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

SARACENO, M.; PROVOST, C.; ZAJACZKOVSKI, U.

DEEP-SEA RESEARCH. PART 1. OCEANOGRAPHIC RESEARCH PAPERS

http://dx.doi.org/10.1016/j.dsr.2009.03.005

Año: 2009 vol. 56 p. 1077 - 1092

0967-0637

The Zapiola Rise (ZR) is a singular sedimentary deposit about 1200 m in height and 1500 km 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 (80 Sv, 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.