Long-term variation in the anticyclonic ocean circulation over the Zapiola Rise as observed by satellite altimetry: Evidence of possible collapses

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2009.03.005 Byline: Martin Saraceno (a)(b), Christine Provost (c), Uriel Zajaczkovski (b)(d) Keywords: Zapiola Rise; South Atlantic circulation; Interannual variability Abstract: The Zapiola Rise (ZR) is a singular sedimentary deposit about 1200m in height and 1500km in width located in the Argentine Basin. In situ and satellite observations have revealed the presence of an intense counterclockwise circulation around the feature, with a volume transport comparable to those of the major ocean currents. The existence of a very low-frequency variability of the transport associated with the anticyclonic circulation is documented for the first time. As the Zapiola anticyclonic circulation plays a significant role in the mixing of the strongly contrasted water masses of the South Atlantic, variations in the anticyclonic transport can have a major impact on the mixing, hence a role in global climate variability. The circulation was clearly anticyclonic in the periods 1993-1999 and 2002-2007. In contrast, the 1999-2001 period did not show evidence of an anticyclonic flow in the mean surface velocity field. Moreover, the analysis of the weekly fields during that period of time revealed a cyclonic pattern from time to time. Previous work has shown that the flow can be considered as purely barotropic over the ZR region. A 15-year time-series of the transport was produced using absolute altimeter-derived geostrophic velocities. The estimated transport presents high-frequency variability associated with mesoscale activity superimposed on a low-frequency signal. The amplitude of the estimated transport is in good agreement with the only in situ-derived estimation available (80Sv, January 1993). The low-frequency signal presents a minimum during the period 1999-2001, further suggesting that at times the Zapiola anticyclonic flow may have significantly decreased in strength or even vanished. Possible causes of the low-frequency variability are discussed. Author Affiliation: (a) Centro de Investigaciones del Mar y la Atmosfera (CIMA), Intendente Guiraldes 2160, Ciudad Universitaria, Pab. II, 2do piso, Ciudad de Buenos Aires C1428EGA, Argentina (b) Departamento de la Atmosfera y los Oceanos, FCEyN, Universidad de Buenos Aires, Intendente Guiraldes 2160, Ciudad Universitaria, Pab. II, 2do piso, Ciudad de Buenos Aires C1428EGA, Argentina (c) Laboratoire d'Oceanographie et du Climat: Experimentation et Approche Numerique (LOCEAN), Universite Pierre et Marie Curie, Tour 45/55, 5eme etage, BoA[R]te 100, 4 Place Jussieu, 75252 Paris cedex 05, France (d) Departamento de Oceanografia, Servicio de Hidrografia Naval, Av. Montes de Oca 2124, Ciudad de Buenos Aires C1270ABV, Argentina Article History: Received 8 October 2008; Revised 3 March 2009; Accepted 11 March 2009