Intrusions of sub-Antarctic surface water across the subtropical convergence southwest of Africa

The terminal region of the Agulhas Current south of Africa is characterized by the complete retroflection of the Current. The region has been shown to be populated by a range of eddies and rings. It has been observed that the spawning of an Agulhas Current ring at the retroflection is preceded by the northward wedging of the Subtropical Convergence (STC) through the retroflection loop to effectively "pinch" off these rings. The resultant entrainment of cold Sub-Antarctic Water Surface (SAASW) behind the displaced STC is of climatic and oceanographic interest in light of the concurrent interruption and eastward retreat of the warm Agulhas Current. The leakage of Agulhas Current water into the South-eastern Atlantic Ocean in the form of filaments may also be temporarily terminated during these SAASW intrusion episodes. In order to investigate intrusions of SAASW into the Agulhas Retroflection region, all available data of any kind have been accessed and analyzed. A serial satellite study, using both METEOSAT and NOAA images, suggests that approximately four intrusions of SAASW are observed per year. These intrusions generally occur between 11° E and 19° E; the westerly intrusions being more prevalent during extreme episodes of SAASW intrusions. The mean temperature and salinity distribution at the retroflection shows that the longitudinal location of SAASW intrusions seems to be geographically invariant suggesting a possible topographic influence by prominent geographical features. Hydrographic analysis of sub-Antarctic water intrusions leads one to believe that they are not just shallow, short-term phenomena but may reach to depths of approximately 800 m to 1000 m, persisting for about 28 days on average. These intrusions introduce low temperature, low salinity (< 35) water into the retroflection region with an average areal geographical coverage of 158 000 ± 118 256 km2. This suggests that these sub-Antarctic water intrusions may have important oceanographic and biologic implications to the dynamics of the Agulhas Retroflection and the oceanic region to the west of it in light of the sheer magnitude of the amount of water involved.

Grantor

Department of Oceanography

Year dc:date.issued

2003

Author dc:creator

• Fillis, C S

Advisor dc:contributor.advisor

• Lutjeharms, J R E

• Oceanography

Handle dc:identifier.uri

http://hdl.handle.net/11427/38367

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/38367