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A guide to understanding the physical oceanographic processes of Algoa Bay

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SAEON’s Elwandle and Egagasini nodes developed a monitoring and research programme at the Algoa Bay Sentinel Site to study the long-term climatic and ocean changes likely to impact on the environment of the region.
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The cover page of the report on the dynamics of Algoa Bay.
- Dr Wayne Goschen, Marine Data Manager and Physical Oceanographer, SAEON Egagasini Node

 

A synopsis of the main results of physical oceanographic research undertaken in and around Algoa Bay up until 2010 has been published1. The report was authored by Dr Wayne Goschen (SAEON) and Dr Eckart Schumann (Nelson Mandela Metropolitan University Research Associate).

Sponsored by SAEON and the Institute for Maritime Technology (IMT) on behalf of the South African Navy, the report describes the coastal ocean off Algoa Bay, a large log-spiral shaped embayment on the southeast Cape coast and how the waters of the bay are located in the transition zone between the Agulhas Current-dominated region to the northeast and the wide Agulhas Bank shelf dynamics to the southwest2.

The report was primarily written for young marine biologists working on the SAEON Algoa Bay Sentinel Site who needed a brief overview of the physical oceanography of the bay.

Within the bay itself, interactions between nearshore, coastal and deep-water oceanographic processes, weather systems and local bathymetry and shoreline contours result in a dynamic ocean. The report was primarily written for young marine biologists working on the SAEON Algoa Bay Sentinel Site who needed a brief overview of the physical oceanography of the bay. The report also provides references to the original publications that the researchers should read and reference. These ocean processes are briefly introduced below.

Agulhas Current

The Agulhas Current flows southwestwards along the continental shelf edge of the east coast of South Africa (about 50 km offshore of Algoa Bay) and dominates large-scale ocean features in the region. Over the continental shelf off Algoa Bay (depth 150 m - 200 m), wind-driven ocean dynamics become more important.

Frequently, in this region, shear-edge eddies and plumes on the inshore edge of the Agulhas Current penetrate over the shelf and may be driven into Algoa Bay by southwesterly winds. Large solitary meanders in the Agulhas Current (“Natal Pulses”) are known to influence the dynamics of the shelf waters from time to time as they propagate southwestward past Algoa Bay. Evidence exists of entrainment of shelf water by the Agulhas Current, sometimes causing predominantly southwestward flows in the deeper reaches of Algoa Bay.

The Agulhas Current may also be the source of cold water entering Algoa Bay, termed current-driven upwelling. The dynamics of the Agulhas Current ensure that isotherms slope upwards at the continental shelf break, by a combination of Ekman veering in the bottom boundary layer and the widening of the Agulhas Bank to the west (which introduces dynamical processes of their own). The colder, deeper water can then be brought onto the shelf and advected towards the coast, which may prime Algoa Bay for wind-driven upwelling.

Water that is uplifted at the shelf break may upwell to the surface sporadically along the inshore edge of the Agulhas Current and at the coast where the shelf is narrow, which at times may be accentuated by local northeasterly winds. Such upwelling has been observed as far north as Port Edward, but commonly occurs south of Mbashe. Previous researchers have proposed the existence of an upwelling cell near Port Alfred, but it is probably just part of an increasing frequency of upwelling southwestward. Such cold water then also rounds Cape Padrone and penetrates into Algoa Bay.

Wind-driven upwelling

Another source of cold water in Algoa Bay is wind-driven upwelling. Along the south coast of South Africa upwelling occurs preferentially along the south side of prominent capes, driven by easterly-component winds, and then progresses westwards. It has been shown that cold, upwelled water (wind-driven) from Cape Padrone can be brought westwards into Algoa Bay during easterly component winds. It has also been found that upwelling off the south side of Cape Recife can be driven into Algoa Bay with the southwesterly winds, after the relaxation of the upwelling favourable northeasterly breezes. Because of the seasonal nature of the winds, such upwelling occurs predominantly in summer.

Within the shallower reaches of Algoa Bay, wind-driven, tidal, geostrophic and inertial currents play a large role in the water circulation, while in the nearshore zone wind and wave action are known to cause the northward transport of sediment. The nearshore is also the zone of large rip-currents. Warm water filaments originating in the upper layers of the Agulhas Current may even penetrate into the shallows of Algoa Bay, changing the currents and temperature structures in the nearshore zone.

Long-term changes

Evidence points to a warming of the Agulhas Current since the 1980’s; associated with this observation is that the coastal waters in and around Algoa Bay have been cooling. There is also evidence of a poleward shift of the westerly winds in the Southern Hemisphere, which may lead to more easterly component winds off the Eastern Cape, and hence more upwelling and colder water in Algoa Bay.

The long-term climatic and ocean changes are likely to impact on the environment of the region. These trends highlight the need to fully understand the weather and ocean processes influencing Algoa Bay before their influence on the environment and biosphere of Algoa Bay can be appreciated.

It was with this in mind that the SAEON Elwandle Node for Coastal and Inshore Ecosystems and the SAEON Egagasini Node for Offshore Marine Systems have developed the Algoa Bay Long-term Monitoring and Research programme at the Algoa Bay Sentinel Site.

1 A PDF copy of the report may be downloaded from the SAEON Data Portal or requested via email from the author at wayne_at_saeon.ac.za. Limited hard copies are available from the SAEON Elwandle (Grahamstown) and Egagasini nodes (Cape Town).

2 References are not used in this summary although the full report contains references.

Related content:

Tracking network project launched to help monitor our oceans

Building capacity for the WIO and Agulhas Current Observing System

 

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