Dynamics and estimation of the Agulhas leakage

Abstract

The Agulhas Current is a powerful boundary current that flows southward on the eastern coast of South Africa. At the southern termination of the continental shelf, most of the current turns back to the Indian Ocean in the Agulhas Return Current, this is called the Agulhas retroflection. In this process some eddies are shed from the current and propagate westward in the Atlantic Ocean. The water that originates from the Indian Ocean and enters the South Atlantic Ocean is called the Agulhas leakage. It is believed that this flux of relatively warm and salty water plays an important role in the Atlantic Meridional Overturning Circulation. In this thesis the dynamics of the retroflection process is studied. A new regime of retroflection is found for highly turbulent flows. In this regime the volume of leakage reaches a maximum and stays unchanged while both the wind stress curl and the Agulhas Current transport increase. This is found to be due to an increase of the interactions with the northern branch of the Antarctic Circumpolar Current. The importance of the Indonesian Throughflow, the equatorial flux of water from the Pacific Ocean to the Indian Ocean, is also explored using numerical models. It is shown that it leads to strengthen the leakage but does not change the proportion of retroflection. On the way from the Indonesian Through flow to the Agulhas Current an important passage is the South East Madagascar Current. Using dynamic topography data from satellite altimetry, dipolar structures are found to form continuously at the southern tip of Madagascar. The perturbations induced by these structures contributed to the two early retroflections of the Agulhas Current observed over the last 20 years. Finally, a method is developed and validated to measure the Agulhas leakage volume transport from satellite altimetry data.