, the stronger the current. In this instance, currents weaker than 30 cm/sec have not been plotted. The loop or meander in the current shows a well-developed Natal Pulse just north of Algoa Bay. The number of Natal Pulses, how fast they move, how big they are and how long they last have an impact on the fishing and shipping industry.

In his poster presentation, Christo Whittle will provide some insight into the physical dynamics driving primary productivity on the broad continental shelf south of Africa known as the Agulhas Bank. Whittle, who is also a physical oceanographer, expands, “The Agulhas Bank is a complex oceanic region, influenced by coastal upwelling processes typical of the Benguela Upwelling System and also shelf-edge dynamic upwelling caused by the Agulhas Current. Upwelling refers to the process whereby cold, nutrient rich waters rise from greater depths to the surface, essentially ‘fertilising’ surface waters. This stimulates the growth of phytoplankton, which in turn supports nursery areas and spawning grounds for an abundant variety of commercially exploitable fish species.”

The physical parameters associated with these upwelling events, i.e. the drop in sea surface temperature and increased chlorophyll-a concentration because of more phytoplankton, can be observed from space, and are used by scientists to describe and understand these systems.

Whittle combines his knowledge of oceanography with earth observation analytical methods to develop various indicators from satellite-derived data to investigate marine and freshwater ecosystems. “Our main objective is to establish an effective way of monitoring the health of these systems for sustainable resource exploitation,” he explains.

In pursuit of his PhD, he is investigating ten years of coincident sea surface temperature and chlorophyll-a concentration data from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard NASA's (National Aeronautics and Space Administration) flagship earth observation system, Aqua.  The data were used to identify different types of upwelling events, their forcing mechanisms and contribution to chlorophyll-a concentration on the Agulhas Bank. Whittle’s poster presentation for the Chapman conference is based on this research and is titled ‘Characterisation of Agulhas Bank upwelling variability from satellite-derived sea surface temperature and ocean colour products’ (see Figure 2).

The Agulhas Current is easily observed on this MODIS sea surface temperature satellite image for 27 October 2005 as a sinuous ribbon of warm (dark red, 22-25oC) water flowing along the shelf edge (500m isobath) of the South African East and South Coast. Lower sea surface temperatures (17-19oC) can be observed between the Agulhas Current and the South Coast on the eastern most part of the Agulhas Bank. In the insert representing co-incident chlorophyll-a concentration data from the same instrument, enhanced primary production (yellow to red), due to the influence of the Agulhas Current, is evident as a plume extending from the south coast towards the central Agulhas Bank.

In June this year, Whittle was invited to join the Group for High Resolution Sea Surface Temperature international science team tasked with providing global high resolution sea surface temperature products to the operational oceanographic, meteorological, climate and general scientific community. This presents a valuable opportunity for South African remote sensing specialists to contribute to quality sea surface temperature products and new experimental data.

Enquiries:
Dr Marjolaine Krug
MKrug@csir.co.za
Christo Whittle
CWhittle@csir.co.za