Eddies and dipoles around South Madagascar: formation, pathways and large-scale impact

摘要

During a hydrographic cruise in March 2001, we encountered a dipole-like vortex structure directly southwest of Madagascar. The cruise formed part of the Dutch-South African Agulhas Current Sources Experiment (ACSEX). Direct current observations with a lowered ADCP showed that the multipole involved a deep-reaching central jet, with over 20 cm/s speeds still at 2000 m depth, with two contra-rotating eddies on either side. Both eddies had a size of about 250km. A subsequent analysis of the combined TOPEX-POSEIDON/ERS altimeter dataset revealed the regular formation of dipoles around south Madagascar, where the East Madagascar Current (EMC) separates from the shelf. Between April 1995 and June 2000, 16 such features could be identified, six of which were formed between August 1996 and May 1997 and six between May 1999 and June 2000. These periods of enhanced dipole formation coincided with the negative phases of the Indian Ocean Dipole and El Nino cycles, suggesting a connection between these climate modes and the interannual variability in the periods of dipole train formation. The temperature-salinity characteristics of the eddies from the ACSEX dipole were identical to those measured in the EMC during the same cruise. The cyclonic eddy appeared to have drawn its waters from the inshore side of the EMC, suggesting it was formed as a lee-eddy on the cyclonic flank of the separating jet. The data indicate that the eddies carry their water along. At the observed scales, one dipole per year would feed 8 Sv (1 Sv= 10~6m~3/s) of EMC-derived water into the Agulhas retroflection region. In the Mozambique Basin, the eddy pairs behaved quite irregularly, with many of them splitting and interacting with other eddies in the region. The anticyclones then propagated mostly westward, while the cyclone tracks diverged more between west and southwest and were influenced by the interaction with the Mozambique Plateau. A very regular train of dipoles started in December 1999 and continued through 2000. They were spawned roughly at 2-month intervals, stayed remarkably coherent and triggered an unusually early retroflection of the Agulhas Current in late 2000. Thus, interannual variability originating in the equatorial climate modes of the Indian Ocean seems to result in variability of the Agulhas retroflection and associated ring shedding.