Shedding of an Agulhas ring observed at sea

Abstract

The formation of current rings by the Gulf Stream through the pinching-oif of elongated meanders1–5is the dominant mechanism by which mass, momentum, vorticity, chemical substances and biota are transported across it4. Gulf Stream rings have an important effect on the ocean regions where they are found6,7, profoundly influencing the energy and biotic distributions and being responsible for significant ventilation of the thermocline8. The formation of such rings has also been observed in other western boundary currents such as the Kuroshio9 and the East Australian current10. The spawning of rings at the Agulhas current retroflection has thus far only been inferred from thermal infrared images obtained by satellite11 as there has been no contemporaneous confirmatory evidence at depths greater than the surface layer. The production of Agulhas rings is of particular importance as it may be one of the primary mechanisms in the exchange of thermocline water between the Indian and Atlantic oceans12,13, a significant climatic factor. We report here the occlusion of an Agulhas ring based on the first extensive set of measurements taken at sea during such an event.