Natal pulses and the formation of Agulhas rings

Large solitary meanders form on the Agulhas Current at irregular intervals as it travels along the east coast of South Africa. These so-called Natal pulses are thought to have a significant effect on the shedding of Agulhas rings downstream at the Agulhas Retrofiection and thereby on the exchange of water properties between the Indian and the Atlantic Ocean. Data from the Geosat, ERS 1, and TOPEX/Poseidon satellite altimeters and Advanced Very High Resolution Radiometer (AVHRR) infrared imagery from the Pathfinder project are analyzed and show that this intuitive idea seems to be correct. Close to the coast, individual altimeter tracks are used to identify the cyclonic Natal pulses as depressions in the sea-surface topography. Using different tracks, the pulses can then be followed from close to Durban to the Agulhas Bank. They show that each shedding of an Agulhas ring is preceded by the appearance of a Natal pulse close to Durban. A significant correlation is found with a time lag of 165 days. Interpolated topography maps are used to follow pulses along the Agulhas Bank to the ring-shedding area. They indicate that sometimes pulses trigger ring shedding by themselves or by merging with Rossby wave-like meanders in the Agulhas Return Current. Infrared imagery supports these interpretations.