A deep-reaching anticyclonic eddy in the subtropical gyre of the eastern South Atlantic Academic Article uri icon

abstract

  • A CTD transect of the South Atlantic at a nominal latitude of 23-degrees-S was made in early 1983. About two-thirds of the way from the Brazil Current in the west to the Benguela Current in the east, we encountered a large-diameter eddy (300-400 km). Its dynamic signature (bowl-shaped isopyenals) penetrated to over 4000 m, with the eddy anticyclonic transport relative to 4000 db over 30 x 10(6) m3 s 1. Various origins for this eddy are conceivable. We feel that the most likely one is the Agulhas Retroflection region: that the eddy was formed by a pinching off of this current elbow, and a subsequent northwest movement of more than 2500 km over a period spanning two winters. There was a superposed double thermostad in the core of the eddy interpreted to the product of two successive winters’ convective overturning. Main thermocline waters from the Agulhas Current, recognizable by the distribution of oxygen, were trapped in the eddy core and carried with it. This trapping could extend as deep as the Antarctic Intermediate Water (AAIW). The propagating eddy distored the local property distributions. A ring of low-oxygen water from lower latitudes surrounded the core of the eddy at the thermocline levels. AAIW of higher salinity and lower oxygen, and Deep Water of higher salinity and lower nutrients were observed at the eddy stations. These characteristics are also found to the north of the eddy. The kinematics of a drifting anticyclonic eddy provides a framework for understanding the trapping, the low-oxygen ring, and the pulling underneath of the AAIW and Deep Water.

publication date

  • 1991