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

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

Nice, Palais des Congrès Acropolis, France.

Congreso; Observing and Forecasting the Ocean, OSTST Meeting; 2008

<!-- /* Font Definitions */ @font-face {font-family:Times; panose-1:2 0 5 0 0 0 0 0 0 0; mso-font-charset:0; mso-generic-font-family:auto; mso-font-pitch:variable; mso-font-signature:3 0 0 0 1 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-ascii-font-family:Times; mso-fareast-font-family:Times; mso-hansi-font-family:Times; mso-bidi-font-family:"Times New Roman"; mso-fareast-language:FR;} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.25in 1.0in 1.25in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> 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. A 15-year time-series of the transport was produced using absolute altimeter-derived geostrophic velocities and considering a purely barotropic flow. 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.