Abstract:

Role of the Durban Eddy on South Africa’s east coast oceanography

Lisa Guastella, Mike Roberts

Oceanography Department, University of Cape Town, Private Bag, Rondebosch, 7700

Bayworld Centre for Research and Education, 5 Riesling Rd, Constantia, 7806

The Durban Eddy is a lee-trapped, semi-permanent, cold core cyclonic eddy circulation that occurs off the east coast of South Africa between Durban in the north and approximately Sezela in the south. The eddy is a mesoscale feature, extending approximately 60-90 km alongshore and 30-50 km offshore and is driven mainly by the strong Agulhas Current moving offshore of the regressing shelf edge near Durban. Acoustic Doppler Current Profilers (ADCP’s) deployed offshore of Durban during the ACEPII KZN Bight research programme from March 2009 to September 2010 indicate consistently strong south-westward flow associated with the Agulhas Current 40 km offshore of Durban, while the inshore regions are characterised by frequent current reversals associated with transient eddies. Analysis of the ADCP data together with satellite imagery shows that the eddy is present offshore of Durban approximately 55% of the time, with an average lifespan of 8.6 days, and “inter-eddy” periods of 4-7 days. A thermistor string deployed in the eddy centre and ship CTD (conductivity, temperature, depth) data indicate upward doming of the thermal structure in the centre of the eddy associated with cooler water and nutrients being advected higher up in the water column, which stimulates primary production. Satellite-tracked surface drifters released in the eddy have shown that the nutrient-rich eddy water can be transported northwards along the inshore regions of the KwaZulu-Natal (KZN) Bight, thus contributing to the KZN Bight ecosystem, and southwards along the South African east coast, both by the eddy migrating downstream and by eddy water getting absorbed into the Agulhas Current itself. The Durban Eddy thus acts as a source of nutrients, feeding the marine ecosystems of both the KZN Bight to the north and the KZN and Eastern Cape coast to the south.

Presentation Topic

Role of the Durban Eddy on South Africa’s east coast oceanography

 

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