Quarterdeck 2.1

Refining Our Knowledge of the Antarctic Circumpolar Current

by Alejandro Orsi

Alejandro Orsi's major research interests have always concerned Antarctic oceanography. In his Argentina home Alejandro completed his undergraduate studies in oceanography at the Instituto Technologico de Buenos Aires in 1985. Shortly after, while working for the Instituto Antartico Argentino, he joined the research group lead by Dr. Worth Nowlin and Dr. Tom Whitworth, who were carrying out the Mapping Abyssal Production and Courses of Outflow from the Weddell Sea (MAPCOWS) experiment in the South Atlantic. By the fall of 1987 Alejandro started his graduate studies at Texas A&M University. His master's thesis was a comprehensive description of the large-scale circulation known as the Weddell Gyre and its characteristic water masses. A variety of features were examined during this analysis of the historical hydrography, some of which sparked enough discussion to become the topics of his dissertation.

An important component of the Southern Ocean oceanography was addressed in Alex's doctoral work, the Antarctic Circumpolar Current (ACC). Pronounced surface gradients of temperature and salinity mark the northern limit of circumpolar waters, separating them from those of the subtropical regime. The location of this Subtropical Front (STF) has been known for many decades, yet there is no well-established poleward boundary to the ACC. Based on water property distributions and geostrophic flow considerations, Alex was able to differentiate this strong circumpolar current from the colder and weakly-sheared circulations found farther to the south. A definition for the southern boundary (Bdy) of the ACC is postulated in this study, and traced around Antarctica as the poleward edge of Upper Circumpolar Deep Water. With its meridional bounds clearly defined, the characteristic frontal structure of the ACC was studied next.

[26K] At Drake Passage, three deep-reaching fronts account for most of the ACC transport. The northern two, the Subantarctic Front (SAF) and Polar Front (PF), already have been traced unbroken around Antarctica by means of their particular indicators. The third deep front observed at Drake Passage also continues with similar characteristics as a circumpolar feature. In this study it is called the southern ACC front. Stations from multiple synoptic transects of the three circumpolar fronts are used to describe the average property structure within each ACC zone, and to infer the meridional mixing of waters along isopycnal surfaces. In addition, it is demonstrated that approximately 100 Sverdrups (1 Sverdrup=106 m3s-1) are carried continuously to the east by the ACC, between the STF and its poleward boundary.

This updated description of the ACC offers potential for new quantitative and theoretical studies of this important current, as well as its many interactions with the adjacent subtropical and subpolar regimes. It is expected that realistic ocean models will produce the observed features presented in this study. In the meantime, Alex will carry out his postdoctoral work at the Pacific Marine Environmental Laboratory in Seattle, Washington. [102K]

Editor's Note: Alejandro Orsi successfully defended his dissertation on October 29, 1993.

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Updated July 20, 1995